ALBERT

Description: Observations of wave and sediment processes collected at two locations on the Atchafalaya inner shelf show that wave dissipation in shallow, muddy environments is strongly coupled to bed-sediment reworking by waves. During an energetic wave event (2 m significant wave height in 5 m water depth), acoustic backscatter records suggest that sediment in the surficial bed layer evolves from consolidated mud through liquefaction, fluid mud formation, and hindered settling to gelled, under-consolidated mud. Net swell dissipation increases steadily during the storm from negligible prestorm values, consistent with bed softening, but shows no correlation with detectable fluid mud layers. Remarkably, the maximum dissipation rate occurs poststorm, when no fluid mud layers are present. In the waning stage of the storm, the contribution of different wave-forcing processes to wave dissipation is analyzed using an inverse modeling approach based on a nonlinear three-wave interaction model. Although wave-mud interaction dominates dissipative processes, nonlinear three-wave interactions control the shape of the frequency distribution of the dissipation rate. In the wake of the storm, the viscosity values predicted by the inverse modeling converge toward measured values characteristic for gelled mud in a trend that is consistent with a fluid mud entering dewatering and consolidation stages.

Description: Causes of the coupled model bias in simulating the zonal sea surface temperature (SST) gradient in the equatorial Atlantic are examined in three versions of the same coupled general circulation model (CGCM) differing only in the cumulus convection scheme. One version of the CGCM successfully simulates the mean zonal SST gradient of the equatorial Atlantic, in contrast to the failure of the Coupled Model Intercomparison Project phase 3 models. The present analysis shows that key factors to be successful are high skills in simulating the meridional location of the Intertropical Convergence Zone, the precipitation over northern South America, and the southerly winds along the west coast of Africa associated with the West African monsoon in boreal spring. Model biases in the Pacific contribute to the weaker precipitation over northern South America. Uncoupled experiments with the atmospheric component further confirm the importance of remote influences on the development of the equatorial Atlantic bias.

Description: Oblique nonlinear interactions based on the Kadomtsev-Petviashvili equation (also known as the two-dimensional Korteweg–de Vries equation) are extended to internal solitary waves (ISWs) to explain why the amplitude does not decrease owing to the geometric spreading of the cylindrical wavefronts in the South China Sea (SCS). This resonance theory is used to explain a satellite image exhibiting special features, and it is proposed that wave arcs of different amplitude resonate, providing a mechanism for reinforcing a wave by boosting the amplitude. The present theory suggests the amplitude of the ISW that propagates across the SCS basin depends on the interaction of ISWs originating from different sources; hence, studying the generation of an ISW from a single source location cannot predict the ISW correctly.

Description: On western margins of ocean basins, such as the eastern continental shelf of the United States, rates of biological productivity are higher than in the open ocean, in spite of the mean downwelling circulation. We use a nonhydrostatic, three-dimensional, process study ocean model with idealized shelf-slope geometry, wind forcing, and tracers to explore the interplay between the circulation and the biogeochemistry of the shelf and slope; the pathways that can transport nutrients from the deep ocean and from the sediments to the surface ocean euphotic zone. Cross-shelf exchange between the open and coastal ocean is regulated by a shelf break front that separates light waters on the shelf from denser waters on the slope. The wind direction and strength influence both the position and slope of the isopycnals at the front, which become more vertical in response to northerly winds and flatten in response to southerly winds. When the wind direction oscillates between northerly and southerly, it pumps nutrient and gas-rich bottom boundary layer water up to the sea surface. Nutrients tend to accumulate in the benthic boundary layer during southerly winds and are pumped to the surface during periods of northerly winds. Stratification of the water column in summertime reduces the shelf break pump by dampening the effect of the winds on the movement of the front. When extrapolated over the northeast coast of the United States, the nutrients supplied by the shelf break pump from the open ocean to the coastal ocean are three times the estimated nitrogen delivered to the shelf from estuaries.

Description: Ocean acidification resulting from increases in present and future atmospheric CO2 levels could seriously affect diverse coastal and oceanic ecosystems. In this work, we determine that a significant trend in ocean acidification is superposed on the large seasonal and interannual variabilities of acidity in surface waters off the south coast of Honshu, Japan, based on our repeated observations of partial pressure of CO2 (pCO2), total inorganic carbon (TCO2), and pH. Multiple regression analysis of TCO2 as a function of temperature, salinity, and timing of observations shows that TCO2 increased at a rate of +1.23 ± 0.40 μmol kg−1 yr−1 for the period 1994–2008, while no long-term change has been determined for total alkalinity calculated from TCO2 and pCO2 in seawater. These results indicate that pH and the aragonite saturation state (Ωarag) are decreasing at a rate of −0.020 ± 0.007 decade−1 and −0.12 ± 0.05 decade−1, respectively. If future atmospheric CO2 levels keep increasing as predicted by the Intergovernmental Panel on Climate Change emission scenario A1FI, which postulates intensive fossil fuel use associated with very rapid economic growth, a further reduction of −0.8 to −1.0 in Ωarag is likely in the next 50 years. Such a rapid reduction of Ωarag could have negative impacts on a variety of calcareous organisms.

Description: A barotropic tidal model, with a parameterization term to account for the internal wave drag energy dissipation, is used to examine areas of possible M2 internal tide generation in the Kerguelen Plateau region. Barotropic energy flux and a distribution of wave drag dissipation are computed. The results suggest important conversion of barotropic energy into baroclinic tide generation over the northern Kerguelen Plateau shelf break, consistent with a theoretical criterion based on ocean stratification, tidal forcing frequency, and bathymetric gradients. The sea surface height signatures of time-coherent internal tides are studied using TOPEX/Poseidon and Jason-1 altimeter data, whose ascending tracks cross nearly perpendicular to the eastern and western Kerguelen Plateau shelf break. Oscillations of a few centimeters associated with phase-locked internal tides propagate away from the plateau over distances of several hundred kilometers with a ∼110 km wavelength. When reaching the frontal area of the Antarctic Circumpolar Current, the internal tide cannot be identified because of the aliasing of mesoscale variability into the same alias band as M2. Finally, using altimeter data, we estimate the M2 barotropic tidal power converted through the internal tide generation process. We find consistent values with the barotropic model parameterization estimation, which is also in good agreement with global internal tide model estimates. Combined with modeling, this study has shown that altimetry can be used to estimate internal tide dissipation.

Description: Variability in sea level at the longest periods observable in modern records has recently been found to be well correlated with local atmospheric pressure. At shorter periods, however, longshore winds are known to be one of the primary mechanisms for forcing sea level variability on the eastern margins of the ocean. There is a remarkable ∼80 mm drop in mean sea level on both the eastern North Pacific and North Atlantic coasts between the late 1800s and early 1900s; it is found here to be in agreement with longshore wind forcing from the equator up to the latitude of the observed tide stations. Better-resolved data beginning in 1960 show that the delay near the annual period between wind forcing and sea level is approximately 1 month. The relative high in sea level in the late 1800s on the west coast of Europe appears to have propagated westward across the Atlantic as a long Rossby wave and then to have been seen on the east coast of the United States. Because many features in long-term sea level variability are correlated with wind forcing on time scales from annual to decades, it will be prudent to base conclusions about long-term sea level rise on the longest records available. The results here are based on forcing by longshore winds; the related issue of the extent to which open ocean wind curl may also be responsible is not addressed here, nor the extent to which coastal sea level observations are representative of the open ocean.

Description: Analysis of long time series of current meter data from a mooring at 77°E and the equator during 2003–2007, along with mean sea level anomaly data, throws light on the occurrence of the lower-frequency (24 to 40 day) Yanai waves in the upper water column of the central equatorial Indian Ocean (EIO) during the positive Indian Ocean dipole (IOD) years of 2003, 2004, 2006, and 2007 and its absence during the negative IOD year 2005. This result is in contrast with the earlier studies that observed only the higher-frequency (biweekly period) Yanai wave in this region. We propose a new notion for the generation of the lower-frequency Yanai wave in the upper central EIO owing to the positive IOD phenomenon. The strong meridional current shear created by the northward shifting and strengthening of the westward flowing south equatorial current associated with positive IOD and the eastward flowing southwest monsoon current provides energy for the generation of lower-frequency Yanai waves. Vertical stratification of the water column appears to be responsible for the trapping of the different frequency of Yanai waves, with only the higher-frequency Yanai wave in the region of lower pycnocline. During positive IOD the strongly stratified upper water column responds to the lower-frequency Yanai wave, while the deeper ocean (4000 m) exhibited a longer-period (47 day) oscillation. The expected surface signature of Madden-Julian oscillation seems to be suppressed by strong easterlies during the positive IOD years.

Description: A numerical model of saline density currents across a triple-bend sinuous submerged channel enclosed by vertical sidewalls is developed. The unsteady, non-Boussinesq, turbulent form of the Reynolds Averaged Navier-Stokes equations is employed to study the flow structure in a quasi-steady state. Recursive tests are performed with axial slopes of 0.08°, 0.43°, 1.5°, and 2.5°. For each numerical experiment, the downstream and vertical components of the fluid velocity, density, and turbulent kinetic energy are presented at four distinct locations within the channel cross section. It is observed that a crucial change in the flow pattern at the channel bends is observed as the axial slope is increased. At low values of the axial slope a typical river-like pattern is found. At an inclination of 1.5°a transition starts to occur. When the numerical test is repeated with an axial slope of 2.5°, a clearly visible river-reversed secondary circulation is achieved. The change in the cross-sectional flow pattern appears to be associated with the spatial displacement of the core of the maximum downstream fluid velocity. Therefore, the axial slope in this series of experiments is linked to the velocity structure of the currents, with the height of the velocity maximum decreasing as a function of increasing slope. As such, the axial slope should be regarded also as a surrogate for flows with enhanced density or sediment stratification and higher Froude numbers. The work unifies the apparently paradoxical experimental and numerical results on secondary circulation in submarine channels.

Description: The recent detection of a central Pacific type of El Niño has added a new dimension to the El Niño-Southern Oscillation climatic puzzle. Sea surface salinity (SSS) observations collected during 1977–2008 in the tropical Pacific are used to contrast the three eastern Pacific (EP) (1982–1983, 1991–1992, 1997–1998) and seven central Pacific (CP) (1977–1978, 1986–1988, 1990–1991, 1992–1995, 2002–2003, 2004–2005, 2006–2007) types of El Niño events, as well as the six EP (1985–1986, 1988–1989, 1995–1996, 1999–2001, 2005–2006, 2007–2008) and two CP (1983–1984, 1998–1999) types of La Niña events. The EP El Niño events result in large (∼30° longitude) eastward displacements of the eastern edge of the low-salinity warm pool waters in the equatorial band, a resulting well-marked SSS freshening (∼−1) near the dateline, and a SSS increase (∼+1) below the mean position of the South Pacific Convergence Zone (SPCZ). The CP El Niño events are characterized by smaller (50%) eastward displacements of the eastern edge, a ∼15° longitude westward shift of the equatorial SSS freshening, and a comparatively reduced (∼50%) SSS increase in the SPCZ. A qualitative analysis indicates that changes in zonal currents and precipitation can account for the observed contrasted signature in SSS. Eastward current anomalies appear over most of the equatorial band during EP El Niño events. In contrast, there is a tendency for zonal current convergence slightly west of the dateline during CP El Niño events, consistent with the confinement of the warm/fresh pool in the western central equatorial basin, the related quasi-inexistent northeastward migration of the SPCZ, and associated heavy precipitation regime.

Description: We investigate the coupling between the dynamics in the Gulf of Aqaba (Gulf of Eilat, northern Red Sea) and the exchange flow through the Straits of Tiran in response to seasonally varying surface fluxes and northern Red Sea hydrographic conditions. Because the gulf is a relatively small basin, winter mixing between the surface and intermediate layers occurs over most places in the gulf including in the vicinity of the straits, and leads to a maximal exchange flow in the strait. During the spring, warming in the Red Sea forces an influx of warm Red Sea water into the gulf, the surface layer is refilled, and consequently the exchange flow in the straits changes from a maximal to a submaximal flow regime. As a result, the dense wintertime water formation in the gulf and the exchange flow through the strait are strongly coupled on seasonal time scales. In addition, the hydrographic conditions in the northern Red Sea undergo strong seasonality. These characteristics prevent the direct application of current theories for strait-marginal sea systems, which commonly assume steady conditions in the “open ocean” side of the strait and/or an annual mean surface flux over the marginal sea. We explain why the exchange of volume and heat between the Gulf of Aqaba (Gulf of Eilat) and the northern Red Sea is larger during spring-summer despite the net surface buoyancy input into the gulf and why it diminishes during fall-winter despite the large buoyancy loss to the atmosphere. The applicability of the results to other systems is discussed.

Description: Ocean tides under the large Weddell Sea ice shelves are among the least well observed on Earth. Here we present new, spatially extensive observations of the vertical tidal motion of the Filchner-Ronne and Larsen C ice shelves using Global Positioning System (GPS) data spanning a few weeks to years. We pay particular attention to the major tidal constituents (M2, S2, O1, K1) as well as important GRACE aliasing periods (K2 and S1). We compare the estimated constituents with recent global and regional tide models and find that no single model is the most accurate across all constituents or ice shelves. The root-sum-square errors are 7–8 cm (CATS2008a and TPXO7.2) and 11–12 cm (GOT4.7 and FES2004) with the energetic M2 (RMSE = 4–8 cm) and S2 (4–5 cm) generally dominating these statistics. The FES2004 K1 is particularly inaccurate near the Larsen C Ice Shelf, with errors approaching 20 cm, meaning that GRACE Release 4 estimates of mass change in the northern Antarctic Peninsula will be biased. We find tidal energy at 3, 4, 5, 6 and, weakly, at 7 cycles per day at all of our sites. The largest amplitudes within these bands are at M4, MO3 and SP3 and approach 30 mm, although significant spatial variations exist. We show that they generally do not appear to originate in areas of reduced water column in ice shelf grounding zones. Comparing model estimates with our M4, MS4 and MN4 values shows that models do not accurately represent these terms.

