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Centurioni, LR, V. Hormann V, Talley LD, Arzeno I, Beal L, Caruso M, Conroy P, Echols R, Fernando HJS, Giddings SN, Gordon A, Graber H, Harcourt R, Jayne SR, Jensen TG, Lee CM, Lermusiaux PFJ, L'Hegaret P, Lucas AJ, Mahadevan A, McClean JL, Pawlak G, Rainville L, Riser S, Seo H, Shcherbina AY, Skyllingstad E, Sprintall J, Subrahmanyan B, Terrill E, Todd RE, Trott C, Ulloa HN, Wang H.  2017.  Northern Arabian Sea Circulation - Autonomous Research (NASCar): a research initiative based on autonomous sensors. Oceanography . 30(2):74-87.
Chen, R, Gille ST, McClean JL.  2017.  Isopycnal eddy mixing across the Kuroshio Extension: Stable vs unstable states in an eddying model. Journal of Geophysical Research: Oceans. 122(5):4329-4345.   10.1002/2016JC012164  
Delman, AS, Sprintall J, McClean JL, Talley LD.  2016.  Anomalous Java cooling at the initiation of positive Indian Ocean Dipole events. Journal of Geophysical Research: Oceans. 121(8):5805-5824.   10.1002/2016JC011635   AbstractWebsite

Anomalous sea surface temperature (SST) cooling south of Java, initiated during May–July, is an important precursor to positive Indian Ocean Dipole (pIOD) events. As shown previously, the Java SST anomalies are spatially and temporally coincident with seasonal upwelling induced locally by southeasterly trade winds. However, we confirm earlier findings that interannual variability of the Java cooling is primarily driven by remote wind forcing from coastal Sumatra and the equatorial Indian Ocean (EqIO); we also find an influence from winds along the Indonesian Throughflow. The wind forcing in the EqIO and along coastal Sumatra does not initiate SST cooling locally due to a deep thermocline and thick barrier layer, but can force upwelling Kelvin waves that induce substantial surface cooling once they reach the seasonally shallower thermocline near the coast of Java. Satellite altimetry is used to obtain a Kelvin wave coefficient that approximates Kelvin wave amplitude variations along the equator. All pIOD years in the satellite record have anomalous levels of upwelling Kelvin wave activity along the equator during April–June, suggesting that upwelling waves during this season are necessary for pIOD event development. However, a change to wind-forced downwelling Kelvin waves during July–August can abruptly terminate cool Java SST anomalies and weaken the pIOD event. Upwelling Kelvin wave activity along the equator and wind stress anomalies west of Sumatra are both robust predictors of the IOD index later in the calendar year, while values of the Kelvin wave coefficient are the most reliable predictor of pIOD events specifically.

Delman, AS, McClean JL, Sprintall J, Talley LD, Yulaeva E, Jayne SR.  2015.  Effects of eddy vorticity forcing on the mean state of the Kuroshio Extension. Journal of Physical Oceanography. 45:1356-1375.   10.1175/JPO-D-13-0259.1   Abstract

Eddy-mean flow interactions along the Kuroshio Extension (KE) jet are investigated using a vorticity budget of a high-resolution ocean model simulation, averaged over a 13-yr period. The simulation explicitly resolves mesoscale eddies in the KE and is forced with air-sea fluxes representing the years 1995-2007. A mean-eddy decomposition in a jet-following coordinate system removes the variability of the jet path from the eddy components of velocity; thus, eddy kinetic energy in the jet reference frame is substantially lower than in geographic coordinates and exhibits a cross-jet asymmetry that is consistent with the baroclinic instability criterion of the long-term mean field. The vorticity budget is computed in both geographic (i. e., Eulerian) and jet reference frames; the jet frame budget reveals several patterns of eddy forcing that are largely attributed to varicose modes of variability. Eddies tend to diffuse the relative vorticity minima/maxima that flank the jet, removing momentum from the fast-moving jet core and reinforcing the quasi-permanent meridional meanders in the mean jet. A pattern associated with the vertical stretching of relative vorticity in eddies indicates a deceleration (acceleration) of the jet coincident with northward (southward) quasi-permanent meanders. Eddy relative vorticity advection outside of the eastward jet core is balanced mostly by vertical stretching of the mean flow, which through baroclinic adjustment helps to drive the flanking recirculation gyres. The jet frame vorticity budget presents a well-defined picture of eddy activity, illustrating along-jet variations in eddy-mean flow interaction that may have implications for the jet's dynamics and cross-frontal tracer fluxes.

