On thermally maintained circulation in a closed ocean basin

A three-dimensional model of a thermohaline-maintained circulation is presented for a closed basin of finite depth on the beta plane. The circulation, which conserves potential vorticity, consists of a westward drift at all levels in the open ocean and a return flow that is accomplished in a swift eastward-flowing current. A comparison is made to Worthington\u27s recent water-mass analysis of deep circulation in the North Atlantic Basin

Shelf-ocean exchange and hydrography west of the Antarctic Peninsula: A review

The West Antarctic Peninsula (WAP) is a highly productive marine ecosystem where extended periods of change have been observed in the form of glacier retreat, reduction of sea-ice cover and shifts in marine populations, among others. The physical environment on the shelf is known to be strongly influenced by the Antarctic Circumpolar Current flowing along the shelf slope and carrying warm, nutrient-rich water, by cold waters flooding into the northern Bransfield Strait from the Weddell Sea, by an extensive network of glaciers and ice shelves, and by strong seasonal to inter-annual variability in sea-ice formation and air–sea interactions, with significant modulation by climate modes like El Niño–Southern Oscillation and the Southern Annular Mode. However, significant gaps have remained in understanding the exchange processes between the open ocean and the shelf, the pathways and fate of oceanic water intrusions, the shelf heat and salt budgets, and the long-term evolution of the shelf properties and circulation. Here, we review how recent advances in long-term monitoring programmes, process studies and newly developed numerical models have helped bridge these gaps and set future research challenges for the WAP system

Statistical properties of near‐surface flow in the California coastal transition zone

The article of record as published may be found at https://doi.org/10.1029/91JC01072During the summers of 1987 and 1988, 77 near-surface satellite-tracked drifters were deployed in or near cold filaments near Point Arena, California (39°N), and tracked for up to 6 months as part of the Coastal Transition Zone (CTZ) program. The drifters had large drogues centered at 15 m, and the resulting drifter trajectory data set has been analyzed in terms of its Eulerian and Lagrangian statistics. The CTZ drifter results show that the California Current can be characterized in summer and fall as a meandering coherent jet which on average flows southward to at least 30°N, the southern end of the study domain. From 39°N south to about 33°N, the typical core velocities are of O(50 cm s−1) and the current meanders have alongshore wavelengths of O (300 km) and onshore-offshore amplitude of O(100–200 km). The lateral movement of this jet leads to large eddy kinetic energies and large eddy diffusivities, especially north of 36°N. The initial onshore-offshore component of diffusivity is always greater than the alongshore component in the study domain, but at the southern end, the eddy diffusivity is more isotropic, with scalar single particle diffusivity (Kxx + Kyy) of O(8 × 107 cm2 s−1). The eddy diffusivity increases with increasing eddy energy. Finally, a simple volume budget for the 1988 filament observed near 37°N off Point Arena suggests that subduction can occur in a filament at an average rate of O (10 m d−1) some 200 km offshore, thus allowing the cold water initially in the filament core to sink below the warmer ambient water by the time the surface velocity core has turned back onshore. This process explains why satellite temperature and color imagery tend to “see” only flow proceeding offshore

The Leeway of Shipping Containers at Different Immersion Levels

The leeway of 20-foot containers in typical distress conditions is established through field experiments in a Norwegian fjord and in open-ocean conditions off the coast of France with wind speed ranging from calm to 14 m/s. The experimental setup is described in detail and certain recommendations given for experiments on objects of this size. The results are compared with the leeway of a scaled-down container before the full set of measured leeway characteristics are compared with a semi-analytical model of immersed containers. Our results are broadly consistent with the semi-analytical model, but the model is found to be sensitive to choice of drag coefficient and makes no estimate of the cross-wind leeway of containers. We extend the results from the semi-analytical immersion model by extrapolating the observed leeway divergence and estimates of the experimental uncertainty to various realistic immersion levels. The sensitivity of these leeway estimates at different immersion levels are tested using a stochastic trajectory model. Search areas are found to be sensitive to the exact immersion levels, the choice of drag coefficient and somewhat less sensitive to the inclusion of leeway divergence. We further compare the search areas thus found with a range of trajectories estimated using the semi-analytical model with only perturbations to the immersion level. We find that the search areas calculated without estimates of crosswind leeway and its uncertainty will grossly underestimate the rate of expansion of the search areas. We recommend that stochastic trajectory models of container drift should account for these uncertainties by generating search areas for different immersion levels and with the uncertainties in crosswind and downwind leeway reported from our field experiments.Comment: 25 pages, 11 figures and 5 tables; Ocean Dynamics, Special Issue on Advances in Search and Rescue at Sea (2012

