Program

Direct turbulence measurements revealed close relation between tropical instability waves (TIWs) and elevated vertical mixing in the upper thermocline of the eastern equatorial Pacific; this relation was confirmed by co-occurrence between shear instabilities and TIWs inferred from high-resolution Argo and TAO observations. However, the mechanism that TIWs induce elevated thermocline mixing is unclear. This work investigates the processes that are associated with the TIWs and are leading to instabilities, based on results of a high resolution Ocean general circulation model (OGCM) that well resolves TIWs and large eddy simulation (LES) experiments that identify small scale processes. The OGCM results suggest that, at intra-seasonal time scale (characteristic period of TIW), TIWs tend to weaken the stratification (represented by the squared buoyancy frequency, N²) in the middle and upper layers of the thermocline through advection of buoyancy, resulting in smaller Richardson number Ri, a proxy of instability. The layer of decreasing N² roughly corresponds to a layer of 60-120 m at 140°W at the equator. LES experiments further demonstrate that when the advective effect of TIWs is added to an typical profile of 140°W, instabilities occur near the center of thermocline 10-30 hrs after start of the experiments, and are well separated from turbulences of the surface layers; whereas, when the advective effect is absent, independent thermocline turbulence hardly occur. Compared to the hypothesis that the TIWs lead to more instabilities and stronger vertical mixing via providing additional shear, the present study proposes a new mechanism of the energy cascade, i.e, via weakening the thermocline. The results should have implications for vertical mixing parameterization at least for the eastern tropical Pacific and Atlantic where TIWs are energetic.