ALBERT

Description: We study the quasi-geostrophic merging dynamics of axisymmetric baroclinic vortices to understand how baroclinicity affects merging rates and the development of the nonlinear cascade of enstrophy. The initial vortices are taken to simulate closely the horizontal' and vertical structure of Gulf Stream rings. A quasigeostrophic model is set with a horizontal resolution of 9 km and 6 vertical levels to resolve the mean stratification of the Gulf Stream region. The results show that the baroclinic merging is slower than the purely barotropic process, The merging is shown to occur in two phases: the tirst, which produces clove-shaped vortices and diffusive mixing of vorticity contours; and the second, which consists of the sliding of the remaining vorticity cores with a second diffusive mixing of the intemal vorticity field. Comparison among Nof, Cushman-Roisin, Polvani et al, and Dewar and Killworth merging events indicates a substantial agreement in the kinematics of the DYOCRSS. Parameter sensitivity experiments show that the decrease of the baroclinicity parameter of the system, Γ^2, [defined as Γ^2 = (D^2 fo^2)/ (No^2 H^2)], increases the speed of merging while its increase slows down the merging. However, the halting elfect of baroclinicity (large Γ^2 or small Rossby radii of deformation) reaches a saturation level where the merging becomes insensitive to larger F2 values. Furthermore, we show that a regime of small Γ^2 exists at which the merged baroclinic vortex is unstable (metastable) and breaks again into two new vortices, Thus, in the baroelinic case the range of Γ^2 detemines the stability of the merged vortex. We analyze these results by local energy and vorticity balances, showing that the horizontal divergence of pressure work term [∇ *(pv)] and the relative-vorticity advection term (v * ∇ (∇ ^2 φ) trigger the merging during the first phase. Due to this horizontal redistribution process, a net kinetic to gravitational energy conversion occurs via buoyancy work in the region external to the cores of the vortices. The second phase of merging is dominated by a direct baroclinic conversion of available gravitational energy into kinetic energy, which in tum triggers a horizontal energy redistribution producing the final fusion of the vortex centers. This energy and vorticity analysis supports the hypothesis that merging is an internal mixing process triggered by a horizontal redistribution of kinetic energy.

Description: The work has been financed by a grant from the Progetto Finalizzato "Calcolo Parallelo"

Description: Published

Description: 1618/1637

Description: 4A. Clima e Oceani

Description: JCR Journal

Description: restricted

Description: This is the first volume of a "final report" that summarizes, often in a speculative vein, what I have learned over the past 35 years or so about large-scale, low-frequency ocean currents, primarily with support from the Office of Naval Research (ONR). I was also fortunate to have been partially supported by the National Science Foundation and, during the preparation of this report, by the Clark Foundation. This report is meant to be an informal, occasionally anecdotal, state-of-the-art summary account of the World Ocean Circulation (WOC). Seemingly simple questions about how ocean currents behave, such as where various brands of sea water are coming from and going to, have been exciting and difficult research topics for many years. This report is not remotely about "all" of the WOC, it is simply a set of comments about what I have looked into. I believe that the results in this report, although presented in a personal way, are consistent with community wisdom. The report is intended to be readable by non-specialists who have a basic scientific/technical background, especially in other oceanographic areas or meteorology or physics or the geophysical disciplines, not just by specialists in physical oceanography. Anyone wishing to get spun up on the observational basis for the WOC could use this report and associated reference lists as a starting point. Volume I concentrates on the North Atlantic Ocean although there is preliminary discussion of global features. Highlights of this global summary are a new type of composite schematic picture of the World Ocean Circulation in its "upper layers" (Figure I-I) and new summaries (Figures 1-12, 21,91) of the global "thermohaline" circulation.

Description: Funding was provided by the Office of Naval Research, Grant Nos. N00014-89-J-1039 and N00014-95-1-0356, and the Clark Foundation.

Description: The Massachusetts Bays Program made bottom pressure and water velocity observations in Massachusetts and Cape Cod Bays during 1990 and 1991. In the Bays, the sea surface elevation appeared to rise and fall in phase with equal amplitudes at each diurnal or semidiurnal tidal frequency. There is some amplification in Boston and Provincetown harbors. The semidiurnal tides (particularly the M2 constituent) dominate. Massachusetts and Cape Cod Bays are part of the Gulf of Maine/Bay of Fundy system which is resonant near the semidiurnal frequency. This resonance amplifies the importance of the semidiurnal tides so that diurnal and higher harmonic tides become negligible. The sea level tides force currents which move with the same frequencies, but whose amplitudes are affected by the bathymetry. The strongest currents exist in the channel between Race Point and Stellwagen Bank where tidal currents exceed 1 knot. Analysis of current records for their tidal signal is complicated by internal tides which contaminate the records. These internal waves at tidal frequency exist on the stratification in the water column, and disappear during winter well-mixed times. At other times they must be considered as a signifcant source of energy for mixing and resuspension of sediments.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1991