Description: Optical remote sensing is used to measure flow patterns in the swash zone. Timestack images are analyzed to measure the asymmetry and the relative duration of the inflow into the swash zone. This varies significantly between individual swashes, contrary to the classical analytical swash model for runup induced by bores, which predicts a similar flow pattern for all events. For swash forced by breaking bores, the gradient of the x-t locus of flow reversal varies over a wide range and flow reversal can occur simultaneously across the whole swash zone. This variation of the gradient of the locus of flow reversal in x-t space can be parameterized in terms of a single free variable in recent solutions to the nonlinear shallow water equations, which fully defines the swash boundary inflow condition. Consistent with the theory, the horizontal runup, the swash period, and the swash similarity parameter were observed to be independent of the swash inflow conditions but the flow asymmetry is not. Only a weak correlation was observed between the swash boundary condition and the Iribarren number and beach slope. Conversely, the analysis suggests that the degree of swash-swash interaction does influence the swash boundary condition and the resulting internal flow kinematics. The variation in inflow conditions is expected to influence the magnitudes of the velocity moments within the swash zone and therefore sediment transport rates.

Description: The global ocean biogeochemical models that are used in order to assess the ocean role in the global carbon cycle and estimate the impact of the climate change on marine ecosystems are getting more and more sophisticated. They now often account for several phytoplankton functional types that play particular roles in marine food webs and the ocean carbon cycle. These phytoplankton functional types have specific physiological characteristics, which are usually poorly known and therefore add uncertainties to model results. Indeed, this evolution in model complexity is not accompanied by a similar increase in the number and diversity of in situ data sets necessary for model calibration and evaluation. Thus, it is of primary importance to develop new methods to improve model performance using existing biogeochemical data sets, despite their current limitations. In this paper, we have optimized 45 physiological parameters of the PISCES global model, using a variational optimal control method. In order to bypass a global 3-D ocean variational assimilation, which would require enormous computation and memory storage, we have simplified the estimation procedure by assimilating monthly climatological in situ observations at five contrasted oceanographic stations of the JGOFS program in a 1-D version of the PISCES model. We began by estimating the weight matrix in the cost function by using heuristic considerations. Then we used this matrix to estimate the 45 parameters of the 1-D version of the PISCES model by assimilating the different monthly profiles (observed profiles at the five stations) in the same variational procedure on a time window of 1 year. This set of optimized parameters was then used in the standard 3-D global PISCES version to perform a 500 year global simulation. The results of both the standard and the optimized versions of the model were compared to satellite-derived chlorophyll-a images, which are an independent and global data set, showing that our approach leads to significant improvements in simulated surface chlorophyll-a in most of the regions of the world ocean. Besides demonstrating that we have improved the accuracy of the PISCES model, this study proposes a sound methodology that could be used to efficiently account for in situ data in biogeochemical ocean models.

Description: Velocity estimation from an image sequence is one of the most challenging inverse problems in computer vision, geosciences, and remote sensing applications. In this paper a nonlinear model has been created for estimating motion field under the constraint of conservation of intensity. A linear differential form of heat or optical flow equation is replaced by a nonlinear temporal integral form of the intensity conservation constraint equation. Iterative equations with Gauss-Newton and Levenberg-Marguardt algorithms are formulated based on the nonlinear equations, velocity field modeling, and a nonlinear least squares model. An algorithm with progressive relaxation of the overconstraint to improve the performance of the velocity estimation is also proposed. The new estimator is benchmarked using a numerical simulation model. Both angular and magnitude error measurements based on the synthetic surface heat flow from the numerical model demonstrate that the performance of the new approach with the nonlinear model is much better than the results of using a linear model of heat or optical flow equation. Four sequences of NOAA Advanced Very High Resolution Radiometer (AVHRR) images taken in the New York Bight fields is also used to demonstrate the performance of the nonlinear inverse model, and the estimated velocity fields are compared with those measured with the Coastal Ocean Dynamics Radar array. The experimental results indicate that the nonlinear inverse model provides significant improvement over the linear inverse model for real AVHRR data sets.

Description: Periods of high astronomically generated tides contribute to the occurrence of extreme sea levels. Over interannual time scales, two precessions associated with the orbit of the Moon cause systematic variation of high tides. A global assessment of when these tidal modulations occur allows for the prediction of periods when the enhanced risk of coastal flooding is likely in different parts of the world. This paper uses modeled tides to assess the influence of the 18.61 year lunar nodal cycle and the 8.85 year cycle of lunar perigee (which affects high tidal levels as a quasi 4.4 year cycle) on high tidal levels on a global scale. Tidal constituents from the TPXO7.2 global tidal model are used, with satellite modulation corrections based on equilibrium tide expectations, to predict multidecadal hourly time series of tides on a one-quarter degree global grid. These time series are used to determine the amplitude and phase of tidal modulations using harmonic analysis fitted to 18.61, 9.305, 8.85, and 4.425 year sinusoidal signals. The spatial variations in the range and phase of the tidal modulations are related to the global distribution of the main tidal constituents and tidal characteristics (diurnal or semidiurnal and tidal range). Results indicate that the 18.61 year nodal cycle has the greatest influence in diurnal regions with tidal ranges of 〉4 m and that the 4.4 year cycle is largest in semidiurnal regions where the tidal range is 〉6 m. The phase of the interannual tidal modulations is shown to relate to the form of the tide.

Description: This study examines the structure and dynamics of wind-forced intraseasonal zonal current variability in the equatorial Indian Ocean. We take advantage of a variety of satellite and in situ data sets, including unprecedented 4–8 year-long velocity time series records from the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) program. Spectral analysis reveals prominent intraseasonal zonal currents variations along the equator with periods of 30–70 days. These oscillations are vertically in phase above the thermocline and propagate eastward with the local zonal winds. In the thermocline, intraseasonal zonal velocity variations also propagate eastward across a broad range of phase speeds expected for low baroclinic equatorial Kelvin waves; amplitudes decrease with depth, with deeper levels leading those near surface. Collectively, these results suggest that the near-surface layer responds directly to intraseasonal zonal wind stress forcing and that subsequently energy radiates downward and eastward in the thermocline in the form of wind-forced equatorial Kelvin waves. In addition, intraseasonal zonal current variability on the equator is coherent with off-equatorial sea surface height fluctuations in the eastern and central of the basin. This coherence is primarily due to the fact that equatorial zonal wind variations are associated with off-equatorial wind stress curls that can generate local Ekman pumping and westward propagating Rossby waves.

Description: Eight autonomous profiling floats equipped with miniaturized radiometers and fluorimeters have collected data in Pacific, Atlantic, and Mediterranean offshore zones. They measured in particular 0–400 m vertical profiles of the downward irradiance at three wavelengths (412, 490, and 555 nm) and of the chlorophyll a fluorescence. Such autonomous sensors collect radiometric data regardless of sky conditions and collect essentially uncalibrated fluorescence data. Usual processing and calibration techniques are no longer usable in such remote conditions and have to be adapted. The proposition here is an interwoven processing by which missing parts of irradiance profiles (due to intermittent cloud occurrence) are interpolated by accounting for possible changes in optical properties (detected by the fluorescence signal) and by which the attenuation coefficient for downward irradiance, used as proxy for [Chl a] (the chlorophyll a concentration), allows the fluorescence signal to be calibrated in absolute units (mg m−3). This method is successfully applied to about 600 irradiance and fluorescence profiles. Validation of the results in terms of [Chl a] is made by matchup with satellite (MODIS-A) chlorophyll (24.3% RMSE, N = 358). Validation of the method is obtained by applying it on similar field data acquired from ships, which, in addition to irradiance and fluorescence profiles, include the [Chl a] HPLC determination, used for final verification.

Description: The statistics of the temperature and its spatial derivatives at a wind-driven air-water interface were obtained from a comprehensive data set of high resolution infrared imagery for wind speeds ranging from 2 ms−1 to 10 ms−1. We focus our effort on considerations of the anisotropy, symmetry, and intermittency of the surface turbulence. The analysis reveals that the root-mean-square surface temperature, when made nondimensional by using the surface heat flux and friction velocity, is nearly independent of Richardson number (Ri, defined in section 2). In addition, the derivatives of the thermal field appear also to converge to a limiting value at low Ri. The skewness of the temperature field, though slightly positive for the lowest wind speed (2 ms−1), is otherwise negative. On the other hand, the skewness of the derivative of the temperature field in the along-wind direction is strictly positive, while the skewness in the cross-wind direction is essentially zero, owing to the spanwise symmetry of the flow. This has the consequence that wind direction can be estimated by computing the skewness of the directional derivative of the temperature field. The flatness of the temperature field is observed to be near the Gaussian value of 3 throughout the wind speed range, while the along-wind and cross-wind derivatives show non-Gaussian behavior, indicating the presence of intermittency in the thermal fields at small scales. All probability density functions of the temperature derivatives are seen to have Gaussian cores, with distinct exponential tails.

Description: Above sand waves on the seafloor, surface short waves, which are responsible for the radiance distribution in remote sensing imagery, are modulated gradually by the submarine topography. The relaxation rate μr characterizes the rate at which the short waves reach their saturation range after being disturbed. It is a key parameter in the weak hydrodynamic interaction theory and is also a most important parameter in the imaging mechanism used for mapping submarine bottom topography. In this study, a robust expression containing intensity and phase (advection effect) modulations of the perturbed action spectrum of short waves was deduced, by using the first-order weak hydrodynamic interaction theory. On the basis of the phase modulation, a method was developed to determine the relaxation rate in the Sun glitter imaging mechanism. The relaxation rates were estimated using in situ data measured on a cruise over the sand waves of the Taiwan Banks, a sea area between the East China Sea and the South China Sea, on 28–29 August 2006. Results showed that, under a wind speed of 5.0 m s−1, the relaxation rate of short waves was about 0.055 s−1 in response to current variations and about 0.025 s−1 equivalently in response to sea bottom topographic variations. The former value could be applied to interpret the amplitude of submarine topography by using satellite imagery, while the latter one (equivalent relaxation rate μ′r) could help to more accurately calibrate the spatial position of the retrieved sea bottom topography.

Description: An idealized mathematical model of tsunami evolution in deep sea and across the continental shelf is proposed. The initial value problem in deep sea is related to the well-known Cauchy- Poisson problem and the tsunami propagation across the continental shelf is derived using the linearized shallow water equations. When analyzing different cases of tsunamis in deep sea, it was found that tsunamis evolve into two basic wave types. One resembles a single wave and the other a wave packet. The analysis of different cases of tsunamis at the shoreline reveals that the continental shelf, due to its geometrical properties, serves as a tsunami amplifier, producing tsunami amplitudes up to 20 times larger than those at the edge of the continental shelf. A comparison with tsunami measurements suggests that the idealized model may be used for a reliable assessment of the principle hydrodynamic properties of the tsunami, such as the tsunami amplitude and its half period.

Description: The dynamic structure of an ocean eddy in the eddy-abundant South China Sea has rarely been captured by measurements and has seldom been discussed in the literature. In the present study, in situ current and hydrographic measurements from a weeklong cruise and concurrent satellite altimeter observations were utilized to examine the three-dimensional structure and physical properties of a cold eddy in the southwestern South China Sea. The underlying forcing mechanism for the formation of this cyclonic cold eddy was found to be tightly associated with the recirculation in a coastal baroclinic jet that had separated off the Vietnamese coast. The eddy was significantly influenced by a coexisting, anticyclonic warm eddy in the separated jet. With relatively steady intensity and radius, the cold eddy endured for two weeks after its swift formation in late August and prior to its quick dissipation in mid-September. This cold eddy was horizontally and vertically heterogeneous. Asymmetric currents with much stronger magnitude were found on its southeastern flank, next to the warm eddy, where a front in the pycnocline was responsible for the sharp decrease in the cold eddy's intensity in the water below. The distributions of temperature, vorticity, and vertical velocity in the cold eddy were spatially asymmetric and not overlapping. The intensity of the cold eddy gradually decreased with the depth and the eddy extended downward for more than 250 m with a vertically tilted central axis. The upward velocities around the center of the eddy and the downward velocities to the southwest and to the east of the center jointly formed the upward domes of isotherms and isohalines in the central part of the cold eddy.

Description: Using the Regional Ocean Model System, the ocean circulation on the East China Sea (ECS) shelf was examined by a fine-resolution model which was nested in a coarse-resolution Pacific Ocean model. The high-resolution simulation shows an accurate volume transport of 2.70 Sv (Sv ≡ 106 m3s−1) through the Tsushima Strait, which is more consistent with the previous 5.5 year observation value (2.64 Sv) than former model results. For the Taiwan Strait it also shows a close volume transport (1.03 Sv) to a recent estimate (1.20 Sv). At the same time the model results reproduced almost all of the known circulation structure on the ECS shelf. In addition, the hindcast of 2009 shows a Kuroshio Bottom Branch Current to the northeast of Taiwan (KBBCNT). The KBBCNT is confirmed by the observational bottom high-salinity water (from 15 August to 2 September 2009) whose distribution is also reproduced by the model results. Tracer and particle experiments were carried out to elucidate the formation of the high-salinity water and the pathway of the KBBCNT. In light of the field observation and numerical experiments, a new pathway of the KBBCNT is proposed: bifurcated from the subsurface water of Kuroshio northeast of Taiwan, it upwells northwestward gradually from 300 to 60 m, then turns to northeast in the region around 27.5°N, 122°E, and finally reaches 31°N off the mouth of the Changjiang River along ∼60 m isobaths, forming the bottom saline water off the coast of Zhejiang province, China.

Description: A strong upwelling off the continental shelf of the northern South China Sea (SCS) in 1998 summer is reinvestigated using a suite of new satellite measurements and numerical modeling. Previous studies indicate that the upwelling in the western SCS, especially off the Vietnam coast, almost disappears during 1998 summer following the El Niño because of the weakened southwest monsoon. This study identifies that the coastal upwelling in the adjacent northern SCS (NSCS) is significantly strengthened during 1998 summer, and the alongshore wind stress is dramatically enhanced over the region. As a result, the offshore Ekman transport in 1998 summer is the strongest, almost twice the average of the other 17 years during 1997–2007. The Chl a concentrations in the representative upwelling regions are much higher than any in other years. Further analysis suggests that two adjacent basin-scale upwellings in the SCS have different responses and maintaining mechanisms because of the anticyclonic atmospheric circulation anomaly over the SCS and northwest Pacific. The northern flank of the atmospheric circulation anomaly intensifies the monsoonal winds off the NSCS coast, while the southern flank suppresses the southwesterly winds along the Vietnam coast.

Description: Ventilation of waters in and around the Sea of Okhotsk was investigated using simulations of chlorofluorocarbons (CFCs) in the northwestern North Pacific. We used an ocean general circulation model coupled with a sea ice model. The model reproduces the distributions of CFCs similar to observed values and indicates the importance of tidal mixing along the Kuril Islands and brine rejection to ventilation of waters in and around the Sea of Okhotsk. To clarify the role of each process, numerical experiments excluding one of the two processes were carried out. Results show that brine rejection transports CFCs into the intermediate layer as deep as 200–400 m along the path of dense shelf water in the western Sea of Okhotsk, but hardly to other areas and layers. On the other hand, tidal mixing transports CFCs into the intermediate and deeper layers throughout the Sea of Okhotsk. We conclude that the tidal mixing has a greater influence than brine rejection on the ventilation of layers below the winter mixed layer.

