Asian Summer Monsoon—ENSO Feedback on the Cane–Zebiak Model ENSO

Abstract

The Asian summer monsoon heating anomalies are parameterized in terms of the concurrent ENSO SST anomalies and used as additional forcing in the Cane–Zebiak (CZ) Pacific ocean–atmosphere anomaly model. The Asian heating parameterization is developed from the rotated principal component analysis of combined interannual variability of the tropical Pacific SSTs, residually diagnosed tropical diabatic heating at 400 mb (from ECMWF’s analyses), and the 1000-mb tropical winds during the 1979–97 summer months of June, July, and August. Analysis of the 95 000-yr-long model integrations conducted with and without the interactive Asian sector heating anomalies reveals that their influence on the Pacific surface winds leads to increased ENSO occurrence—an extra ENSO event every 20 yr or so. An examination of the ENSO distribution w.r.t. the peak SST anomaly in the eastern equatorial Pacific shows increased El Niño occurrence in the 2.2–3.6 K range (and −1.0 to −1.6 K range in case of cold events) along with a modest reduction in the 0.6–1.2 K range, that is, a population shift due to the strengthening of weak El Niños in the monsoon run. The interaction of ENSO-related Asian summer monsoon heating with the CZ model’s ocean–atmosphere also results in a wider period distribution of ENSO variability, but with the El Niño peak phase remaining seasonally locked with the northern winter months. The above modeling results confirm the positive feedback between Asian summer monsoon and ENSO suggested by previous empirical and diagnostic modeling studies; the feedback is generated primarily by the diabatic heating changes in the Asian Tropics.