Schonau, MC, Rudnick DL, Cerovecki I, Gopalakrishnan G, Cornuelle BD, McClean JL, Qiu B.  2015.  The Mindanao Current: Mean structure and connectivity. Oceanography . 28(4):34-45.
Chen, R, Gille ST, McClean JL, Flierl GR, Grisesel A.  2015.  A multi-wavenumber theory for eddy diffusivities and its application to the southeast Pacific (DIMES) region. Journal of Physical Oceanography. 45:1877-1896.   10.1175/JPO-D-14-0229.1  
Qiu, B, Rudnick DL, Cerovecki I, Cornuelle BD, Chen S, Schonau MC, McClean JL, Gopalakrishnan G.  2015.  The Pacific North Equatorial Current: New insights from the Origins of the Kuroshio and Mindanao Currents (OKMC) Project. Oceanography. 28(4):24-33.
Li, L, Miller AJ, McClean JL, Eisenman I, Hendershott MC.  2014.  Processes driving sea ice variability in the Bering Sea in an eddying ocean/sea ice model: anomalies from the mean seasonal cycle. Ocean Dynamics. 64:1693-1717.: Springer Berlin Heidelberg   10.1007/s10236-014-0769-7   AbstractWebsite

A fine-resolution (1/10°) ocean/sea ice model configured in the Community Earth System Model framework is compared with observations and studied to determine the basin-scale and local balances controlling the variability of sea ice anomalies from the mean seasonal cycle in the Bering Sea for the time period 1980–1989. The model produces variations in total Bering Sea ice area anomalies that are highly correlated with observations. Surface air temperature, which is specified from reanalysis atmospheric forcing, strongly controls the ice volume variability in this simulation. The thermodynamic ice volume change is dominated by surface energy flux via atmosphere-ice sensible heat flux, except near the southern ice edge where it is largely controlled by ocean-ice heat flux. While thermodynamic processes dominate the variations in ice volume in the Bering Sea on the large scale, dynamic processes are important on the local scale near ice margins (both oceanic and land), where dynamic and thermodynamic ice volume changes have opposite signs and nearly cancel each other. Ice motion is generally consistent with winds driving the flow, except near certain straits in the north where ice motion largely follows ocean currents. Two key climate events, strong ice growth with cold air temperature and northerly wind anomalies in February 1984 and weak ice growth with warm air temperature and southerly wind anomalies in February 1989, are studied here in detail. While the processes controlling the ice changes are generally similar to those in other years, these large events help reveal the characteristic spatial patterns of ice growth/melt and transport anomalies.

Li, L, McClean JL, Miller AJ, Eisenman I, Hendershott MC, Papadopoulos CA.  2014.  Processes driving sea ice variability in the Bering Sea in an eddying ocean/sea ice model: Mean seasonal cycle. Ocean Modelling. 84:51-66.   AbstractWebsite

The seasonal cycle of sea ice variability in the Bering Sea, together with the thermodynamic and dynamic processes that control it, are examined in a fine resolution (1/10°) global coupled ocean/sea-ice model configured in the Community Earth System Model (CESM) framework. The ocean/sea-ice model consists of the Los Alamos National Laboratory Parallel Ocean Program (POP) and the Los Alamos Sea Ice Model (CICE). The model was forced with time-varying reanalysis atmospheric forcing for the time period 1970–1989. This study focuses on the time period 1980–1989. The simulated seasonal-mean fields of sea ice concentration strongly resemble satellite-derived observations, as quantified by root-mean-square errors and pattern correlation coefficients. The sea ice energy budget reveals that the seasonal thermodynamic ice volume changes are dominated by the surface energy flux between the atmosphere and the ice in the northern region and by heat flux from the ocean to the ice along the southern ice edge, especially on the western side. The sea ice force balance analysis shows that sea ice motion is largely associated with wind stress. The force due to divergence of the internal ice stress tensor is large near the land boundaries in the north, and it is small in the central and southern ice-covered region. During winter, which dominates the annual mean, it is found that the simulated sea ice was mainly formed in the northern Bering Sea, with the maximum ice growth rate occurring along the coast due to cold air from northerly winds and ice motion away from the coast. South of St Lawrence Island, winds drive the model sea ice southwestward from the north to the southwestern part of the ice-covered region. Along the ice edge in the western Bering Sea, model sea ice is melted by warm ocean water, which is carried by the simulated Bering Slope Current flowing to the northwest, resulting in the S-shaped asymmetric ice edge. In spring and fall, similar thermodynamic and dynamic patterns occur in the model, but with typically smaller magnitudes and with season-specific geographical and directional differences.

