Simulations and analysis of mesoscale to turbulence scale process models to facilitate observational process deployments in the Equatorial Pacific Cold Tongue
PIs: D. Whitt (NCAR; now at NASA/ARC), R.-C. Lien (UW/APL), D. Cherian (NCAR), S. Bachman (NCAR), R. Holmes (UNSW), W. Large (NCAR)
Due to the far-reaching societal impacts, developing models and observing systems that enable reliable forecasts of the tropical Pacific Ocean in general and the Equatorial Cold Tongue (ECT; see map) in particular are a high priority. However, global numerical models used for this purpose have significant deficiencies. Several of these deficiencies may result from poorly-constrained parameterizations in the ocean model and/or coarse grid resolution (usually 10-100 km in the horizontal and 10 m in the vertical). For example, upwelling and vertical mixing are two processes that are crucial components of the heat budget of the ECT, but these processes have traditionally been difficult to observe and depend significantly on physics that occurs at scales much smaller than a typical model grid cell. In addition, previous studies have demonstrated that these processes are sensitive to model resolution and parameterization scheme.
This schematic shows modelled sea surface temperatures in the eastern tropical Pacific Cold Tongue, as well as outlines of the regional model domains that were developed to study the ocean dynamics of the region, including a regional box and large eddy simulations circles outlined in magenta.
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).
Daily averaged fields for 05 Dec 1995, as modelled in the MITgcm regional simulation of the ECT by Cherian et al. (2021). (a) Net surface heat flux Qnet (negative means heat enters the ocean). (b) SST. (c) Mean zonal velocity between 50- and 250-m. (d) Integrated turbulent heat flux in the low Ri layer (zRi < z < zMLD) normalized by a depth of 50m (e) Thickness of the low Ri layer (where gradient Richardson number is low, below the MLD). The black rectangle in (d) marks the TIW that is studied in detail in Cherian et al.
Our meetings were moved to zoom in 2020, but we kept going nearly weekly, and we also had some nice interactions with other groups. The team includes Ryan Holmes, Dan Whitt, Deepak Cherian, Bill Large, Scott Bachman, and Ren-Chieh Lien. The zoom meeting was a collaborative meeting with another team funded by the CVP, including Frank Bryan, Billy Kessler, LuAnne Thompson, and Anna-Lena Deppenmeier.