ALBERT

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 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.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1999

Description: A new, global inversion is used to estimate the large scale oceanic circulation based on the World Ocean Circulation Experiment and Java Australia Dynamic Experiment hydrographic data. A linear inverse "box" model is used to combine consistently the transoceanic sections. The circulation is geostrophic with an Ekman layer at the surface and oceanic layers defined by neutral surfaces. Near-conservation of mass, salt and top-to-bottom silica is required and, in addition, heat and the phosphate-oxygen combination (170[P04]+[02]) are conserved in layers that are not in contact with the surface. A globally-consistent solution is obtained for a depth-independent adjustment to the thermal wind field, freshwater flux divergenees, the Ekman transport, and the advective and diffusive dianeutral fluxes between layers. A detailed error budget permits calculation of statistical uncertainties, taking into account both the non-resolved part of the solution and the systematic errors due to the temporal oceanic variability. The estimated water mass transports during the WOCE period (1985-1996) are generally similar to previous published estimates. However, important differences are found. In particular, the inflow of bottom waters into the Pacific Ocean is smaller than in most previous estimates. Utilization of property anomaly conservation constraints allows the estimation of significant dianeutral diffusivities in deep layers, with a global average of 3 ± lcm2s- 1 north of 30°S. Dianeutral transfers indicate that about 20 Sv of bottom water is formed in the Southern Ocean. Significant ocean-atmosphere heat fluxes are found, with a global heating of 2.3 ± 0.4PW in the tropical band and a corresponding cooling at high latitudes. The signature of a large-scale average export production is found for nutrients in several temperate regions. Despite the large uncertainties, the production magnitudes are consistent with independent measurements from sediment traps and isotopic data. Net nutrient sources or sinks are found in several regions, suggesting either transport of dissolved organic matter or a seasonal alias. Oxygen indicates large exchanges with the atmosphere, with intake at high latitudes and outgassing/remineralization at low latitudes.

Description: This work was supported in part by the Jet Propulsion Laboratory/CALTECH (contract #958125), and by gifts from Ford, General Motors, and Daimler-Chrysler to MIT's Climate Modelling Initiative.

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 1999

Description: This thesis studies the problems of generation and maintenance of recirculations by Gulf Stream instabilities. Observations show that the horizontal structure of the jet and its recirculations suffer significant changes in time. Here, the role of internal dynamics of the jet is isolated as one of the possible sources of such variability, and the differences between barotropic and baroclinic instabilities are investigated. The problem of recirculation development is considered in a framework of a free spin down of the 2-layer and the 1-layer, zonally symmetric, quasi-geostrophic jets. Linear stability analysis shows that in strongly baroclinic basic flows, eddies are capable of driving recirculations in the lower layer through the residual meridional circulation. In strongly barotropic jets, the linearly most unstable wave simply diffuses the jet. Nonlinear stability analysis indicates that recirculations are robust features of the 2-layer model. The strength of recirculations is a function of the model’s parameters. It increases with a decrease in the value of the nondimensional /3 due to potential vorticity homogenization constrained by enstrophy conservation. The recirculation strength is a non-monotonic function of the baroclinic velocity parameter; it is the strongest for strongly baroclinic basic flows, weakest for flows with intermediate baroclinic structure and of medium strength for strongly barotropic basic flows. Such non-monotonic behavior is the result of two different processes responsible for the recirculation development: linear eddy-mean flow interactions for strongly baroclinic basic flows and strongly nonlinear eddy-eddy and eddy-mean flow interaction for strongly barotropic flows. In the case of the reduced-gravity model, recirculations develop only for infinite deformation raduis. Basic flows with finite deformation radius are only weakly supercritical and therefore produced negligible recirculations after equilibration. The problem of maintenance of the recirculations is coupled to the questions of existence of low frequency variability and of multiple dynamical regimes of a system consisting of a quasi-geostrophic jet and its recirculations. The problem is studied in a framework of a 2-layer or a reduced-gravity colliding jets model which has no windforcing. Instead, it is forced by inflows and outflows through the open boundaries. Oniy the western boundary of the domain is closed, and a free slip boundary condition is used there. The results of the numerical experiments show that when oniy the mechanism of barotropic instability is present, the model has two energy states for a wide range of interfacial friction coefficients. The high energy state is characterized by well-developed recirculations and displays strong variability associated with either large recirculating gyres and a weak eddy field or small recirculations and a strong eddy field. The iow energy state is characterized by large meridional excursions in the separation point and large amplitude, westward propagating meanders that produce strong rings after interacting with the western wall. For physically relevant bottom friction values, the presence of baroclinic in stability in the recirculation regions of the 2-layer model allows for a unique dynamical regime characterized by well-developed recirculations in both layers. The low-frequency variability associated with the regime is weak and is related to meridional shifts in the position of the jet, to wrapping of the recirculations around each other, and to pulsations in their zonal extent. For strong bottom friction, the 2-layer model has only the mechanism of barotropic instability which reduces it to a 1 1/2-layer configuration; the model displays two dynamical regimes and strong low frequency variability in the upper layer, while the lower layer is strongly frictional.

