ALBERT

Description: New light is shed on Worthington's concept of the North Atlantic circulation, postulating the existence of two anticyclonic gyres. This concept, which seems to have been laid to rest in the last decade, has now been reinforced by the results of a simple linear Sverdrup circulation model yielding a band of westward transport all across the North Atlantic at about the Azores latitude. This narrow band is called the Azores Countercurrent (AzCC) and matches the position of westward flow required by Worthington's “northern gyre.” An anomaly in the meridional change of the wind-stress curl in the eastern North Atlantic has been identified as the driving mechanism. A comparison with observations shows that the AzCC is verified in many analyses of historical datasets and synoptic surveys. A lack of the AzCC in other analyses is probably due to missing meridional sections, strong smoothing, and the superimposed Ekman flow close to the sea surface directed to the southeast. The AzCC has not been verified in low-resolution general circulation models applying simplified wind-stress fields and large friction coefficients, but there is evidence for its existence in recent high-resolution models driven by realistic wind stresses. Based on these findings, a new pattern for the wind-driven upper ocean circulation of the midlatitude North Atlantic is presented.

Description: We examined the physiological responses of steady-state iron (Fe)-replete and Fe-limited cultures of the biogeochemically critical marine unicellular diazotrophic cyanobacterium Crocosphaera at glacial (19 Pa; 190 ppm), current (39 Pa; 380 ppm), and projected year 2100 (76 Pa; 750 ppm) CO2 levels. Rates of N2 and CO2 fixation and growth increased in step with increasing partial pressure of CO2 (pCO2), but only under Fe-replete conditions. N2 and carbon fixation rates at 75 Pa CO2 were 1.4-1.8-fold and 1.2-2.0-fold higher, respectively, relative to those at present day and glacial pCO2 levels. In Fe-replete cultures, cellular Fe and molybdenum quotas varied threefold and were linearly related to N2 fixation rates and to external pCO2. However, N2 fixation and trace metal quotas were decoupled from pCO2 in Fe-limited Crocosphaera. Higher CO2 and Fe concentrations both resulted in increased cellular pigment contents and affected photosynthesis vs. irradiance parameters. If these results also apply to natural Crocosphaera populations, anthropogenic CO2 enrichment could substantially increase global oceanic N2 and CO2 fixation, but this effect may be tempered by Fe availability. Possible biogeochemical consequences may include elevated inputs of new nitrogen to the ocean and increased potential for Fe and/or phosphorus limitation in the future high-CO2 ocean, and feedbacks to atmospheric pCO2 in both the near future and over glacial to interglacial timescales.

Description: A sigma-coordinate, primitive equation ocean circulation model is used to explore the problem of the remnant generation of trapped waves about a tall, circular, isolated seamount by an incident oscillatory barotropic current. The numerical solutions are used to extend prior studies into the fully nonlinear regime, and in particular to quantify and interpret the occurrence of residual circulation. Specific attention is also devoted to the dependence of the resonance and rectification mechanisms on stratification, forcing frequency, and choice of subgrid-scale viscous closure. Resonantly generated trapped waves of significant amplitude are found to occur broadly in parameter space; a precise match between the frequency of the imposed incident current and the frequency of the trapped free wave is not necessary to produce substantial excitation of the trapped wave. The maximum amplification factors produced in these numerical solutions, O(100) times the strength of the incident current, are consistent with previous studies. In the presence of nonlinear advection, strong residual currents are produced. The time-mean circulation about the seamount is dominated by a strong bottom-intensified, anticyclonic circulation closely trapped to the seamount. Maximum local time-mean current amplitudes are found to be as large as 37% of the magnitude of the propagating waves. In addition to the strong anticyclonic residual flow, there is a weaker secondary circulation in the vertical-radial plane characterized by downwelling over the top of the seamount at all depths. Maximum vertical downwelling rates of several tens of meters per day occur at the summit of the seamount. The vertical mass flux implied by this systematic downwelling is balanced by a slow radial flux of mass directed outward along the flanks of the seamount. Time-mean budgets for the radial and azimuthal components of momentum show that horizontal eddy fluxes of momentum are responsible for transporting net radial and azimuthal momentum from the far field to the upper flanks of the seamount. There, Coriolis and pressure gradient forces provide the dominant balances in the radial direction. However, the Coriolis force and viscous effects provide the primary balance for the azimuthal component.

