Mixing Across the Pacific Equatorial Cold Tongue

Oregon State University, Corvallis OR

Investigators

Abstract

The cold tongue in the Equatorial Pacific is an expansive region of net ocean heat uptake. Maintaining these cool sea surface temperatures requires a combination of subsurface mixing or lateral/vertical advection to transport surface heat downward. Whether mixing plays the dominant role is not yet clear, but it has been established that a rich set of fluid dynamical processes in the upper equatorial ocean drives intense turbulence. Evidence from limited observations on the Equator at 140W during the passage a Tropical Instability Wave (TIW) revealed a ten-fold increase in subsurface turbulent heat flux, sufficient to cool surface waters by 2 degree Celsius per month. This suggests the possibility that mixing alone could maintain the equatorial cold tongue, a conjecture supported by climatological evidence that sea surface cooling is enhanced by Tropical Instability Waves. Intellectual Merit: Since 2005, moored mixing measurements on the Equator at 140W using a novel self-contained instrument (chi-pod) provide a more complete temporal assessment of the mixing. These moored records indicate not only considerable variability on short time scales but also significant seasonal variations in turbulence levels that are roughly 90-180° out-of-phase with the annual cycle in Sea Surface Temperature. However, the time-varying part of the spatial structure is completely unknown. Strong cross-equatorial gradients in currents, temperature and salinity imply that gradients in mixing are likely to be strong as well. At present, it is impossible to distinguish temporal cycles from spatial gradients. This project will expand the measurements of mixing on the Tropical Atmosphere-Ocean mooring array along the equator between 140W and 95W. Each of these 4 moorings will be outfitted with 5 chi-pods and the analysis of the data obtained from these moorings will be supplemented by existing data from past process experiments at 140W and a detailed, complementary process experiment at 102.5W. The main goals of he project are to quantify (1) the time- and spatially-varying structure of mixing along and across the cold tongue; (2) the role of mixing in maintaining the cold tongue; (3) the role of TIWs in equatorial mixing; and (4) the role of turbulence in consuming TIW energy. Broader Impacts: A post-doctoral associate will participate in a broad analysis of the Tropical Atmosphere Ocean Array and mixing measurements and be trained in equatorial dynamics. College graduate students will participate in the process experiment. Undergraduate engineering students will be employed in the summer months to help with both electronics and mechanical construction and testing of instrumentation. An undergraduate atmospheric science student will be employed in the summer of 2013 to review and document archived data from the Mary's Peak webcam (http://marycam.coas.oregonstate.edu/index.html) for public dissemination. This project complements ongoing theoretical analyses of Tropical Instability Waves conducted by the lead investigators and several colleagues. Data obtained in the project will be shared with the Climate Process Team on ocean mixing and become part of the archived data sets of the Tropical Atmosphere Ocean Array Program managed by the National Oceanic and Atmospheric Administration. This project is a contribution to the U.S. CLIVAR (CLImate VARiability and predictability) Porgram.