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Title: Can ENSO-Like Convection Force an ENSO-Like Extratropical Response on Subseasonal Time Scales?
Author(s) / Creator(s):
 ;  ;  ;  
Publisher / Repository:
American Meteorological Society
Date Published:
Journal Name:
Journal of Climate
Page Range / eLocation ID:
8339 to 8349
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    Observational and modeling studies show that the relative frequency of El Niño and La Niña varies in association with El Niño–Southern Oscillation (ENSO)‐like tropical Pacific decadal variability (TPDV), but the causality of the linkage remains unclear. This study presents evidence that ENSO‐like TPDV affects the frequency of ENSO events, particularly of El Niño, through a set of climate model experiments. During the positive phase of TPDV, tropical Pacific warming relative to the Indian and Atlantic Oceans increases the occurrence of anomalous westerly winds over the western equatorial Pacific in late boreal winter‐spring, triggering more El Niño and fewer La Niña events. The opposite happens for the negative TPDV phase. The La Niña frequency is also influenced by oceanic adjustments following El Niño, which tends to counteract the effect of wind changes. The mean state control of ENSO offers a potential opportunity for decadal predictions of climate extremes.

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  2. Abstract

    A cyclostationary linear inverse model (CSLIM) is used to investigate the seasonal growth of tropical Pacific Ocean El Niño–Southern Oscillation (ENSO) events with canonical, central Pacific (CP), or eastern Pacific (EP) sea surface temperature (SST) characteristics. Analysis shows that all types of ENSO events experience maximum growth toward final states occurring in November and December. ENSO events with EP characteristics also experience growth into May and June, but CP events do not. A single dominant “ENSO mode,” growing from an equatorial heat content anomaly into a characteristic ENSO-type SST pattern in about 9 months (consistent with the delayed/recharge oscillator model of ENSO), is essential for the predictable development of all ENSO events. Notably, its seasonality is responsible for the late-calendar-year maximum in ENSO amplification. However, this ENSO mode alone does not capture the observed growth and evolution of diverse ENSO events, which additionally involve the seasonal evolution of other nonorthogonal Floquet modes. EP event growth occurs when the ENSO mode is initially “covered up” in combination with other Floquet modes. The ENSO mode’s slow seasonal evolution allows it to emerge while the other modes rapidly evolve and/or decay, leading to strongly amplifying and more predictable EP events. CP events develop when the initial state has a substantial contribution from Floquet modes with meridional mode–like SST structures. Thus, while nearly all ENSO events involve the seasonally varying ENSO-mode dynamics, the diversity and predictability of ENSO events cannot be understood without identifying contributions from the remaining Floquet modes.

    Significance Statement

    The purpose of this study is to identify structures that lead to seasonal growth of diverse types of El Niño–Southern Oscillation (ENSO) events. An important contribution from this study is that it uses an observationally constrained, empirically derived seasonal model. We find that processes affecting the evolution of diverse ENSO events are strongly seasonally dependent. ENSO events with eastern equatorial Pacific sea surface temperature (SST) characteristics are closely related to a single “ENSO mode” that resembles theoretical models of ENSO variability. ENSO events that have central equatorial Pacific SST characteristics include contributions from additional “meridional mode” structures that evolve via different physical processes. These findings are an important step in evaluating the seasonal predictability of ENSO diversity.

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