Entrainment, Rayleigh friction, and boundary layer winds over the tropical Pacific

Abstract

Winds over the tropical Pacific are interpreted using mixed-layer theory. The theory-which posits that the surface winds can be derived interms of a force balance among surface drag, pressure gradients, Coriolis forces, and the vertical mixing of momentum into the boundary layer (entrainment)-is very successful in predicting the seasonal climatology of the surface winds. The model is also used as a basis for interpreting previous results. In particular the model illustrates why studies that model the momentum flux divergence as a Rayleigh damping find optimal damping coefficients that are anisotropic. A linear variant of the model, which also incorporates entrainment but neglects the quadratic relation between the wind speed and the surface stress, is also found to predict the surface winds skillfully. In addition to improving the representation of the winds, it leads to realistic representations of the divergence of the vector wind. If the key parameters of the model (the entrainment rate and the boundary layer depth) are assumed to have uniform climatological mean values over the Pacific basin, optimal parameter values can be derived by matching the model winds to the climatology. Such a procedure leads to boundary layer depths between 300 and 400 m and entrainment rates slightly less than 1 cm s-1. A somewhat more general model of the boundary layer winds (the so-called K-profile parameterization), is used to show that accounting for the vertical structure of the wind profiles yields somewhat larger optimal estimates of w, and h. Overall, the incorporation of the entrainment effect is critical, indicating that the acceleration of the near-surface winds by momentum mixing with the free atmosphere is a first-order effect that should not be neglected in simple models. In physical terms, this effect is one of resisting the turning of the winds.