Mechanism of ENS0-Monsoon Interaction During the Mature-Decay Phases of ENSO Cycle
Bin Wang, R. Wu, X. Fu, R.Lukas, and S.-I. An International Pacific Research Center
School of Ocean and Earth Science and Technology, University of Hawaii
ABSTRACT
Observational evidence is presented to show ENS0-East Asian monsoon interaction during the mature and decay phases of ENS0. During the mature phase of major El Nino episodes, an anomalous low-level anticyclone emerges over the Philippine Sea. The anticyclone develops rapidly in late fall of the ENSO development year and persists until the following spring or early summer, causing anomalous wet (dry) conditions along the East Asian front stretching from the southern China northeastward to the Kuroshio extension. Why does the mature phase ENSO have a delayed impact on East Asian summer monsoon? We found that the development of the anomalous subtropical high over the Philippine Sea is concurrent with the enhancement of the in situ sea surface cooling. It is argued that the development and persistence of the Philippine Sea anticyclone is primarily attributed to a positive feedback between the atmospheric Rossby wave response and associated sea surface cooling. However, this interaction occurs only in the presence of cold seasonal mean northeasterly trades and the strong warming in the central Pacific. The sudden establishment of the anticyclone appears to be triggered by the early winter cold surges.
The Philippine Sea anticyclonic anomalies establish prior to the peaks of major warm event, inducing anomalous easterlies at the equatorial in the western Pacific. By forcing equatorial upwelling Kelvin waves, the easterly anomalies can provide a negative feedback to the eastern Pacific warming. The proposed mechanism is different from, but complimentary to the delayed oscillator mechanism. The latter relies only on oceanic wave adjustment process. Since the formation and maintenance of the Philippine anticyclone depend on the existence of mean circulation during the boreal cold season, the proposed negative feedback favors ENSO turning around in boreal winter. This may partially explain why the reversal of warming or cooling trends of ENSO cycles occurs most frequently toward the end of the calendar year.
