Open AccessOn the Influence of Buoyancy Fluxes on Wind Drift Currents
Author(s) -
Signild Nerheim,
Anders Stigebrandt
Publication year - 2006
Publication title -
journal of physical oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.706
H-Index - 143
eISSN - 1520-0485
pISSN - 0022-3670
DOI - 10.1175/jpo2928.1
Subject(s) - buoyancy , mixed layer , wind stress , ekman layer , stratification (seeds) , convection , geology , mechanics , buoy , atmospheric sciences , meteorology , physics , climatology , boundary layer , oceanography , seed dormancy , germination , botany , dormancy , biology
Data from a moored buoy in the Baltic proper have been analyzed to study the ageostrophic wind-driven (Ekman) transport accounting for buoyancy fluxes in a stratified ocean. A model considering different dynamical regimes governed by wind stress τ, buoyancy fluxes, stratification, and rotation is used to determine the thickness of the mixed layer. For shallow layers in the regime of positive buoyancy fluxes (35% of the time) a transport of about 0.77τ/ρf directed about 30° to the right of the wind is observed. This is far from 1.0τ/ρf and 90°, given by the classical Ekman solution for homogeneous water. The result can be understood qualitatively as caused by drag between the well-mixed surface layer and the underlying layer if the time scale of decay is about 2 h−1. For negative buoyancy fluxes through the sea surface (53% of the time) the mean observed transport was 1.69τ/ρf directed about 60° to the right of the wind. Finite-depth equations for homogeneous water cannot explain this result. No simple explanation of this observational result is offered, but it should be connected to the simultaneously occurring thermohaline convection, which efficiently transmits momentum vertically to the whole mixed layer. The computed mean energy transfer to the wind current is about 12 mW m−2.