Climate change in the Northwest Pacific during the 21st century is investigated as manifested in the SRES A1B scenario of several AR-4 models using cyclostationary EOF (CSEOF) analysis. In case of the high-resolution MIROC model, 12 atmospheric variables including t2 (2 m air temperature) and 5 oceanic variables (sea surface height and 4 ocean variables defined at 47 vertical levels) were subjected to the analysis. Generally, CSEOF decomposition of each variable identifies two main modes—the annual cycle and the clime change signal. Then, atmospheric and oceanic variables (predictor variable) are regressed onto the pc time series of t2 (target variable) to find exact physical relationship between 2 m air temperature and other variables.

In the MIROC high-resolution model, climate change mode exhibits a linear trend with slight natural variability superimposed on it. The linear trend indicates that temperature increase is, on average, ~4oC over the 100 years, but higher on land and lower over the ocean. Oceanic variables are also highly correlated with t2. Spatially, high ocean temperatures are found to the east of Japan between 35°-45°N and are vertically barotropic in the upper 500 m. Salinity is generally oppositely correlated with temperature with warming leading to freshening of the upper level of the ocean. Amidst general warming of the upper ocean, a sign of stronger meandering of the Kurishio extension is seen, which alters the dynamic topography of the sea surface. This meandering is also clearly seen in the vertical section of the velocity field.

Regression analysis of the ocean temperature reveals two depth ranges of higher correlation with t2 separated by a minimum correlation layer at ~2-km depth. The upper layer may indicate that atmospheric temperature is crucially affected by the upper-ocean temperature. High correlation in the lower layer, however, cannot be due to direct exchange of energy between the ocean and the atmosphere and thus other mechanism should be responsible for it. Salinity appears less correlated with t2 than ocean temperature, but correlation with t2 is still significant. Interestingly, salinity has two minimum correlation depths and henceforth there are three high-correlation layers. Detailed physical changes in the Northwest Pacific will be addressed together with plausible physical mechanisms of these minimum/maximum correlation layers.