Effect of the Tropical Pacific-Indian Ocean Temperature Anomaly Mode on the South Asia High
YANG Hui and LI Chong-YinState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029
The east-west difference of tropical Indian Ocean temperature anomaly (dipole) is closely related to the ENSO in the Pacific Ocean through the Walker circulation and other processes. Thus the ENSO and Indian dipole should be regarded as an air-sea coupled system in the tropical Pacific and Indian Ocean. The tropical Pacific-Indian Ocean temperature anomaly mode is presented,and its impact on the South Asia high is studied. The positive phase of the tropical Pacific-Indian Ocean temperature anomaly mode (positive SSTAs in the western Indian Ocean and eastern Pacific, negative SSTAs in the eastern Indian Ocean-western Pacific) is favorable for a weaker South Asia high with southeastwards shift. The negative phase (reverse SSTAs of the positive phase) contributes to a stronger South Asia high with northwestwards shift. The physical mechanisms that the tropical Pacific-Indian Ocean temperature anomaly mode influences the South Asia high are proposed. 1) The tropical Pacific-Indian Ocean temperature anomaly mode greatly influences the Asian summer monsoon, resulting in the rainfall anomaly. The analyses of the vertically integrated heat sources and moisture sinks, and precipitation reveal that the release of latent heat of condensation is the primary heat source. When the tropical Pacific-Indian Ocean temperature anomaly mode is in a positive phase, the heat source over the Tibetan Plateau is smaller than normal with less precipitation there. However, when the tropical Pacific-Indian Ocean temperature anomaly mode is in a negative phase, more abundant precipitation appears,indicating larger heat source there. The negative (positive) heat source anomaly maintains the negative (positive) ascent anomaly over the Tibetan Plateau, which provide a key mechanism for the distinct change of intensity of the South Asia high. The positive (negative) phase is favorable for the stronger (weaker) South China Sea summer monsoon and the intertropical convergence zone. Thus positive (negative) heat source anomaly is found from the South China Sea to the western Pacific via the Philippines, resulting in an anticyclonic (cyclonic) circulation to the southeast of the Tibetan Plateau at the high level. So the South Asia high is located in the southeast (northwest) position. 2) The tropical Pacific-Indian Ocean temperature anomaly mode greatly influences the zonal vertical (Walker) circulation. The positive (negative) phase drives the westerly (easterly) current anomaly at the high level over the Indian Ocean, contributing to the weaker (stronger) easterlies in the south part of South Asia high. 3) The positive (negative) phase drives a clear local Hadley cell anomaly with ascent (descent) over the western Indian Ocean and descent (ascent) over the Iran Plateau, resulting in the weaker (stronger) South Asia high with southeastwards (northwestwards) shift.