Lagrangian turbulence in the Adriatic Sea as computed from drifter data: effects of inhomogeneity and nonstationarity

The properties of mesoscale Lagrangian turbulence in the Adriatic Sea are studied from a drifter data set spanning 1990-1999, focusing on the role of inhomogeneity and nonstationarity. A preliminary study is performed on the dependence of the turbulent velocity statistics on bin averaging, and a preferential bin scale of 0.25 is chosen. Comparison with independent estimates obtained using an optimized spline technique confirms this choice. Three main regions are identified where the velocity statistics are approximately homogeneous: the two boundary currents, West (East) Adriatic Current, WAC (EAC), and the southern central gyre, CG. The CG region is found to be characterized by symmetric probability density function of velocity, approximately exponential autocorrelations and well defined integral quantities such as di usivity and time scale. The boundary regions, instead, are significantly asymmetric with skewness indicating preferential events in the direction of the mean flow. The autocorrelation in the along mean flow direction is characterized by two time scales, with a secondary exponential with slow decay time of 11-12 days particularly evident in the EAC region. Seasonal partitioning of the data shows that this secondary scale is especially prominent in the summer-fall season. Possible physical explanations for the secondary scale are discussed in terms of low frequency fluctuations of forcings and in terms of mean flow curvature inducing fluctuations in the particle trajectories. Consequences of the results for transport modelling in the Adriatic Sea are discussed.Comment: 45 pages, 18 figure

Ocean mixedlayer depth: A subsurface proxy for ocean-atmosphere variability

A new criterion, based on the shallowest extreme curvature of near surface layer density or temperature profiles, is established for demarking the mixed layer depth, h mix. Using historical global hydrographic profile data, including conductivity-temperature-depth and expendable bathythermograph data obtained during World Ocean Circulation Experiment, its seasonal variability and monthly to interannual anomalies are computed. Unlike the more commonly used Δ criterion, the new criterion is able to deal with both different vertical resolutions of the data set and a large variety of observed stratification profiles. For about two thirds of the profiles our algorithm produces an h mix/c that is more reliable than the one of the Δ criterion. The uncertainty for h mix/c is ±5 m for high- (<5 m) and ±8 m for low- (<20 m) resolution profiles. A quality index, QImix, which compares the variance of a profile above h mix to the variance to a depth of 1.5 × h mix, shows that for the 70% of the profile data for which a clearly recognizable well-mixed zone exists near the surface, our criterion identifies the depth of the well-mixed zone in all cases. The standard deviation of anomalous monthly h mix/c is typically 20–70% of the long-term mean h mix/c . In the tropical Pacific the monthly mean anomalies of h mix/c are not well correlated with anomalies of sea surface temperature, which indicate that a variety of turbulent processes, other than surface heat fluxes, are important in the upper ocean there. Comparisons between observed h mix/c and Massachusetts Institute of Techonology/ocean general circulation model/Estimating the Circulation and Climate of the Ocean model simulated mixed layer depth indicate that the KPP algorithm captures in general a 30% smaller mixed layer depth than observed

