Intensified Deep Ocean Variability Induced by Topographic Rossby Waves at the Pacific Yap‐Mariana Junction

In a deep channel at the Pacific Yap-Mariana Junction, intensified variability of flow and isotherm displacements over 3,000-4,600 m is revealed by two separate observations: the recent one during 2014-2016 and the two 20 years ago during 1996-1998. Observed velocity and isotherm displacements can reach 45 cm s(-1) and similar to 600 m at 4,200 m, respectively. Such intensified variability is demonstrated to arise from topographic Rossby waves (TRWs) since vertical profiles of observed currents reasonably conform to TRW features of hyperbolic intensification with depth and highly vertical coherence in phase. Mercator Ocean model outputs reproduce observed TRWs and are used to discuss their energy sources. Both the energetic surface eddy moving across rough topography and mixed barotropic-baroclinic instability of the background flow appear to be the candidates to generate TRWs. Plain Language Summary Traditionally, the deep sea has been viewed as quiet, with weak horizontal and vertical motions; however, results from three subsurface moorings in a deep channel at the Pacific Yap-Mariana Junction show that the fluctuations of deep current speed and isotherm displacement intensify significantly with increasing depth. The observed velocity and isotherm displacements can reach 45 cm s(-1) and similar to 600 m at 4,200 m, comparable to that in the upper ocean. We interpret these deep fluctuations as topographic Rossby waves (TRWs) since vertical distributions of observed velocities obey TRW features of hyperbolic intensification with depth and highly vertical coherence in phase. With the aid of Mercator Ocean model outputs, the TRWs are deemed to arise from the energetic surface eddy moving across the rough topography and barotropic and baroclinic instabilities of the background flow.