Subduction of the North Pacific Mode Waters in a Global High-Resolution GCM

Abstract

The annual subduction rate in the North Pacific is estimated using five-day outputs from a high-resolution general circulation model (GCM). Two maxima (>200 m yr?1) are found in the western North Pacific: one is responsible for the formation of the subtropical mode water (STMW) and the other for the formation of the central mode water (CMW). A local maximum (>75 m yr?1) is also found in the formation region of the eastern subtropical mode water (ESMW). These results are compared with a calculation using the winter mixed layer depth and annual mean velocity fields to examine the effect of mesoscale eddies. Although the mesoscale eddies do not markedly affect the general subduction pattern, they enhance the annual subduction rate by up to 100 m yr?1 in the formation region of the STMW/CMW, a 34% increase in a regional average (30°?44°N, 140°E?170°W). Further analysis shows that the effects of the mean seasonal cycle and smaller-scale (<30 days) eddies are generally small. The authors suggest that the two peaks in the subduction rate are related to a double-front structure on the intergyre boundary in the western North Pacific.