Statistical mechanics of Fofonoff flows in an oceanic basin

We study the minimization of potential enstrophy at fixed circulation and energy in an oceanic basin with arbitrary topography. For illustration, we consider a rectangular basin and a linear topography h=by which represents either a real bottom topography or the beta-effect appropriate to oceanic situations. Our minimum enstrophy principle is motivated by different arguments of statistical mechanics reviewed in the article. It leads to steady states of the quasigeostrophic (QG) equations characterized by a linear relationship between potential vorticity q and stream function psi. For low values of the energy, we recover Fofonoff flows [J. Mar. Res. 13, 254 (1954)] that display a strong westward jet. For large values of the energy, we obtain geometry induced phase transitions between monopoles and dipoles similar to those found by Chavanis and Sommeria [J. Fluid Mech. 314, 267 (1996)] in the absence of topography. In the presence of topography, we recover and confirm the results obtained by Venaille and Bouchet [Phys. Rev. Lett. 102, 104501 (2009)] using a different formalism. In addition, we introduce relaxation equations towards minimum potential enstrophy states and perform numerical simulations to illustrate the phase transitions in a rectangular oceanic basin with linear topography (or beta-effect).Comment: 26 pages, 28 figure

FRONTS 80 : preliminary results from an investigation of the wintertime North Pacific subtropical front

The FRONTS-80 experiment is a joint investigation supported by the Office of Naval Research (ONR) as the lead agency with additional contributions from the National Oceanic and Atmospheric Administration (NOAA), the National Space Administration (NASA), the U.S. Navy and the Canadian Forces. Support from these agencies is gratefully acknowledged. We also gratefully acknowledge the assistance and cooperation provided by the officers and crews of the NOAA Ship OCEANOGRAPHER, the R/V THOMAS WASHINGTON, the HMCS PROVIDER, the HMCS GATINEAU, the HMCS KOOTENAY, the HMCS RESTIGOUCHE and the aircraft of the Commander Patrol Wing Two, U.S. Navy. It should be emphasized that this report is preliminary. It is a report of the observational phase of FRONTS-80 and preliminary results from individual investigators. The report is intended to aid and encourage an integrated analysis of the observations. Individual contributions should not be referenced without consent of the authors. Investigators are expected to provide more comprehensive reports and publications which will. be suitable for referencing. Specific acknowledgment of support by agency and grant or contract number will be given in these reports and publications

Observations of the cold wake of Typhoon Fanapi (2010)

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 40 (2013): 316–321, doi:10.1029/2012GL054282.Several tens of thousands of temperature profiles are used to investigate the thermal evolution of the cold wake of Typhoon Fanapi, 2010. Typhoon Fanapi formed a cold wake in the Western North Pacific Ocean on 18 September characterized by a mixed layer that was >2.5 °C cooler than the surrounding water, and extending to >80 m, twice as deep as the preexisting mixed layer. The initial cold wake became capped after 4 days as a warm, thin surface layer formed. The thickness of the capped wake, defined as the 26 °C–27 °C layer, decreased, approaching the background thickness of this layer with an e-folding time of 23 days, almost twice the e-folding lifetime of the Sea Surface Temperature (SST) cold wake (12 days). The wake was advected several hundreds of kilometers from the storm track by a preexisting mesoscale eddy. The observations reveal new intricacies of cold wake evolution and demonstrate the challenges of describing the thermal structure of the upper ocean using sea surface information alone.This work is primarily supported by the U.S. Office of Naval Research, with additional support from the National Science Foundation and the National Science Council, Taiwan

Typhoon-ocean interaction in the western North Pacific : Part 1

Author Posting. © The Oceanography Society, 2011. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 24 no. 4 (2011): 24–31, doi:10.5670/oceanog.2011.91.The application of new technologies has allowed oceanographers and meteorologists to study the ocean beneath typhoons in detail. Recent studies in the western Pacific Ocean reveal new insights into the influence of the ocean on typhoon intensity.This work is supported by grants from the Office of Naval Research, N00014- 10-WX-20203 (Black), N00014-08-1- 0656 (Centurioni), N00014-08-1-0577 (D’Asaro), N00014-09-1-0816 (D’Asaro), N00014-10-WX-21335 (Harr), N00014-08-1-0614 (Jayne), N00014- 09-1-0133 (Lee), N00014-08-1-0560 (Lien), N00014-10-1-0313 (student support), N00014-08-1-0658 (Rainville), N00014-08-1-0560 (Sanford); the National Oceanic and Atmospheric Administration NA17RJ1231 (Centurioni); and the National Science Foundation OCE0549887 (D’Asaro)