Evans, KJ, Mahajan S, Branstetter M, McClean JL, Caron J, Maltrud ME, Hack JJ, Bader DC, Neale R, Leifeld JK.  2014.  A spectral transform dynamical core option within the Community Atmosphere Model (CAM4). Journal of Advances in Modeling Earth Systems. 6:902-922.   10.1002/2014MS000329   AbstractWebsite

An ensemble of simulations covering the present day observational period using forced sea surface temperatures and prescribed sea-ice extent is configured with an 85 truncation resolution spectral transform dynamical core (T85) within the Community Atmosphere Model (CAM), version 4 and is evaluated relative to observed and model derived data sets and the one degree finite volume (FV) dynamical core. The spectral option provides a well-known base within the climate model community to assess climate behavior and statistics, and its relative computational efficiency for smaller computing platforms allows it to be extended to perform high-resolution climate length simulations. Overall, the quality of the T85 ensemble is similar to FV. Analyzing specific features of the T85 simulations show notable improvements to the representation of wintertime Arctic sea level pressure and summer precipitation over the Western Indian subcontinent. The mean and spatial patterns of the land surface temperature trends over the AMIP period are generally well simulated with the T85 ensemble relative to observations, however the model is not able to capture the extent nor magnitude of changes in temperature extremes over the boreal summer, where the changes are most dramatic. Biases in the wintertime Arctic surface temperature and annual mean surface stress fields persist with T85 as with the CAM3 version of T85, as compared to FV. An experiment to identify the source of differences between dycores has revealed that the longwave cloud forcing is sensitive to the choice of dycore, which has implications for tuning strategies of the physics parameter settings.

Xu, LX, Xie SP, McClean JL, Liu QY, Sasaki H.  2014.  Mesoscale eddy effects on the subduction of North Pacific mode waters. Journal of Geophysical Research-Oceans. 119:4867-4886.   10.1002/2014jc009861   AbstractWebsite

Mesoscale eddy effects on the subduction of North Pacific mode waters are investigated by comparing observations and ocean general circulation models where eddies are either parameterized or resolved. The eddy-resolving models produce results closer to observations than the noneddy-resolving model. There are large discrepancies in subduction patterns between eddy-resolving and noneddy-resolving models. In the noneddy-resolving model, subduction on a given isopycnal is limited to the cross point between the mixed layer depth (MLD) front and the outcrop line whereas in eddy-resolving models and observations, subduction takes place in a broader, zonally elongated band within the deep mixed layer region. Mesoscale eddies significantly enhance the total subduction rate, helping create remarkable peaks in the volume histogram that correspond to North Pacific subtropical mode water (STMW) and central mode water (CMW). Eddy-enhanced subduction preferentially occurs south of the winter mean outcrop. With an anticyclonic eddy to the west and a cyclonic eddy to the east, the outcrop line meanders south, and the thermocline/MLD shoals eastward. As eddies propagate westward, the MLD shoals, shielding the water of low potential vorticity from the atmosphere. The southward eddy flow then carries the subducted water mass into the thermocline. The eddy subduction processes revealed here have important implications for designing field observations and improving models.