Description: A general discussion of possible techniques for observation of near-surface currents indicates that the surface-following frame of reference will provide several advantages over the Eulerian or Lagrangian frames. One problem with surface-following measurements is the biasing effects of the waves. A technique for making unbiased measurements is developed. This technique requires that both the sensor velocity and the fluid velocity be measured. A sensor platform, the Surface Acoustic Shear Sensor (SASS), which makes the required measurements is described. The processing scheme for interpreting the measurements from the SASS is described at length. The data that SASS has obtained from two deployments in the Shelf Mixed Layer Experiment (SMILE) is presented. This data shows clearly that the biasing effects of waves can not, in general, be ignored. In the summary of the data we find surprisingly little shear in the downwind direction in the top 4m of the water column. In the crosswind direction observed, observed shear seems to be indicative of an across shelf pressure gradient and intense near-surface mixing.

Description: Financial support for my work was from NSF grant OCE-87-16937.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2008

Description: The subtidal circulation of the southeast Greenland shelf is described using a set of highresolution hydrographic and velocity transects occupied in summer 2004. The main feature present is the East Greenland Coastal Current (EGCC), a low-salinity, highvelocity jet with a wedge-shaped hydrographic structure characteristic of other surface buoyancy-driven currents. The EGCC was observed along the entire Greenland shelf south of Denmark Strait, while the transect north of the strait showed only a weak shelf flow. This observation, combined with evidence from chemical tracer measurements that imply the EGCC contains a significant Pacific Water signal, suggests that the EGCC is an inner branch of the polar-origin East Greenland Current (EGC). A set of idealized laboratory experiments on the interaction of a buoyant current with a submarine canyon also supported this hypothesis, showing that for the observed range of oceanic parameters, a buoyant current such as the EGC could exhibit both flow across the canyon mouth or into the canyon itself, setting the stage for EGCC formation. Repeat sections occupied at Cape Farewell between 1997 and 2004 show that the alongshelf wind stress can also have a strong influence on the structure and strength of the EGCC and EGC on timescales of 2-3 days. Accounting for the wind-induced effects, the volume transport of the combined EGC/EGCC system is found to be roughly constant (~2 Sv) over the study domain, from 68°N to Cape Farewell near 60°N. The corresponding freshwater transport increases by roughly 60% over this distance (59 to 96 mSv, referenced to a salinity of 34.8). This trend is explained by constructing a simple freshwater budget of the EGCC/EGC system that accounts for meltwater runoff, melting sea-ice and icebergs, and net precipitation minus evaporation. Variability on interannual timescales is examined by calculating the Pacific Water content in the EGC/EGCC from 1984-2004 in the vicinity of Denmark Strait. The PW content is found to correlate significantly with the Arctic Oscillation index, lagged by 9 years, suggesting that the Arctic Ocean circulation patterns bring varying amounts of Pacific Water to the North Atlantic via the EGC/EGCC.

Description: Funding for the cruise and analysis was provided by National Science Foundation grant OCE-0450658, which along with NSF grant OCE- 0095427 provided funds for my tuition and stipend as well.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2003

Description: Inertial terms dominate the single-gyre ocean model and prevent western-intensification when the viscosity is small. This occurs long before the oceanically-appropriate parameter range. It is demonstrated here that the circulation is controlled if a mechanism for ultimate removal of vorticity exists, even if it is active only in a narrow region near the boundary. Vorticity removal is modeled here as a viscosity enhanced very near the solid boundaries to roughly parameterize missing boundary physics like topographic interaction and three dimensional turbulence over the shelf. This boundary-enhanced viscosity allows western-intensified mean flows even when the inertial boundary width, is much wider than the frictional region because eddies flux vorticity from within the interior streamlines to the frictional region for removal. Using boundary-enhanced viscosity, western-intensified calculations are possible with lower interior viscosity than in previous studies. Interesting behaviors result: a boundary-layer balance novel to the model, calculations with promise for eddy parameterization, eddy-driven gyres rotating opposite the wind, and temporal complexity including basin resonances. I also demonstrate that multiple-gyre calculations have weaker mean circulation than single-gyres with the same viscosity and subtropical forcing. Despite traditional understanding, almost no inter-gyre flux occurs if no-slip boundary conditions are used. The inter-gyre eddy flux is in control only with exactly symmetric gyres and free slip boundaries. Even without the inter-gyre flux, the multiple-gyre circulation is weak because of sinuous instabilities on the jet which are not present in the single-gyre model. These modes efficiently flux vorticity to the boundary and reduce the circulation without an inter-gyre flux, postponing inertial domination to much smaller viscosities. Then sinuous modes in combination with boundary-enhanced viscosity can control the circulation.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2009

