Diagnosing diathermal velocities in the eastern tropical Pacific

Anna-Lena Deppenmeier, National Center for Atmospheric Research, Boulder, CO, United States, Frank Bryan, National Center for Atmospheric Research, Climate and Global Dynamics, Boulder, CO, United States, William S. Kessler, NOAA/PMEL/OCRD, Seattle, WA, United States and LuAnne Thompson, University of Washington, Oceanography, Seattle, WA, United States
Upwelling in the equatorial Pacific drives sea surface temperature (SST) changes across many time scales, which impact the large scale climate through coupled feedbacks. Vertical motion across isotherms is responsible for a large portion of SST variability in the eastern tropical Pacific, but the connection between vertical mixing and larger structures of velocity and temperature remains poorly understood. These processes are a source of uncertainty in most climate models. In this study, we analyze vertical motion and exchanges in the upper 200m of a high resolution (10km horizontal, 10m vertical) forced ocean model to characterize the scales of variability that affect air-sea interaction and control coupled dynamics. We investigate mechanisms of cross-isothermal transport and their consequences for SST variability.

Diapycnal mixing, eddy driven vertical transport from tropical instability waves, wind-driven horizontal transport, and remotely forced thermocline heave all contribute to local changes in diathermal velocity. We approach this from two different perspectives, a kinematic approach that includes locally-driven isopycnal layer thickness changes and horizontal divergence, and a thermodynamic approach in which mixing, solar heating, eddy transports and local temperature tendency are calculated. Controls on the seasonal cycle and ENSO evolution are identified, describing the balance among vertical mixing, upwelling, and eddy fluxes. We connect the mechanisms controlling diathermal velocities to SST variability and the impact on the tropical climate.