Off-Equatorial Deep-Cycle Turbulence Forced by Tropical Instability Waves in the Equatorial Pacific Journal of Physical Oceanography

The equatorial Pacific cold tongue is a site of large heat absorption by the ocean. This heat uptake is enhanced by a daily cycle of shear turbulence beneath the mixed layer—“deep-cycle turbulence”—that removes heat from the sea surface and deposits it in the upper flank of the Equatorial Undercurrent. Deep-cycle turbulence results when turbulence is triggered daily in sheared and stratified flow that is marginally stable (gradient Richardson number Ri ≈ 0.25). Deep-cycle turbulence has been observed on numerous occasions in the cold tongue at 0°, 140°W, and may be modulated by tropical instability waves (TIWs). Here we use a primitive equation regional simulation of the cold tongue to show that deep-cycle turbulence may also occur off the equator within TIW cold cusps where the flow is marginally stable. In the cold cusp, preexisting equatorial zonal shear uz is enhanced by horizontal vortex stretching near the equator, and subsequently modified by horizontal vortex tilting terms to generate meridional shear υz off of the equator. Parameterized turbulence in the sheared flow of the cold cusp is triggered daily by the descent of the surface mixing layer associated with the weakening of the stabilizing surface buoyancy flux in the afternoon. Observational evidence for off-equatorial deep-cycle turbulence is restricted to a few CTD casts, which, when combined with shear from shipboard ADCP data, suggest the presence of marginally stable flow in TIW cold cusps. This study motivates further observational campaigns to characterize the modulation of deep-cycle turbulence by TIWs both on and off the equator. Grant no. NA18OAR4310408

赤道太平洋冷舌是海洋发生大量吸热的区域。该区域的海洋吸热过程因混合层下方剪切湍流的日循环——即“深循环湍流(deep-cycle turbulence)”——而得到增强，这类湍流会将海洋表层的热量移除并输送至赤道潜流(Equatorial Undercurrent)的上层侧翼。当在临界稳定（梯度理查森数Ri≈0.25）的剪切层结流中每日触发湍流时，便会产生深循环湍流。此前已有多次在0°、140°W处的冷舌区域观测到深循环湍流的记录，且该现象可能受热带不稳定波(Tropical Instability Waves, TIWs)调控。本研究借助冷舌区域的原始方程区域模拟结果，证实深循环湍流也可发生在赤道外的TIW冷尖峰区域，该区域的流场同样处于临界稳定状态。在冷尖峰区域，预先存在的赤道纬向剪切uz会因赤道附近的水平涡拉伸作用而增强，随后经水平涡倾斜项改造，在赤道外生成经向剪切υz。冷尖峰区域剪切流中的参数化湍流，会因午后地表稳定浮力通量减弱引发的表层混合层下沉而每日被触发。目前针对赤道外深循环湍流的观测证据仅局限于少量CTD站位(CTD casts)，若将此类数据与船载声学多普勒流速剖面仪(Acoustic Doppler Current Profiler, ADCP)测得的剪切数据相结合，便可推断TIW冷尖峰区域存在临界稳定流场。本研究旨在推动后续观测研究工作，以明确热带不稳定波对赤道内外深循环湍流的调控机制。资助编号：NA18OAR4310408