Turbulence From Tropical Instability Waves in the Equatorial Cold Tongues: Quantification From Multiyear Moored Records

Tropical instability waves (TIWs) are identified in three multiyear equatorial mooring records in Pacific and Atlantic cold tongues to evaluate how TIWs modulate turbulence. At 0 degrees, 140 degrees W in the Pacific, TIWs are present in 43% of observations, and are associated with elevated vertical shear and a 40% average increase in turbulence dissipation rates (epsilon) above the Equatorial Undercurrent. Zonal shear is greatest when currents are southward while buoyancy is greatest later in the TIW cycle, leading to greater potential for instability and elevated turbulence before and during the southward flow maximum. This suggests that TIW vortex stretching contributes to enhanced shear and turbulence. In the Atlantic, TIWs are found in 38% of observations at 0 degrees, 23 degrees W and 16% of observations at 0 degrees, 10 degrees W. TIWs at 23 degrees W increase epsilon by 18% where turbulence is likely modulated by vortex stretching and, near the surface, by the seasonally wind-forced equatorial roll. At 23 degrees W and 140 degrees W, TIWs with strong meridional velocity fluctuations are associated with the strongest turbulence. Contributions of seasonal variations are removed by considering only periods when TIWs are climatically active. During these periods, mean values of epsilon in the presence of strong TIWs are elevated by 61% at 140 degrees W, 29% at 23 degrees W, and 36% at 10 degrees W. At 10 degrees W, where our identification scheme may include wind-forced oscillations in the same frequency band, increases in epsilon are not consistent in the presence of TIWs and do not contribute significantly to multiyear averages. Tropical instability waves (TIWs) are 1,000-km scale westward-propagating features that modify currents, temperature, and turbulence in the Pacific and Atlantic equatorial cold tongues, areas of relatively cold water on the equator that exist due to equatorial upwelling. The purpose of this study is to evaluate the role of TIWs in modulating turbulence, which by enhancing vertical mixing drives heat transport from the surface to the ocean interior. The results show that TIWs increase multiyear averages of turbulence by a large amount in the Pacific cold tongue, by a smaller amount in the central Atlantic cold tongue, and by a negligible amount in the eastern Atlantic cold tongue. Because the cold tongues are regions of exceptionally high vertical heat fluxes, these findings suggest that TIWs play an important role in the global climate system. Three multiyear mooring records in the equatorial Atlantic and Pacific were used to identify and characterize tropical instability waves (TIWs) TIWs are associated with enhanced vertical shear and turbulence dissipation rates above the equatorial undercurrent TIWs elevate long-term averages of turbulence significantly in the Pacific cold tongue and less in the Atlantic