Indian Ocean Warming Trend Reduces Pacific Warming Response to Anthropogenic Greenhouse Gases: An Interbasin Thermostat Mechanism

Abstract

A greater warming trend of sea surface temperature in the tropical Indian Ocean than in the tropical Pacific is a robust feature found in various observational data sets. Yet this interbasin warming contrast is not present in climate models. Here we investigate the impact of tropical Indian Ocean warming on the tropical Pacific response to anthropogenic greenhouse gas warming by analyzing results from coupled model pacemaker experiments. We find that warming in the Indian Ocean induces local negative sea level pressure anomalies, which extend to the western tropical Pacific, strengthening the zonal sea level pressure gradient and easterly trades in the tropical Pacific. The enhanced trade winds reduce sea surface temperature in the eastern tropical Pacific by increasing equatorial upwelling and evaporative cooling, which offset the greenhouse gas warming. This result suggests an interbasin thermostat mechanism, through which the Indian Ocean exerts its influence on the Pacific response to anthropogenic greenhouse gas warming.

Indian Ocean warming as a driver of the North Atlantic warming hole

“…Then what could have caused the strengthening of surface westerlies? We now turn attention to the tropical Indian Ocean (TIO) whose temperature has increased by about 1°C since the mid-1950s [17][18][19][20][21][22][23][24][25][26][27] , at a faster rate than that of the other two tropical oceans ( Fig. 1a).…”

Tropical Pacific cold tongue mode triggered by enhanced warm pool convection due to global warming

“…Our hypothesis is aimed at explaining the CTM formation in the context of global warming by exploring the stepwise physical processes of air-sea interaction within the tropical Pacific basin. Moreover, rapid warming in the tropical Atlantic and Indian Oceans also contributes to the La Niña-like response (Li et al 2015a;Luo et al 2012, Mcgregor et al 2014Zhang et al 2019aZhang et al , 2019b. The ENSO and other natural low-frequency variability signals are treated as noises in our study.…”

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

“…The net result is a suppression of equatorial Pacific warming in response to increasing GHGs (Luo et al, ). Zhang et al () also performed several atmosphere‐ocean coupled model experiments with and without the Indian Ocean warming to confirm their hypothesis. They still invoke the ocean thermostat mechanism as the primary initiator of the interbasin SST warming contrast between the Indian Ocean and the equatorial Pacific Ocean; thus, their hypothesis is referred to as an interbasin ocean thermostat mechanism.…”

“…They still invoke the ocean thermostat mechanism as the primary initiator of the interbasin SST warming contrast between the Indian Ocean and the equatorial Pacific Ocean; thus, their hypothesis is referred to as an interbasin ocean thermostat mechanism. Zhang et al (2019) used three observational SST data sets, all of which show larger warming in the Indian Ocean than in the equatorial Pacific since 1920, thus strengthening the interbasin SST warming contrast between the Indian Ocean and the equatorial Pacific. Figures 1a and 1b show tropical SST trends over the period 1920-2005 derived from the composite mean of the three observational SST data sets and from the 41-member ensemble mean of the National Center for Atmospheric Research (NCAR) Community Earth System Model Large-Ensemble Simulation (CESM-LENS; Kay et al, 2015) under the historical scenario, respectively.…”

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A recent study by Zhang et al (2019, https://doi.org/10.1029 proposes "an interbasin ocean thermostat mechanism" to support the hypothesis that enhanced Indian Ocean warming during the past century has reduced the Pacific warming response to increasing greenhouse gases. In this commentary, we review the proposed mechanism in a broader perspective and discuss a potential role of Atlantic Ocean in modulating the future Pacific warming response to emphasize the interactive response of the pantropical atmosphere-ocean to increasing greenhouse gases.

Plain Language SummaryDuring the past century, the tropical Pacific Ocean has warmed significantly less than the other tropical oceans in response to increasing greenhouse gases.

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