Griesel, A, McClean JL, Gille ST, Sprintall J, Eden C.  2014.  Eulerian and Lagrangian isopycnal eddy diffusivities in the Southern Ocean of an eddying model. Journal of Physical Oceanography. 44:644-661.   10.1175/JPO-D-13-039.1  
Chen, R, McClean JL, Gille ST, Griesel A.  2014.  Isopycnal eddy diffusivities in the Kuroshio Extension from an eddying ocean circulation model. Journal of Physical Oceanography. 44:2191-2211.   10.1175/JPO-D-13-0258.1  
Wang, JB, Flierl GR, LaCasce JH, McClean JL, Mahadevan A.  2013.  Reconstructing the ocean's interior from surface data. Journal of Physical Oceanography. 43:1611-1626.   10.1175/jpo-d-12-0204.1   AbstractWebsite

A new method is proposed for extrapolating subsurface velocity and density fields from sea surface density and sea surface height (SSH). In this, the surface density is linked to the subsurface fields via the surface quasigeostrophic (SQG) formalism, as proposed in several recent papers. The subsurface field is augmented by the addition of the barotropic and first baroclinic modes, whose amplitudes are determined by matching to the sea surface height (pressure), after subtracting the SQG contribution. An additional constraint is that the bottom pressure anomaly vanishes. The method is tested for three regions in the North Atlantic using data from a high-resolution numerical simulation. The decomposition yields strikingly realistic subsurface fields. It is particularly successful in energetic regions like the Gulf Stream extension and at high latitudes where the mixed layer is deep, but it also works in less energetic eastern subtropics. The demonstration highlights the possibility of reconstructing three-dimensional oceanic flows using a combination of satellite fields, for example, sea surface temperature (SST) and SSH, and sparse (or climatological) estimates of the regional depth-resolved density. The method could be further elaborated to integrate additional subsurface information, such as mooring measurements.

Gopalakrishnan, G, Cornuelle BD, Gawarkiewicz G, McClean JL.  2013.  Structure and evolution of the cold dome off northeastern Taiwan: A numerical study. Oceanography. 26(1):66-79.   10.5670/oceanog.2013.06   AbstractWebsite

Numerous observational and modeling studies of ocean circulation surrounding Taiwan have reported occurrences of cold water and doming of isotherms (called the cold dome) that result in the formation of coastal upwelling on the northeastern Taiwan shelf. We use a high-resolution (1/24°) ocean model based on the Massachusetts Institute of Technology general circulation model to study the evolution of this distinct shelf-slope circulation phenomenon. We performed a number of model simulations spanning a five-year period (2004–2008) using realistic atmospheric forcing and initial and open boundary conditions. The model solutions were compared with satellite measurements of sea surface height (SSH), sea surface temperature (SST), and historical temperature and salinity observations. The model showed a realistically shaped cold dome with a diameter of ~ 100 km and temperature of ~ 3°C below the ambient shelf waters at 50 m depth. The occurrences of simulated cold dome events appeared to be connected with the seasonal variability of the Kuroshio Current. The model simulations showed more upwelling events during spring and summer when the core of the Kuroshio tends to migrate away from the east coast of Taiwan, compared to fall and winter when the core of the Kuroshio is generally found closer to the east coast of Taiwan. The model also reproduced weak cyclonic circulation associated with the upwelling off northeastern Taiwan. We analyzed the spatio-temporal variability of the cold dome using the model solution as a proxy and designed a "cold dome index" based on the temperature at 50 m depth averaged over a 0.5° × 0.5° box centered at 25.5°N, 122°E. The cold dome index correlates with temperature at 50 m depth in a larger region, suggesting the spatial extent of the cold dome phenomenon. The index had correlation maxima of 0.78 and 0.40 for simulated SSH and SST, respectively, in and around the cold dome box region, and we hypothesize that it is a useful indicator of upwelling off northeastern Taiwan. In addition, both correlation and composite analysis between the temperature at 50 m depth and the East Taiwan Channel transport showed no cold dome events during low-transport events (often in winter) and more frequent cold dome events during high-transport events (often in summer). The simulated cold dome events had time scales of about two weeks, and their centers aligned roughly along a northeastward line starting from the northeastern tip of Taiwan.

