Wind-Driven Mid-Depth Pacific Cooling in a Dynamically Consistent Ocean State Estimate

A comparison of 19th century HMS Challenger hydrography with modern observations indicated the mid-depth Pacific has cooled at a rate of $5\pm 3$ centikelvin (cK, $1\mathrm{cK}=0.01\mathrm{kelvin}$) per century and could be explained as a slow ongoing adjustment to the Little Ice Age. The historical hydrographic data was sparse, however, and limited by 19th century technology. Observations from recent decades are more plentiful, with $O\left(10^3\right)$ times more observations are used to constrain the dynamically consistent ocean state estimate produced by the Estimating the Circulation and Climate of the Ocean (ECCO) Consortium, for example. We find the ECCO mid-depth Pacific cooling trend is similar in magnitude (5 cK/century) and spatial structure to HMS Challenger data. Additionally, cooling in the mid-depth North Pacific only emerges after the model is optimized to observations. Here, sensitivity experiments are used to isolate the effects of data-constrained model parameters on temperature trends in this region. The optimized wind stress achieves most (3 cK/century) of the cooling in ECCO through anomalous vertical transport and adiabatic isopycnal upwelling. Optimized initial conditions and mixing coefficients play a less important role, causing cooling rates of 2 cK/century and 1 cK/century, respectively. Furthermore, the ECCO heat budget is found to be kinematically similar to that expected from a centuries-long simulation of pre-industrial anomalies being transported into the ocean interior. Thus, mid-depth cooling in ECCO can be physically interpreted and is within the dynamical expectations of an ocean out of equilibrium with the atmosphere.