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Seasonal sea surface cooling in the equatorial Pacific cold tongue controlled by ocean mixing


Sea surface temperature (SST) is a critical control on the atmosphere1, and numerical models of atmosphere–ocean circulation emphasize its accurate prediction. Yet many models demonstrate large, systematic biases in simulated SST in the equatorial ‘cold tongues’ (expansive regions of net heat uptake from the atmosphere) of the Atlantic2 and Pacific3 oceans, particularly with regard to a central but little-understood feature of tropical oceans: a strong seasonal cycle. The biases may be related to the inability of models to constrain turbulent mixing realistically4, given that turbulent mixing, combined with seasonal variations in atmospheric heating, determines SST. In temperate oceans, the seasonal SST cycle is clearly related to varying solar heating5; in the tropics, however, SSTs vary seasonally in the absence of similar variations in solar inputs6. Turbulent mixing has long been a likely explanation, but firm, long-term observational evidence has been absent. Here we show the existence of a distinctive seasonal cycle of subsurface cooling via mixing in the equatorial Pacific cold tongue, using multi-year measurements of turbulence in the ocean. In boreal spring, SST rises by 2 kelvin when heating of the upper ocean by the atmosphere exceeds cooling by mixing from below. In boreal summer, SST decreases because cooling from below exceeds heating from above. When the effects of lateral advection are considered, the magnitude of summer cooling via mixing (4 kelvin per month) is equivalent to that required to counter the heating terms. These results provide quantitative assessment of how mixing varies on timescales longer than a few weeks, clearly showing its controlling influence on seasonal cooling of SST in a critical oceanic regime.

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Figure 1: Six-year record of mixing at the TAO mooring at 0, 140° W.
Figure 2: Annual cycles of upper-ocean vertical structure at 0, 140° W over the period 2005–11.
Figure 3: Annual cycles of SST and turbulence at 0, 140° W.
Figure 4: Seasonally averaged vertical profiles of turbulence heat flux.

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This work was funded by the National Science Foundation (grants 0424133, 0728375 and 1256620). We thank M. Neeley-Brown and R. Kreth, who were primarily responsible for construction, testing and maintenance of χ-pods, and P. Freitag and NOAA's PMEL mooring group, who helped us to get started with these measurements. We also thank NOAA's NDBC group, who have continued to deploy our χ-pods on TAO moorings. E. Shroyer, S. de Szoeke, K. Benoit-Bird and D. Chelton provided comments on the paper. This is PMEL contribution no. 3970. We dedicate this paper to the memory of our colleague and co-author A. Perlin, who passed away during final revisions, and to the memory of lab engineer R. Kreth.

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J.N.M. wrote the paper. A.P. and J.N.M. did the analysis. J.D.N. has been part of this project since its inception and provided suggestions for analysis. M.J.M. provided advice on the large-scale context of these measurements. All authors contributed suggestions and text at the writing stage.

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Correspondence to James N. Moum.

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The authors declare no competing financial interests.

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Supplementary Information

This file contains Supplementary Text and Supplementary Figures 1-4. (PDF 627 kb)

Supplementary Data

A MATLAB data file containing spectral estimates of turbulence quantities derived from raw voltages. (TXT 25372 kb)

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Moum, J., Perlin, A., Nash, J. et al. Seasonal sea surface cooling in the equatorial Pacific cold tongue controlled by ocean mixing. Nature 500, 64–67 (2013).

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