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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. 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. D. Wang

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No abstract available.

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J. Y. Wang and S. C. Wang

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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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Andrew Schepen and Q. J. Wang

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The majority of international climate modeling centers now produce seasonal rainfall forecasts from coupled general circulation models (GCMs). Seasonal rainfall forecasting is highly challenging, and GCM forecast accuracy is still poor for many regions and seasons. Additionally, forecast uncertainty tends to be underestimated meaning that forecast probabilities are statistically unreliable. A common strategy employed to improve the overall accuracy and reliability of GCM forecasts is to merge forecasts from multiple models into a multimodel ensemble (MME). The most widely used technique is to simply pool all of the forecast ensemble members from multiple GCMs into what is known as a superensemble. In Australia, seasonal rainfall forecasts are produced using the Predictive Ocean–Atmosphere Model for Australia (POAMA). In this paper, the authors demonstrate that mean corrected superensembles formed by merging forecasts from POAMA with those from three international models in the ENSEMBLES dataset remain poorly calibrated in many cases. The authors propose and evaluate a two-step process for producing MMEs. First, forecast calibration of the individual GCMs is carried out by using Bayesian joint probability models that account for parameter uncertainty. The calibration leads to satisfactory forecast reliability. Second, the individually calibrated forecasts of the GCMs are merged through Bayesian model averaging (BMA). The use of multiple GCMs results in better forecast accuracy, while maintaining reliability, than using POAMA only. Compared with using equal-weight averaging, BMA weighting produces sharper and more accurate forecasts.

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Yuqing Wang and Greg J. Holland

Abstract

The beta drift of diabatic vortices is investigated with a three-dimensional primitive equation model with simple physical parameterizations. The vertical coupling mechanism discussed in Part I is extended to include the effects of diabatic heating and moist processes. The results show that the motion and evolution of the diabatic vortices can substantially differ from those of adiabatic vortices.

The anticyclone at the upper troposphere tends to propagate equatorward and westward due to the Rossby wave dispersion. But the continuous regeneration of an anticyclonic PV anomaly by diabatic heating keeps the upper-level anticyclone in a band stretching from the vortex core to several hundred kilometers equatorward and westward. Downward penetration of the circulation associated with these anticyclonic PV anomalies reduces the westward motion of the diabatic vortices by the vertical coupling mechanism discussed in Part I. This also rotates the lower-level beta-gyres anticyclonically, resulting in a more poleward asymmetric flow over the lower-level vortex core. As a result, diabatic vortices with a deeper and stronger outflow-layer anticyclone move in a more poleward direction than do the equivalent adiabatic baroclinic or barotropic vortices.

The asymmetric divergent flow associated with convective asymmetries within the vortex core region deflects the vortex center toward the region with maximum convection. Evolution of both the asymmetric convection and the vertical coupling may result in meandering vortex tracks.

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Lei Wang and Paul J. Kushner

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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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Weimin Wang and Michael J. McPhaden

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The surface-layer heat balance in the equatorial Pacific is examined in order to determine the processes responsible for the mean seasonal cycle of sea surface temperature (SST). Principal datasets include multiyear time series of surface winds, upper-ocean temperature, and velocity obtained from Tropical Atmosphere Ocean (TAO) buoy array at four locations along the equator in the western (165°E), central (170°W), and eastern (140° and 110°W) Pacific. A blended satellite–in situ SST product and climatological 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, of the terms that can be directly estimated, the net surface heat flux is generally the largest term in heat balance. Zonal heat advection is important at all locations and is generally a cooling term except in the eastern Pacific where the springtime reversal of the South Equatorial Current leads to warming. Meridional heat advection is largest in the eastern Pacific where it is dominated by seasonally varying tropical instability waves, which tend to warm the equator. The inferred vertical heat fluxes out the base of the mixed layer are comparable in magnitude to the surface fluxes, except in the western Pacific where they are close to zero. From these inferred vertical fluxes, the authors estimate the mean seasonal cycles in vertical eddy diffusivities and entrainment velocities, which, in the eastern Pacific, mimic the mean seasonal cycle of the surface winds. Implications for modeling and predicting SST are addressed.

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Greg J. Holland and Yuqing Wang

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The mechanisms associated with tropical cyclone recurvature are investigated using a five-level primitive equation model and an idealized environment with characteristics observed in cyclone recurvature conditions. All cyclones moved generally with the flow in the lower and middle troposphere, but the precise motion occurs by a combination of divergence and of advection in both the horizontal and the vertical. The horizontal advection arises from a combination of the initial environmental flow and local changes from rearrangement of the potential vorticity field by cyclone-environment interaction (the so-called,β effect). The balance between these mechanisms changes as the vortex recurves. Since the gradients of potential vorticity increase sharply poleward of the subtropical ridge, this is the preferred region for development of an anticyclonic gyre. This gyre is advected eastward and becomes the dominant anticyclonic system. Recurvature is aided by horizontal deformation of the cyclone in the vicinity of this gyre, and by the manner in which the vertical tilt of the vortex and local divergence fields vary as it moves through a changing vertical wind shear of the environment. Recurvature is sensitive to the degree of diabatic heating and to small meridional changes in the initial vortex location.

It is shown that recurvature can occur through an initially unbroken subtropical ridge, but that the presence of a midlatitude trough substantially enhances the potential for recurvature. However, while changes in the upper troposphere are indicative of recurvature potential, recurvature is accomplished largely by lower-tropospheric changes. An important component of this change is the development of a major anticyclone poleward and eastward of the cyclone. A recent observational study by Ford et al. concurs with this finding.

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