Description: North Atlantic air-sea heat and freshwater flux data from several sources are used to estimate the conversion rate of water from one density to another throughout the range of sea surface density. This cross-isopycnal mass flux varies greatly over the ocean, with a maximum of 32.2 × 106 m3 s−1 at σ = 26.1 kg m−3 (toward greater densities) and a minimum of −7.6 × 106 m3 s−1 (toward lesser densities) at σ = 23.0 kg m−3. The air-sea fluxes force water to accumulate in three density bands: one at the lowest sea surface densities generated by heating; one centered near the density of subtropical mode water; and one spanning subpolar mode water densities. The transfer of water to the highest and lowest densities is balanced by mixing, which returns water to the middle density range, and also by boundary sources or sinks. Integrating the cross-isopycnal flux over all densities gives an annual average sinking of about 9 × 1O6 m3 s−1, which presumably escapes across the equator and must be balanced by a similar inflow. Comparison with estimates from tracer studies suggests that the renewal of tracer characteristics at a given density may occur without the existence of an annual average mass source at that density, because along- and cross-isopycnal mixing can renew a tracer without supplying mass.

Description: Antarctic Bottom Water flows into the western North Atlantic across the equator, shifting from the western side to the eastern side of the trough between the American continents and the Mid-Atlantic Ridge as it continues north. This is puzzling because such large-scale motion is thought to be controlled by dynamics that disallows an eastern boundary current. Previous explanations for the transposition involve a (necessarily small-scale) density current that changes sides because of the change in sign of rotation across the equator, or a topographic effect that changes the sign of the effective mean vorticity gradient and thus requires an eastern boundary current. Here an alternative explanation for the overall structure of bottom flow is given. A source of mass to a thin bottom layer is assumed to upwell uniformly across its interface into a less dense layer at rest. A simple formula for the magnitude of the upwelling and thickness of the layer is derived that depends on the source strength to the bottom layer. For a strong enough source, the bottom layer thickness is zero along a grounding curve that separates the bottom water from the western boundary and confines it to the east. A band of recirculating interior flow occurs, supplied by an isolated northern and western boundary current. Similar structures appear to exist in the Antarctic Bottom Water of the western North Atlantic.

Description: Results of a three-dimensional primitive equation model are presented simulating turbulent mesoscale motions in the seasonal thermocline on an f plane. The model is based on a hybrid vertical coordinate scheme and conserves isopycnic potential vorticity. Mesoscale turbulence is modeled in terms of an unstable potential vorticity front. The model integration starts from a purely zonal, 60-km-wide geostrophically balanced jet, on which is superimposed a small initial perturbation. The most unstable mode exhibits a wavelength of 85 km and is driven by a mixed type of instability. Characteristic dynamical ingredients of the wave are enhanced cyclonic and anticyclonic relative vorticity in the troughs and the ridges, respectively, due to the curvature of the flow. Vertical motion of up to 10 m d−1 occurring downstream of the ridges (downwelling) and downstream of the troughs (upwelling) is driven by geostrophic advection of relative vorticity. The contrast of static stability across the front is changing during amplification of the instability: in troughs the stability is decreasing whereas in ridges it is increasing. The density field exhibits local anomalies of the isopycnals' depths (bumps) due to the ageostrophic cross-jet advection of potential vorticity streamers wound up in cyclones and anticyclones. Locally, the potential vorticity gradients are enhanced, creating a multiple front structure. The model results support observations and findings of earlier atmospheric and oceanic models. It is emphasized that mesoscale turbulent structures may have a profound influence on primary productivity, mixed-layer, and internal wave dynamics.

Description: The circulation of the northeastern Atlantic Ocean at intermediate depths is characterized by watermass transformation processes that involve Iceland–Scotland Overflow Water (ISOW) from the northeast, Labrador Sea Water (LSW) from the west, and Mediterranean Water from the south. Field observations were carried out with 89 eddy-resolving floats (RAFOS and MARVOR types). The data coverage achieved is remarkably high and enables a comprehensive study of the eastern basins between Iceland and the Azores. The trajectories show typical pathways of the water masses involved and the role that the complex bottom topography plays in defining them. The ISOW paths tend to lean against the slopes of the Reykjanes Ridge and Rockall Plateau. Westward escapes through multiple gaps in the ridge are possible, superimposed on a sustained southward flow in the eastern basin along the Mid-Atlantic Ridge. LSW pathways leading to the eastern basins are subject to high variability in flow direction and eddy activity. In addition to a selection of characteristic trajectories, maps of the horizontal distributions of Lagrangian eddy kinetic energy and integral time scales are presented. These reveal distinct areas of intensified mixing in the Iceland Basin, as well as the sharp contrast between the subpolar and subtropical dynamics. A self-contained eddy detection scheme is applied to obtain statistics on individual eddy properties and their abundance. It is suggested that much of the intensified mixing can be related to cyclonic activity, particularly in the subpolar region.

