Adjustment of Near-Equatorial Wind Stress with Four-Dimensional Variational Data Assimilation in a Model of the Pacific Ocean

Abstract

A four-dimensional variational scheme is described in which ocean observations are assimilated into an ocean general circulation model using wind stress forcing fields as control variables. Idealized (?twin?) experiments are performed to evaluate the possibility of reconstructing wind stress variability and its oceanic response from synthetic observations of the ocean state. Two types of wind stress errors are considered: time-varying errors associated with a wind burst and constant errors associated with a wind stress bias. Both sets of experiments demonstrate that the spatial structure of the wind stress variations is well reconstructed, while the estimation of their amplitude and time evolution is less accurate. Sparser equatorial sampling, similar to that of the Tropical Atmosphere?Ocean array, only slightly degrades the analysis. Omitting velocity and salinity observations leads to a less accurate amplitude and time evolution of the wind stress increment. Still, general features are captured in the analysis when only temperature observations are assimilated. Additional twin experiments point out that errors in thermal structure due to errors in the model and initial conditions can only partly be corrected by modifying the wind stress forcing. To change the shape and position of the thermocline through a wind stress correction requires an adjustment time scale of several weeks. The temperature gradient can be changed by the correction of wind stress errors, but controlling model error, initial conditions, or both may be necessary to correct for this type of systematic error more effectively.