The Nature of the Stochastic Wind Forcing of ENSO

Capotondi, Antonietta; Sardeshmukh, Prashant D.; Ricciardulli, Lucrezia

doi:10.1175/jcli-d-17-0842.1

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The Nature of the Stochastic Wind Forcing of ENSO

2018

By Capotondi, Antonietta ; Sardeshmukh, Prashant D. ; Ricciardulli, Lucrezia

Source: Journal of Climate, 31(19), 8081-8099.

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Journal Title:

Journal of Climate

Personal Author:

Capotondi, Antonietta ; Sardeshmukh, Prashant D. ; Ricciardulli, Lucrezia

NOAA Program & Office:

OAR (Oceanic and Atmospheric Research) ; CIRES (Cooperative Institute for Research in Environmental Sciences)

Description:

El Nino-Southern Oscillation (ENSO) is commonly viewed as a low-frequency tropical mode of coupled atmosphere-ocean variability energized by stochastic wind forcing. Despite many studies, however, the nature of this broadband stochastic forcing and the relative roles of its high- and low-frequency components in ENSO development remain unclear. In one view, the high-frequency forcing associated with the subseasonal Madden-Julian oscillation (MJO) and westerly wind events (WWEs) excites oceanic Kelvin waves leading to ENSO. An alternative view emphasizes the role of the low-frequency stochastic wind components in directly forcing the low-frequency ENSO modes. These apparently distinct roles of the wind forcing are clarified here using a recently released high-resolution wind dataset for 1990-2015. A spectral analysis shows that although the high-frequency winds do excite high-frequency Kelvin waves, they are much weaker than their interannual counterparts and are a minor contributor to ENSO development. The analysis also suggests that WWEs should be viewed more as short-correlation events with a flat spectrum at low frequencies that can efficiently excite ENSO modes than as strictly high-frequency events that would be highly inefficient in this regard. Interestingly, the low-frequency power of the rapid wind forcing is found to be higher during El Nino than La Nina events, suggesting a role also for state-dependent (i.e., multiplicative) noise forcing in ENSO dynamics.

Source:

Journal of Climate, 31(19), 8081-8099.

DOI:

https://doi.org/10.1175/jcli-d-17-0842.1

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Journal Article

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