Description: The α- and γ-hexachlorocyclohexanes (HCHs) are being scavenged from the atmosphere by falling snow, with the average total scavenging ratios (WT) of 3.8 × 104 and 9.6 × 103, respectively. After deposition, HCH snow concentrations can decrease by 40% because of snowpack ventilation and increase by 50% because of upward migration of brine from the ice. HCH vertical distribution in sufficiently cold winter sea ice, which maintains brine volume fractions

Description: In August 2007, three long-lived anticyclonic eddies (ACE1, ACE2, and ACE3) were detected by both satellite sea level anomaly (SLA) map and in situ measurements in the northern South China Sea (SCS). ACE3 had a two-core (ACE3(1) and ACE3(2)) structure. In situ stations along 18°N almost cut through the centers of ACE2 and ACE3(2). Near the centers of ACE2 and ACE3(2), mean temperature and sound velocity are ∼0.65°C and ∼2 m s−1 larger than those in their surrounding areas, respectively, while mean salinity and density are ∼0.02 psu and ∼0.15 m3 s−1 smaller than those in their surrounding areas due to downwelling near the eddy cores. The depths of maximum and minimum salinity near the eddy cores are ∼65 m and ∼35 m larger than those in their surrounding areas. The vertical depth with current speed larger than 0.05 m s−1 can reach ∼900 m. Their detailed evolutionary processes were depicted based on the variation of geostrophic currents and the trajectories of five drifting buoys. ACE1 lasted 147 days, while ACE2 and ACE3 lasted 168 days and 210 days, respectively. ACE1 had a smaller mean SLA (18.8 cm) in its lifetime than ACE2 (21.8 cm) and ACE3 (25.3 cm) but had a larger negative mean vorticity (−7.7 × 10−6 s−1) than ACE2 (−7.0 × 10−6 s−1) and ACE3 (−7.0 × 10−6 s−1). One short-lived anticyclonic eddy that split from ACE2 and another one that merged with ACE3 both had a smaller SLA, negative vorticity, and diameter than ACE2 and ACE3, respectively.

Description: The marginal ice zone (MIZ) is the boundary between the open ocean and ice-covered seas, where sea ice is significantly affected by the onslaught of ocean waves. Waves are responsible for the breakup of ice floes and determine the extent of the MIZ and floe size distribution. When the ice cover is highly fragmented, its behavior is qualitatively different from that of pack ice with large floes. Therefore, it is important to incorporate wave-ice interactions into sea ice–ocean models. In order to achieve this goal, two effects are considered: the role of sea ice as a dampener of wave energy and the wave-induced breakup of ice floes. These two processes act in concert to modify the incident wave spectrum and determine the main properties of the MIZ. A simple but novel parameterization for floe breaking is derived by considering alternatively ice as a flexible and rigid material and by using current estimates of ice critical flexural strain and strength. This parameterization is combined with a wave scattering model in a one-dimensional numerical framework to evaluate the floe size distribution and the extent of the MIZ. The model predicts a sharp transition between fragmented sea ice and the central pack, thus providing a natural definition for the MIZ. Reasonable values are found for the extent of the MIZ given realistic initial and boundary conditions. The numerical setting is commensurate with typical ice-ocean models, with the future implementation into two-dimensional sea ice models in mind.

Description: The Stokes interfacial edge wave in a viscous rotating two-layer system is studied theoretically. The mean wave-induced Lagrangian drift velocity is obtained from the vertically integrated Eulerian equations of momentum and mass, correct to second order in wave steepness. The analysis is valid for shallow-water waves in the case when the upper layer is much thicker than the lower layer. In the lower layer the effect of viscosity is confined to a frictional boundary layer at the bottom. The waves are trapped by the bottom slope and can propagate in either direction along the bottom contours (in the y direction). Assuming that the waves attenuate in space as they propagate, this yields a Stokes drift velocity and a mean energy density E that decay exponentially in y. In this problem −∂E/∂y is the relevant radiation stress forcing in the wave propagation direction. It is explained why this differs from the radiation-stress forcing of −${^3\!/\!_2}$∂E/∂y for plane waves in an unbounded nonrotating shallow ocean. The bottom stress acting on the mean Eulerian wave-induced flow is modeled by a turbulent friction coefficient. The results show that the maximum mean Eulerian drift current is considerably larger than the maximum Stokes drift velocity. Since the Eulerian current becomes negative at larger seaward distances, the total mean Lagrangian drift current is confined to a rather narrow wedge in the lower layer.

Description: The assimilative capacities of estuaries and coastal seas for effluent discharges are predominantly determined by the rates at which pollutant-bearing effluents are first dispersed and then flushed from the coastal region into the open ocean. The dispersion coefficients and flushing, as measured by the water residence time in the Persian Gulf (Arabian Gulf), were investigated using the three-dimensional numerical model Estuary, Lake and Coastal Ocean Model (ELCOM). The model was first validated using the R/V Mt. Mitchell expedition profile data, collected from 27 January to 26 February 1992 and from 13 May to 12 June 1992. The validated model was then used to compute the geographic variability of the horizontal dispersion coefficients Kx throughout the gulf. Model results revealed that dispersion was principally driven by the shear associated with the tides, but along the Arabian coast, wind was an additional significant energy source for dispersion. The water residence time was found to be more than 3 years along the Arabian coast, but shorter along the Iranian coast.

Description: The circulation over the continental shelf break of the western and southwestern Gulf of Mexico is inferred from the analysis of drifter trajectories and 12–19 months of continuous current measurements at seven different locations. The interpretation of the data is backed up by satellite altimetry, coastal sea level from tide gauges and wind model outputs. In accordance with previous numerical results, subinertial surface currents are driven by the wind along the shelves of the states of Tamaulipas and Veracruz. Northern wind regimes would force southward currents, whereas southern wind regimes would force northward currents at the surface but southward near the bottom, through a process involving Ekman drift and geostrophic balance. Our results show, however, that alongshore current variations are not correlated with the wind over the Western Campeche Bank. In addition, we identify other sources of current forcing. The transient eddies that collapse along the continental shelf can force strong alongshore currents and overwhelm the influence of established wind regimes. Their erratic occurrence is likely to be a major factor of interannual variability of the alongshore currents. Also, we point out the existence of coastally trapped waves generated by the wind in the northern shelf of Tamaulipas and propagating down to the Western Campeche Bank. The period of these waves ranges between 6 and 10 days, with phase speeds in the 4 m/s range.

Description: A high-resolution numerical model study of the Canary Basin in the northeast subtropical Atlantic Ocean is presented. A long-term climatological solution from the Regional Oceanic Modeling System (ROMS) reveals mesoscale variability associated with the Azores and Canary Current systems, the northwest African coastal upwelling, and the Canary Island archipelago. The primary result concerns the Canary Current (CanC) which, in the solution, transports ∼3 Sv southward in line with observations. The simulated CanC has a well-defined path with pronounced seasonal variability. This variability is shown to be mediated by the westward passage of two large annually excited counterrotating anomalous structures that originate at the African coast. The anomalies have a sea surface expression, permitting their validation using altimetry and travel at the phase speed of baroclinic planetary (Rossby) waves. The role of nearshore wind stress curl variability as a generating mechanism for the anomalies is confirmed through a sensitivity experiment forced by low-resolution winds. The resulting circulation is weak in comparison to the base run, but the propagating anomalies are still discernible, so we cannot discount a further role in their generation being played by annual reversals of the large-scale boundary flow that are known to occur along the African margin. An additional sensitivity experiment, where the Azores Current is removed by closing the Strait of Gibraltar presents the same anomalies and CanC behavior as the base run, suggesting that the CanC is rather insensitive to upstream variability from the Azores Current.

Description: The relationship between oceanic phytoplankton and climate variability has been given increasing attention with the accumulation of satellite-derived chlorophyll data over the past decade. Here we examine the dominant variability of phytoplankton and its associated tropical climate systems; in particular, the El Niño–Southern Oscillation (ENSO). The analysis, using 148 months of chlorophyll data, reveals that the first two leading modes of tropical chlorophyll anomalies are linked to the mature phase and the decaying phase of the ENSO cycle. It is also found that when El Niño events occur, the reduced surface solar radiation (enhanced convective activity), as well as the insufficient nutrient supply (suppressed equatorial upwelling), can also play a significant role in reducing chlorophyll concentration. The effect of reduced surface solar radiation on chlorophyll is larger in the central Pacific than in the eastern and western Pacific, and this regional difference of the impact induces a distinctly asymmetric response of ocean chlorophyll to El Niño and La Niña in the central Pacific. A linear statistical analysis shows that the dominant variability of chlorophyll associated with ENSO contributes radiant feedback to the equatorial Pacific by altering the surface shortwave albedo. The decreased chlorophyll concentration during El Niño tends to induce radiant cooling at the ocean surface.

Description: Mesoscale eddies are observed each year in the South China Sea (SCS); however, their contributions to the biogeochemical cycles have never been systematically quantified. Here, we use a coupled three-dimensional physical-biogeochemical model to evaluate the eddy impact. We first track the modeled mesoscale eddies in the SCS and then analyze the biogeochemical responses to these eddies individually. Compared with the SCS basin mean, modeled depth-integrated (0–125 m) chlorophyll, zooplankton, new production, and silicate uptake are significantly enhanced in the cyclonic eddies and reduced in the anticyclonic eddies. Following the movements of the eddy center, temporal variations of phytoplankton community structure suggest that diatoms respond to cyclonic eddies strongly first and the responses last longer; then picoplankton grow after the diatoms. In the cyclonic eddies, modeled new production is 1.87 ± 0.37 mmol N m−2 d−1, which is 28% higher than the SCS basin-averaged value, while in the anticyclonic eddies, modeled new production is about 32% lower than the SCS basin mean. As a consequence, modeled detrital nitrogen export for cyclonic eddies is 41% higher than the SCS basin mean, and that for anticyclonic eddies is 31% lower than the SCS basin mean. These values experience strong interannual variations with anomalously low magnitudes found during El Niño conditions for both of the eddies and the SCS basin mean. Our results indicate that cyclonic eddies in the SCS are important sources of nutrients to the euphotic zone and therefore play a significant role in regulating biological productivity and the carbon cycle.

Description: The validity and accuracy of approaches used to determine hurricane surge hazard risk received much attention following the hurricane seasons in mid- to late-2000, which caused record surge-related damage along the Gulf of Mexico coastline. Following Hurricane Katrina in 2005, research showed that most extreme-value statistics approaches underestimated the risk associated with this surge event. In this paper, two of the most popular methods for determining hurricane surge extreme-value statistics are reviewed: the historical surge population approach and the joint probability method. Here, it is demonstrated that both limited historical record length and random along-coast variability in hurricane landfall location can introduce significant errors into surge estimates. For example, the historical surge population approach gives errors of 9% to 17% for return periods between 50 and 1000 years when a surge record of 100 years is considered. In contrast, it is shown that the joint probability method yields significantly more reliable surge estimates, with errors of 2% to 3% for return periods between 50 and 1000 years when a storm record of 100 years is considered. Finally, we show that both methods remain robust when decadal-scale climate variability in the storm rate of occurrence is considered, so long as the hurricane history is long enough to capture the full decadal cycle. When used in conjunction with continuous surge response information, it can be concluded that the joint probability method is a practical and reliable approach for determining extreme-value hurricane surge statistics.

Description: A comprehensive numerical study of oscillatory wave boundary layers on spatially varying bottom roughness is presented. The study utilizes a model solving incompressible Reynolds-averaged Navier-Stokes equations coupled with k-ω turbulence closure, modified in a simple way to incorporate anisotropy in turbulent normal stresses. The model is first validated via comparison with existing oscillating tunnel measurements involving sudden bottom roughness transitions. It is then used to parametrically study oscillatory boundary layer flows, wherein the bed shear stress amplifications and period-averaged streaming characteristics induced by bottom roughness variations are systematically assessed. The effects of variable roughness ratio, gradual roughness transitions, as well as changing flow orientation in plan are all considered. As part of the latter, roughness-induced secondary flows are predicted to occur as the oscillatory flow becomes oriented parallel to a line of roughness transition. This phenomenon is proposed as a natural transverse grain sorting mechanism for coastal flows over graded sediments. Subsequent model testing demonstrates potential generation of secondary circulation cells having characteristic size the order of the wave boundary layer thickness. Analogy is made to similar features known to develop within steady flows, having characteristic size the order of the flow depth.

Description: A high-resolution sea surface temperature (SST) data derived from several satellites is used to investigate the variability of the thermal front off northeastern Taiwan. Hidden by a dominant annual cycle, the SST data cannot reveal the thermal front fluctuation in the form of Hovmöller diagram. An innovative methodology has been applied to the SST satellite imagery to derive the SST Standardized Index (SSTSI), capable of revealing the frontal variability with multiple time scales. Principal component analysis shows that the SSTSI variation consists mainly of two modes. Mode 1 represents a strong annual cycle related to the seasonal reversal of the monsoonal winds. The temperature gradient is enhanced in winter and a cold dome is observed off northern Taiwan in summer. Mode 2 is highly correlated with the upstream Kuroshio variability. The shoreward (seaward) migration of the thermal front takes place when the Kuroshio transport weakens (strengthens). The results are consistent with transports estimated by tidal gauge measurements, satellite altimeter-based sea level anomaly, and surface flow patterns derived from high-frequency radars. Mode 2 is coherent with the Kuroshio transport through the East Taiwan Channel at periods of 120 and 45 d with a time lag of 40 and 11 d, respectively. This 120 d fluctuation is due to the interaction between westward-propagating eddies and the Kuroshio east of Taiwan, while the 45 d signal arises from the Kuroshio's self-instability. The interannual variations of the SST pattern in winter and summer are also discussed.