Description: Observations and inverse models suggest that small-scale turbulent mixing is enhanced in the Southern Ocean in regions above rough topography. The enhancement extends 1 km above the topography suggesting that mixing is supported by breaking of gravity waves radiated from the ocean bottom. In other regions, gravity wave radiation by bottom topography has been primarily associated with the barotropic tide. In this study, we explore the alternative hypothesis that the enhanced mixing in the Southern Ocean is sustained by internal waves generated by geostrophic motions flowing over bottom topography. Weakly-nonlinear theory is used to describe the internal wave generation and the feedback of the waves on the zonally averaged flow. A major finding is that the waves generated at the ocean bottom at finite inverse Froude numbers drive vigorous inertial oscillations. The wave radiation and dissipation at equilibrium is therefore the result of both geostrophic flow and inertial oscillations and differs substantially from the classical lee wave problem. The theoretical predictions are tested versus two-dimensional and three-dimensional high resolution numerical simulations with parameters representative of the Drake Passage region. Theory and fully nonlinear numerical simulations are used to estimate internal wave radiation from LADCP, CTD and topography data from two regions in the Southern Ocean: Drake Passage and the Southeast Pacific. The results show that radiation and dissipation of internal waves generated by geostrophic motions reproduce the magnitude and distribution of dissipation measured in the region.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 2001

Description: A comparison of monthly biogeochemical measurements made from 1993 to 1995, combined with hydrography and satellite altimetry, was used to observe the impacts of nine eddy events on primary productivity and particle flux in the Sargasso Sea. Measurements of primary production, thorium-234 flux, nitrate+nitrite, and photosynthetic pigments made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site were used. During the three years of this study, four out of six high thorium- 234 flux events over 1000 dpm/m2/d occurred during the passage of an eddy. Primary production nearly as high as the spring bloom maximum was observed in two modewater eddies (May 1993 and July 1995). The 1994 spring bloom at BATS was suppressed by the passage of an anticyclone. Distinct phytoplankton community shifts were observed in mode-water eddies, which had an increased percentage diatoms and dinoflagelletes, and in cyclones, which had an increased percentage cyanobacteria (excluding Prochlorococcus). The difference in the observations of mode-water eddies and cyclones may result from the age of the eddy, which was very important to the biological response. In general, eddies that were one to two months old elicited a large biological response; eddies that were three months old may show a biological response and were accompanied by high thorium flux measurements; eddies that were four months old or older did not show a biological response or high thorium flux. Our conceptual model depicting the importance of temporal changes during eddy upwelling and decay fit the observations well in all 7 upwelling eddies. Additional information is needed to determine the importance of deeper mixed layers and winter mixing to the magnitude of the eddy impacts. Also, sampling generally captured only the beginning, end, and lor edge of an eddy due to the monthly to semi-monthly frequency of the measurements made at BATS. Lagrangian studies, higher resolution time-series, and/or more spatial coverage is needed to provide additional information for improved C and N budgets in the Sargasso Sea and to complete our understanding of the temporal changes that occur in an eddy.

Description: Funding for this work was provided by NASA and NSF through the JGOFS Synthesis and Modeling Program.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2009

Description: Interaction between the Antarctic Circumpolar Current and the continental slope/shelf in the Marguerite Bay and west Antarctic Peninsula is examined as interaction between a wind-driven channel flow and a zonally uniform slope with a bay-shaped shelf to the south. Two control mechanisms, eddy advection and propagation of topographic waves, are identified in barotropic vortex-escarpment interactions. The two mechanisms advect the potential vorticity (PV) perturbations in opposite directions in anticyclone-induced interactions but in the same direction in cyclone-induced interactions, resulting in dramatic differences in the two kinds of interactions. The topographic waves become more nonlinear near the western(eastern if in the Northern Hemisphere) boundary of the bay, where strong cross-escarpment motion occurs. In the interaction between a surface anticyclone and a slope penetrating into the upper layer in a two-layer isopycnal model, the eddy advection decays on length scales on the order of the internal deformation radius, so shoreward over a slope that is wider than the deformation radius, the wave mechanism becomes noticeably significant. It acts to spread the cross-isobath transport in a much wider range while the transport directly driven by the anticyclone is concentrated in space. A two-layer wind-driven channel flow is constructed to the north of the slope in the Southern Hemisphere, spontaneously generating eddies through baroclinic instability. A PV front forms in the first layer shoreward of the base of the topography due to the lower-layer eddy-slope interactions. Perturbed by the jet in the center of the channel, the front interacts with the slope/shelf persistently yet episodically, driving a clockwise mean circulation within the bay as well as crossisobath transport. Both the transports across the slope edge and out of the bay are comparable with the maximum Ekman transport in the channel, indicative of the significance of the examined mechanism. The wave-boundary interaction identified in the barotropic model is found essential for the out-of-bay transport and responsible for the heterogeneity of the transport within the bay. Much more water is transported out of the bay from the west than from the east, and the southeastern area is the most isolated region. These results suggest that strong out-of-bay transport may be found near the western boundary of the Marguerite Bay while the southeastern region is a retention area where high population of Antarctic krill may be found.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1984

