Abstract
In this study, the time-varying wind stress drag coefficient in the Ekman model was inverted by the cubic spline interpolation scheme based on the adjoint method. Twin experiments were carried out to investigate the influences of several factors on inversion results, and the conclusions were (1) the inverted distributions with the cubic spline interpolation scheme were in good agreement with the prescribed distributions of the wind stress drag coefficients, and the cubic spline interpolation scheme was superior to direct inversion by the model scheme and Cressman interpolation scheme; (2) the cubic spline interpolation scheme was more advantageous than the Cressman interpolation scheme even if there is moderate noise in the observations. The cubic spline interpolation scheme was further validated in practical experiments where Ekman currents and wind speed derived from mooring data of ocean station Papa were assimilated. The results demonstrated that the variation of the time-varying wind stress drag coefficient with time was similar to that of wind speed with time, and a more accurate inversion result could be obtained by the cubic spline interpolation scheme employing appropriate independent points. Overall, this study provides a potential way for efficient estimation of time-varying wind stress drag coefficient.
Highlights
Wind stress over the sea surface is a primary driving force of upper ocean circulation and ocean surface waves and is an important mechanism by which the atmosphere conveys momentum to the ocean
The twin experiments proved that wind stress drag coefficient (WSDC) can be inverted successfully by the adjoint method and that the cubic spline interpolation (CSI) scheme is an advantageous scheme compared with other two schemes
Current velocity and wind speed data were obtained from the ocean mooring station (Papa, 50.1◦ N,144.9◦ W) in the mooring system of the Ocean Climate Station Project (OCS) of NOAA, which has been instrumented with upper ocean and surface sensors since
Summary
Wind stress over the sea surface is a primary driving force of upper ocean circulation and ocean surface waves and is an important mechanism by which the atmosphere conveys momentum to the ocean. The exact estimation of wind stress is important for the simulation and forecasting of atmospheric and oceanic processes [1]. Wind stress is a key parameter for understanding physical processes in both the atmosphere and ocean, whose determination is important for coupled ocean–atmosphere modeling [2]. The WSDC was usually assumed to be a constant [5], considered as a linear function with respect to wind speed [6], or further believed to be a nonlinear function with respect to wind speed [7,8]. It is widely accepted that wind stress coefficients vary nonlinearly with wind speed (and with time)—especially at high wind speeds [9,10]. The determination of the WSDC is a difficult problem in the field of marine science and atmospheric science because of the interaction between air and sea; this difficulty stems from the problem of turbulence
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