Ivanova, DP, McClean JL, Hunke EC.  2012.  Interaction of ocean temperature advection, surface heat fluxes and sea ice in the marginal ice zone during the North Atlantic Oscillation in the 1990s: A modeling study. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007532   AbstractWebsite

A moderately fine-resolution (0.4 degrees, 40 vertical levels), global, coupled ice-ocean model was configured and run for 24 years (1979-2002), forced with high-frequency National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) atmospheric fluxes. The model consists of the Los Alamos National Laboratory Parallel Ocean Program (POP) and sea ice model (CICE). The fidelity of the simulated mean climatological state and variability of key variables such as ice concentration, total ice area, ice thickness and drift were compared to observational data sets from satellite and ice drift buoy measurements. Basin-scale changes in the lower atmosphere/surface ocean/sea-ice in the simulated Arctic and Nordic Seas before and after the North Atlantic Oscillation (NAO) phase switch in 1995 were examined using winter composite analyses over the period 1990-1999. Ice cover changes between the two NAO phases were consistent with observations in that reduced concentrations were found in the Nordic and Barents Seas and increased values occurred in the Labrador Sea. Next we regionally evaluated the relative importance of winter anomalies of upper-ocean mixed layer net heat fluxes and of ocean temperature advection on marginal ice zone variability in the Irminger, Greenland, and Barents Seas for this ten-year period. We found that the net heat flux winter anomaly was at least four times more important than the winter anomaly of ocean temperature advection in the Greenland and Barents Seas, while it was twice as important in the Irminger Sea. The Ekman ocean temperature advection component generally dominated the geostrophic component in all three regions.

Lenn, YD, Chereskin TK, Sprintall J, McClean JL.  2011.  Near-surface eddy heat and momentum fluxes in the Antarctic Circumpolar Current in Drake Passage. Journal of Physical Oceanography. 41:1385-1407.   10.1175/jpo-d-10-05017.1   AbstractWebsite

The authors present new estimates of the eddy momentum and heat fluxes from repeated high-resolution upper-ocean velocity and temperature observations in Drake Passage and interpret their role in the regional Antarctic Circumpolar Current (ACC) momentum balance. The observations span 7 yr and are compared to eddy fluxes estimated from a 3-yr set of output archived from an eddy-resolving global Parallel Ocean Program (POP) numerical simulation. In both POP and the observations, the stream-averaged cross-stream eddy momentum fluxes ((u'v') over bar) correspond to forcing consistent with both a potential vorticity flux into the axis of the Subantarctic Front (SAF) and a sharpening of all three main ACC fronts through Drake Passage. Further, the POP analysis indicates that the mean momentum advection terms reflect the steering of the mean ACC fronts and are not fully balanced by the eddy momentum forcing, which instead impacts the strength and number of ACC fronts. The comparison between POP and observed eddy heat fluxes was less favorable partly because of model bias in the water mass stratification. Observed cross-stream eddy heat fluxes ((v'T') over bar) are generally surface intensified and poleward in the ACC fronts, with values up to approximately -290 +/- 80 kW m(-2) in the Polar and Southern ACC Fronts. Interfacial form stresses F(T), derived from observed eddy heat fluxes in the SAF, show little depth dependence below the Ekman layer. Although F(T) appears to balance the surface wind stress directly, the estimated interfacial form stress divergence is only an order of magnitude greater than the eddy momentum forcing in the SAF. Thus, although the eddy momentum forcing is of secondary importance in the momentum balance, its effect is not entirely negligible.

Gawarkiewicz, G, Jan S, Lermusiaux PFJ, McClean JL, Centurioni L, Taylor K, Cornuelle B, Duda TF, Wang J, Yang YJ, Sanford T, Lien RC, Lee C, Lee MA, Leslie W, Haley PJ, Niiler PP, Gopalakrishnan G, Velez-Belchi P, Lee DK, Kim YY.  2011.  Circulation and intrusions northeast of Taiwan: Chasing and predicting uncertainty in the cold dome. Oceanography. 24:110-121.   10.5670/oceanog.2011.99   AbstractWebsite

