Role of Transients in the Dynamics of East Asian Summer Seasonal Mean Circulation Anomalies—A Study of 1993 and 1994

Abstract

By using the National Centers for Environmental Prediction reanalysis data and a nonlinear barotropic model, the transient activities and the possible role of transient vorticity fluxes in the maintenance of east Asian summer seasonal mean circulation anomalies associated with Japan?s extremely cool and wet summer of 1993 and hot and dry summer of 1994 are examined. Data analysis shows that the summertime atmospheric circulation and the transient activity anomalies exhibit nearly opposite patterns over east Asia during these two years. Vorticity budget calculations indicate that the anomalies of transient activities in these two years produced strong transient vorticity flux forcing anomalies that are comparable in magnitude to the divergent forcing anomalies over east Asia. The nonlinear barotropic model, when forced by the anomalous divergence and transient vorticity flux forcing together, produces simulations that bear resemblance to the observed summertime seasonal mean circulation anomalies of 1993 and 1994. The response to the anomalous transient vorticity flux forcing is found to be similar in magnitude with the response to the anomalous divergence. The solution deteriorates if the transient vorticity flux forcing anomaly is not included. Both the vorticity budget analysis and model simulations in the present paper show that the transient vorticity flux forcing anomaly plays an important role in maintaining the observed circulation anomalies over east Asia during the 1993 and 1994 summers.