Indo-Pacific variability on seasonal to multidecadal timescales. Part II: Multiscale atmosphere-ocean linkages

Abstract

The coupled atmosphere-ocean variability of the Indo-Pacific on interannual to multidecadal timescales is investigated in a millennial control run of CCSM4 and in observations using a family of modes recovered in Part~I of this work from unprocessed SST data through nonlinear Laplacian spectral analysis (NLSA). It is found that ENSO and combination modes of ENSO with the annual cycle exhibit a seasonally synchronized southward shift of equatorial surface zonal winds and thermocline adjustment consistent with terminating El Nino and La Nina events. The surface wind patterns associated with these modes also generate teleconnections between the Pacific and Indian Oceans, leading to a pattern of SST anomalies characteristic of the Indian Ocean dipole. Fundamental and combination modes representing the tropospheric biennial oscillation (TBO) in CCSM4 are also found to be consistent with mechanisms for seasonally synchronized biennial variability of the Asian-Australian monsoon and Walker circulation. On longer timescales, the leading multidecadal pattern recovered by NLSA from Indo-Pacific SST data in CCSM4, referred to as west Pacific multidecadal mode (WPMM), is found to significantly modulate ENSO and TBO activity with periods of negative SST anomalies in the western tropical Pacific favoring stronger ENSO and TBO variability. Physically, this behavior is attributed to the fact that cold WPMM phases feature anomalous decadal westerlies in the tropical central Pacific and easterlies in the tropical Indian Ocean, as well as an anomalously deep thermocline in the eastern Pacific cold tongue. Moreover, despite the relatively low SST variance explained by this mode, the WPMM is found to correlate significantly with decadal precipitation over Australia in CCSM4.