Description: Increased sea ice melt and decreased surface albedo have changed the near-surface water mass structure of the Canada Basin. From 1993–2009, the near-surface temperature maximum (NSTM) and remnant of the previous winter's mixed layer (rWML) warmed by about 1.5°C and 0.5°C and freshened by about 4 and 2 practical salinity units, respectively. Results from a 1-D model suggest rWML warming can be explained by heat diffusion from both the NSTM and Pacific Summer Water (PSW). The same model predicts salinization of the rWML, whereas freshening was observed. This suggests that changes to the rWML are from both diffusion and the accumulation of freshwater. The rWML's salinity was associated with distance from the center of the Beaufort Gyre; the rWML at stations inside the gyre was on average 1.9 salinity units fresher than at stations outside. In addition, the salinity of PSW in the Canada Basin - defined by its local temperature maximum - freshened from about 30–32 in 1993 to 28–32 in 2008. Order of magnitude calculations suggest that neither changes in PSW source waters nor changes in advection pathways of PSW explain this freshening. Our model suggests that salt diffused from PSW to the freshening rWML; this diffusion increased (and freshened the PSW salinity range) as the rWML freshened. These results show that surface effects through warming and ice melt are felt to at least the depth of PSW. Observations from 2009 show the appearance of a third temperature maximum from an as yet unknown source.

Description: The dynamics of near-inertial motions, and their relation to mixing, is investigated here with an extensive data set, including turbulence and high-resolution velocity observations from two cruises conducted in 2008 (summer) and 2010 (winter) in the Bornholm Basin of the Baltic Sea. In the absence of tides, it is found that the basin-scale energetics are governed by inertial oscillations and low-mode near-inertial wave motions that are generated near the lateral slopes of the basin. These motions are shown to be associated with persistent narrow shear-bands, strongly correlated with bands of enhanced dissipation rates that are the major source of mixing inside the permanent halocline of the basin. In spite of different stratification, near-inertial wave structure, and atmospheric forcing during summer and winter conditions, respectively, the observed dissipation rates were found to scale with local shear and stratification in a nearly identical way. This scaling was different from the Gregg-Henyey-type models used for the open ocean, but largely consistent with the MacKinnon-Gregg scaling developed for the continental shelf.

Description: The spectral polarized radiance distribution provides the most complete description of the light field that can be measured. However, this is a very difficult parameter to measure, particularly near the surface, because of its large dynamic range, changes in the skylight illumination, and waves at the air-sea interface. To measure the Stokes vector of the downwelling light field, which contains the polarization information, requires the combination of four images acquired simultaneously. To achieve this, we used the downwelling polarized radiance distribution camera system (DPOL) during the Radiance in a Dynamic Ocean (RaDyO) program Santa Barbara Channel and Hawaiian experiments. DPOL consists of four fisheye lenses and a spectral filter changer that allow us to capture the downwelling hemisphere of the polarized radiance distribution at seven wavelengths. Our measurements show that very near the surface, for clear sky conditions, the dominant source of polarization is the refracted sky light. As one progresses in the water column the polarization due to light scattering by the water increases and polarization due to light scattering in the water becomes dominant.

Description: The tropical Pacific and Atlantic Oceans have similar mean states - easterly winds and a zonally sloping thermocline which shoals in the east - but strikingly different sea surface temperature variability. Seasonal and interannual variations have comparable amplitudes, and correspond to different modes of oscillation in the Pacific. In the Atlantic, the seasonal cycle is dominant and has properties of both those modes. The Bjerknes feedback between the wind and SST, and its associated delayed, negative feedback constitute a free (interannual) mode of the Pacific, but in the Atlantic influence the seasonal cycle. This difference between the two oceans is attributable to the smaller dimensions of the Atlantic. An energetic analysis shows a circular relationship between available potential energy and the work done by the wind from April until September in the Atlantic. This energetics perspective suggests that a seasonally excited thermocline mode of coupled variability plays an active role in the seasonal cycle during these months, after which point seasonal forcing regains control.

Description: We monitored the spatiotemporal progression of dissolved organic carbon (DOC) and carbon monoxide (CO), along with general meteorological, hydrographic, and biological variables, in first-year sea ice in the western Canadian Arctic between mid-March and early July 2008. DOC and CO concentrations fluctuated irregularly in surface ice, but followed the concentration of ice algae in bottom ice, i.e., low at the start of ice algal accumulation, highly enriched during the peak-bloom and early post-bloom, and depleted again during sea ice melt. Vertical profiles of DOC and CO typically decreased downward in early spring and were variable in the melting season. In the presence of high bottom ice algal biomass in mid-spring, DOC and CO exhibited high concentrations in the bottom (DOC: 563 ± 434 μmol L−1; CO: 82.9 ± 84 nmol L−1) relative to the surface (DOC: 56 ± 26 μmol L−1; CO: 16.8 ± 7 nmol L−1). Landfast ice contained higher levels of DOC and CO than drifting ice. Cruise-mean DOC and CO inventories in sea ice were 87 ± 51 mmol m−2 and 13.9 ± 10 μmol m−2, respectively. Net productions of DOC and CO linked to the ice algal bloom were assessed to be 75 mmol m−2 and 13.2 μmol m−2. Sea ice in the study area was estimated to contribute 7.4 × 107 moles of CO a−1 to the atmosphere. This study suggests that sea ice plays important roles in the cycling of organic carbon and trace gases.

Description: We investigate differences of the ocean response in the Amazon domain to the seasonal variability of the river discharge that are either introduced via assimilating climatological temperature and salinity or by specifying seasonally varying river runoff. The role of the seasonal cycle of the Amazon freshwater discharge for the evolution of the barrier layer (BL) in the western tropical Atlantic and on the freshwater budget is estimated. During the experiments, three different runoff fields are being applied, including a time-mean runoff, a seasonally varying runoff, and one that results from the GECCO assimilation approach. The simulation forced with a seasonal Amazon discharge appears to be closer to the constrained solution and moves away from the run with a constant runoff, demonstrating that the seasonal variability of the Amazon is an essential contributor in the freshwater forcing of the western tropical Atlantic. The modeled time-mean BL thickness seems to be overestimated by the model relative to the data. On the seasonal timescale, the simulated spatial mean BL is found to vary between 13 and 30 m, with a maximum occurring in July, following the Amazon high discharge period in May. Analyzing the freshwater content balance, we find integrated near-surface freshwater import from the western tropical Atlantic interior of around 0.20 Sv in October–November at 38°W and cumulative freshwater export out of the domain with a maximum of around 0.4 Sv in June as an effect of the Amazon flood in May.

Description: Tidal constituents and datums are computed on a high resolution grid of the northwestern Atlantic Ocean, including the Gulf of Mexico and the Caribbean Sea. A global model is used to determine tidal parameters on a grid with a nominal resolution of 800 × 800. The global model includes self-attraction and loading, drag in shallow marginal seas, and internal tide drag in the deep ocean. Simulations are performed at 1000 year intervals during the Holocene (10,000 calibrated years before present (10 ka)) in combination with changes in bathymetry and coastline location derived from a glacial isostatic adjustment model. The global model results are then used to force a regional barotropic tidal model. The regional model uses an unstructured finite element grid, with very high resolution at the coastline. The model results reveal significant variations in tidal constituent amplitudes throughout the Holocene. In the northwestern Atlantic, semi-diurnal components show a strong amplification at around 9 ka while in the Gulf of Mexico, the response is much more muted. Variations in diurnal tidal parameters are found to be less significant than semi-diurnal parameters throughout the model domain. Changes in tidal range, of great relevance to changes in relative sea level (RSL), are also investigated throughout the Holocene. The overall structure is similar to the patterns observed in the M2 tide, with peak increases of 200–300%, relative to present-day, being observed along the east coast of the United States from 9 to 8 ka. Finally, the high spatial resolution of the regional model allows for the investigation of tidal changes at spatial scales (e.g., individual bays) much smaller than in previous studies.

Description: The Southern Ocean Gas Exchange Experiment (SO GasEx) is the third in a series of U.S.-led open ocean process studies aimed at improving the quantification of gas transfer velocities and air-sea CO2 fluxes. Two deliberate 3He/SF6 tracer releases into relatively stable water masses selected for large ΔpCO2 took place in the southwest Atlantic sector of the Southern Ocean in austral fall of 2008. The tracer patches were sampled in a Lagrangian manner, using observations from discrete CTD/Rosette casts, continuous surface ocean and atmospheric monitoring, and autonomous drifting instruments to study the evolution of chemical and biological properties over the course of the experiment. CO2 and DMS fluxes were directly measured in the marine air boundary layer with micrometeorological techniques, and physical, chemical, and biological processes controlling air-sea fluxes were quantified with measurements in the upper ocean and marine air. Average wind speeds of 9 m s−1 to a maximum of 16 m s−1 were encountered during the tracer patch observations, providing additional data to constrain wind speed/gas exchange parameterizations. In this paper, we set the stage for the experiment by detailing the hydrographic observations during the site surveys and tracer patch occupations that form the underpinning of observations presented in the SO GasEx special section. Particular consideration is given to the mixed layer depth as this is a critical variable for estimates of fluxes and biogeochemical transformations based on mixed layer budgets.

Description: A mechanism for the formation of ice bands is proposed as a coupled response of ice edge and lee waves to wind under the hydrostatic approximation. A high-resolution ice-ocean coupled model is used in an x-z domain with grid sizes (x, z) = (250 m, 1 m). Under an along-ice-edge wind, such that the Ekman transport is away from the ice edge, the nearly discontinuous surface stress between the ice-covered and open seas generates lee waves. A thin layer of high-potential vorticity fluid under the ice is produced by the Ekman forcing, enabling the ice edge to rapidly slip over less stratified water. This is favorable for supercritical conditions when lee waves are generated. Ice bands are formed by the corresponding convergences and divergences. The flow becomes subcritical farther behind the ice-edge but secondary lee waves and ice bands form because of the secondary stress discontinuity behind the lead ice band. An analytical solution is derived to show that ice bands have longer widths than the lee-wavelengths because the ice-ocean stress creates the smoothing effect. Vertical motions associated with the lee waves have speed of the order of 10 m/day, extend to the bottom (300 m), and contribute to deep vertical mixing and the subsequent melting of the ice. These small-scale features are not modeled well with horizontal grids coarser than approximately 2.5 km.

Description: Recent studies in lakes have shown evidence for a strong influence of shear-induced stratification on mixing in turbulent bottom boundary layers (BBLs) on sloping topography. These observations suggest that the periodic near-bottom shear resulting from internal wave motions may lead to alternating periods of gravitationally stable and unstable stratification in the BBL with relevant implications for turbulence and mixing in the entire basin. The impact of these processes for basin-scale mixing is investigated here in a three-dimensional processes-oriented modeling study, using a well-investigated system (Lake Alpnach, Switzerland) as an example. Consistent with available observations, our results indicate that the BBL becomes gravitationally unstable in areas with upslope flow, covering a substantial fraction of the total bottom area of the lake. While near-bottom convection associated with the unstable stratification in these areas results in strong turbulence, its contribution to net mixing is negligible since the BBL is already well mixed. Conversely, in areas with downslope flow the near-bottom shear generates stable stratification, leading to a suppression of turbulence but also to larger mixing rates due to an enhanced mixing efficiency. Overall, BBL mixing is found to dominate basin-scale mixing. These mechanisms are likely to be important for a large class of stratified natural waters, in which boundary layer mixing is energized by periodic internal waves or basin-scale motions.

Description: Measured turbulence power spectra, cospectra, and ogive curves from a shallow tidal flow were scaled using Monin-Obukhov similarity theory to test the applicability to a generic tidal flow of universal curves found from a uniform, neutrally stable atmospheric boundary layer (ABL). While curves from individual 10 min data bursts deviate significantly from similarity theory, averages over large numbers of sufficiently energetic bursts follow the general shape. However, there are several differences: (1) Variance in the measured curves was shifted toward higher frequencies, (2) at low frequencies, velocity spectra were significantly more energetic than theory while cospectra were weaker, and (3) spectral ratios of momentum flux normalized by turbulent kinetic energy (TKE) indicate decreased fluxes and/or elevated TKE levels. Several features of the turbulence structure may explain these differences. First, turbulent dissipation exceeded production, indicating nonequilibrium turbulence, possibly from advection of TKE. Indeed, using the production rate rather than dissipation markedly improves agreement in the inertial subrange. Second, spectral lag of the largest eddies due to inhomogeneous boundary conditions and decaying turbulence could explain spectral deviations from theory at low frequencies. Finally, since the largest eddies dominate momentum transfer, the consequence of the cospectra difference is that calculated ogive curves produced smaller total momentum fluxes compared to theory, partly because of countergradient fluxes. While ABL similarity scaling applied to marine bottom boundary layers (MBBLs) will produce curves with the general shape of the universal curves, care should be taken in determining details of turbulent energy and stress estimates, particularly in shallow and inhomogeneous MBBLs.

Description: Within the Radiance in a Dynamic Ocean (RaDyO) program, we have created and deployed a high dynamic range camera that can resolve the full spherical radiance distribution at the ocean surface and at depth. We present here the first results from deployments of the camera in near-surface water in eutrophic, mesotropic, and oligotropic environments. The instrument resolves the dynamics and fine structure of both the downwelling and upwelling radiance distribution and its variation with depth in these optically diverse water types. The various irradiances (Ed, Eu, Eo, Eou, and Eod) are computed by integration. The distribution functions (e.g., the average cosines) are computed directly, as are the various diffuse attenuation coefficients. The fully specified radiance field therefore provides all the pertinent information to derive not only all of the apparent optical properties but, in principle, the inherent optical properties such as the absorption coefficient and the phase function as well. Comparison of the measured radiance field to independent measurements has shown very good agreement.

Description: A novel aerial observational method for studying internal features in the coastal ocean is developed and tested in a study of large nonlinear internal solitary-like waves. Photogrammetrically rectified oblique photo images from a circling aircraft are used to track a number of internal wave packets for periods of up to one hour in the Strait of Georgia, British Columbia, Canada. Combining these sequences with coincident water column data allows us to obtain a more complete view of the spatial structure of internal waves. Highly accurate measurements of wave propagation speeds and directions are possible. The applicability of various weakly nonlinear theories in modeling propagation of the observed large-amplitude waves is tested. The measured wave speeds enable us to differentiate between classic internal wave models. The linear, KdV (Korteweg-de Vries), and BO (Benjamin-Ono) models are applied with and without background shear. After background shear effects are included, it is found that a continuously stratified BO equation can predict propagation speeds within observational error, and that this is not true for other theories. The technique may be useful in future studies of oblique internal wave interactions.

