A Concise and Effective Expression Relating Subsurface Temperature to the Thermocline in the Equatorial Pacific.

The temperature of the subsurface water entrained into the surface mixed layer plays a key role in controlling the sea surface temperature (SST) and its interannual variability in the equatorial Pacific. In this paper, we combine a hyperbolic tangent function bounded by the warm pool SST and centered at the thermocline depth with a variable sharpness parameter to describe the time‐space evolutions of the subsurface temperature. Under simple approximations of the sharpness parameter, this concise expression becomes remarkably efficient in capturing the observed and climate‐model simulated subsurface temperature variability in terms of anomalies of the thermocline depth and SST of the El Niño‐Southern Oscillation (ENSO) phenomenon. The formulations for the subsurface temperature and thermocline sharpness developed in this work should be useful tools for evaluating and understanding the role of the thermocline feedback in ENSO behaviors in both theoretical and comprehensive climate models. Plain Language Summary: While significant advances have been made in our ability to understand, simulate, and predict the El Niño‐Southern Oscillation (ENSO), its remarkable complexity is still not well understood. The subsurface ocean temperature in the equatorial Pacific is known to play a key role in controlling sea surface temperature and its interannual variations. An accurate description of its relationship with thermocline depth variability is of essential importance to understanding ENSO's complex dynamics in mechanistic studies. In this paper, we develop a concise expression which captures the observed relationship between anomalies of the subsurface temperature and thermocline depth remarkably well. This expression should be useful for ENSO mechanistic model studies and diagnostic investigations into climate models' performances in their ENSO simulations. Key Points: The equatorial Pacific subsurface temperature is expressed as a hyperbolic tangent function of thermocline depth and sharpnessEffective parameterizations for the subsurface temperature are derived by simple approximations of the thermocline sharpnessThese parameterizations can be useful tools for evaluating the impacts of thermocline feedback on ENSO [ABSTRACT FROM AUTHOR]