Description: This thesis addresses several aspects of the problem of determining the effect of the low-frequency eddy variability on the mean circulation of the Western North Atlantic. A framework for this study is first established by scale analysis of the eddy and mean terms in the mean momentum, vorticity, and heat balances in three regions of the Western North Atlantic -- the northern recirculation, the southern recirculation, and the mid-ocean. The data from the last decade of field experiments suggest somewhat different conclusions from the earlier analysis of Harrison (1980). In the momentum balance we confirm that the eddy terms are negligible compared to the lowest order mean geostrophic balance. The eddy term may be an 0(1) term in the vorticity balance only in the northern recirculation region where the mean flow is anisotropic. In the mean heat balance, if the mean temperature advection is scaled using the thermal wind relation, then the eddy heat flux is negligible in the mid-ocean, but it may be important in the recirculation areas. For all the balances the eddy terms are comparable to or an order of magnitude larger than the mean advective terms. We conclude from the scale analysis that the eddy field is most likely to be important in the Gulf Stream recirculation region. These balances are subsequently examined in more detail using data from the Local Dynamics Experiment (LDE). Several inconsistencies are first shown in McWilliams' (1983) model for the mean dynamical balances in the LDE. The sampling uncertainties do not allow us to draw conclusions about the long-term dynamical balances. However, it is shown that if we assume that the linear vorticity balance holds between the surface and the thermocline for a finite record, then the vertical velocity induced by the eddy heat flux divergence is non-zero. The local effect of the mesoscale eddy field on the mean potential vorticity distribution of the Gulf Stream recirculation region is determined from the quasigeostrophic eddy potential vorticity flux. This flux is calculated by finite difference of current and temperature time series from the Local Dynamics Experiment. This long-term array of moorings is the only experimental data from which the complete eddy flux can be calculated. The total eddy flux is dominated by the term due to the time variation in the thickness of isopycnal layers. This thickness flux is an order of magnitude larger than the relative vorticity flux. The total flux is statistically significant and directed 217° T to the southwest with a magnitude of 1.57 x 10 -5 cm/2s. The direction of the eddy flux with respect to the mean large scale potential vorticity gradient from hydrographic data indicates that eddies in this region tend to reduce the mean potential vorticity gradient. The results are qualitatively consistent with numerical model results and with other data from the Gulf Stream recirculation region. We find that the strength of the eddy transfer in the enstrophy cascade is comparable to the source terms in the mean enstrophy balance. The Austauch coefficient for potential vorticity mixing is estimated to be 0(107cm2/sec). An order of magnitude estimate of the enstrophy dissipation due only to the internal wave field shows that other processes must be important in enstrophy dissipation. The measured eddy potential vorticity fluxes are compared to the linear stability model of Gill, Green, and Simmons (1974). An earlier study (Hogg, 1984) has shown agreement between the empirical orthogonal modes of the data and the predicted wavenumbers, growth rates, and phase speeds of the most unstable waves. However, we show substantial disagreement in a comparison of the higher moments the eddy heat and potential vorticity fluxes. Because the critical layer of the model is located near the surface, the model predicts that most of the eddy potential vorticity and eddy heat flux should occur within about 300 meters of the surface. The data show much greater deep eddy heat flux than predicted by the model. It is suggested that the unstable modes in the ocean have a longer vertical scale because of the reduction in the buoyancy frequency near the surface. The evidence for in situ instability is also examined in the decay region of the Gulf Stream from an array of current and temperature recorders. Although there is vertical phase propagation in the empirical orthogonal modes for some of the variables at some of the moorings, there is not much evidence for a strong ongoing process of wave generation.

Description: This research has been conducted under NSF contract numbers OCE 77-19403, ATM 79-21431, and OCE 82-00154.