Description: Financial support for this research was provided by NSF grant number OCE 9617848.

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 September 2011

Description: Remote sensing and in situ observations are used to investigate the ocean response to the Tokar Wind Jet in the Red Sea. The wind jet blows down the pressure gradient through the Tokar Gap on the Sudanese coast, at about 18°N, during the summer monsoon season. It disturbs the prevailing along-sea (southeastward) winds with strong cross-sea (northeastward) winds that can last from days to weeks and reach amplitudes of 20-25 m/s. By comparing scatterometer winds with along-track and gridded sea level anomaly observations, it is shown that an intense dipolar eddy spins up in less than seven days in response to the wind jet. The eddy pair has a horizontal scale of 140 km. Maximum ocean surface velocities can reach 1 m/s and eddy currents extend at least 200 m into the water column. The eddy currents appear to cover the width of the sea, providing a pathway for rapid transport of marine organisms and other drifting material from one coast to the other. Interannual variability in the strength of the dipole is closely matched with variability in the strength of the wind jet. The dipole is observed to be quasi-stationary, although there is some evidence for slow eastward propagation—simulation of the dipole in an idealized high-resolution numerical model suggests that this is the result of self-advection. These and other recent in situ observations in the Red Sea show that the upper ocean currents are dominated by mesoscale eddies rather than by a slow overturning circulation.

Description: This work is supported by Award Nos. USA 00002, KSA 00011 and KSA 00011/02 made by King Abdullah University of Science and Technology (KAUST).

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1989

Description: When ocean waves in deep water interact with a current, the direction of propagation and characteristics of the waves such as height and length are affected. Swell in the open ocean can undergo significant refraction as it passes through major current systems like the Gulf Stream or Antarctic Circumpolar Current. Remote sensing techniques such as synthetic aperture radars (SAR) have the potential to detect wave systems over a wide geographical area. Combining a model for wave refraction in the presence of currents with SAR measurements, the inverse problem of using the measured wave data can be solved to determine the direction and magnitude of the intervening currents. In this study the behavior of swell measured by SAR on a satellite pass over the Gulf Stream is examined. The refraction predicted by a numerical model under conditions of varying current profiles and velocities is compared to SAR generated wave spectra. By matching the current profile which results in the best correlation of wave refraction to the SAR data, the tomographic problem of measuring the Gulf Stream current is solved. The best correlation between the model and SAR data is obtained when a current is modeled by a top hat velocity profile with a direction of 75° and a current speed of 2 m/s. The direction agrees with that visually observed from the SAR images, and the direction and speeds are close to the Coast Guard estimates for the Gulf Stream at the time of the SEASAT,pass. The current profiles used did not take into account a possible widening of the Gulf Stream at the position of the satellite overpass. There is a great deal of scatter in the SAR data, both before and in the Gulf Stream, so it is difficult to correlate every point with specific current behavior, but the increase in wave length and change in wave angle in the center of the Gulf Stream seem to indicate that there may be a non-uniform feature such as the formation of an eddy or other lateral variability near the current's edge.

Description: I was supported by the U. S. Navy.

