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Sea surface cooling at the Equator by subsurface mixing in tropical instability waves


Changes in sea surface temperature of equatorial waters have critical effects on the large-scale atmospheric circulation1,2,3. So far, large-scale, energetic tropical instability waves in equatorial waters have been thought to warm the sea surface through both meridional and zonal advection4,5. Here, we present shipboard profiling measurements of turbulence kinetic-energy dissipation rate that reveal unanticipated vigorous mixing associated with tropical instability waves. The meridional tropical instability-wave shear increases the shear above the core of the Equatorial Undercurrent, which is already large, nudging the flow toward instability. As a consequence, turbulence dissipation rates and heat fluxes are many times greater than previous measurements at the same location but in the absence of tropical instability waves. The vertical divergence of turbulence heat flux is sufficient to cool the upper layer by 2 K per month, and heat the core of the Equatorial Undercurrent by 10 K per month. Long-term records at 140 W further reveal that cooling of the sea surface is significantly correlated to tropical-instability-wave kinetic energy. Thus, seasonal surface cooling in the central equatorial Pacific may be largely caused by mixing induced by tropical instability waves.

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Figure 1: Measurements from 16-day profiling time series at 0,140 W in boreal autumn 2008.
Figure 2: Historical comparison of turbulence and heat flux at 0,140 W.
Figure 3: Nudging of flow towards instability through enhanced meridional shear.
Figure 4: Relation of SST to TIW energy at 140 W.


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This research was funded by the National Science Foundation (0424133, 0728375). Thanks to M. Neeley-Brown, R. Kreth, K. Chabuk, J. Johnson, A. Schmittner, M.-H. Chang and J. Milliard for their assistance in obtaining these measurements, to the captain and crew of the R/V Wecoma for provision of an experimental platform and to NOAA’s TAO Project office for the equatorial mooring data. A. Schmittner, E. Shroyer, D. Chelton, M. McPhaden and B. Smyth provided comments.

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Correspondence to J. N. Moum.

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Moum, J., Lien, RC., Perlin, A. et al. Sea surface cooling at the Equator by subsurface mixing in tropical instability waves. Nature Geosci 2, 761–765 (2009).

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