LETTERS PUBLISHED ONLINE: 11 OCTOBER 2009 | DOI: 10.1038/NGEO657 Sea surface cooling at the Equator by subsurface mixing in tropical instability waves J. N. Moum 1 * , R.-C. Lien 2 , A. Perlin 1 , J. D. Nash 1 , M. C. Gregg 2 and P. J. Wiles 1 Changes in sea surface temperature of equatorial waters have critical effects on the large-scale atmospheric circulation 1–3 . So far, large-scale, energetic tropical instability waves in equato- rial waters have been thought to warm the sea surface through both meridional and zonal advection 4,5 . Here, we present ship- board profiling measurements of turbulence kinetic-energy dissipation rate that reveal unanticipated vigorous mixing as- sociated 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 dis- sipation 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 turbu- lence 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. The equatorial current system excites a complex hierarchy of small-scale geophysical fluid dynamics that contribute to mix- ing processes there 6,7 . From experiments conducted at 0 ◦ , 140 ◦ W in 1984 (refs 8, 9), 1987 (ref. 10) and 1991 (ref. 11) were derived mixing parameterizations used in numerical models of the equatorial ocean, yet indications are that these parame- terizations do not work particularly well 12 . One reason for inaccuracies in mixing parameterizations is the paucity of ex- isting measurements relative to the natural range of variability, given the large intraseasonal, seasonal and interannual cycles of equatorial flows 13 . In particular, none of these short field experiments sampled the passage of tropical instability waves (TIWs). Recent numerical simulations show that intensifica- tion of mixing by TIWs occurs owing to enhanced shear at the mixed layer base 14 ; turbulence-heat-flux estimates from a Lagrangian float that encountered a TIW in September 2005 support this 15 . Energetic meanders in the equatorial ocean with periods of 13–40 days and wavelengths of 700–1,600 km are documented in satellite images of sea surface temperature (SST; ref. 16), height 17 and ocean colour 18 as well as subsurface measurements of temperature 19 and velocity 20 . These signals are a combination of a surface-trapped Yanai wave on the Equator (at shorter periods) and a first-meridional-mode Rossby wave just north of the Equator (and at longer period; ref. 21), and are loosely referred to as TIWs. TIWs are characterized by large meridional velocities, which provide the potential for vigorous interactions with the already energetic equatorial current system. For the first time, we were 1 College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331-5503, USA, 2 Applied Physics Laboratory, University of Washington, Seattle, Washington 98105-6698, USA. *e-mail:moum@coas.oregonstate.edu. able to measure the effect of an energetic TIW at 0 ◦ , 140 ◦ W as it passed in autumn 2008. Intensive shipboard profiling was conducted at 0 ◦ , 140 ◦ W from 24 October 2008 to 09 November 2008, a period coinciding with the passage of a TIW with zonal wavelength ≈ 10 ◦ (about 1,100 km) as indicated by SST (see Supplementary Fig. S1). The TIW was accompanied by strong meridional velocities (amplitude 1 m s −1 ; Fig. 1b) confined to the depth range above the core of the Equatorial Undercurrent (EUC; the core is defined as the depth of the eastward velocity maximum; Fig. 1a). Enhanced current shear (quantified by the squared vertical gradient of velocity, S 2 0 = u 2 z + v 2 z ; u is zonal and v meridional velocity, subscript z represents vertical gradient) was observed immediately above the EUC core (Fig. 1c), coincident with high stratification (N 2 =−g ρ z /ρ , ρ is fluid density and g the acceleration due to gravity; Fig. 1d). To isolate TIW effects, we define S 2 zonal = u 2 z , which excludes meridional shear, and with which we compare S 2 0 . Measurements of turbulence kinetic-energy (KE) dissipation rate (ε) revealed three distinct regimes above the EUC core (Fig. 1e). Within the mixed layer, ε was high. The mixed layer deepened every night as the sea surface cooled, contributing to a daily cycle in ε near the surface 8,9 . Below the mixed layer is the deep cycle layer of turbulence 11 , which also shows a daily cycle in ε, thought to be associated with a narrowband internal gravity-wave field 22 . Both have been observed in previous experiments. The unique, new regime is the 20–40-m-thick layer of strong mixing immediately above the EUC core (28–31 October, 01–08 November). Although the magnitude of ε changed over these periods, it did not cycle on a daily basis. For extended periods (01–02 November, 04–07 November), a region of weak shear, weak stratification and low ε existed above this layer, indicating decoupling from surface forcing. The presence of weakly stratified layers is surprising because, in a one-dimensional sense, these should be rapidly eroded by strong mixing above and below. Their existence must be associated with unresolved three-dimensional processes. Because this new, intensely mixing layer resides within the EUC but above the core, we refer to it as the upper core layer. Above 110 m, experiment-averaged values of ε from autumn 2008 (Fig. 2a) were at least five times larger than found in previous experiments at the same location and time of year in 1984 (ref. 6) and 1991 (ref. 11). High values of ε also extend deeper, to the EUC core. This is an unanticipated enhancement of turbulence, presumably brought on by two distinct, though related, factors. One factor is the additional current shear due to the TIW meridional velocity, which nudges the system towards instability. A necessary condition for instability in stratified shear flows is defined by the gradient Richardson number, Ri = N 2 /S 2 < 1/4 (ref. 23). Averaged over our observation period, S 2 0 /S 2 zonal = 1.15 in the deep-cycle regime; in the upper core layer, S 2 0 /S 2 zonal = 1.32, a 32% increase NATURE GEOSCIENCE | ADVANCE ONLINE PUBLICATION | www.nature.com/naturegeoscience 1 © 2009 Macmillan Publishers Limited. All rights reserved.