Ocean surface wind variability under the Pliocene warmth

During the mid-Pliocene, carbon dioxide (CO2) concentrations were comparable to current levels, ~350-450 p.p.m., making the mid-Pliocene a valuable analog for current and potentially future climates. However, the global average temperature in the mid-Pliocene is estimated to have been ~3 ºC higher than today, implying factors beyond greenhouse gases contributed to the warmth. Surface wind velocity, a key driver of ocean mixing and air-sea turbulent heat flux, significantly affects ocean heat content and global heat distribution. Understanding the role of surface wind speed in warm climates is therefore important to uncover the causes of the Pliocene warmth. In this study, we utilized a set of climate models from the Coupled Model Intercomparison Project (CMIP) and the Paleoclimate Modeling Intercomparison Project (PMIP) archives to analyze surface wind variability in the tropics during the mid-Pliocene and the Pre-industrial periods. Over the tropics, surface wind velocity is strongly influenced by sea surface temperature (SST) patterns, predominantly El Niño-Southern Oscillation. To explore the underlying mechanisms of surface wind variability, we applied Empirical Orthogonal Functions (EOF) to tropical SST data to extract SST patterns and decompose surface wind speed into SST-dependent and SST-independent components. Our results revealed that the ratio of SST-dependent and SST-independent wind variability could vary substantially in space and with season, and it could differ between the mid-Pliocene and the Pre-industrial period with a large inter-model spread. Implications for understanding the mid-Pliocene warmth and constraining future climate projection will be discussed.