Description: Surface currents measured by High Frequency (HF) radar are used to investigate the dynamics in the coastal waters of Qingdao, China, on the western coast of the Yellow Sea. Different factors affecting the surface currents are revealed by dynamical analysis. Harmonic tidal analysis shows that the coastal tidal currents are barotropic and their temporal evolution is mainly influenced by the semidiurnal tidal constitutes, topography and geometry. It is also found that their horizontal distribution represents the effect of local topography. At sub-tidal frequencies, the high correlation between winds and the sub-tidal surface currents indicates a crucial role of wind-forcing in driving the currents. Varying wind direction coupled with small-scale features of the coastal geometry results in complicated sub-mesoscale circulation. In addition, the monthly coastal circulation is characterized by an eddy structure and is primarily determined by the outflow which is closely associated with the sea level slope along the coast. The variability of the residual currents is studied by analyzing the cross-shore momentum equation with wind stress, sea level records, and HF radar currents. It is shown that both the barotropic pressure gradient and the nonlinear tidal stress contribute to the variations of the residual currents near the bay mouth and further govern the coastal monthly circulation in August, 2008.

Description: A detailed comparison of results from a numerical three-dimensional hydrostatic lake model with high-resolution observations of the vertical structure of the turbulent bottom boundary layer (BBL) in a medium-size lake (Lake Alpnach, Switzerland) is provided. The focus of this study is on the shear-induced generation and destruction of stratification in the BBL that may ultimately lead to unstable layers (convection). The model was shown to provide a reliable description of the internal seiching dynamics, as well as the local BBL properties, including the generation of shear-induced convection in two data sets from 2003 and 2007. Basin-scale mixing parameters, inferred from the simulations, are closely connected to the seiching motions, with the hypolimnetic mixing reacting almost immediately to the variable wind-forcing and seiching activity. During upslope flow, the BBL becomes convectively turbulent, causing low mixing efficiency on a basin-scale, whereas during downslope flow, the BBL is restratifying and shear-induced turbulence is weak but leads to a higher mixing efficiency. The overall deep-water mixing efficiency varied in the range of 5 to 10% in this system dominated by turbulent boundary processes.

Description: We study the polarization properties of the light field under a dynamic ocean surface using realistic linear and nonlinear ocean surface waves. The three-dimensional polarized radiative transfer of the dynamic ocean–atmosphere system is considered using a Monte Carlo vector radiative transfer simulation for arbitrary depth. The program is validated with measurement data taken in Hawaii during the Radiance in a Dynamic Ocean project. The main focus of this study is the influence of the wind-driven ocean waves on the polarization patterns and statistics at different optical depths under various conditions of light wavelength and solar incidence. Of special interest is the effect of the nonlinearity of the surface waves on the polarization statistics. To facilitate the study, phase-resolved direct simulations of the linear and nonlinear surface wavefields are performed using a high-order spectral method. The results show that the time-averaged degree of polarization within the Snell's window is dependent on the mean square slope of the ocean surface. Higher mean square slope, or wind speed, leads to a smaller degree of polarization. At the same time, the variability of the degree of polarization has a strong dependence on the surface roughness. A rougher ocean surface induces higher variability of the degree of polarization. The effect of wave nonlinearity can be neglected for the mean value of polarization, but is manifested in the variability of the degree of polarization, with a general increase in the variance with increasing wave nonlinearity. The present findings provide possible mechanisms for characterizing the dynamic ocean surface based on underwater polarized light measurements.

Description: An accurate bottom pressure sensor has been moored at different sites varying from a shallow sea strait via open ocean guyots to a 1900 m deep Gulf of Mexico. All sites show more or less sloping bottom topography. Focusing on frequencies (σ) higher than tidal, the pressure records are remarkably similar, to within the 95% statistical significance bounds, in the internal gravity wave continuum (IWC) band up to buoyancy frequency N. The IWC has a relatively uniform spectral slope: log(P(σ)) = −αlog(σ), α = 2 ± 1/3. The spectral collapse is confirmed from independent internal hydrostatic pressure estimate, which suggests a saturated IWC. For σ 〉 N, all pressure-spectra transit to a bulge that differs in magnitude. This bulge is commonly attributed to long surface waves. For the present data it is suggested to be due to stratified turbulence–internal wave coupling, which is typically large over sloping topography. The bulge drops off at a more or less common frequency of 2–3 × 10−2 Hz, which is probably related with typical turbulent overturning scales.

Description: We study the impact of decadal inversions of the Ionian upper layer circulation (denominated as Adriatic-Ionian Bimodal Oscillation System) on thermohaline properties of the Levantine and Cretan Seas. Lagrangian drifter data and surface geostrophic currents show that the Atlantic Water (AW) flow is well organized and most intense when the Ionian circulation is cyclonic. During the Ionian anticyclonic phase, the AW spreading pathway is the longest, contributing to its prolonged mixing and higher salinity once it reaches the Levantine. Thus, the Levantine basin is subject to less dilution by AW during the anticyclonic surface circulation phase. Empirical orthogonal function analysis of the sea level shows a large-amplitude circular feature in the northern Ionian which matches the cyclonic/anticyclonic gyre obtained from Lagrangian measurements. Furthermore, it reveals the out-of-phase variability of the North Ionian Gyre and the Aegean and Levantine sea levels. We further show that the surface salinity of the Levantine basin variation is out of phase with that of the Ionian surface layers. Salinity variations of the deepwater column in the Aegean are out of phase with the Ionian surface salinity values, owing probably to a fast transfer of the surface salinity changes via winter deep convection. The changing of the Levantine and Cretan Seas' salinity parallel to the Ionian circulation inversions suggests that the preconditioning for the eastern Mediterranean transient (EMT) is driven by internal processes. As the Ionian inversions are cyclical events, we conclude that the EMT is not an isolated episode but potentially a recurrent phenomenon.

Description: Sea surface range Doppler velocities from nearly 1200 Envisat Advanced Synthetic Aperture Radar (ASAR) acquisitions between 2007 and 2011, covering the Norwegian Sea, the North Sea, and the Skagerrak Sea, have been examined. After systematic corrections, the inflow of Atlantic Water to the Norwegian Sea, via the two branches of the Norwegian Atlantic Current, is investigated. Distinct expressions of the eastern branch, the Norwegian Atlantic Slope Current, are revealed with a speed of 20–40 cm/s and a clear manifestation of topographic steering along the 500 m isobath. The western branch, the Norwegian Atlantic Front Current, is also depicted but with lower surface velocities. Moreover, parts of the Norwegian Coastal Current are also detected with time-averaged speed reaching up to 40 cm/s. At a spatial resolution of 10 km, the root mean square errors of these velocities are estimated to be less than 5 cm/s. The range Doppler velocity retrievals are assessed and compared to other direct and indirect estimates of the upper ocean current, including surface Lagrangian drifters, moored recording current meter measurements, and surface geostrophic current inverted from several mean dynamic topography fields. The results are promising and demonstrate that the synthetic aperture radar based range Doppler velocity retrieval method is applicable to monitoring the temporal and spatial variations of ocean surface circulation, provided the imaging geometry is favorable.

Description: The seasonal and spatial variability of dissolved Barium (Ba) in the Amundsen Gulf, southeastern Beaufort Sea, was monitored over a full year from September 2007 to September 2008. Dissolved Ba displays a nutrient-type behavior: the maximum water column concentration is located below the surface layer. The highest Ba concentrations are typically observed at river mouths, the lowest concentrations are found in water masses of Atlantic origin. Barium concentrations decrease eastward through the Canadian Arctic Archipelago. Barite (BaSO4) saturation is reached at the maximum dissolved Ba concentrations in the subsurface layer, whereas the rest of the water column is undersaturated. A three end-member mixing model comprising freshwater from sea-ice melt and rivers, as well as upper halocline water, is used to establish their relative contributions to the Ba concentrations in the upper water column of the Amundsen Gulf. Based on water column and riverine Ba contributions, we assess the depletion of dissolved Ba by formation and sinking of biologically bound Ba (bio-Ba), from which we derive an estimate of the carbon export production. In the upper 50 m of the water column of the Amundsen Gulf, riverine Ba accounts for up to 15% of the available dissolved Ba inventory, of which up to 20% is depleted by bio-Ba formation and export. Since riverine inputs and Ba export occur concurrently, the seasonal variability of dissolved Ba in the upper water column is moderate. Assuming a fixed organic carbon to bio-Ba flux ratio, carbon export out of the surface layer is estimated at 1.8 ± 0.45 mol C m−2 yr−1. Finally, we propose a climatological carbon budget for the Amundsen Gulf based on recent literature data and our findings, the latter bridging the surface and subsurface water carbon cycles.

Description: El Niño Modoki is a variant of El Niño characterized by warming around the dateline flanked by anomalous cooling in the east and west. However, the opposite phase (La Niña Modoki) has received little attention because the prominent cooling of sea surface temperature (SST) during major La Niña events is observed in the central Pacific, and thus, it is difficult to define the two different types of cold events from the SST anomaly pattern. Here we demonstrate that cold events in 2000 and 2008 can be clearly distinguished from traditional La Niña events using surface currents derived from satellite observations. During 2000 and 2008, anomalous zonal currents in the equatorial Pacific demonstrate divergence, with westward currents west of the dateline and eastward currents east of it. These currents are opposite to the circulation pattern during the 2004 El Niño Modoki event. An empirical orthogonal function (EOF) analysis of surface currents for the period 1993–2009 shows a circulation anomaly pattern similar to that in 2000, 2004, and 2008 in the second EOF. The first EOF is consistent with traditional El Niño/La Niña events and does not exhibit a current reversal along the equator. Our results also indicate that strong cyclonic (anticyclonic) circulation anomalies occur in the tropical western Pacific around 5°N–15°N during the 2004 (2000 and 2008) El Niño (La Niña) Modoki events and during the strong traditional El Niño (La Niña) events of 1997 (1998). These circulation anomalies are related to an SST gradient in the western and central Pacific.

Description: Ensemble predictions are performed using the LDEO5 model for the period from 1856 to 2003 based on a well developed El Niño–Southern Oscillation (ENSO) ensemble system. Information-based and ensemble-based potential predictability measures of ENSO are explored using ensemble predictions and the recently developed framework of predictability. Relationships of these potential predictability measures and actual predictability measures are investigated on multiple time scales from interannual to decadal. Results show that among three information-based potential predictability measures, relative entropy (RE) is better than predictive information (PI) and predictive power (PP) in quantifying correlation-based prediction skill, whereas PI and PP are better indicators in estimating mean square error (MSE)-based prediction skill. The primary reason for these relationships is analyzed and the control factors of the potential predictability measures are identified. It is found that RE is dominated by the signal component, but the dispersion component has a comparable contribution during weak ENSO periods.

Description: The physical and biological processes controlling surface mixed layer pCO2 and O2 were evaluated using in situ sensors mounted on a Lagrangian drifter deployed in the Atlantic sector of the Southern Ocean (∼50°S, ∼37°W) during the austral fall of 2008. The drifter was deployed three times during different phases of the study. The surface ocean pCO2 was always less than atmospheric pCO2 (−50.4 to −76.1 μatm), and the ocean was a net sink for CO2 with fluxes averaging between 16.2 and 17.8 mmol C m−2 d−1. Vertical entrainment was the dominant process controlling mixed layer CO2, with fluxes that were 1.8 to 2.2 times greater than the gas exchange fluxes during the first two drifter deployments, and was 1.7 times greater during the third deployment. In contrast, during the first two deployments the surface mixed layer was always a source of O2 to the atmosphere, and air-sea gas exchange was the dominant process occurring, with fluxes that were 2.0 to 4.1 times greater than the vertical entrainment flux. During the third deployment O2 was near saturation the entire deployment and was a small source of O2 to the atmosphere. Net community production (NCP) was low during this study, with mean fluxes of 3.2 to 6.4 mmol C m−2 d−1 during the first deployment and nondetectable (within uncertainty) in the third. During the second deployment the NCP was not separable from lateral advection. Overall, this study indicates that in the early fall the area is a significant sink for atmospheric CO2.

Description: The distribution and biological cycling of the climate active trace gas dimethylsulfide (DMS) and its algal precursor dimethylsulfoniopropionate (DMSP) were characterized at 20 stations across the Canadian High Arctic during fall 2007. Transformation rates of DMSP and production rates of DMS from dissolved DMSP (DMSPd) were measured during 3 h onboard incubations with radioactively labeled 35S-DMSP. Particulate DMSP (DMSPp) in surface waters varied between 2 and 39 nmol L−1 and increased with chlorophyll a (Chl a) concentrations (r = 0.84). DMS concentrations in surface waters ranged from 0.05 to 0.8 nmol L−1 and were positively correlated with DMSPp (r = 0.89) and Chl a (r = 0.74). The DMSPd loss rate constant varied from 0.01 to 0.14 h−1 and was also positively correlated with Chl a concentrations (r = 0.67). The turnover time of the DMSPd pool varied between 0.3 and 3.4 days (mean = 0.96 day). Bacterial DMS production varied between 0.01 and 0.51 nmol L−1 d−1 (mean = 0.14 nmol L−1 d−1). Assuming local steady state conditions at the time scale of a day, the turnover time of the DMS pool based only on production from DMSPd was ∼6 days at the sampling stations. This long turnover time suggests that DMS production was dominated by nonbacterial processes during our study. Our results show that DMS production could persist at low rates in late fall under ice-free conditions. The magnitude of this production appears to be limited by the low algal and bacterial production prevailing at that time.

Description: Five surf zone dye tracer releases from the HB06 experiment are simulated with a tracer advection diffusion model coupled to a Boussinesq surf zone model (funwaveC). Model tracer is transported and stirred by currents and eddies and diffused with a breaking wave eddy diffusivity, set equal to the breaking wave eddy viscosity, and a small (0.01 m2 s−1) background diffusivity. Observed and modeled alongshore parallel tracer plumes, transported by the wave driven alongshore current, have qualitatively similar cross-shore structures. Although the model skill for mean tracer concentration is variable (from negative to 0.73) depending upon release, cross-shore integrated tracer moments (normalized by the cross-shore tracer integral) have consistently high skills (≈0.9). Modeled and observed bulk surf zone cross-shore diffusivity estimates are also similar, with 0.72 squared correlation and skill of 0.4. Similar to the observations, the model bulk (absolute) cross-shore diffusivity is consistent with a mixing length parameterization based on low-frequency (0.001–0.03 Hz) eddies. The model absolute cross-shore dispersion is dominated by stirring from surf zone eddies and does not depend upon the presence of the breaking wave eddy diffusivity. Given only the bathymetry and incident wave field, the coupled Boussinesq-tracer model qualitatively reproduces the observed cross-shore absolute tracer dispersion, suggesting that the model can be used to study surf zone tracer dispersion mechanisms.

