Numerical case studies for oceanic thermal anomalies with a dynamic model

Abstract

Numerical investigations to identify the physical processes responsible for the generation, evolution, and dissipation of oceanic thermal anomalies (OTA) were carried out using the numerical dynamic model of the North Pacific Experiment (Norpax). The Norpax model is based on time‐integrations of the finite‐difference forms of the primitive equations. It possesses an actual coastal configuration and 10 vertical layers, with a constant maximum depth of 4 km. The horizontal grid spacing, both longitudinal and latitudinal, is 2.5°. The seasonally varying model climatology is generated by integrating the model over 80 years of simulations under the climatological atmospheric forcing, of which the first 60 years are the long‐term annual mean conditions and the last 20 years vary with the seasonal cycle. Large‐scale features of the model ocean climatology compare favorably with observed large‐scale motions and structure in the North Pacific Ocean. The model is used for oceanic thermal anomaly studies. Two simulated cases are presented: one demonstrates the generation and evolution of OTA's under anomalous atmospheric wind forcing of winter 1949–1950, and the other portrays the evolution and dissipation of the OTA's under climatological atmospheric conditions in winter 1971–1972. The resulting model simulations are compared with observational data to examine to what extent change of the oceanic thermal structure is accounted for by the anomalous wind forcing and how much by the internal adjustments in the ocean. The model indicates that OTA's are generated by anomalous atmospheric winds and that thermal advection, both horizontal and vertical, plays the most important role in the generation. During winter, anomalous wind‐induced upwelling has more influence on a cold anomaly in the tropic and subtropic regions than anomalous downwelling has on a warm anomaly in the subarctic region under a weak stable state. The behavior of an initial anomaly under climatological conditions is closely related to large‐scale features, such as the circulation pattern and the thermal gradient in the ocean, and is also subject to modifications due to the presence of OTA's. Aside from limitations of the present course‐grid model, most discrepancies found between the simulated and observed anomalies can generally be attributed to the lack of reliable and accurate meteorological and subsurface data. As better oceanic and atmospheric data become available, further studies of anomaly dynamics through numerical experiments will lead to understanding anomalous heat distributions in the upper layers of the ocean and hence to better ocean predictions.