18 October 2019

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

Science

Here we investigate the impact of tropical Indian Ocean warming on the tropical Pacific response to anthropogenic greenhouse gas (GHG) warming by analyzing results from coupled model pacemaker experiments. We find that warming in the Indian Ocean induces local negative sea level pressure (SLP) anomalies, which extend to the western tropical Pacific, strengthening the zonal SLP gradient and easterly trades in the tropical Pacific. The enhanced trade winds reduce SST in the eastern tropical Pacific by increasing equatorial upwelling and evaporative cooling, which offset the GHG warming. This result suggests an interbasin thermostat mechanism, through which the Indian Ocean exerts its influence on the Pacific response to anthropogenic GHG warming.

Impact

The spatial pattern of the centennial (1920-2013) SST warming trend in the tropical Pacific under influences of anthropogenic forcing is still uncertain, due to the cross-data and model-data differences. In spite of these uncertainties, we find a robust signal of interbasin warming contrast from observations, with a larger rate of SST warming in the tropical Indian Ocean than that in the tropical Pacific. This interbasin warming contrast, however, is not captured by climate model simulations. By analyzing the ensemble mean of Indian Ocean-Global Atmosphere (IOGA) pacemaker experiments and the CESM Large-Ensemble (CESM-LE), we isolate and compare the impacts of tropical Indian Ocean warming and external forcing on Pacific SST and Walker circulation under global warming.

Summary

The comparisons between observations and climate models, however, have not fully taken into account the discrepancies of internal climate variabilities between observations and climate models. Previous studies have at least partly attributed to the SST cooling trend in the tropical Pacific since the late 1990s to the negative phase of the IPO, yet the IPO amplitude is generally underestimated by CMIP5 models. Therefore, the significant model-data differences of the interbasin warming contrast could be partly associated with model biases in simulating the natural internal climate variabilities. Given the prominent role of the tropical Indian Ocean warming in contributing to Pacific changes under global warming, it is important to explore in detail the causes for the large discrepancies in the Indian Ocean warming magnitude between observations and climate models, and a future study targeting the relative roles of external forcing and internal climate variability in causing the interbasin warming contrast is warranted. 

Contact
Aixue Hu
National Center for Atmospheric Research (NCAR)
Publications
Zhang, L, W Han, K Karnauskas, G Meehl, A Hu, N Rosenbloom, and T Shinoda.  2019.  "Indian Ocean Warming Trend Reduces Pacific Warming Response to Anthropogenic Greenhouse Gases: An Interbasin Thermostat Mechanism."  Geophysical Research Letters.  https://doi.org/10.1029/2019gl084088.