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Thomas J. Weingartner and Robert H. Weisberg

Abstract

Temperature and velocity time series, obtained by surface moorings during the Seasonal Response of the Equatorial Atlantic Experiment, are used to investigate the role of ocean dynamics upon the annual cycle of equatorial sea surface temperature (SST) and upper ocean heat. The annual cycle in SST is explained by different mechanisms, each operant at different phases of the cycle. The boreal springtime decrease in SST results from upwelling in response to the seasonal intensification of easterly wind stress. This upwelling causes the seasonal formation of the cold tongue along the equator in the central and eastern portions of the basin. An early summer increase in SST is attributed to the meridional convergence of Reynolds' heat flux associated with surface current instability-generated waves. After the instability waves abate, SST and mixed layer depth remain relatively steady from late summer through fall when the advective terms are small and cancelling, suggesting that surface heating is then balanced by a diffusive flux at the base of the mixed layer. SST increases in wintertime following the seasonal relaxation in easterly wind stress, thus, completing its annual cycle. This increase is attributed to the concentration of the surface flux over a mixed layer that is shoaling due to both the basin-wide adjustment of the thermocline and the local reduction in turbulent energy production. Thus, SST variations are found to be most closely controlled by ocean dynamics during those times when the thermocline is adjusting basin-wide to the seasonal changes in wind stress; either directly by large advective fluxes (boreal spring-summer) or indirectly by mediating mixed layer depth (boreal winter). Analyses at 75 m depth show zonal and vertical advection to be important, and within a control volume over the upper 150 m all of the advective terms are important.

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Thomas J. Weingartner and Robert H. Weisberg

Abstract

The annual cycle of the upper ocean's vertical velocity component (w) on the equator at 28°W is examined by integrating the continuity equation using current meter data from the Seasonal Response of the Equatorial Atlantic Experiment. The annual cycle consists in part of an intense, but brief (∼1 month), upwelling season beginning with the onset of strong easterly wind stress in boreal spring. This upwelling is followed by weaker downwelling during the summer despite the persistence of strong easterly wind stress. The record-length averaged w profile shows that maximum upwelling (0.6 × 10−3 cm s−1) is located slightly above the core of the Equatorial Undercurrent and downwelling is located below the base of the thermocline. The standard deviations are about tenfold the magnitude of the means. Independent evidence supporting these results are that 1) sea surface temperature (SST) is related to w during the springtime changes in easterly wind stress with the observed and computed isotherm displacements in agreement, 2) temperature and w are coherent and in quadrature within the thermocline over a broad range of frequencies exclusive of the instability wave band, 3) during the instability wave season, upwelling is associated with increasing SST and the vertical Reynolds' heat flux is maximum and divergent in the thermocline, and 4) after the instability waves abate, w and easterly wind stress are coherent and out-of-phase. The observed evolution of w differs from that implied by climatology, and these differences are attributed to the ocean's response to rapidly varying winds that are observed in-situ versus slowly varying winds characteristic of climatology.

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Robert H. Weisberg and Thomas J. Weingartner

Abstract

Evidence is presented for the generation of planetary waves by barotropic instability within the cyclonic shear region of the Atlantic Ocean's South Equatorial Current (SEC). Immediately following the springtime intensification of the southeast trade wind, which accelerates the SEC westward, a packet of waves with central periodicity of around 25 days is observed lasting for about three cycles. Independent wavenumber analyses on 1983 and 1984 data give newly identical zonal wavelengths and phase speed estimates of around 1100 km and −50 cm s−1. The waves are anisotropic and spatially inhomogeneous with generation confined primarily to the mixed layer.

An energetics analysis using 1983 data centered upon the equator at 28°W shows a rapid increase in total perturbation energy (TPE) reaching values of 2000 erg cm−3 within two weeks. The subsequent decrease in TPE at this location is due primarily to meridional pressure-work divergence. Baroclinic instability is negligible because both the meridional and zonal components are small and cancelling.

Thermodynamically, the waves effect a southward heat transport during the period when the North Equatorial Countercurrent (NECC) is most rapidly gaining heat, suggesting that the waves act to regulate the heat stored in the NECC. Also the Reynolds' heat flux convergence upon the equator appears to halt the upwelling induced cooling and to increase sea surface temperature. In 1983 this convergence was equal to the climatological atmosphere-ocean net heat flux.

Dynamically, the waves decelerate the SEC north of the equator and reduce its shear. This occur simultaneously with a deceleration of the SEC by the basinwide adjustment of the zonal pressure gradient (ZPG). The seasonal modulation of the waves is therefore a consequence of both the ZPG response to seasonally varying wind stress as well as the instability itself since both are stabilizing. The basin size and hence the ZPG adjustment time differences between the Atlantic and Pacific Oceans would thus account for the observed differences in wave season durations between these two oceans.

