Interannual covariance between Japan summer precipitation and Western North Pacific SST

A statistical analysis is conducted of the covariance between summer precipitation in Japan and concurrent variations of sea surface temperature (SST) in the Western North Pacific (WNP; 120-160degreesE, 0-60degreesN). To this end, a singular value decomposition (SVD) is applied to observed monthly anomalies of the two fields for the period 1961-1998 (38 summer seasons). The first coupled mode of the SVD explains as much as 71% of the total cross-covariance between WNP SST and Baiu precipitation. It is characterized by a zonally elongated dipole in the SST anomaly with opposite loadings on either side of about 30degreesN and a precipitation anomaly with its maximum in southern Japan. The patterns indicate a negative correlation between Japan's summer precipitation and the sea surface temperature in the East China Sea, Japan Sea and Kuroshio-Oyashio extension. Simpler composite analyses confirm the robustness of the coupling. In a linear regression analysis, the score of the WNP SST explains 30% of the domain mean summer precipitation variance, which is larger than the variance accounted for by lagged and concurrent ENSO-indices. Composites related to the mode's precipitation score were found to support both directions of the atmosphere-ocean interaction. On the one hand, the anomalies in radiation and evaporation at the sea surface would be sufficiently large to induce the observed temperature variations in the northern pole of the SST pattern. On the other hand, the possibility for an atmospheric response to the SST is supported by the consistency of anomalies in lower-tropospheric baroclinicity and moisture flux divergence with the underlying SST distribution.