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The Oyashio is a western boundary current of the subarctic circulation in the North Pacific. In recent years an anomalous southward intrusion of the Oyashio has been observed frequently from winter to late spring that is closely associated with the atmospheric temperature and other climatic elements of the eastern part of Japan. The anomalous southward intrusion of the Oyashio east of Japan was observed in 1963, 1974, 1981, and 1984. The average southward limit of the Oyashio water in the northern area of the Kuroshio Extension is approximately 39°–39.5°N in spring, but the Oyashio intrudes south to about 36°N in these anomalous years. The relationship between the southward intrusion of the Oyashio and the variation in wind stress over the North Pacific is examined by use of wind stress data. It has been demonstrated that prior to occurrences of the anomalous southward intrusion of the Oyashio in late spring, the southward shift of the latitude with no wind stress curl was commonly observed in the North Pacific by the development of the prominent Aleutian low. If a Sverdrup balance is assumed, the subarctic circulation shifts southward about 300–500 km from its annual mean latitude. During the Oyashio intrusion periods, stronger southeastward wind stress by the Siberian high were observed in the western region of the North Pacific, which suggests that the occurrence of the Oyashio intrusion is associated with the global change in the atmospheric circulation. However, analysis of historical data shows that if the southward shift of the wind system occurs, a warm eddy detached from the Kuroshio Extension and a large eastward expansion of the Tsugaru warm current suppress the development of the anomalous southward intrusion of the Oyashio east of Japan. A simple method to predict the anomalous intrusion of the Oyashio in late spring is proposed on the basis of these results.

Outgoing long-wave radiation (OLR), objectively analyzed daily winds ( u , v ) and geopotential height data for 5 northern winters are used to study the relationship between low-level equatorward (meridional) winds and tropical convection on the 30–60 day time scale. The general tendency of equatorial 30–60 day OLR perturbations is to propagate eastward. Occasionally, these perturbations exhibit irregular movement. This study is limited to the periods when the eastward propagation is clearly defined. Correlation maps indicate that the low-frequency modes are of global-scale character with zonal wavenumber one. The low-level 30–60 day northerly wind along the eastern edge of the Siberian high (anomaly) does not contribute to convection and monsoon activity in the Indonesian-northern Australian region. However, 30–60 day northeasterly flows of subtropical Pacific origin play a significant role in monsoon activity. An active monsoon condition with strong upper-level southerly outflow from the monsoon region and an intense westerly jet near Japan, is observed approximately 10 days prior to the maximum low-level northerly currents from Siberia. Fluctuations with periods less than 30 days are considered as transient disturbances. The square of transient meridional winds shows pronounced spectral peaks in the 30–60 day period range. This is associated with an amplitude modulation of synoptic-scale transient disturbances, which is most pronounced in both the NH and SH extratropics. Perhaps, transient disturbances, as a group, can make a contribution toward the enhancement and/or maintenance of extratropical low-frequency perturbations. DOI: 10.1111/j.1600-0870.1988.tb00342.x