Interannual Variability of the Summertime North Pacific Subtropical High and its Relation to Atmospheric Convection over the Warm Pool.

Abstract

Using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis data and satellite-observed outgoing long-wave radiation (OLR) data, we examined the westward extension and eastward contraction of the North Pacific subtropical high in summer (NPSH). It was found that the NPSH shows a great variability in its western extent, both on the seasonal and interannual time scales. In order to examine the interannual variations of NPSH, we defined a NPSH index as the June-July-August (JJA) mean geopotential height anomalies at 850 hPa averaged over the west edge (110∼150°E, 10∼30°N) of NPSH. This index describes the year-to-year zonal displacement of NPSH. Composite analysis based on this NPSH index showed that there is a significant relation between zonal displacement of NPSH and intensity of atmospheric convection over the warm pool. A low-level cyclonic (anticyclonic) anomaly that is closely associated with the zonal shift of NPSH appears north of enhanced (weakened) atmospheric convection, i.e., the vorticity anomaly is found north of the divergence one. Climatologically, the NPSH contracts eastward swiftly after pentad 40 (July 15 to 19). Such an eastward contraction is closely associated with the poleward shift of both NPSH and atmospheric convection over the tropical western Pacific warm pool. However, such seasonal variations of both NPSH and convection show distinct features between the summers with positive and negative NPSH indexes. During summers with positive NPSH index, NPSH and convection over the warm pool do not show an appreciable seasonal evolution. During summers with negative index, by contrast, they show a swift seasonal evolution after pentad 40.Finally, we performed a vorticity analysis to explain the relation between the divergence and vorticity anomalies on the interannual time scale. The analysis shows that in the lower troposphere (925 hPa), the advection of relative vorticity is comparable to the stretching and is responsible for the northward shift of the circulation anomaly relative to anomalous atmospheric convection. The difference from the theory of Gill (1980) is discussed. In the upper troposphere (200 hPa), the advection is slightly smaller than the stretching with opposite signs in East Asia and the western North Pacific, and thus the position of the vorticity anomaly is consistent with that of the stretching anomaly.