The Potential Influence of Falling Ice Radiative Effects on Central Pacific El Niño Variability under Progressive Global Warming

AbstractThis study examines the potential influence of inserting falling ice (snow) radiative effects (FIREs) in GCMs on simulated surface wind stress and sea surface temperature (SST) in the evolution of Central Pacific El Niño (CP-El Niño) events under a progressive warming climate at 1% per year increase of atmospheric CO2 concentration. Using controlled simulations with the CESM1 model, it is shown that the exclusion of FIREs (NOS) generates persistent westerly anomalies both in surface wind stress and low-level winds relative to those with FIREs (SON). These anomalies subsequently lead to a weakening of the easterly trade winds associated with warmer SST anomalies in modeled life cycle. Results over three independent 40-year intervals (P1: 20-60 yeas; P2: 61-100 years; P3: 101-140 years) are compared with Coupled Model Intercomparison Project phase 5 (CMIP5) models without FIREs. Both NOS and CMIP5 models simulate longer life cycles of CP-El Niño events with weakening easterlies and warmer SST anomalies on the equator, persistently propagating eastward from the mature phases to dissipating phases. However, SON (with FIREs) produces a shorter El Niño life cycle together with stronger easterlies and colder SSTs over the eastern to central equatorial Pacific, and stronger variability and strength of CP-El Niño compared to NOS. The magnitudes of the simulated westerlies and warm SST anomalies after the peak tend to diminish without eastward shifting compared to NOS, following the peak of the CP-El Niño activity. Moreover, the decadal variability for an abrupt warming behavior over for P2 and P3 discovered in CESM1 simulations was also found in CMIP5 simulations. The differences between NOS and SON are pronounced for P1 but become weaker for P2 and P3. 1. IntroductionThe El Niño-Southern Oscillation (ENSO) is characterized by increases of central to eastern equatorial Pacific sea surface temperatures (SSTs) during the El Niño phase and vice versa during the La Niña phase, with significant impacts on worldwide climate systems through global atmospheric and oceanic teleconnections [McPhaden et al., 2006; Guilyardi et al., 2009]. ENSO tends to have two distinct types of regime, which are often referred to as “EP-El Niño” and “CP-El Niño”, with its largest anomalous SST warming in the eastern equatorial Pacific (EP) and central equatorial Pacific (CP), respectively [Larkin & Harrison, 2005; Ashok et al., 2007; Kao & Yu, 2009; Kug et al., 2009; Kim et al., 2012]. In the past few decades, studies using Coupled Model Intercomparison Project Phase 3 (CMIP3) and Phase 5 (CMIP5: Taylor et al. [2012]) model outputs do not find consensus projections in the twenty-first century responses of El Niño events to anthropogenic forcing [Ashok et al., 2007; Meehl et al., 2007; Wang & Hendon, 2007; Kao & Yu, 2009; Kim et al., 2009; Kug et al., 2009; Yeh et al., 2009; Lee & McPhaden, 2010; Yu & Kim, 2010; Ding et al., 2011; Ham & Kug, 2011; Ham et al., 2012; Kim & Yu, 2012; Cai et al., 2014, 2015; Kohyama et al., 2017]. However, there are some common features from CMIP3 and CMIP5 model simulations under greenhouse gas (GHG) forcing: (1) a weakening of the Pacific trade winds [Chen et al., 2018], (2) a shoaling of the equatorial thermocline in the central and western Pacific [e.g., Yeh et al., 2009; Collins et al., 2010], (3) a clear trend towards an increased occurrence of CP-El Niño [Ashok et al., 2007; Kao & Ye, 2009; Kug et al., 2009; Lee & McPhaden, 2010], and (4) an increased intensity of CP-El Niño that results in distinct teleconnection patterns and hence climatic impacts [e.g., Wang & Hendon, 2007; Kim et al., 2009; Yeh et al., 2009; Cai et al., 2014, 2015]. Previous studies have documented that, under GHG forcing, the various flavors of ENSO are associated with the tropical Pacific background states with increasing occurrences of CP‐El Niño events due to a weakened surface wind stress. A weakened Walker circulation is also found favorable for a flatter oceanic thermocline and thus a more frequent CP‐El Niño events [Yeh et al., 2009; Collins et al., 2010]. A great deal of uncertainty, however, still exists on how the CP-El Niño might change in the future climate with no clear consensus being reached thus far on, for example, the location and magnitude of the strongest SST anomalies and the temporal evolution among most models participating in CMIPs [e.g., Power et al., 2013; Taschetto et al., 2014; Yeh et al., 2014]. Given various model biases for the 20th century simulations and the lack of sufficient model agreements for the 21st century projection, whether the projected changes for CP‐El Niño behaviors would actually take place remains largely uncertain. One of the probable reasons is that all the CMIP3 and most CMIP5 models do not include the falling ice (snow) radiative effects (FIREs). The potential importance of the FIREs on the tropical Pacific has been studied in a number of studies for present-day climate [Gettelman and Morrison, 2015; Li et al., 2014a, b, 2018, 2020a, b; Michibata et al., 2019]. These studies showed a distinct Pacific Ocean pattern of radiation-circulation changes related to FIREs. In brief, when FIREs are excluded, models simulated persistent westerly anomalies of surface wind stress and low-level flow typically opposing the trade winds, decreasing surface wind stress leading to warmer SST over the central Pacific trade-wind regions. The lack of the FIREs biases simulations of radiation, SST and circulation in the Pacific present-day climate [Li et al., 2014a, 2020a] and produces an unrealistically seasonal cycle of surface wind and SST and a persistent eastward propagation of warm SST anomalies following the peak in CP-El Niño activity in the present-day climate [Li et al., 2018]. These biases in the seasonal variations are reduced with the inclusion of the FIREs, which would increase the confidence in simulating their future behaviour. Chen et al. [2018] explored how FIREs contribute to simulated Pacific climate change via a pair of sensitivity experiments with (SON) and without FIREs (NOS) using 1pctCO2 simulations of the CESM1 climate model, in which the atmospheric CO2 concentration increases at 1 % per year for 140 years and compared the results with the CMIP5 ensemble mean. They found stronger changes in convective activity and its eastward shift and a stronger zonal gradient of SST warming in the SON simulation than NOS. They also pointed out that the NOS patterns of change are similar to those in CMIP5 models that exclude FIREs, hinting that the future warming-driven changes in precipitation and circulation over the tropical south-central Pacific might be underestimated by most CMIP5 models. The present study uses the same CESM1-CAM5 output to extend the studies of Chen et al. [2018] and Li et al. [2018] to the seasonal variations as well as the occurrence and evolution of CP-El Niño under progressive global warming. This study aims at exploring the potential influence of FIREs from progressive warming simulations on the changes of SSTs and surface wind stress in mean state and seasonal variability and on CP-El Niño evolution from SON to NOS. The results are compared with the ensemble mean of CMIP5 simulations under the same 1pctCO2 protocol.We describe the methodology for determining CP-El Niño in Section 2. In Section 3, model data are described. Results follow in Section 4 and further discussion of the results is presented in Section 5. Conclusions are drawn in Section 6.