ENSO-Related Precipitation and Its Statistical Relationship with the Walker Circulation Trend in CMIP5 AMIP Models

Bo Yim,Sang-Wook Yeh,Byung-Ju Sohn

doi:10.3390/atmos7020019

Abstract

Observational evidence shows that the Walker circulation (WC) in the tropical Pacific has strengthened in recent decades. In this study, we examine the WC trend for 1979–2005 and its relationship with the precipitation associated with the El Niño Southern Oscillation (ENSO) using the sea surface temperature (SST)-constrained Atmospheric Model Intercomparison Project (AMIP) simulations of the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models. All of the 29 models show a strengthening of the WC trend in response to an increase in the SST zonal gradient along the equator. Despite the same SST-constrained AMIP simulations, however, a large diversity is found among the CMIP5 climate models in the magnitude of the WC trend. The relationship between the WC trend and precipitation anomalies (PRCPAs) associated with ENSO (ENSO-related PRCPAs) shows that the longitudinal position of the ENSO-related PRCPAs in the western tropical Pacific is closely related to the magnitude of the WC trend. Specifically, it is found that the strengthening of the WC trend is large (small) in the CMIP5 AMIP simulations in which the ENSO-related PRCPAs are located relatively westward (eastward) in the western tropical Pacific. Therefore, the zonal shift of the ENSO-related precipitation in the western tropical Pacific, which is associated with the climatological mean precipitation in the tropical Pacific, could play an important role in modifying the WC trend in the CMIP5 climate models.

Highlights

The climate models simulate the magnitude of the Walker circulation (WC) trend differently even in the same sea surface temperature (SST)-constrained Atmospheric Model Intercomparison Project (AMIP) simulations
Our result indicates that the dryness of the climatological mean precipitation in the off-equatorial western Pacific and the central-to-eastern tropical Pacific could contribute to the westward shift of a center of the El Niño Southern Oscillation (ENSO)-related precipitation anomalies (PRCPAs) in the western tropical Pacific, which is associated with the strengthening of the WC trend
We investigated how the WC trend for 1979–2005 was related to the ENSO-related PRCPA on interannual timescales by analyzing the same SST-constrained AMIP simulations of the 29 Coupled Model Intercomparison Project Phase 5 (CMIP5) models

Summary

Introduction

Walker circulation (WC), which is the large-scale zonal overturning circulation with ascent over the western and descent over the eastern tropical Pacific, is an important factor of the global climate system, and its variability is strongly associated with the El Niño Southern Oscillation (ENSO) [1,2].It is of great importance to understand how the WC has changed and how it might change due to greenhouse warming, because the variations in its intensity and structure have a significant impact on the tropical climate, and the monsoonal circulations over adjacent continents [3] and the global climate [3,4].Much observational evidence showed that the WC has strengthened and its western edge has shifted westward in recent decades [5,6,7,8,9,10,11,12,13,14]. Walker circulation (WC), which is the large-scale zonal overturning circulation with ascent over the western and descent over the eastern tropical Pacific, is an important factor of the global climate system, and its variability is strongly associated with the El Niño Southern Oscillation (ENSO) [1,2]. It is of great importance to understand how the WC has changed and how it might change due to greenhouse warming, because the variations in its intensity and structure have a significant impact on the tropical climate, and the monsoonal circulations over adjacent continents [3] and the global climate [3,4]. Sohn and Park [5] showed that the WC has intensified since 1979 from both the reanalysis data and satellite-based water budget data. They found a significant strengthening of both the lower-level inward transports and the mid-level outward transports in the two most recent decades (1989–2008), suggesting that the strengthening of WC was primarily caused by an intensification of the wind-related circulation strength

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