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Jeremy L. Kasper and Thomas J. Weingartner

Abstract

Idealized numerical simulations using the Regional Ocean Modeling System demonstrate the effects of an immobile landfast ice cover that is frictionally coupled to an underice buoyant plume established by river discharge. The discharge rapidly increases and decreases over a 30-day period and has a maximum of 6000 m3 s−1. This study examined the response to a landfast ice cover of 26-km width and one that encompasses the entire shelf width. The model setting mimics spring conditions on the Alaskan Beaufort Sea (ABS) shelf. In comparison with the ice-free case subject to the same discharge scenario, underice plumes are broader and deeper, and the downwave freshwater flux is substantially decreased and delayed. Multiple anticyclonic bulges form in the ice-free case, but only a single, large bulge forms when ice is present. These differences are because of the frictional coupling between the ice and plume, which results in an Ekman-like underice boundary layer, a subsurface velocity maximum, and frictional shears that enhance vertical mixing and entrainment. For a partially ice-covered shelf, the plume spreads across the ice edge to form a swift, buoyant, ice-edge jet, whose width accords with the scale of Yankovsky and Chapman for a surface-advected plume. For a fully ice-covered shelf, the buoyant water spreads 60 km offshore over a 30-day period. For a steady discharge of 6000 m3 s−1 and a completely ice-covered shelf, the plume spreads offshore at a rate of ~1.5 km day−1 and extends ~95 km offshore after 60 days.

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William J. Williams, Thomas J. Weingartner, and Albert J. Hermann

Abstract

The cross-shelf structure of a buoyancy-driven coastal current, such as produced by a river plume, is modeled in a two-dimensional cross-shelf slice as a “wide” geostrophically balanced buoyancy front. Downwelling-favorable wind stress applied to this front leads to advection in the surface and bottom boundary layers that causes the front to become steeper so that it eventually reaches a steep quasi-steady state. This final state is either convecting, stable and steady, or stable and oscillatory depending on D/δ * and by /f2, where D is bottom depth, δ * is an Ekman depth, by is the cross-shelf buoyancy gradient, and f is the Coriolis parameter. Descriptions of the cross-shelf circulation patterns are given and a scaling is presented for the isopycnal slope. The results potentially apply to the Alaska Coastal Current, which experiences strong, persistent downwelling-favorable wind stress during winter, but also likely have application to river plumes subjected to downwelling-favorable wind stress.

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Andreas Münchow, Thomas J. Weingartner, and Lee W. Cooper

Abstract

During the ice-free summer season in 1995 the authors deployed and subsequently tracked 39 surface drifters to test the hypothesis that the discharge from the Kolyma River forces a buoyancy-driven coastal current from the East Siberian Sea toward Bering Strait. The observed mean flow is statistically significant at the 95% level of confidence, but its direction contradicts their initial hypothesis. Instead of a coastally trapped eastward flow, the authors find a laterally sheared westward flow with maximum velocities offshore that correlate only weakly with the local winds. At a daily, wind-dominated timescale the drifter data reveal spatially coherent flows of up to 0.5 m s−1. The Lagrangian autocorrelation scale is about 3 days and the Lagrangian eddy length scale reaches 40 km. This spatial scale exceeds the nearshore internal deformation radius by a factor of 3; however, it more closely corresponds to the internal deformation radius associated with the offshore ice edge. Bulk estimates of the horizontal mixing coefficient resemble typical values of isotropic open ocean dispersion at midlatitudes. Hydrographic observations and oxygen isotope ratios of seawater indicate a low proportion of riverine freshwater relative to sea ice melt in most areas of the East Siberian Sea except close to the Kolyma Delta. The observations require a reevaluation of the conceptual view of the summer surface circulation of the East Siberian Sea. Eastward buoyancy-driven coastal currents do not always form on this shelf despite large river discharge. Instead, ice melt waters of a retreating ice edge act as a line source of buoyancy that in 1995 forced a westward surface flow in the East Siberian Sea.

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Ying-Chih Fang, Thomas J. Weingartner, Rachel A. Potter, Peter R. Winsor, and Hank Statscewich

Abstract

This study investigates the applicability of the optimal interpolation (OI) method proposed by Kim et al. for estimating ocean surface currents from high-frequency radar (HFR) in the northeastern Chukchi Sea, where HFR siting is dictated by power availability rather than optimal locations. Although the OI technique improves data coverage when compared to the conventional unweighted least squares fit (UWLS) method, biased solutions can emerge. The quality of the HFR velocity estimates derived by OI is controlled by three factors: 1) the number of available incorporating radials (AR), 2) the ratio of the incorporating radials from multiple contributing site locations [ratio of overlapping radial velocities (ROR) or radar geometry], and 3) the positive definiteness [condition number (CN)] of the correlation matrix. Operationally, ROR does not require knowledge of the angle covariance matrix used to compute the geometric dilution of precision (GDOP) in the UWLS method and can be computed before site selection to optimize coverage or after data processing to assess data quality when applying the OI method. The Kim et al. method is extended to examine sensitivities to data gaps in the radial distribution and the effects on OI estimates.

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