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Abstract Understanding how the tropical Pacific responds to rising greenhouse gases in recent decades is of paramount importance given its central role in global climate systems. Extensive research has explored the long-term trends of tropical Pacific sea surface temperatures (SSTs) and the overlying atmosphere, yet the historical change in the upper ocean has received far less attention. Here, we present compelling evidence of a prominent subsurface cooling pattern along the thermocline in the central-to-eastern tropical Pacific since 1958. This subsurface cooling has been argued to be contributing to the observed cooling or lack of warming of the equatorial cold tongue SST. We further demonstrate that different mechanisms are responsible for different parts of the subsurface cooling. In the central-to-eastern equatorial Pacific and the southeastern off-equatorial Pacific, where zonal wind stress strengthens, a pronounced subsurface cooling trend emerges just above the thermocline that is closely tied to increased Ekman pumping. In the eastern equatorial Pacific where zonal wind stress weakens, the westward surface current and eastward Equatorial Undercurrent weaken as well, resulting in reduced vertical current shear and increased ocean stability, which suppresses vertical mixing and leads to local cooling. We conclude that the historical subsurface cooling is primarily linked to dynamical adjustments of ocean currents to tropical surface wind stress changes. 

Abstract The equatorial cold tongue region has not warmed up in response to historical radiative forcing in the real world, contrary to the strong warming often simulated by climate models. Here we demonstrate that climate models fail to represent one or both of the key processes driving observed sea surface temperature (SST) pattern formation: a realistic surface wind stress pattern shaping subsurface cooling through wind‐driven circulation changes, and effective connectivity between subsurface and surface temperatures via upwelling and mixing. Consequently, none of the models approximate the observed lack of cold tongue SST warming and strengthening of zonal SST gradient across the equatorial Pacific. Furthermore, those that come closest achieve this due to interhemispheric warming differences rather than equatorial dynamics as observed. Addressing different origins of subsurface cooling in observations and simulations, and how they connect to SST, will lead to improved understanding of tropical Pacific SST changes to date and how they will evolve in the future. 

Abstract The early‐to mid‐Pliocene (5.3–3 Ma), characterized by warmer temperatures and similar CO₂concentrations to present day, is considered a useful analog for future warming scenarios. Geological evidence suggests that during the Pliocene, many modern‐day desert regions received higher levels of rainfall and supported large perennial lakes and wetter vegetation types. These wetter conditions have been difficult to reconcile with model predictions of 21st century drying over most subtropical land regions. Using an atmospheric General Circulation Model, we show that underestimates of Pliocene rainfall over certain areas in models may be related to insufficient sea surface temperature (SST) warmth simulated over relatively local eastern boundary current regions. When SSTs off the coast of California are raised to more closely match some proxy reconstructions, rainfall increases over much of adjacent western North America. Over the southwestern USA, this increased rainfall is mainly due to a convergent monsoonal circulation that develops over late boreal summer. A smaller wintertime increase in precipitation also occurs due to differences in rainfall associated with midlatitude cyclones. Wetter land conditions are expected to weaken upwelling‐favorable coastal winds, so that increased rainfall caused by coastal SST warming suggests a positive feedback that could help sustain wet, Pliocene‐like conditions.