Role of internal variability in recent decadal to multidecadal tropical Pacific climate changes

Abstract

While the Earth's surface has considerably warmed over the past two decades, the tropical Pacific has featured a cooling of sea surface temperatures in its eastern and central parts, which went along with an unprecedented strengthening of the equatorial trade winds, the surface component of the Pacific Walker Circulation (PWC). Previous studies show that this decadal trend in the trade winds is generally beyond the range of decadal trends simulated by climate models when forced by historical radiative forcing. There is still a debate on the origin of and the potential role that internal variability may have played in the recent decadal surface wind trend. Using a number of long control (unforced) integrations of global climate models and several observational data sets, we address the question as to whether the recent decadal to multidecadal trends are robustly classified as an unusual event or the persistent response to external forcing. The observed trends in the tropical Pacific surface climate are still within the range of the long‐term internal variability spanned by the models but represent an extreme realization of this variability. Thus, the recent observed decadal trends in the tropical Pacific, though highly unusual, could be of natural origin. We note that the long‐term trends in the selected PWC indices exhibit a large observational uncertainty, even hindering definitive statements about the sign of the trends.

Enhanced warming constrained by past trends in equatorial Pacific sea surface temperature gradient

“…1). This may not happen due solely to an extreme internal fluctuation at interdecadal time scales 6 . However, additional analyses show that the failure to reproduce the observed trend only spans over the recent two decades, but not the entire 60-year period ( Supplementary Figs.…”

“…1), consistent with a strengthening of the Walker circulation as suggested from atmospheric data 21 . There exist observational uncertainty 22 and a dependence of the trend magnitude on chosen period 6 , but the discrepancy between observations and climate models is obvious and critical not only for assessing the reliability of future climate changes in models but also for estimating the Earth's climate sensitivity (ECS) from historical records. Recent studies show that cloud and lapse rate feedbacks to doubling of CO2 concentration depend on spatial inhomogeneity in sea surface warming 8,9,23 , called the 'pattern effect' 24 .…”

Pacific variability reconciles observed and modelled global mean temperature increase since 1950

“…It has further been suggested that anthropogenic aerosols play a role during the 'global warming hiatus' by altering the trade winds and the state of the PDO (Smith et al 2016;Takahashi and Watanabe 2016), but the robustness of these results has been questioned (Kuntz and Schrag 2016;Oudar et al 2018). Further, observational uncertainties exist, and these are particularly large in the pre-satellite era (Bordbar et al 2017;Kajtar et al 2018). Reanalyses have been argued to overestimate the intensification of the Pacific Walker circulation during the past decades when compared to satellite-derived estimates (Chung et al 2019), but de Boisséson et al (2014 found the tropical Pacific trade wind trends, which are the surface expression of the Walker circulation cell, to be robust in different observations including reanalyses.…”

Pantropical Response to Global Warming and the Emergence of a La Niña‐Like Mean State Trend

“…However, other observational SST data sets and some reanalysis products show warming trend with La Niña‐like zonal structure during the instrumental period (Karnauskas et al, ) and a strengthening of the Walker circulation over the past several decades (Chung et al, ; Sohn et al, ). Due to strong natural variability, these trends are sensitive to the time period considered, and there is uncertainty in determining long‐term trends from observations (Bordbar et al, ). Nevertheless, there is a physically consistent hypothesis to explain suppressed warming in the eastern equatorial Pacific and the associated strengthening of the Walker circulation in response to increasing greenhouse gases (GHGs): an ocean dynamic thermostat mechanism (Clement et al, ).…”