Description: Sea ice motion is an important element in mass balance calculations, ice thermodynamic modeling, ice management plans for industry, and ecosystems studies. In the historical literature, sea ice motion in the Beaufort Sea was characterized by a predominantly anticyclonic motion during winter months, with episodic reversals to cyclonic activity during summer. However, recent studies have shown an increase in cyclonic activity throughout the annual cycle. In this paper we examine circulation in the Beaufort Sea based on the trajectories of 22 ice beacons launched in the Franklin Bay area during the International Polar Year - Circumpolar Flaw Lead (IPY-CFL) study during an over-wintering experiment in 2007–2008. Dispersion characteristics of ice motion show that absolute zonal dispersion follows a t2 scaling law characteristic of advection associated with Beaufort Gyre circulation, whereas absolute meridional dispersion follows a scaling law of t5/4 characteristic of floaters and dispersion in 2-D turbulence. Temporal autocorrelations of ice velocity fluctuations highlight definitive timescales with values of 1.2 (0.7) days in the zonal (meridional) direction. Near-Gaussian behavior is reflected in higher-order moments for ice velocity fluctuation probability density functions (pdfs). Non-Gaussian behavior for absolute displacement pdfs indicates spatial heterogeneity in the ice motion fields. Atmospheric forcing of sea ice is explored through analysis of daily North American Regional Reanalysis and in situ wind data, where it is shown that ice in the CFL study region travels with an average speed of approximately 0.2% and an average angle of 51.5° to the right of the surface winds during the 2007–2008 winter. The results from this analysis further demonstrate seasonality in ice drift to wind ratios and angles that corresponds to stress buoy data indicative of increases in internal ice stress and connectivity due to consolidation of the seasonal ice zone to the coast and perennial ice pack during winter in the Beaufort Sea region.

Description: The horizontal dispersion of ocean tracers due to the interaction of vertical diffusion and the vertical shear from internal waves is examined using a Lagrangian particle-tracking model. For idealized, sinusoidal horizontal velocity profiles of unbounded vertical extent, the irreversible horizontal dispersion of particle clouds is maximal when the timescale of particle diffusion across the velocity variations and the period of shear oscillation are comparable. For measured velocity profiles from the thermocline in the Gulf of Aqaba (Red Sea), the total dispersion and irreversible dispersion of particle clouds both tend to increase with vertical diffusion, cloud height, and cloud age. The total and irreversible horizontal cloud variances typically grow with time (t) like t0 to t3 and t1 to t3, respectively. Estimates of the irreversible horizontal dispersivity from an existing analytical model based on vertically constant, oscillating shear agree remarkably well with estimates from the particle-tracking model, with differences dependent on the vertical variability in vertical diffusion and shear that are not incorporated in the analytical model.

Description: Warm salty Atlantic Water is the main source water for the Arctic Ocean and thus plays an important role in the mass and heat budget of the Arctic. This study explores interannual to decadal variability of Atlantic Water properties in the Nordic Seas area where Atlantic Water enters the Arctic, based on a reexamination of the historical hydrographic record for the years 1950–2009, obtained by combining multiple data sets. The analysis shows a succession of four multiyear warm events where temperature anomalies at 100 m depth exceed 0.4°C, and three cold events. Three of the four warm events lasted 3–4 years, while the fourth began in 1999 and persists at least through 2009. This most recent warm event is anomalous in other ways as well, being the strongest, having the broadest geographic extent, being surface-intensified, and occurring under exceptional meteorological conditions. Three of the four warm events were accompanied by elevated salinities consistent with enhanced ocean transport into the Nordic Seas, with the exception of the event spanning July 1989–July 1993. Of the three cold events, two lasted for 4 years, while the third lasted for nearly 14 years. Two of the three cold events are associated with reduced salinities, but the cold event of the 1960s had elevated salinities. The relationship of these events to meteorological conditions is examined. The results show that local surface heat flux variations act in some cases to reinforce the anomalies, but are too weak to be the sole cause.

Description: Tropical cyclones (TC) represent a powerful, albeit highly transient forcing able to redistribute ocean heat content locally. Recent studies suggest that TC-induced ocean mixing can have global climate impacts as well, including changes in poleward heat transport, ocean circulation, and thermal structure. In several previous modeling studies devoted to this problem, the TC mixing was treated as a permanent (constant in time) source of additional vertical diffusion in the upper ocean. In contrast, this study aims to explore the highly intermittent character of the mixing. We present results from a series of coupled climate experiments with different durations of the imposed intermittent mixing but where each has the same annual mean diffusivity. All simulations show robust changes in sea surface temperature and ocean subsurface temperature, independent of the duration of the mixing that varies between the experiments from a few days to a full year. Simulated temperature anomalies are characterized by a cooling in the subtropics, a moderate warming in middle to high latitudes, a pronounced warming of the equatorial cold tongue, and a deepening of the tropical thermocline. These effects are paralleled by substantial changes in ocean and atmosphere circulation and heat transports. While the general patterns of changes remain the same from one experiment to the next, their magnitude depends on the relative duration of the mixing. Stronger mixing, but of a shorter duration, has less of an impact. These results agree with a simple model of heat transfer for the upper ocean with a time-dependent vertical diffusivity.

Description: Direct measurements of air-sea heat, momentum, and mass (including CO2, DMS, and water vapor) fluxes using the direct covariance method were made over the open ocean from the NOAA R/V Ronald H. Brown during the Southern Ocean Gas Exchange (SO GasEx) program. Observations of fluxes and the physical processes associated with driving air-sea exchange are key components of SO GasEx. This paper focuses on the exchange of CO2 and the wind speed dependency of the transfer velocity, k, used to model the CO2 flux between the atmosphere and ocean. A quadratic dependence of k on wind speed based on dual tracer experiments is most frequently encountered in the literature. However, in recent years, bubble-mediated enhancement of k, which exhibits a cubic relationship with wind speed, has emerged as a key issue for flux parameterization in high-wind regions. Therefore, a major question addressed in SO GasEx is whether the transfer velocities obey a quadratic or cubic relationship with wind speed. After significant correction to the flux estimates (primarily due to moisture contamination), the direct covariance CO2 fluxes confirm a significant enhancement of the transfer velocity at high winds compared with previous quadratic formulations. Regression analysis suggests that a cubic relationship provides a more accurate parameterization over a wind speed range of 0 to 18 m s−1. The Southern Ocean results are in good agreement with the 1998 GasEx experiment in the North Atlantic and a recent separate field program in the North Sea.

Description: Many studies have investigated bubble size distributions in the ocean, but the measured size range does not normally extend to bubbles with a radius below 20 μm. Bubbles smaller than this are thought to have a significant effect on the optical properties of the ocean, potentially affecting remotely sensed measurements of ocean color and the optical detection of particulates and dissolved matter. Such optical data are becoming the major source of oceanic information about algal blooms, primary productivity, sediment loading and the spread of pollutants. The challenges associated with measuring these bubbles are difficulty of calibrating sensors with independent bubble size measurements and lack of knowledge about the organic coating on the bubbles. This paper describes simultaneous oceanic measurements of these small bubbles using independent optical and acoustical techniques. These measurements agree well, and an investigation of the bubble coating parameters was made. Both the optical and acoustical properties of bubbles are affected by this organic coating, and a comparison of these measurements narrows down the choice of possible coating parameters. Our results suggest that the bubbles measured in this study were likely to have a coating with a thickness of 10 nm and a refractive index of 1.18, and that the coating thickness is the more important parameter for optical inversions. The research described here is the first attempt to constrain these parameters in the ocean using two independent techniques and suggests that further studies of this type could result in significant insight into oceanic bubble coatings.

Description: An ocean reanalysis that covers the period from 1871 to 2008 is used to analyze the interannual variability of sea surface salinity (SSS) in the tropical Indian Ocean. The reanalysis SSS and the SSS anomaly patterns during Indian Ocean Dipole (IOD) and El Niño–Southern Oscillation (ENSO) events are compared with patterns from Argo SSS data. The mean seasonal SSS variation is large in the northern Bay of Bengal compared with variations in the Arabian Sea and equatorial Indian Ocean. During a positive IOD event, positive SSS anomalies are found along the Sumatra coast that are due to the combination of wind-driven upwelling of subsurface high-salinity waters, enhanced evaporation, and anomalous surface circulation. The opposite is true, to a lesser extent, during negative IOD events. A dipole mode index for salinity (DMIS) based on SSS data and a new index based on the average of salinity in a region off the coast of Sumatra are introduced to monitor SSS variability during IOD and ENSO events. The impact of concomitant El Niño events on a positive IOD event is large with freshening (a negative SSS anomaly) in the equatorial Indian Ocean and salting (positive SSS anomaly) off the southern Sumatra coast. The (impact of) intense freshening reaches into the southwestern tropical Indian Ocean. The impact of concomitant La Niña with negative IOD is also large with an intense freshening in the southeastern Arabian Sea and salting off the northern Sumatra coast.

Description: Southern Ocean (SO) sea ice simulations are particularly sensitive to wind forcing. Two real-time wind data sets covering the same period are employed to force SO sea ice in a sea ice–ocean general circulation model. Both data sets are analysis products, featuring the same temporal resolution but differing in their horizontal resolution and their source. Even in simulations where the upper ocean temperature is constrained by satellite-derived sea ice concentration, the sea ice simulations and associated surface buoyancy fluxes reveal pronounced differences along the Antarctic coastline. While the discrepancies cannot unambiguously be related to the different resolution of the wind forcing, their concentration along the coastline is indicative of being related to the representation of orography, such as coastal steep slopes and mountain ranges, including their ruggedness. Along the coast of the Weddell Sea, the net ice production rate increases by about a factor of 3 with the higher-resolution winds. On the other hand, along east Antarctica, the lower-resolution winds result in higher ice production, due to a generally stronger (overestimated) offshore component, presumably related to the smoother orography extending seaward beyond the coastline. This regionally opposite behavior leads to a relatively weak difference in total dense water formation around Antarctica and thus global deep ocean properties and circulation. Overall, the results indicate that long-term climate model projections are likely to be highly sensitive to model resolution in the Antarctic coastal zone.

Description: The Arctic Ocean is an optically complex environment and presents unique challenges for ocean color satellite remote sensing. Phytoplankton pigment packaging, high concentrations of chromophoric dissolved organic matter (CDOM), and the frequent presence of subsurface chlorophyll a (Chl a) maxima (SCM), complicate satellite measurement of surface Chl a. However, the impact of likely errors in surface Chl a on satellite-based estimates of depth-integrated daily net primary production (NPP) have yet to be quantified. Here we use a large in situ Chl a and primary production database (ARCSS-PP) to calculate the magnitude of the error that likely results from both omission of SCM and overestimated phytoplankton biomass when satellite-based Chl a is used as input to an NPP algorithm. Results show that errors in pan-Arctic NPP, due to omission of the SCM, increase from 0.2% in January to 16% in July and are largest in the Beaufort and Chukchi Sea. Over an annual cycle, the error is approximately 8%. Errors in regional NPP resulting from overestimates of surface Chl a by Sea-viewing Wide Field-of-view Sensor can be larger, particularly when surface Chl a and NPP are low, but are partially offset by underestimates in NPP due to omission of NPP at the SCM. As a result, the combined effect of underestimates in NPP due to omission of the SCM and overestimates in NPP due to high satellite Chl a yields a total error in annual pan-Arctic, depth-integrated NPP of

Description: An atmosphere-ocean mixed layer coupled model is developed to predict the diurnal variability of sea surface temperature (SST). For this purpose, a new mixed layer model is developed, which is able to reproduce realistic temperature profiles under the various atmospheric conditions, ranging from the formation of a diurnal thermocline under strong wind to the appearance of strong near surface stratification under weak wind. The predicted diurnal warming of SST (ΔSST) from the model is compared with satellite and buoy data in various aspects, including scatterplots, time series, and probability density functions of ΔSST, in order to examine the predictability. The model performance is also compared with other model results. In addition the diurnal variation of temperature profiles below the sea surface, whose information is not available from satellite data, is investigated based on model output.

Description: Phytoplankton chlorophyll-a (Chl) seasonal cycles of the North Atlantic are described using satellite ocean color observations covering the 1980s and the 2000s. The study region is where warmer SST and higher Chl in the 2000s as compared to the 1980s have been reported. It covers latitudes from 30°N–50°N and longitudes from 60°W–0°W, where two phytoplankton blooms take place: a spring bloom that follows stratification of upper layers, and a fall bloom due to nutrient entrainment through deepening of the mixed layer. In the 1980s, spring and fall blooms were of similar amplitude over the entire study region. In the 2000s, the fall bloom was weaker in the eastern Atlantic (east of 40°W), because of a delayed deepening of the mixed layer at the end of summer (mixed layer depth (MLD) determined from in situ data). Conversely, the spring bloom of the eastern Atlantic was stronger in the 2000s than it was in the 1980s, because of a deeper MLD and stronger winds in winter. In the Northwestern Atlantic (northwest of 38°N–40°W), little differences are observed for spring and fall blooms, and for the wintertime MLD. Our results show that the links between upper layer stratification, SST changes, and biological responses are more complex than the simple paradigm that sequentially relates higher stratification with warmer SST and an enhanced (weakened) growth of the phytoplankton population in the subpolar (subtropical) region.

Description: In this paper we report measurements from the first year-round mooring underneath sea ice in McMurdo Sound, Antarctica, which we combine with full-depth ocean profiles to identify the incremental appearance of potentially supercooled ice shelf water (ISW). We investigate the effects of ISW on sea ice using observations of sea ice growth and crystal structure together with under-ice photography. We show that the appearance of ISW at the surface leads to a disruption in the columnar texture of the sea ice, but that persistent growth enhancement occurs only once the entire water column has cooled to the surface freezing point. In doing so, we demonstrate the possibility of inferring the presence of ISW beneath sea ice through crystallographic analysis of cores. These findings will be useful for both modeling and observing the extent of ISW-enhanced ice growth. In addition, we found that the local growth of first-year landfast sea ice only accounted for half of the observed increase in salinity over the water column, which indicates that polynyas are responsible for approximately half of the salt flux into McMurdo Sound.

Description: Spaceborne observations of sea surface topography have revealed a significant interannual variability of the Azores Current strength and eddy energy. The objective of this paper is to establish the relationship between these variations and atmospheric forcing over the subtropical North Atlantic. Based on satellite altimetry, hydrography, and atmospheric reanalysis products, it is demonstrated that the interannual variability of the Azores Current eastward velocity and eddy energy may be driven by the adjustment of the ocean to the strength of westerly and trade winds, modulated by the North Atlantic Oscillation. Surface intensification (frontogenesis), which is mainly due to the wind-driven meridional Ekman current convergence, is found significant, but not sufficient to explain the observed interannual variability of the Azores Current strength.