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 September 2012

Description: Interactions between the ocean circulation in sub-ice shelf cavities and the overlying ice shelf have received considerable attention in the context of observed changes in flow speeds of marine ice sheets around Antarctica. Modeling these interactions requires parameterizing the turbulent boundary layer processes to infer melt rates from the oceanic state at the ice-ocean interface. Here we explore two such parameterizations in the context of the MIT ocean general circulation model coupled to the z-coordinates ice shelf cavity model of Losch (2008). We investigate both idealized ice shelf cavity geometries as well as a realistic cavity under Pine Island Ice Shelf (PIIS), West Antarctica. Our starting point is a three-equation melt rate parameterization implemented by Losch (2008), which is based on the work of Hellmer and Olbers (1989). In this form, the transfer coefficients for calculating heat and freshwater fluxes are independent of frictional turbulence induced by the proximity of the moving ocean to the fixed ice interface. More recently, Holland and Jenkins (1999) have proposed a parameterization in which the transfer coefficients do depend on the ocean-induced turbulence and are directly coupled to the speed of currents in the ocean mixed layer underneath the ice shelf through a quadratic drag formulation and a bulk drag coefficient. The melt rate parameterization in the MITgcm is augmented to account for this velocity dependence. First, the effect of the augmented formulation is investigated in terms of its impact on melt rates as well as on its feedback on the wider sub-ice shelf circulation. We find that, over a wide range of drag coefficients, velocity-dependent melt rates are more strongly constrained by the distribution of mixed layer currents than by the temperature gradient between the shelf base and underlying ocean, as opposed to velocity-independent melt rates. This leads to large differences in melt rate patterns under PIIS when including versus not including the velocity dependence. In a second time, the modulating effects of tidal currents on melting at the base of PIIS are examined. We find that the temporal variability of velocity-dependent melt rates under tidal forcing is greater than that of velocity-independent melt rates. Our experiments suggest that because tidal currents under PIIS are weak and buoyancy fluxes are strong, tidal mixing is negligible and tidal rectification is restricted to very steep bathymetric features, such as the ice shelf front. Nonetheless, strong tidally-rectified currents at the ice shelf front significantly increase ablation rates there when the formulation of the transfer coefficients includes the velocity dependence. The enhanced melting then feedbacks positively on the rectified currents, which are susceptible to insulate the cavity interior from changes in open ocean conditions.

Description: National Science and Engineering Research Council of Canada

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 1996

Description: This thesis addresses the question of how a highly energetic eddy field could be generated in the interior of the ocean away from the swift boundary currents. The energy radiation due to the temporal growth of non-trapped (radiating) disturbances in such a boundary current is thought to be one of the main sources for the described variability. The problem of stability of an energetic current, such as the Gulf Stream, is formulated. The study then focuses on the ability of the current to support radiating instabilities capable of significant penetration into the far-field and their development with time. The conventional model of the Gulf Stream as a zonal current is extended to allow the jet axis to make an angle to a latitude circle. The linear stability of such a nonzonal flow, uniform in the along-jet direction on a beta-plane, is first studied. The stability computations are performed for piece-wise constant and continuous velocity profiles. New stability properties of nonzonal jets are discussed. In particular, the destabilizing effect of the meridional tilt of the jet axis is demonstrated. The radiating properties of nonzonal currents are found to be very different from those of zonal currents. In particular, purely zonal flows do not support radiating instabilities, whereas flows with a meridional component are capable of radiating long and slowly growing waves. The nonlinear terms are then included in the consideration and the effects of the nonlinear interactions on the radiating properties of the solution are studied in detail. For these purposes, the efficient numerical code for solving equation for the QG potential vorticity with open boundary conditions of Orlanski's type is constructed. The results show that even fast growing linear solutions, which are trapped during the linear stage of developement, can radiate energy in the nonlinear regime if the basic current is nonzonal. The radiation starts as soon as the initial fast exponential growth significantly slows. The initial trapping of those solutions is caused by their fast temporal growth. The new mechanism for radiation is related to the nonzonality of a current.

Description: This work was supported by NSF Grant OCE 9301845.