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.
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.
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