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

Abstract

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°, 140°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 (ϵ) 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°, 23°W and 16% of observations at 0°, 10°W. TIWs at 23°W increase ϵ by 18% where turbulence is likely modulated by vortex stretching and, near the surface, by the seasonally wind‐forced equatorial roll. At 23°W and 140°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 ϵ in the presence of strong TIWs are elevated by 61% at 140°W, 29% at 23°W, and 36% at 10°W. At 10°W, where our identification scheme may include wind‐forced oscillations in the same frequency band, increases in ϵ are not consistent in the presence of TIWs and do not contribute significantly to multiyear averages.