The Formation of Precipitation Anomaly Patterns during the Developing and Decaying Phases of ENSO

This study proposes a new explanation for the formation of precipitation anomaly patterns in the boreal summer during the El Nino-Southern Oscillation (ENSO) developing and decaying phases. During the boreal summer June-July-August (JJA) (0) of the El Nino (La Nina) developing phase, the upper level (300-100 hPa) positive potential temperature anomalies resemble a Matsuno- Gill-type response to central Pacific heating (cooling), and the lower level (1000-850 hPa) potential temperature anomalies are consistent with local SST anomalies. During the boreal summer JJA(1) of the El Nino (La Nina) decaying phase, the upper level potential temperature warms over the entire tropical zone and resembles a Matsuno-Gill-type response to Indian Ocean heating (cooling), and the lower level potential temperature anomalies follow local SST anomalies. The vertical heterogeneity of potential temperature anomalies influences the atmospheric stability, which in turn influences the precipitation anomaly pattern. The results of numerical experiments confirm our observations.