An important element of present oceanographic research is the assessment and quantification of uncertainty. These studies are challenging in the coastal ocean due to the wide variety of physical processes occurring on a broad range of spatial and temporal scales. In order to assess new methods for quantifying and predicting uncertainty, a joint Taiwan-US field program was undertaken in August/September 2009 to compare model forecasts of uncertainties in ocean circulation and acoustic propagation, with high-resolution in situ observations. The geographical setting was the continental shelf and slope northeast of Taiwan, where a feature called the "cold dome" frequently forms. Even though it is hypothesized that Kuroshio subsurface intrusions are the water sources for the cold dome, the dome's dynamics are highly uncertain, involving multiple scales and many interacting ocean features. During the experiment, a combination of near-surface and profiling drifters, broad-scale and high-resolution hydrography, mooring arrays, remote sensing, and regional ocean model forecasts of fields and uncertainties were used to assess mean fields and uncertainties in the region. River runoff from Typhoon Morakot, which hit Taiwan August 7-8, 2009, strongly affected shelf stratification. In addition to the river runoff, a cold cyclonic eddy advected into the region north of the Kuroshio, resulting in a cold dome formation event. Uncertainty forecasts were successfully employed to guide the hydrographic sampling plans. Measurements and forecasts also shed light on the evolution of cold dome waters, including the frequency of eddy shedding to the north-northeast, and interactions with the Kuroshio and tides. For the first time in such a complex region, comparisons between uncertainty forecasts and the model skill at measurement locations validated uncertainty forecasts. To complement the real-time model simulations, historical simulations with another model show that large Kuroshio intrusions were associated with low sea surface height anomalies east of Taiwan, suggesting that there may be some degree of predictability for Kuroshio intrusions.

Carman, JC, McClean JL.  2011.  Investigation of IPCC AR4 coupled climate model North Atlantic mode water formation. Ocean Modelling. 40:14-34.   10.1016/j.ocemod.2011.07.001   AbstractWebsite

The formation of mode waters in the North Atlantic was examined in the suite of ocean models that comprise the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3). We constructed model climatologies for 1980-1999 from the 20th century simulations, and compared their mode water properties (temperature, salinity, formation rate, volume, turnover time, heat content) with data. In these models, we found biases in both the properties of the mode waters and their formation rates. For Subpolar Mode Water (SPMW), property biases principally involved salinity errors; additionally, some models form SPMW in an anomalous region west of the British Isles, shifting the source location of waters entering the overturning cell and altering the Nordic Seas' involvement in the Meridional Overturning Circulation. For Subtropical Mode Water (STMW), property biases involved both salinity and temperature errors, while positioning of heat and water fluxes relative to the Gulf Stream and northwest Sargasso Sea influenced STMW formation rate. Deficiencies in STMW formation rate and volume produced a turnover time of 1-2 years, approximately half of that observed; these variations in mode water bulk properties imply variation in ocean heat storage and advection, and hence deficiencies in all the models' abilities to adequately respond to changes in climatic forcing. Published by Elsevier Ltd.

McClean, JL, Bader DC, Bryan FO, Maltrud ME, Dennis JM, Mirin AA, Jones PW, Kim YY, Ivanova DP, Vertenstein M, Boyle JS, Jacob RL, Norton N, Craig A, Worley PH.  2011.  A prototype two-decade fully-coupled fine-resolution CCSM simulation. Ocean Modelling. 39:10-30.   10.1016/j.ocemod.2011.02.011   AbstractWebsite

A fully coupled global simulation using the Community Climate System Model (CCSM) was configured using grid resolutions of 0.1 degrees for the ocean and sea-ice, and 0.25 degrees for the atmosphere and land, and was run under present-day greenhouse gas conditions for 20 years. It represents one of the first efforts to simulate the planetary system at such high horizontal resolution. The climatology of the circulation of the atmosphere and the upper ocean were compared with observational data and reanalysis products to identify persistent mean climate biases. Intensified and contracted polar vortices, and too cold sea surface temperatures (SSTs) in the subpolar and mid-latitude Northern Hemisphere were the dominant biases produced by the model. Intense category 4 cyclones formed spontaneously in the tropical North Pacific. A case study of the ocean response to one such event shows the realistic formation of a cold SST wake, mixed layer deepening, and warming below the mixed layer. Too many tropical cyclones formed in the North Pacific however, due to too high SSTs in the tropical eastern Pacific. In the North Atlantic anomalously low SSTs lead to a dearth of hurricanes. Agulhas eddy pathways are more realistic than in equivalent stand-alone ocean simulations forced with atmospheric reanalysis. (C) 2011 Elsevier Ltd. All rights reserved.

