This goal of this project, funded by the CVP pre-field modelling program in support of TPOS, is to conduct process-oriented numerical simulations and analysis to elucidate the spatio-temporal characteristics and dynamics of upwelling, vertical mixing, and the associated vertical fluxes of heat in the Pacific ECT. Computational constraints dictate that a model hierarchy is needed to study the full range of relevant spatial and temporal scales (from global to less than meter scale).This project addresses processes occurring at a restricted range of oceanic scales from turbulent scales to mesoscales (1 m to 500 km and minutes to months), and aims to achieve three primary goals: 1) building sufficient process-level understanding to develop robust, physically based parameterizations for global climate models, 2) creation of a hierarchy of process models to inform and supplement observational campaigns in the ECT, and 3) using process model results to participate in the design of observational process studies. Further, the project aims to develop informed hypotheses (via analysis of model results) that can be tested and refined through observational process studies within TPOS.
This project was funded by NOAA’s Climate Variability and Predictability (CVP) Program as one of its eight new projects in support of TPOS Process Studies.
The creation of a hierarchy of regional model simulations of the mesoscale to microscale processes in the equatorial Pacific cold tongue is a key accomplishment. These include a relatively long 25-year-long regional ocean simulation at 6-km horizontal resolution, a 2-year integration at 2 km resolution, as well as 35-day-long large eddy simulations (LES) spanning an entire tropical instability wave period at 1-m horizontal resolution at both 0 N and 3 N along 140W.
The analysis of both the regional ocean models and LES suggests a new hypothesis: that deep-cycle turbulence, which has previously been observed to play an important role in cooling the sea surface temperature above the highly-sheared undercurrent on the equator, also contributes significantly to sea-surface temperature cooling over much of the cold tongue area. However, sea-surface temperature cooling by ocean mixing is more intermittent off of the equator due to the modulation of conditions that favor mixing by tropical instability waves. An important caveat is that the factors that control the strength of upper-ocean mixing remain crudely parameterized in the regional ocean models. And, the relationship between vertical mixing and the larger-scale pre-conditions is not well-constrained by observations off of the equator.
It would be valuable to conduct a process study of the complex combination of factors by which ocean circulation and mixing drives sea-surface temperature cooling across the entire cold tongue. However, experimental design must account for the more-intermittent nature of ocean mixing and its driving processes off of the equator. In any case, the modeling suggests that ocean mixing may cool the off-equatorial surface temperature of the ECT more strongly than previously thought, although observational data about the nature of upper-ocean mixing off of the equatorial undercurrent in the ECT is limited.
Cherian, D. et al. (2021) Off-equatorial deep-cycle turbulence forced by Tropical Instability Waves in the equatorial Pacific. J. Phys. Oceanogr. https://doi.org/10.1175/JPO-D-20-0229.1
The MITgcm simulation output used for Cherian et al. (2021) is available on NCAR’s Digital Asset Services Hub (DASH) repository (https://dashrepo.ucar.edu/). We have provided daily averages for much of the domain (www.doi.org/10.5065/6qa0-v481); 4-hourly output along latitudinal sections at longitudes 110°W, 125°W, 140°W, and 155°W (www.doi.org/10.5065/6qa0-v481); and hourly fields for the TIW that is the focus of this paper (www.doi.org/10.5065/3kb5-g350).