Description: The Vasco—Cirene field experiment, in January—February 2007, targeted the Seychelles—Chagos thermocline ridge (SCTR) region, with the main purpose of investigating Madden—Julian Oscillation (MJO)-related SST events. The Validation of the Aeroclipper System under Convective Occurrences (Vasco) experiment (Duvel et al. 2009) and Cirene cruise were designed to provide complementary views of air—sea interaction in the SCTR region. While meteorological balloons were deployed from the Seychelles as a part of Vasco, the Research Vessel (R/V) Suroît was cruising the SCTR region as a part of Cirene. more: The Vasco—Cirene program explores how strong air—sea interactions promoted by the shallow thermocline and high sea surface temperature in the Seychelles—Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceanographic cruise collected oceanic, atmospheric, and air—sea flux observations in this region in January—February 2007. The contemporaneous Vasco field experiment complemented these measurements with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a mooring and 12 Argo profilers. Unusual conditions prevailed in the Indian Ocean during January and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles—Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward currents prevailed down to 800 m. These anomalous conditions had a major impact on tuna fishing in early 2007. Our dataset also sampled the genesis and maturation of Tropical Cyclone Dora, including high surface temperatures and a strong diurnal cycle before the cyclone, followed by a 1.5°C cooling over 10 days. Balloonborne instruments sampled the surface and boundary layer dynamics of Dora. We observed small-scale structures like dry-air layers in the atmosphere and diurnal warm layers in the near-surface ocean. The Cirene data will quantify the impact of these finescale features on the upper-ocean heat budget and atmospheric deep convection.

Description: Data from a surface mooring located in the Sargasso Sea at 34°N, 70°W between May 1982 and May 1984 were compared with satellite data to investigate large diurnal sea surface temperature changes. Mooring and satellite measurements are in excellent agreement for those days on which no clouds covered the site at the time of the satellite pass. During the summer half-year at this site, there is a 20% charm of diurnal warming of more than 0.5°C, with values of up to 3.5°C observed in the two-year period. Diurnal warming observed at the mooring has been simulated well by a one-dimensional model driven by local beat and momentum fluxes. Under the conditions of very light wind and strong insolation that produce the Largest surface warming, the surface mixed-layer depth reduces to the convection depth, and wind-mixing becomes unimportant. The thermal response is then limited to depths between 1 and 2 m, making it likely that such events have been underreported in routine ship observations. In all cases observed, the spatial extent of warming events as determined by satellite data are well correlated with the corresponding atmospheric pressure patterns. Conditions giving rise to the largest diurnal warming events are often associated with a westward-extending ridge of the Bermuda high. In the region studied, 57°–75°W and 29°–43°N, diurnal warming of more than 1°C was found on occasion to cover areas in excess of 300 000 km2, with warming of more than 2°C coveting areas in excess of 130 000 km2.

Description: In this paper we use the historical hydrographic data base for the South Atlantic Ocean to investigate (i) the hydrographic boundary between the subtropical gyre and the Antarctic Circumpolar Current (ACC), the Sub-tropical Front (STF), and (ii) the southern current band of the gyre, which is called the South Atlantic Current (SAC). The STF begins in the west in the Brazil-Falkland (Malvinas) confluence zone, but at locations at and west of 45°W this front is often coincident with the Brazil Current front. East of 45°W the STF appears to be a distinct feature to at least the region south of Africa, whereupon it continues into the Indian Ocean. The associated current band of increased zonal speed is the SAC, which, except for one instance, is found at or north of the surface STF until Indian Ocean water from the Agulhas retroflection is reached. A reversal of baroclinicity in the STF is observed south of a highly saline Agulhas ring, causing the SAC to separate from the STF and turn north into the Benguela Current. Zonal flow south of the STF is generally weak and serves to separate the South Atlantic and circumpolar currents. In the Argentine Basin, the SAC has a typical volume transport of 30 Sv (1 Sv = 106m3s−1) in the upper 1000 m relative to a deep potential density surface (σ4 = 45.87 kg m−3), and can be as high as 37 Sv. It is thus comparable to, or stronger than, the Brazil Current. In the Cape Basin, the transport of the SAC is reduced to about 15 SY before it turns north to feed the Benguela Current. In late 1983 this flow was joined by about 8 Sv of water from the Agulhas Current.