Linking the Western North Pacific Subtropical High and the two types of ENSO

Abstract:

The recent studies showed that the summertime Western North Pacific Subtropical High (WPSH) experienced an interdecadal shift to the 2–3-yr band of the variability after around 1990, which coincides with the time that the El Niño shifted from predominantly the EP type to predominantly the CP type. For weather forecast and climate projection in the East-Asia, it is important for an AGCM to simulate the WPSH variability associated with the two types of ENSO. By analyzing the CMIP5-AMIP model output, we find that one model group successfully simulates the observed quasi-biennial (2–3-yr) band but not the low-frequency (3–5-yr) band while the other group produces the observed low-frequency band better than the quasi-biennial band of the variability. We also find the SST anomalies in the tropical central Pacific (i.e., CP ENSO) responsible for forcing the quasi-biennial WPSH variability and the anomalies in the tropical eastern Pacific (i.e., EP ENSO) responsible for the low-frequency variability. This study suggests that the relative strengths of the Hadley Circulation and Walker Circulation, which link the two bands of the WPSH variability to the SST variations in the central and eastern Pacific, are crucial to a realistic simulation of the summer WPSH variability in AGCMs.