American Geophysical Union Become an AGU Member
Subscribe to AGU Journals		 AGU Home AGU Publications

Read Full Article (file size: 2309105 bytes)    Cited by

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, C09038, doi:10.1029/2005JC002964, 2006

Two-dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large-scale deformation

T. Nagai

Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA


A. Tandon

Department of Physics and School for Marine Science and Technology, University of Massachusetts Dartmouth, North Dartmouth, Massachusetts, USA


D. L. Rudnick

Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA


Abstract

Ageostrophic secondary circulation (ASC) in frontal data sets have been previously diagnosed using only adiabatic versions of Omega equations. This study includes the effect of vertical mixing on such diagnostics. Modified quasi-geostrophic and semigeostrophic balance equations are examined for the existing Azores front data sets and idealized fronts in the presence of diabatic vertical mixing as well as a large-scale deformation field. The relevant dimensionless parameter representing the relative importance of large-scale deformation and vertical mixing is the inverse of the Ekman number based on the deformation rate (αH 2/A o ), where α is large-scale deformation rate, H is vertical scale of the front, and A o is the maximum vertical eddy viscosity; when this parameter is smaller than O(102), the influence of vertical mixing may be important. Other relevant parameters include Burger number, Bu, and Prandtl number, Pr. The vertical velocity in ASCs caused by diabatic mixing in all diagnostics solutions except for large Bu cases shows good agreement with the Garrett-Loder scaling. The structure of ASCs is found to be sensitive to Pr. The diagnostics of ASCs for the Azores front data set indicates that the vertical mixing intensifies the ASC in the upper ocean (∼100 m) and should not be neglected, when the large-scale deformation rate, α, is O(10−6 s−1) or smaller. The magnitude of vertical velocity in ASCs due to mixing contributions reaches ∼O(10 m d−1) with downwelling on the dense side and upwelling on the less dense side of Azores front.

Received 21 March 2005; accepted 18 May 2006; published 28 September 2006.

Keywords: oceanic fronts; ageostrophic secondary circulation; vertical mxing.

Index Terms: 4528 Oceanography: Physical: Fronts and jets; 4520 Oceanography: Physical: Eddies and mesoscale processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes (4490); 4279 Oceanography: General: Upwelling and convergences (4964); 4572 Oceanography: Physical: Upper ocean and mixed layer processes.

Read Full Article (file size: 2309105 bytes)    Cited by

Citation: Nagai, T., A. Tandon, and D. L. Rudnick (2006), Two-dimensional ageostrophic secondary circulation at ocean fronts due to vertical mixing and large-scale deformation, J. Geophys. Res., 111, C09038, doi:10.1029/2005JC002964.