Disentangling the mechanisms of equatorial Pacific climate change

Abstract

Most state-of-art models project a reduced equatorial Pacific east-west temperature gradient and a weakened Walker circulation under global warming. However, the causes of this robust projection remain elusive. Here, we devise a series of slab ocean model experiments to diagnostically decompose the global warming response into the contributions from the direct carbon dioxide (CO 2 ) forcing, sea ice changes, and regional ocean heat uptake. The CO 2 forcing dominates the Walker circulation slowdown through enhancing the tropical tropospheric stability. Antarctic sea ice changes and local ocean heat release are the dominant drivers for reduced zonal temperature gradient over the equatorial Pacific, while the Southern Ocean heat uptake opposes this change. Corroborating our model experiments, multimodel analysis shows that the models with greater Southern Ocean heat uptake exhibit less reduction in the temperature gradient and less weakening of the Walker circulation. Therefore, constraining the tropical Pacific projection requires a better insight into Southern Ocean processes.

Two Competing Drivers of the Recent Walker Circulation Trend

“…The effect of the uniform warming trend is then extracted by the difference between the AMIP and AMIP+4Ktrend, showing the pattern of precipitation trend that has a meridional, but not zonal, contrast unlike the trend in the AMIP and observations (Figure 3d). In the global warming response, enhanced radiative cooling and tropospheric stability both act to counteract the increased condensational heating (Kang et al., 2023; Knutson & Manabe, 1995; Ma et al., 2012), the features also identified in our experiments (Figures S6 and S7 in Supporting Information ). However, those effects are relatively zonally homogeneous besides are not yet visible in the past, and therefore the PWC strengthening is reproduced solely by the precipitation pattern change using a simple linear dynamical computation (Figure S8 and Text S3 in Supporting Information ).…”

“…However, it is becoming clear that the climate model simulations are systematically biased to generate a weakening of the zonal contrast in SST and SLP over the past decades (Wills et al., 2022), leading to suspicion that the models do not adequately represent a forced response to GHGs or aerosols due perhaps to errors in the coupled mean state in the Pacific (Seager et al., 2019, 2022). There are several mechanisms to explain the observed strengthening, for example, aerosol forcing (Smith et al., 2016; Takahashi & Watanabe, 2016), remote effect of the Southern Ocean cooling (Dong et al., 2022; England et al., 2020; Kang et al., 2023; Kim et al., 2022), and coupling with tropical Atlantic and Indian Oceans (Li et al., 2016; McGregor et al., 2014; O’Reilly et al., 2023), but they are based on the potentially biased coupled model simulations, hindering a robust attribution of the past PWC intensification.…”

Fast and Slow Responses of Atlantic Meridional Overturning Circulation to Antarctic Meltwater Forcing

“…The non‐monotonic response is attributed to the differing timescales of atmospheric and oceanic teleconnections. The SO cooling could affect the tropical climate within a few years through near‐surface propagation, resulting in convection redistribution (Kang et al., 2023; Kim et al., 2022). Then, the suppressed convection immediately triggers an upper‐level cyclonic flow, which initiates Rossby wave propagation to the extratropics.…”

Contribution of Surface Radiative Effects, Heat Fluxes and Their Interactions to Land Surface Temperature Variability

“…The surface energy budgets over ocean and land are distinct (Donohoe & Battisti, 2013; Sejas et al., 2021). The heat capacity of ocean mixed layer is large, and the horizontal heat transport convergence is significant in oceans, so that the energy budget of sea surface is not always balanced (Hansen et al., 2005; Kang et al., 2023; Liu et al., 2018; Roberts et al., 2017; von Schuckmann et al., 2020). In contrast, the heat capacity of the land surface is small, and the heat content of land surface is not significantly affected by horizontal heat transport over land on the monthly scale, so that the imbalance term of the land surface energy budget is small.…”