Meeting Documents

Diagnosis of Equatorial Pacific Richardson Numbers in High-Resolution Global-Ocean Simulations

Abstract

At the Equator in the upper ocean in the Pacific Ocean, the South Equatorial Current flows westward at the surface at 0.25-0.5 m s^-1, the Equatorial Undercurrent (EUC) flows eastward with a core speed greater than 1.5 m s^-1 at ~ 100-m depth, and a myriad of propagating waves maintain an intense vertical gradient of current above the depth of the EUC core speed. Because the onset of vertical mixing is dependent on the gradient Richardson Number (Ri), the modeled vertical shear will determine the utility of a model for studies of physical and biogeochemical processes. It is a tenet of faith that the smaller the meridional grid size of an ocean general circulation model, the more accurate would be the modeled EUC core speed because of the strong north-south current shear between the Equator and 1.5° latitude. This hypothesis is tested with global ocean model simulations with horizontal grid dimensions of 1/12° (LLC1080), 1/24° (LLC2160), and 1/48° (LLC4320) made available by the Estimating the Circulation and Climate of the Ocean (ECCO) project. In-situ current measurements were obtained by the Tropical Ocean-Atmosphere (TAO) moored array. Hourly data were analyzed for the 1-year period, 1 November 2011 to 31 October 2012. At 0°, 140°W the 12-month mean LLC4320 and TAO depths and magnitudes of EUC core speeds were 125 m and 0.86 m s^-1 and 85 m and 1.15 m s^-1, respectively. LLC4320 and TAO standard deviations were 0.19 and 0.34 m s^-1, respectively, indicating time variations with considerable amplitudes. The LLC4320 core speed occurred 40 m deeper than that observed and with 0.29 m s^-1 smaller speed. The uppermost depth of TAO currents was 35 m; we used the same depth for the uppermost depth of LLC4320 currents. At 35-m depth, the LLC4320 and TAO mean zonal currents were 0.05 and 0.66 m s^-1, respectively. At all depths, mean LLC4320 and TAO meridional currents were < 0.05 m s^-1. Magnitude of TAO zonal shear between 35 and 85 m was 0.01 s^-1. Magnitude of LLC4320 zonal shear between 35 and 125 m was 0.009 s^-1, which was 10% smaller than TAO shear and indicated a good correspondence with TAO. This would produce an approximate 20% smaller LLC4320 Ri compared to TAO Ri. Comparison of hourly Ri generated with TAO currents and LLC1080, LLC2160, and LLC4320 simulations, including results at 0°, 170°W and 0°, 110°W, will be described.