Description: Frictional sliding plays a fundamental role in the deformation of the Arctic Ocean ice cover and in ice-structure interactions. To examine its character, sliding experiments were performed along freshly created Coulombic faults in first-year S2 sea ice harvested from the Beaufort Sea during the winters of 2003, 2007, and 2009. For comparison, and to complement experiments performed earlier, we also examined freshwater ice of similar microstructure. The principal variables were sliding velocity (from 8 × 10−7 m s−1 to 4 × 10−3 m s−1), temperature (−40°C, −10°C, and −3°C) and normal stress (from 0.02 to 2.7 MPa). Over the ranges explored, the coefficient of friction varies by a factor of four, from ∼0.4 to ∼1.6. The coefficient for both materials reaches a maximum value at an intermediate velocity, which is an order of magnitude higher for sea ice (∼10−4 m s−1) than for freshwater ice (∼10−5 m s−1). At lower velocities, sliding is characterized by velocity strengthening where, for a given velocity, the coefficient of friction of sea ice is lower than that of freshwater ice. At higher velocities, sliding is characterized by velocity weakening, where, for a given velocity, the coefficients of friction for sea ice is closer to that of freshwater ice. Velocity strengthening is explained in terms of creep deformation within contact zones along the fault. Velocity weakening is tentatively attributed to a combination of fracture and localized melting. An implication of stable versus unstable sliding is discussed for sea ice mechanics.

Description: Data are presented from a survey by airborne scanning laser profilometer and an AUV-mounted, upward looking swath sonar in the spring Beaufort Sea. The air-snow (surface elevation) and water-ice (draft) surfaces were mapped at 1 × 1 m resolution over a 300 × 300 m area. Data were separated into level and deformed ice fractions using the surface roughness of the sonar data. The relation (R = d/f) between draft, d, and surface elevation, f, was then examined. Correlation between top and bottom surfaces was essentially zero at full resolution, requiring averaging over patches of at least 11 m diameter to constrain the relation largely because of the significant error (∼15 cm) of the laser instrument. Level ice points were concentrated in two core regions, corresponding to level FY ice and refrozen leads, with variations in R attributed primarily to positive snow thickness variability. Deformed ice displayed a more diffuse “cloud,” with draft having a more important role in determining R because of wider deformed features underwater. Averaging over footprints similar to satellite altimeters showed the mean surface elevation (typical of ICESat) to be stable with averaging scale, with R = 3.4 (level) and R = 4.2 (deformed). The “minimum elevation within a footprint” characteristic reported for CryoSat was less stable, significantly overestimating R for level ice (R 〉 5) and deformed ice (R 〉 6). The mean draft difference between measurements and isostasy suggests 70 m as an isostatic length scale for level ice. The isostatic scale for deformed ice appears to be longer than accessible with these data (〉300 m).

Description: During the 2007 UK SOLAS Deep Ocean Gas Exchange Experiment in the northeast Atlantic Ocean, we conducted the first ever study of the effect of a deliberately released surfactant (oleyl alcohol) on gas transfer velocities (kw) in the open ocean. Exchange rates were estimated with the 3He/SF6 dual tracer technique and from measured sea-to-air DMS fluxes and surface water concentrations. A total of seven kw estimates derived from 3He/SF6 were made, two of which were deemed to be influenced by the surfactant. These exhibited suppression from ∼5% to 55% at intermediate wind speeds (U10) in the range 7.2–10.7 m s−1. Similarly, kw determined from DMS data (kDMS) was also depressed by the surfactant; suppression ranged from ∼39% at 5.0 m s−1 to ∼24% at 10.8 m s−1. Surfactant thus has the potential to measurably suppress gas exchange rates even at moderate to high wind speeds.

Description: A new global ocean-atmosphere model has been developed to determine the penetration of ultraviolet (UV) radiation through the water column. This is accomplished by combining an atmospheric UV irradiance model, taking into consideration the effects of aerosols, clouds, and the air-sea interface, with empirical in-water diffuse attenuation coefficient (Kd(λUV)) relationships. These empirical relationships are derived from simultaneous in situ profiles of visible wavelength inherent optical properties and downwelling UV irradiances. The combined model is applied to global data sets using a look-up table approach to speed up calculation time. The atmospheric model compared against ∼3000 data points gave a root-mean-square error (RMSE) of between 10% and 15% at wavelengths of 305, 325, 340, and 380 nm; the coupled global model compared against 30 independent in-water irradiance profiles gave a logarithmic RMSE of between 0.15 and 0.35 at these wavelengths. On the global scale the 10% irradiance levels were found to be deepest in the oceanic gyres (∼18, 32, 44, and 70 m at 305, 325, 340 and 380 nm, respectively) and shallowest in the optically complex continental shelf regions. The calculated UV doses were shown to be spectrally and seasonally variable, with the highest values being encountered in the eastern Mediterranean during July, with values of ∼0.5, 4, 7, and 10 kJ m−2 d−1 nm−1 at 305, 325, 340, and 380 nm, respectively.

Description: A recent hydrographic section at 24.5°N in the Atlantic and 6 months of observations from a moored array show that Antarctic Bottom Water (AABW), the densest and deepest water mass in the world oceans, has been warming. While Johnson et al. (2008) showed that northward AABW transport at 24.5°N has been declining from 1981 to 2004, suggesting that the lower cell of the overturning circulation could halt in the near future, estimates from the latest hydrographic section in 2010 indicate a partial recovery of northward AABW transport. From 6 months of temperature and salinity observations at a deep moored array at 24–26°N, we find that short-term variability between April and November 2009 is of the same magnitude as the changes observed from hydrographic sections between 1981 and 2004. These observations highlight the possibility that transport changes estimated from hydrographic sections may be aliased by short-term variability. The observed AABW transport variability affects present estimates of the upper meridional overturning circulation by ±0.4 Sv (1 Sv = 106 m3 s−1).

Description: The mesoscale, submesoscale, and microscale structure of a front in the California Current was observed using a towed vehicle outfitted with microconductivity sensors. Thirteen 〉60 km cross-front sections from 0 to 350 m in depth were covered in 3.5 days. Objectively mapped data are fit via the Omega (ω) equation to obtain vertical velocity. A composite cross-front section shows elevated mixing on the dense side within 10–20 km of the front. Water downwells and gradients are elevated there: i.e., Rossby number (Ro), horizontal strain (α), spice gradients, and microscale thermal dissipation (χ). Thermal eddy diffusivity (KT) reaches 10−3 m2 s−1 and increases 3–10× from the anticyclonic to the cyclonic side with a depth mean of ∼10−4 m2 s−1. The spatial structure of KT, Ro, and α are similar on the dense side, suggesting an energy cascade from the mesoscale via the submesoscale to the microscale. However, it is unclear whether frontogenesis, internal wave blocking by elevated vorticity, or internal wave trapping by large α produces the elevated mixing. The mean turbulent heat flux opposes the mean restratifying, mesoscale heat flux of 10 W m−2 and may allow the front to persist. Turbulent nitrate fluxes are 0.1–0.3 mmol m−2 s−1. Chlorophyll fluorescence and beam transmission reveal a

Description: The Antarctic Circumpolar Current (ACC), with its associated three-dimensional circulation, plays an important role in global climate. This study concentrates on surface signatures of recent climate change in the ACC region and on mechanisms that control this change. Examination of climate model simulations shows that they match the observed late 20th century sea-surface temperature (SST) trends averaged over this region quite well, despite underestimating the observed surface-wind increases. Such wind increases, however, are expected to lead to significant cooling of the region, contradicting the observed SST trends. Motivated by recent theories of the ACC response to variable wind and radiative forcing, the authors used two idealized models to assess contributions of various dynamical processes to the SST evolution in the region. In particular, a high-resolution channel model of the ACC responds to increasing winds by net surface ACC warming due to enhanced mesoscale turbulence and associated heat transports in the mixed layer. These fluxes, modeled, in a highly idealized fashion, via increased lateral surface mixing in a coarse-resolution hybrid climate model, substantially offset zonally non-uniform surface cooling due to air-sea flux and Ekman-transport anomalies. These results suggest that the combination of these opposing effects must be accounted for when estimating climate response to any external forcing in the ACC region.

Description: We implement the effects of gravitational self-attraction and loading (SAL) into a global baroclinic ocean circulation model and investigate effects on sea level patterns, ocean circulation, and density distributions. We compute SAL modifications as an additional force on the water masses at every time step by decomposing the field of ocean bottom pressure anomalies into spherical harmonic functions and then applying Love numbers to account for the elastic properties of the solid Earth. Considering SAL in the postprocessing turns out to be insufficient, especially in coastal waters and on subweekly time scales, where SAL modifies local sea level by around 0.6–0.8 cm on average; in the open ocean, changes mostly remain around 0.3 cm. Modifications of water velocities as well as of heat and salt distributions are modeled, yet they are small. Simple parameterizations of SAL effects currently used in a number of ocean circulation models suffer from the process's inhomogeneity in space and time. These parameterizations improve the modeled sea level patterns but fail to reproduce SAL impacts on circulation and density distributions. We therefore suggest to explicitly consider the full SAL effect in ocean circulation models, especially when investigating sea level variations faster than around 4 days.

Description: Recent works show that multichannel seismic (MCS) systems are able to provide detailed information on the oceans' fine structure. The aim of this paper is to analyze whether 1-D full waveform inversion algorithms are suitable to recover the extremely weak acoustic impedance contrasts associated to the oceans' fine structure, as well as their potential to image meso-scale objects such as meddies. We limited our analysis to synthetic, noise-free data, in order to identify some methodological issues related to this approach under idealistic conditions (e.g., 1-D wave propagation, noise-free data, known source wavelet). We first discuss the influence of the starting model in the context of the multi-scale strategy that we have implemented. Then we show that it is possible to retrieve not only sound speed but also salinity and temperature contrasts within reasonable bounds from the seismic data using Neural Network relationships trained with regional oceanographic data sets. Potentially, the vertical resolution of the obtained models, which depends on the maximum frequency inverted, is of the order of 5–10 m, whereas the root mean square error of the inverted properties is shown to be ∼0.5 m/s for sound speed, 0.1°C for temperature, and 0.06 for salinity. To conclude this study, we have inverted synthetic data simulated along an oceanographic transect acquired during the EU-funded Geophysical Oceanography (GO) project. The results demonstrate the applicability of the method for synthetic data, as well as its potential to define oceanographic features along 2-D transects at full ocean depth with excellent lateral resolution.

Description: Ocean evaporation (E) and precipitation (P) are the fundamental components of the global water cycle. They are also the freshwater flux forcing (i.e., E-P) for the open ocean salinity. The apparent connection between ocean salinity and the global water cycle leads to the proposition of using the oceans as a rain gauge. However, the exact relationship between E-P and salinity is governed by complex upper ocean dynamics, which may complicate the inference of the water cycle from salinity observations. To gain a better understanding of the ocean rain gauge concept, here we address a fundamental issue as to how E-P and salinity are related on the seasonal timescales. A global map that outlines the dominant process for the mixed-layer salinity (MLS) in different regions is thus derived, using a lower-order MLS dynamics that allows key balance terms (i.e., E-P, the Ekman and geostrophic advection, vertical entrainment, and horizontal diffusion) to be computed from satellite-derived data sets and a salinity climatology. Major E-P control on seasonal MLS variability is found in two regions: the tropical convergence zones featuring heavy rainfall and the western North Pacific and Atlantic under the influence of high evaporation. Within this regime, E-P accounts for 40–70% MLS variance with peak correlations occurring at 2–4 month lead time. Outside of the tropics, the MLS variations are governed predominantly by the Ekman advection, and then vertical entrainment. The study suggests that the E-P regime could serve as a window of opportunity for testing the ocean rain gauge concept once satellite salinity observations are available.

Description: We calculate net community production (NCP) during summer 2005–2006 and spring 2006 in the Ross Sea using multiple approaches to determine the magnitude and consistency of rates. Water column carbon and nutrient inventories and surface ocean O2/Ar data are compared to satellite-derived primary productivity (PP) estimates and 14C uptake experiments. In spring, NCP was related to stratification proximal to upper ocean fronts. In summer, the most intense C drawdown was in shallow mixed layers affected by ice melt; depth-integrated C drawdown, however, increased with mixing depth. ΔO2/Ar-based methods, relying on gas exchange reconstructions, underestimate NCP due to seasonal variations in surface ΔO2/Ar and NCP rates. Mixed layer ΔO2/Ar requires approximately 60 days to reach steady state, starting from early spring. Additionally, cold temperatures prolong the sensitivity of gas exchange reconstructions to past NCP variability. Complex vertical structure, in addition to the seasonal cycle, affects interpretations of surface-based observations, including those made from satellites. During both spring and summer, substantial fractions of NCP were below the mixed layer. Satellite-derived estimates tended to overestimate PP relative to 14C-based estimates, most severely in locations of stronger upper water column stratification. Biases notwithstanding, NCP-PP comparisons indicated that community respiration was of similar magnitude to NCP. We observed that a substantial portion of NCP remained as suspended particulate matter in the upper water column, demonstrating a lag between production and export. Resolving the dynamic physical processes that structure variance in NCP and its fate will enhance the understanding of the carbon cycling in highly productive Antarctic environments.

Description: Determination of the relative inputs of aquatic autochthonous and terrestrial allochthonous organic matter into marine and lacustrine environments is essential to understanding the global carbon budget. A variety of proxies are used for this purpose, including the Branched and Isoprenoid Tetraether (BIT) index. This is calculated from the concentrations of branched glycerol dialkyl glycerol tetraethers (GDGTs), derived from unidentified terrestrial bacteria, and crenarchaeol, a marker for aquatic mesophile Thaumarchaeota (Crenarchaeota group I). As the index is a ratio, its value depends on both the crenarchaeol aquatic in situ production and the soil-derived branched GDGT input. Therefore, the BIT index reflects not only changes in the input of terrestrial or soil organic matter but also relative variations in aquatic Thaumarchaeota abundance in the water column. In fact, we show that in oceanic and lacustrine settings, the BIT index can be dominated by the aquatic end-member of the ratio. Consequently, the BIT index by itself can be an unreliable proxy to compare the input of terrestrial matter between sites and over time, and we propose that the quantification of branched GDGT fluxes or concentrations may instead be a better indicator of soil terrestrial inputs.

