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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.
Copyright 2006 by the American Geophysical Union.
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