Abstract
Abstract This study examines the slow modulation of El Niño–Southern Oscillation (ENSO) intensity and its underlying mechanism. A 10–15-yr ENSO intensity modulation cycle is identified from historical and paleoclimate data by calculating the envelope function of boreal winter Niño-3.4 and Niño-3 sea surface temperature (SST) indices. Composite analyses reveal interesting spatial asymmetries between El Niño and La Niña events within the modulation cycle. In the enhanced intensity periods of the cycle, El Niño is located in the eastern tropical Pacific and La Niña in the central tropical Pacific. The asymmetry is reversed in the weakened intensity periods: El Niño centers in the central Pacific and La Niña in the eastern Pacific. El Niño and La Niña centered in the eastern Pacific are accompanied with basin-scale surface wind and thermocline anomalies, whereas those centered in the central Pacific are accompanied with local wind and thermocline anomalies. The El Niño–La Niña asymmetries provide a possible mechanism for ENSO to exert a nonzero residual effect that could lead to slow changes in the Pacific mean state. The mean state changes are characterized by an SST dipole pattern between the eastern and central tropical Pacific, which appears as one leading EOF mode of tropical Pacific decadal variability. The Pacific Walker circulation migrates zonally in association with this decadal mode and also changes the mean surface wind and thermocline patterns along the equator. Although the causality has not been established, it is speculated that the mean state changes in turn favor the alternative spatial patterns of El Niño and La Niña that manifest as the reversed ENSO asymmetries. Using these findings, an ENSO–Pacific climate interaction mechanism is hypothesized to explain the decadal ENSO intensity modulation cycle.
Highlights
El Nino–Southern Oscillation (ENSO) is known to undergo decadal variations in its frequency, intensity, and propagation pattern (e.g., Wang and Wang 1996; An and Wang 2000; Fedorov and Philander 2000; Timmermann 2003; An and Jin 2004; Yeh and Kirtman 2004)
By analyzing historical and paleo-proxy climate datasets, we investigated the decadal modulation of ENSO intensity and the ENSO residual effects on the Pacific mean state
The modulation cycle is characterized by evident spatial asymmetry between El Nino and La Nina events, which allows the ENSO cycle to leave a nonzero residual effect onto the mean state changes in the tropical Pacific
Summary
El Nino–Southern Oscillation (ENSO) is known to undergo decadal (and interdecadal) variations in its frequency, intensity, and propagation pattern (e.g., Wang and Wang 1996; An and Wang 2000; Fedorov and Philander 2000; Timmermann 2003; An and Jin 2004; Yeh and Kirtman 2004). The decadal ENSO variability and its potential influences on global climate and weather have prompted extensive research (e.g., Torrence and Webster 1999; Power et al 1999). Various hypotheses have been put forward to explain the origin. Ó 2009 American Meteorological Society Brought to you by UNIVERSITY OF CALIFORNIA Irvine | Unauthenticated | Downloaded 10/18/21 08:59 PM UTC
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