Description: Models are routinely used to remove the effects of global ocean tides from GRACE data during processing to reduce temporal aliasing into monthly GRACE solutions. These models have typically been derived using data from satellite altimeters such as TOPEX/Poseidon. Therefore the Arctic ocean components of tide models are not constrained by altimetry data, potentially resulting in errors that are likely to alias into monthly GRACE gravity fields at all latitudes. Seven years of GRACE inter-satellite accelerations are inverted to solve for corrections to the amplitude and phase of major solar and lunar ocean tides at latitudes north of 50°N using a mascon approach. The tide model originally applied to our data was FES2004, truncated to maximum degree lmax = 90. Simulations are performed to verify that our inversion algorithm works as designed. Uncertainty estimates are derived from tidal solutions on land, and by subtracting two independent solutions that each use 3.5 years of data. Features above the noise floor in the M2, K1, S2, and O1 solutions likely represent errors in FES2004. Errors due to truncating the spherical harmonic expansion of FES2004 are too small, and errors in the land mask model (needed to transform sea surface heights into mass) only affect coastal areas and do not produce similar relative amplitudes for any examined tides. In the oceans north of 50°N, these residuals tend to reduce the FES2004 amplitudes for M2, K1, S2, and O1. Reductions in the variance of accelerations not used in our inversion suggest that our results can be used to improve GRACE processing.

Description: We studied the nitrogen biogeochemistry of the ice-covered eastern Bering Sea shelf using the isotope ratios (15N/14N and 18O/16O) of NO3− and other N species. The 15N/14N of late winter NO3− on the shelf decreases inshore and is inversely correlated with bottom water [NH4+], consistent with an input of low-15N/14N NO3− from partial nitrification of NH4+ remineralized from the sediments. An inshore 15N/14N increase in total dissolved N (TDN) suggests that (1) the sediment-derived NH4+ is elevated in 15N due to the same partial nitrification that yields the low-15N/14N NO3−, and (2) 15N-deplete NO3− from partial nitrification within the sediments is denitrified to N2. The proportion of newly nitrified NO3− on the shelf, evidenced by an inshore decrease in NO3− 18O/16O, is correlated with the N deficit, further implicating nitrification coupled to denitrification; however, a simple N isotope budget indicates a comparable rate of denitrification supported by diffusion of NO3− into the sediments. The isotopic impact of benthic N loss is further demonstrated by a correlation between the 15N/14N of shelf surface sediment and the N deficit of the overlying water column, both of which increase inshore and northward, as well as by Arctic NO3− isotope data indicating that the fixed N transported through Bering Strait has a 15N/14N higher than is found in the open Bering Sea. The significant net isotope effect of benthic N loss on the Bering shelf, 6–8 ‰, is at odds with previous assumptions regarding the global ocean's N isotope budget.

Description: This study assesses the accuracy of tide model predictions in the Amundsen Sea sector of West Antarctica. Tide model accuracy in this region is poorly constrained, yet tide models contribute to simulations of ocean heat transfer and to the removal of tidal signals from satellite observations of ice shelves. We use two satellite-based interferometric synthetic aperture radar (InSAR) methods to measure the tidal motion of the Dotson Ice Shelf at multiple epochs: a single-difference technique that measures tidal displacement and a double-difference technique that measures changes in tidal displacement. We use these observations to evaluate predictions from three tide models (TPXO7.1, CATS2008a_opt, and FES2004). All three models perform comparably well, exhibiting root-mean-square deviations from the observations of ∼9 cm (single-difference technique) and ∼10 cm (double-difference technique). Care should be taken in generalizing these error statistics because (1) the Dotson Ice Shelf experiences relatively small semidiurnal tides and (2) our observations are not sensitive to all tidal constituents. An error analysis of our InSAR-based methods indicates measurement errors of 7 and 4 cm for the single- and double-difference techniques, respectively. A model-based correction for the effect of fluctuations in atmospheric pressure yields an ∼6% improvement in the agreement between tide model predictions and observations. This study suggests that tide model accuracy in the Amundsen Sea is comparable to other Antarctic regions where tide models are better constrained. These methods can be used to evaluate tide models in other remote Antarctic waters.

Description: The Eastern Weddell Sea is characterized by narrow continental shelves and Warm Deep Water (WDW) is located in close proximity to the ice shelves in this region. The exchange of WDW across the Antarctic Slope Front (ASF) determines the rate of basal ice shelf melting. Here, we present a unique data set consisting of 2351 vertical profiles of temperature and salinity collected by southern elephant seals (Mirounga leonina) and a profile beneath the Fimbul Ice Shelf (FIS), obtained via drilling through 395 m of ice. This data set reveals variations in salinity and temperature through winter, and using a conceptual model of the coastal salt budget we quantify the main exchange processes. Our data show that modified WDW, with temperatures below −1.5°C, is advected onto the shelf and into the ice shelf cavities by an eddy overturning of the ASF. The onshore Ekman flux of surface waters during summer is the main source of freshwater that leads to the formation of low salinity shelf waters in the region. The modified WDW that reaches beneath the ice shelves is too cold for basal ice shelf melting to create such low salinity water. A high-resolution model of an idealized ASF–continental shelf–ice shelf system supports the conclusions from the data analysis. The inflow of WDW onto the continental shelf and into the ice shelf cavity occurs within a bottom boundary layer where the eddy advection in the model is particularly strong, in close agreement with the observed vertical profile of temperature beneath the FIS.

Description: During the spring of 2009, an ultrawideband microwave radar was deployed as part of Operation IceBridge to provide the first cross-basin surveys of snow thickness over Arctic sea ice. In this paper, we analyze data from three ∼2000 km transects to examine detection issues, the limitations of the current instrument, and the regional variability of the retrieved snow depth. Snow depth is the vertical distance between the air-snow and snow-ice interfaces detected in the radar echograms. Under ideal conditions, the per echogram uncertainty in snow depth retrieval is ∼4–5 cm. The finite range resolution of the radar (∼5 cm) and the relative amplitude of backscatter from the two interfaces limit the direct retrieval of snow depths much below ∼8 cm. Well-defined interfaces are observed over only relatively smooth surfaces within the radar footprint of ∼6.5 m. Sampling is thus restricted to undeformed, level ice. In early April, mean snow depths are 28.5 ± 16.6 cm and 41.0 ± 22.2 cm over first-year and multiyear sea ice (MYI), respectively. Regionally, snow thickness is thinner and quite uniform over the large expanse of seasonal ice in the Beaufort Sea, and gets progressively thicker toward the MYI cover north of Ellesmere Island, Greenland, and the Fram Strait. Snow depth over MYI is comparable to that reported in the climatology by Warren et al. (1999). Ongoing improvements to the radar system and the utility of these snow depth measurements are discussed.

Description: The near seabed mean and turbulent processes on the continental slope were measured for a three week period using an array of acoustic-Doppler velocimeters and thermistors over the bottom 30 m at the 400 m isobath. Baroclinic motions with characteristics similar to internal bores or boluses propagated onshore during the flood phase of both spring and neap tides. The arrival time of these internal bores at our measurement site varied amongst tidal cycles and their characteristics were not highly correlated with the amplitude of the barotropic tidal forcing. The passage of the internal bores was associated with large turbulent overturns, enhanced turbulent kinetic energy dissipation ($\epsilon$ 〉 10−6 W kg−1) and intensified currents (〉6 times the barotropic forcing) within meters of the seabed. During the deployment, stratification and shear competed to govern our observed overturning length scale ($\lesssim$4 m) that were characterized by the Ellison length scale LE. Only measurements closest to the seabed (1.7 m) were described by the log law-of-the-wall; generally both buoyancy and the presence of the bottom boundary influenced LE, while sometimes flow-induced shear determined LE. As the distance of our measurements from the seabed increased, the influence of buoyancy became more pronounced. These results highlight that a more general descriptor of the overturning length scale is necessary for complex stratified shear flows.

Description: The advance and retreat of sea ice produces seasonal convection and stratification, dampens surface waves and creates a separation between the ocean and atmosphere. These are all phenomena that can affect the air-sea gas transfer velocity (k660), and therefore it is not straightforward to determine how sea ice cover modulates air-sea flux. In this study we use field estimates k660 to examine how sea ice affects the net gas flux between the ocean and atmosphere. An inventory of salinity, 3He, and CFC-11 in the mixed layer is used to infer k660 during the drift of Ice Station Weddell in 1992. The average of k660 is 0.11 m d−1 across nearly 100% ice cover. In comparison, the only prior field estimates of k660 are disproportionately larger, with average values of 2.4 m d−1 across 90% sea ice cover, and 3.2 m d−1 across approximately 70% sea ice cover. We use these values to formulate two scenarios for the modulation of k660 by the fraction of sea ice cover in a 1-D transport model for the Southern Ocean seasonal ice zone. Results show the net CO2 flux through sea ice cover represents 14–46% of the net annual air-sea flux, depending on the relationship between sea ice cover and k660. The model also indicates that as much as 68% of net annual CO2 flux in the sea ice zone occurs in the springtime marginal ice zone, which demonstrates the need for accurate parameterizations of gas flux and primary productivity under partially ice-covered conditions.

Description: A numerical model is used to investigate the time and space extent of the near-surface temperature maximum (NSTM) of the Canadian Basin of the Arctic Ocean over the years 2000–2009. The NSTM is formed from local summertime absorption of solar radiation which, in some circumstances, descends through the fall and early winter to form a warm subsurface layer just below the winter mixed layer. We find that winter survival of this layer is confined largely to the Beaufort Gyre of the Canadian Basin, where Ekman convergence and downwelling push the summer warm layer down below the winter mixing depth. In recent years, summer stratification has increased, downwelling has accelerated, and the NSTM has warmed as the sea ice cover in the Beaufort Gyre has thinned. The result is a strengthening NSTM which contained enough heat by the end of winter 2007/2008 to melt about 20 cm of sea ice. Northwest of Alaska the model also simulates a second, deeper temperature maximum layer that forms from advection of saltier summer Pacific water. However, this layer is difficult to adequately resolve and maintain given the model's resolution.

Description: A model that accurately simulates surf zone waves, mean currents, and low-frequency eddies is required to diagnose the mechanisms of surf zone tracer transport and dispersion. In this paper, a wave-resolving time-dependent Boussinesq model is compared with waves and currents observed during five surf zone dye release experiments. In a companion paper, Clark et al. (2011) compare a coupled tracer model to the dye plume observations. The Boussinesq model uses observed bathymetry and incident random, directionally spread waves. For all five releases, the model generally reproduces the observed cross-shore evolution of significant wave height, mean wave angle, bulk directional spread, mean alongshore current, and the frequency-dependent sea surface elevation spectra and directional moments. The largest errors are near the shoreline where the bathymetry is most uncertain. The model also reproduces the observed cross-shore structure of rotational velocities in the infragravity (0.004 〈 f 〈 0.03 Hz) and very low frequency (VLF) (0.001 〈 f 〈 0.004 Hz) bands, although the modeled VLF energy is 2–3 times too large. Similar to the observations, the dominant contributions to the modeled eddy-induced momentum flux are in the VLF band. These eddies are elliptical near the shoreline and circular in the mid surf zone. The model-data agreement for sea swell waves, low-frequency eddies, and mean currents suggests that the model is appropriate for simulating surf zone tracer transport and dispersion.

Description: The mean vertical structure of mesoscale eddies in the Peru-Chile Current System is investigated by combining the historical records of Argo float profiles and satellite altimetry data. A composite average of 420 (526) profiles acquired by Argo floats that surfaced into cyclonic (anticyclonic) mesoscale eddies allowed constructing the mean three-dimensional eddy structure of the eastern South Pacific Ocean. Key differences in their thermohaline vertical structure were revealed. The core of cyclonic eddies (CEs) is centered at ∼150 m depth within the 25.2–26.0 kg m−3 potential density layer corresponding to the thermocline. In contrast, the core of the anticyclonic eddies (AEs) is located below the thermocline at ∼400 m depth impacting the 26.0–26.8 kg m−3 density layer. This difference was attributed to the mechanisms involved in the eddy formation. While intrathermocline CEs would be formed by instabilities of the surface equatorward coastal currents, the subthermocline AEs are likely to be shed by the subsurface poleward Peru-Chile Undercurrent. In the eddy core, maximum temperature and salinity anomalies are of ±1°C and ±0.1, with positive (negative) values for AEs (CEs). This study also provides new insight into the potential impact of mesoscale eddies for the cross-shore transport of heat and salt in the eastern South Pacific. Considering only the fraction of the water column associated with the fluid trapped within the eddies, each CE and AE has a typical volume anomaly flux of ∼0.1 Sv and yields to a heat and salt transport anomaly of ±1–3 × 1011 W and ±3–8 × 103 kg s−1, respectively.

Description: We present observations of high-frequency current variability on the continental shelf and the slope of the Antarctic Peninsula using Lagrangian surface drifters deployed as part of the Antarctic Drifter Experiment: Links to Isobaths and Ecosystems (ADELIE) project. Here we focus on high-frequency processes such as tides and inertial oscillations that are typically smoothed out of large-scale spatially averaged, and/or temporally averaged, observed current fields. We investigate the role that this class of motion plays in the transport of physical or biogeochemical properties. Lateral displacements on the shelf and slope are found to be larger than displacements in deeper waters where tidal currents are negligible. We apply this result in a parameterization of the lateral dispersion during an off-line drifter modeling study. The outcome is an improvement on the modeling of Lagrangian drifting particles compared with a standard random walk scheme.

Description: A unique time series of moored bio-optical measurements documented the 2004 spring-summer bloom in the southern Labrador Sea. In situ and satellite chlorophyll data show that chlorophyll levels in the 2004 bloom were at the upper end of those typically observed in this region. Satellite chlorophyll and profiling float temperature/salinity data show that the main bloom, which typically peaks in June/July, is often preceded by ephemeral mixed layer shoaling and a lesser, short-lived bloom in May; this was the case in 2004. The particulate backscatter to beam attenuation ratio (bbp[470 nm]/Cp[660 nm]) showed peaks in the relative abundance of small particles at bloom initiation and during the decline of the bloom, while larger particles dominated during the bloom. Chlorophyll/Cp and bbp/chlorophyll were correlated with carbon export and dominated by changes in the pigment per cell associated with lower light levels due to enhanced attenuation of solar radiation during the bloom. An NPZ (nutrients, phytoplankton, zooplankton) model captured the phytoplankton bloom and an early July peak in zooplankton. Moored acoustic Doppler current profiler (ADCP) data showed an additional mid-June peak in zooplankton biomass which was attributed to egg-laying copepods. The data reported here represent one of the few moored time series of Cp, bbp and chlorophyll extending over several months in an open ocean region. Interpretation of data sets such as this will become increasingly important as these deployments become more commonplace via ocean observing systems. Moreover, these data contribute to the understanding of biological-physical coupling in a biogeochemically important, yet poorly studied region.