Griesel, A, Gille ST, Sprintall J, McClean JL, LaCasce JH, Maltrud ME.  2010.  Isopycnal diffusivities in the Antarctic Circumpolar Current inferred from Lagrangian floats in an eddying model. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc005821   AbstractWebsite

Lagrangian subsurface isopycnal eddy diffusivities are calculated from numerical floats released in several regions of the Antarctic Circumpolar Current (ACC) of the 0.1 degrees Parallel Ocean Program. Lagrangian diffusivities are horizontally highly variable with no consistent latitudinal dependence. Elevated values are found in some areas in the core of the ACC, near topographic features, and close to the Brazil-Malvinas Confluence Zone and Agulhas Retroflection. Cross-stream eddy diffusivities are depth invariant in the model ACC. An increase of Lagrangian eddy length scales with depth is masked by the strong decrease with depth of eddy velocities. The cross-stream diffusivities average 750 +/- 250 m(2) s(-1) around the Polar Frontal Zone. The results imply that parameterizations that (only) use eddy kinetic energy to parameterize the diffusivities are incomplete. We suggest that dominant correlations of Lagrangian eddy diffusivities with eddy kinetic energy found in previous studies may have been due to the use of too short time lags in the integration of the velocity autocovariance used to infer the diffusivities. We find evidence that strong mean flow inhibits cross-stream mixing within the ACC, but there are also areas where cross-stream diffusivities are large in spite of strong mean flows, for example, in regions close to topographic obstacles such as the Kerguelen Plateau.

Bryan, FO, Tomas R, Dennis JM, Chelton DB, Loeb NG, McClean JL.  2010.  Frontal scale air-sea interaction in high-resolution coupled climate models. Journal of Climate. 23:6277-6291.   10.1175/2010jcli3665.1   AbstractWebsite

The emerging picture of frontal scale air sea interaction derived from high resolution satellite observations of surface winds and sea surface temperature (SST) provides a unique opportunity to test the fidelity of high resolution coupled climate simulations Initial analysis of the output of a suite of Community Climate System Model (CCSM) experiments indicates that characteristics of frontal scale ocean atmosphere interaction such as the positive correlation between SST and surface wind stress are realistically captured only when the ocean component is eddy resolving The strength of the coupling between SST and surface stress is weaker than observed however as has been found previously for numerical weather prediction models and other coupled climate models The results are similar when the atmospheric component model grid resolution is doubled from 0 5 degrees to 0 25 degrees an indication that shortcomings in the representation of subgrid scale atmospheric planetary boundary layer processes rather than resolved scale processes are responsible for the weakness of the coupling In the coupled model solutions the response to mesoscale SST features is strongest in the atmospheric boundary layer but there is a deeper reaching response of the atmospheric circulation apparent in free tropospheric clouds This simulated response is shown to be consistent with satellite estimates of the relationship between mesoscale SST and all sky albedo

Arbic, BK, Shriver JF, Hogan PJ, Hurlburt HE, McClean JL, Metzger EJ, Scott RB, Sen A, Smedstad OM, Wallcraft AJ.  2009.  Estimates of bottom flows and bottom boundary layer dissipation of the oceanic general circulation from global high-resolution models. Journal of Geophysical Research-Oceans. 114   10.1029/2008jc005072   AbstractWebsite

