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J. H. LaCasce and J. Wang

Abstract

A previously published method by Wang et al. for predicting subsurface velocities and density from sea surface buoyancy and surface height is extended by incorporating analytical solutions to make the vertical projection. One solution employs exponential stratification and the second has a weakly stratified surface layer, approximating a mixed layer. The results are evaluated using fields from a numerical simulation of the North Atlantic. The simple exponential solution yields realistic subsurface density and vorticity fields to nearly 1000 m in depth. Including a mixed layer improves the response in the mixed layer itself and at high latitudes where the mixed layer is deeper. It is in the mixed layer that the surface quasigeostrophic approximation is most applicable. Below that the first baroclinic mode dominates, and that mode is well approximated by the analytical solution with exponential stratification.

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J. D. Wang

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No abstract available.

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J. Y. Wang

Abstract

Information on atmospheric constituents is contained in the remotely measured spectral radiances. Two iteration methods, linear and nonlinear, are presented to demonstrate the possibility of inferring the water vapor profile from ground-based measurements. The linear inversion method which linearizes the radiative transfer equation is found to have a narrow range of convergence. A study of the vertical resolution of the inferred profile through the linear inversion technique indicates that fine-scale detailed structure of the profile cannot be reconstructed. The nonlinear iteration procedure, which minimizes the root-mean-squares residual of the random noise along the direction of “steepest” descent, is found capable of inferring a reasonably stable solution with wide range of convergence and is proven in numerical stability superior to the linear technique. The effects of the errors both in radiance measurements and in temperature profile on the inferred profile are also presented.

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J. Y. Wang and S. C. Wang

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Weimin Wang and Michael J. McPhaden

Abstract

The surface-layer heat balance on interannual timescales in the equatorial Pacific has been examined in order to determine the processes responsible for sea surface temperature (SST) variability associated with warm and cold phases of the ENSO cycle (El Niño and La Niña). Principal datasets include multiyear time series of surface winds, upper-ocean temperature, and velocity obtained from the Tropical Atmosphere Ocean buoy array at four locations along the equator in the western (165°E), central (170°W), and eastern (140°W and 110°W) Pacific. A blended satellite/in situ SST product and surface heat fluxes based on the Comprehensive Ocean–Atmosphere Data Set are also used. Changes in heat storage, horizontal heat advection, and heat fluxes at the surface are estimated directly from data; vertical fluxes of heat out of the base of the mixed layer are calculated as a residual.

Results indicate that all terms in the heat balance contribute to SST change on interannual timescales, depending on location and time period. Zonal advection is important everywhere, although relative to other processes, it is most significant in the central Pacific. The inferred vertical heat flux out of the base of the mixed layer is likewise important everywhere, especially so in the eastern equatorial Pacific where the mean thermocline is shallow. Meridional advection (primarily due to instability waves in this analysis) is a negative feedback term on SST change in the eastern equatorial Pacific, tending to counteract the development of warm and cold anomalies. Likewise, the net surface heat flux generally represents a negative feedback, tending to damp SST anomalies created by ocean dynamical processes. The implications of these results for ENSO modeling and theory are discussed.

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Weimin Wang and Michael J. McPhaden

Abstract

The surface layer temperature balance in the equatorial Pacific has been examined for the period of 1996–99 by using data obtained from Tropical Atmosphere–Ocean buoy array at four locations along the equator, namely, 165°E, 170°W, 140°W, and 110°W. Time rate change of SST, horizontal temperature advection, and heat fluxes at the sea surface are estimated directly from the data. Vertical fluxes of heat out of the base of the mixed layer, related to changes in vertical mixing and upwelling, are calculated as a residual. Results indicate that all terms in the temperature balance contributed to SST variations during different stages of the 1997–98 El Niño and the 1998–99 La Niña. Zonal advection was particularly important during the onset of the El Niño in the central Pacific. The inferred vertical heat flux out of the base of the mixed layer was important most of the time, and was especially pronounced in the eastern Pacific during the termination phase of the El Niño. Meridional advection due to eddies was a negative feedback on SST change in the eastern Pacific during both the El Niño and La Niña, tending to counteract the development of warm and cold anomalies. Likewise, the net surface heat flux generally represented a negative feedback, tending to damp SST anomalies created by ocean dynamical processes. The relevance of these results to recent theoretical developments concerning ENSO timescale physical processes is discussed.

