A Pre-Field Modeling Study of Scales, Variability and Processes in the Near Surface Eastern Equatorial Pacific Ocean in Support of TPOS

PI’s: Frank Bryan (NCAR), William S. Kessler (NOAA/PMEL), LuAnne Thompson (UW)

Description

We characterize the dynamics and the space- and time-scales of upwelling through analysis of high resolution ocean model output. We focus on the connection of upwelling to mixing. This work informs the design of a prospective Pacific Upwelling and Mixing Physics (PUMP) process study in the context of available observing technologies.

We address the following issues:
1. The dynamics that control the near equator horizontal divergence and upwelling.
2. The partitioning of upwelling into adiabatic (along isotherms) and diabatic (across isotherms) components.
3a. The linkage between the equatorial three dimensional circulation and the regional heat budget,
3b. including the dependence on surface forcing across a range of time scales from synoptic to inter-annual and in different locations within the tropical Pacific.

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.

Accomplishments

We use a water mass transformation framework to partition upwelling into adiabatic and diabatic components. Using this framework we are able to attribute the diabatic, or cross-isothermal, upwelling to specific physical processes represented in the model heat budget as a function of time and space. We find that in the mean one third of the total upwelling in the central and eastern tropical Pacific occurs diabatically.

We identify two regimes with differing controls of water mass transformation and associated diabatic upwelling.
1. In the mixed layer solar penetration promotes cross-isothermal motion toward warmer isotherms with partial compensation by vertical mixing driving cross-isothermal motion toward cooler temperatures.
2. In the thermocline cross-isothermal upwelling is driven by vertical mixing.

We also show that modulation of shear related vertical mixing occurs on inter-annual time scales resulting in a corresponding inter-annual modulation in cross-isothermal velocities. Ongoing work is focused on modulation of diabatic upwelling on timescales of weeks to months, and in variations in its geographical distribution.

Lessons Learned

1. Diabatic upwelling in the mean and on interannual timescales is smaller than adiabatic upwelling by a factor of two.

2. The physical drivers behind water mass transformation in the thermocline differ depending on the time scale under investigation.

3. Vertically resolved profiles of turbulent heat fluxes, both on the equator and off, are necessary to corroborate the model results and to adequately measure the variations of diabatic upwelling in the mixed-layer and the thermocline.

Publications

Deppenmeier, A-L, Bryan, FO, Kessler, WS, Thompson, L: Modulation of cross-isothermal velocities with ENSO in the tropical Pacific cold tongue (2021) Journal of Physical Oceanography, doi: 10.1175/JPO-D-20-0217.1