This paper (1) compares the bottom flows of three existing high-resolution global simulations of the oceanic general circulation to near-bottom flows in a current meter database and (2) estimates, from the simulations, the global energy dissipation rate of the general circulation by quadratic bottom boundary layer drag. The study utilizes a data-assimilative run of the Naval Research Laboratory Layered Ocean Model (NLOM), a nonassimilative run of NLOM, and a nonassimilative run of the Parallel Ocean Program z-level ocean model. Generally speaking, the simulations have some difficulty matching the flows in individual current meter records. However, averages of model values of |u(b)|(3) (the time average of the cube of bottom velocity, which is proportional to the dissipation rate) computed over all the current meter sites agree to within a factor of 2.7 or better with averages computed from the current meters, at least in certain depth ranges. The models therefore likely provide reasonable order-of-magnitude estimates of areally integrated dissipation by bottom drag. Global dissipation rates range from 0.14 to 0.65 TW, suggesting that bottom drag represents a substantial sink of the similar to 1 TW wind-power transformed into geostrophic motions.

Jayne, SR, Hogg NG, Waterman SN, Rainville L, Donohue KA, Watts DR, Tracey KL, McClean JL, Maltrud ME, Qiu B, Chen SM, Hacker P.  2009.  The Kuroshio Extension and its recirculation gyres. Deep-Sea Research Part I-Oceanographic Research Papers. 56:2088-2099.   10.1016/j.dsr.2009.08.006   AbstractWebsite

This paper reports on the strength and structure of the Kuroshio Extension and its recirculation gyres. In the time average, quasi-permanent recirculation gyres are found to the north and south of the Kuroshio Extension jet. The characteristics of these recirculations gyres are determined from the combined observations from the Kuroshio Extension System Study (KESS) field program (June 2004-June 2006) and include current meters, pressure and current recording inverted echo sounders, and subsurface floats. The position and strength of the recirculation gyres simulated by a high-resolution numerical model are found to be consistent with the observations. The circulation pattern that is revealed is of a complex system of multiple recirculation gyres that are embedded in the crests and troughs of the quasi-permanent meanders of the Kuroshio Extension. At the location of the KESS array, the Kuroshio Extension jet and its recirculation gyres transport of about 114Sv. This represents a 2.7-fold increase in the transport of the current compared to the Kuroshio's transport at Cape Ashizuri before it separates from the coast and flows eastward into the open ocean. This enhancement in the current's transport comes from the development of the flanking recirculation gyres. Estimates from an array of inverted echo sounders and a high-resolution ocean general circulation model are of similar magnitude. (C) 2009 Elsevier Ltd. All rights reserved.

Griesel, A, Gille ST, Sprintall J, McClean JL, Maltrud ME.  2009.  Assessing eddy heat flux and its parameterization: A wavenumber perspective from a 1/10° ocean simulation. Ocean Modelling. 29:248-260.   10.1016/j.ocemod.2009.05.004   AbstractWebsite

Diffusivities diagnosed from eddy heat fluxes in eddying models tend to show unphysically large positive and negative values. These have been attributed to rotational components that do not influence the heat budget. The rotational component of the horizontal eddy heat flux is diagnosed in the Southern Ocean of the 1/10 degrees Parallel Ocean Program as a function of averaging lengthscale. At the scales at which most of the eddy heat flux energy occurs, the rotational component, as measured by its curl, accounts for more than 95% of the total flux, increasing to more than 99% with increasing lengthscale. Hence, rotational components are the dominant component of eddy fluxes even when they are averaged over scales larger than the eddy scales. The rotational component can be approximated by its projection along temperature variance contours; however this only accounts for a fraction of the total rotational component, leaving a residual that is still more rotational than divergent. The eddy heat flux can be parameterized as a function of averaging lengthscale using coherence analysis. The meridional eddy heat flux is most coherent with its parameterizations on lengthscales larger than 50 degrees, but this coherence is due to the rotational component. The divergence of the eddy heat flux is most coherent with the divergence of the parameterizations on scales less than 4 degrees, where the curl of the eddy heat flux has a minimum. Diffusivities estimated from the eddy heat flux show a high wavenumber dependence and are as high as 15,000 m(2)s(-1), reflecting the presence of the rotational component. Using the divergence, more realistic, less wavenumber dependent values are estimated, ranging from 500 m(2)s(-1) within the ACC to 1000 m(2)s(-1) in the near surface ocean in the Agulhas Retroflection region. (C) 2009 Elsevier Ltd. All rights reserved.