Seasonal and Interannual Variability in Temperature of the Upper Layer of the Northwest Pacific, 1964–1983

Abstract

Available BT data in the northwest Pacific were analyzed to reveal seasonal and interannual variability in thermal structure of the upper 400 m layer in the northwest Pacific. Bimonthly temperature averaged over a 2° ? 2° square data in the area within 0°?44°N, 120°E?180° were interpolated to a spatially uniform data grid by combination of the Laplace and spline methods. Temporal variability of sea surface temperature (SST) is the largest in the seasonal change due to surface heating (July?October) and winter cooling (January?April). The variability at deeper layers below 100 m is primarily interannual in nature. The climatological seasonal cycle of the variation of SST shows that heating and cooling of the surface water start from the western tropical boundary area, extending towards the east and north, and that a certain time lag exists between SST and temperature at a deeper layer. The seasonal change of temperature in the upper layer in the midlatitudes between 15° and 35°N might be explained by the seasonal variation of the thermocline depth, induced by the seasonal change of the wind forcing over the study area. Horizontal maps of interannual root mean square deviation of temperature, which is the largest at 200 m in the upper 400 m, show a zonal minimum between 15° and 35°N and large interannual variability in the tropical area and high latitudes. Most of the high variability in the tropical area can be explained by the first complex empirical orthogonal function (CEOF) which varies in time with a period of 3?4 years. The CEOF analysis also showed that variability in the tropical area propagates to the northwest with a phase speed of order of 10 cm s?1, which might correspond to the first baroclinic Rossby wave with approximate wavelength of 16 000 km.