Abnormal Meridional Temperature Gradient and its Relation to the Okhotsk High

Abstract

The climatological meridional atmospheric temperature structure in the Okhotsk region in summer is characterized by a poleward increase of the surface air temperature. Under this anomalous temperature gradient, the stationary anticyclone, referred to as the Okhotsk high, occasionally occurs. The relationship of the interannual variation of the meridional temperature gradient anomaly is statistically investigated, with those of the Okhotsk high, the sea surface temperature, and the global atmosphere mainly using NCEP reanalysis data. The correlation between the anomaly and the Okhotsk high is quite high. That is, in years when the meridional temperature gradient is positive, the Okhotsk high appears more than normal. The anomaly is composed of two independent factors, the warmness of eastern Siberia and the coldness of the northwestern North Pacific. The vertical structures of the anticyclone, related to the Siberian warmness, are different from those related to the Pacific coldness. The former anticyclone has deep structure, whereas the latter has shallow structure. Therefore, there are two types of the Okhotsk High. The warmness of Siberia is connected to the Rossby wave propagating along the northern coast of the Eurasian continent. The coldness of the North Pacific is, on the other hand, influenced by the variation of the tropical Pacific. Consequently, the interannual variation of the Okhotsk high is influenced by completely different remote sources; one is the tropical Pacific, and the other is the highlatitude areas facing the Arctic Ocean. To understand the interannual variation of the Asian summer monsoon, which is closely related to the occurrence of the Okhotsk high, both the Arctic and the tropics should be considered.