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Xu Wang and Guang J. Zhang

Abstract

Low-frequency intraseasonal oscillations in the tropical atmosphere in general circulation models (GCMs) were studied extensively in many previous studies. However, the simulation of the quasi-biweekly oscillation (QBWO), which is an important component of the intraseasonal oscillations, in GCMs has not received much attention. This paper evaluates the QBWO features over the South China Sea in early [May–June (MJ)] and late [August–September (AS)] summer in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5.3 (CAM5), using observations and reanalysis data. Results show that the major features of the spatial distribution of the QBWO in both MJ and AS are simulated reasonably well by the model, although the amplitude of the variation is overestimated. CAM5 captures the local oscillation in MJ and the westward propagation in AS of the QBWO. Although there are important biases in geographical location and intensity in MJ, the model represents the QBWO horizontal and vertical structure qualitatively well in AS. The diagnosis of the eddy vorticity budget is conducted to better understand the QBWO activities in the model. Both horizontal advection of relative vorticity and that of planetary vorticity (Coriolis parameter) are important for the local evolution of the QBWO in MJ in observations as well as model simulation, whereas advection of planetary vorticity contributes to the westward propagation of QBWO vorticity anomalies in AS. Since the Coriolis parameter f only changes with latitude, this suggests that the correct simulation of anomalous meridional wind is a key factor in the realistic simulation of the QBWO in the model.

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Liping Wang and Chester J. Koblinsky

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Using sea surface height data collected by Geosat and Topex/Poseidon, the seasonal (annual) gyre circulations in the regions of the Gulf Stream and the Kuroshio Extension were studied. The seasonal gyre circulation is roughly confined to the regions where the annual mean subtropical recirculations exist. Associated with this seasonal gyre circulation, the surface transports of the Kuroshio and the Gulf Stream are found to be maximum in the late fall and minimum in the late spring. Using historical data, the authors demonstrated that these seasonal gyre circulations are mostly confined to the mixed layer. A simple diagnostic calculation of the buoyancy balance associated with the seasonal gyre circulations shows that they are driven primarily by local buoyancy flux (heating and cooling), while contribution from advection by large-scale ocean circulation is negligible. Even though the seasonal gyre circulation is primarily driven by local buoyancy forcing, it is in the opposite sense to that originally proposed by Worthington for the annual mean subtropical recirculation. The buoyancy balance within the study region suggests that dynamics associated with the mean recirculation and the seasonal gyre are fundamentally different.

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J. A. Whitehead and Wei Wang

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A model of deep ocean circulation driven by turbulent mixing is produced in a long, rectangular laboratory tank. The salinity difference is substituted for the thermal difference between tropical and polar regions. Freshwater gently flows in at the top of one end, dense water enters at the same rate at the top of the other end, and an overflow in the middle removes the same amount of surface water as is pumped in. Mixing is provided by a rod extending from top to bottom of the tank and traveling back and forth at constant speed with Reynolds numbers >500. A stratified upper layer (“thermocline”) deepens from the mixing and spreads across the entire tank. Simultaneously, a turbulent plume (“deep ocean overflow”) from a dense-water source descends through the layer and supplies bottom water, which spreads over the entire tank floor and rises into the upper layer to arrest the upper-layer deepening. Data are taken over a wide range of parameters and compared to scaling theory, energetic considerations, and simple models of turbulently mixed fluid. There is approximate agreement with a simple theory for Reynolds number >1000 in experiments with a tank depth less than the thermocline depth. A simple argument shows that mixing and plume potential energy flux rates are equal in magnitude, and it is suggested that the same is approximately true for the ocean.

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Lei Wang and Paul J. Kushner

Abstract

Stationary wave nonlinearity describes the self-interaction of stationary waves and is important in maintaining the zonally asymmetric atmospheric general circulation. However, the dynamics of stationary wave nonlinearity, which is often calculated explicitly in stationary wave models, is not well understood. Stationary wave nonlinearity is examined here in the simplified setting of the response to localized topographic forcing in quasigeostrophic barotropic dynamics in the presence and absence of transient eddies. It is shown that stationary wave nonlinearity accounts for most of the difference between the linear and full nonlinear response, particularly if the adjustment of the zonal-mean flow to the stationary waves is taken into account. The separate impact of transient eddy forcing is also quantified. Wave activity analysis shows that stationary wave nonlinearity in this setting is associated with Rossby wave critical layer reflection. A nonlinear stationary wave model, similar to those used in baroclinic stationary wave model studies, is also tested and is shown to capture the basic features of the full nonlinear stationary wave solution.

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