Effect of Tropical SST on the Northwest Pacific Subtropical Anticyclone. Part I: Linear Rossby Wave Propagation

Abstract

The development of anomalous subtropical anticyclone around East Asia, influences the regional climate of Japan and Korea in the summer. The effect of tropical sea surface temperature (SST) is studied using a regional atmospheric model (MM5 ver. 3.5) with realistic lateral, and surface boundary forcing. The following results were obtained. (1) An uniform increases of SST around the Philippine Islands generates the propagation of a Rossby wave, resulting in the formation of an anomalous subtropical anticyclone around Japan/Korea. (2) This Rossby wave propagation is linear in that the model response to negative SST anomalies reverses polarity. The linearity suggests that the response to the SST forcing around the Philippine Islands is a deterministic problem, with its magnitude depending on the size of the SST anomaly. Nonlinearity increases if there is a tropical depression, or typhoon in the model domain. (3) There are a zonally weakened upper-level jet, and a meridionally intensified low-level jet, when the Rossby wave is excited by SST changes. A positive SST anomaly intensifies the meandering of the upper-level jet. (4) The time scale for the atmospheric circulation to respond to such an SST forcing, is about 5 days. (5) In the vertical, the model response is barotropic over 40°N∼55°N, 100°E∼160°E, and is baroclinic to the south over 10°N∼40°N, 100°E∼140°E. (6) The effect of SST forcing is about four times stronger than model internal variability. (7) Only positive SST forcing affects the precipitation pattern around Japan and Korea, causing rainfall to decrease and increase in these regions, respectively. Both positive and negative SST forcing affect the precipitation around the Philippine Islands; a positive SST forcing increases precipitation, and vice versa. (8) The response of above Rossby wave is sensitive to the geographical location of SST forcing, reaching a maximum in the region (110°E∼140°E, 10°N∼20°N) among the experiments carried out. The study results of meridionally propagating Rossby wave induced anomalous subtropical anticyclone contribute to the understanding of regional climate around East Asia in summer.