A Series of Middepth Zonal Flows in the Pacific Driven by Winds

Abstract

A series of middepth zonal flows observed in the Pacific is produced in a World Ocean model with the horizontal resolution of 1° ? 1° and 40 vertical levels. It is demonstrated that the middepth zonal flows are driven by the wind and reach down to several thousand meters. The surface wind gyres appear to shift poleward with depth, leaving behind the gyres that originate from the equatorial response. In meridional sections, the pattern of the zonal flows slants poleward with increasing depth. The formation mechanism for the middepth zonal flows is clarified using an idealized basin model and a semianalytical model of vertical normal mode decomposition. In the models, the inclusion of vertical diffusion is essential. The zonal flows at low latitudes are formed as the equatorial response to uniform zonal winds. The response at middle and high latitudes is accounted for as follows. Quasigeostrophic (QG) dynamics with vertical diffusion reproduces the reversal of the zonal flows with depth. The slanting pattern of the zonal flows is due to the non-QG effect for the response of the vertical higher modes. Then the inclusion of horizontal diffusion reduces the response of the gravest modes. The third to fifth vertical modes are very important for forming the middepth wind-driven circulation. The wind-driven circulation at middepths is very weak compared to that in the surface layer, but this is sufficient to overcome the weak thermohaline circulation in the middepth Pacific.