Symmetric and asymmetric modes of the thermohaline circulation

Authors: MOHAMMAD, REZWAN; NILSSON, JOHAN

Source: Tellus A, Volume 58, Number 5, October 2006 , pp. 616-627(12)

Buy & download fulltext article:

Price: $48.00 plus tax (Refund Policy)

Abstract:

On the basis of a zonally averaged two-hemisphere ocean model, this study investigates how the asymmetric thermohaline circulation depends on the equator-to-pole as well as the pole-to-pole density difference. Numerical experiments are conducted with prescribed surface density distributions as well as with mixed boundary conditions. Further, two different representations of the small-scale vertical mixing are considered, viz. constant and stability-dependent vertical diffusivity. The two mixing representations yield the opposite overturning responses when the equator-to-pole density difference is changed, keeping the shape of the surface density field invariant. However, the overturning responses of the two representations are qualitatively similar when the degree of asymmetry of the surface density field is changed, keeping the density difference invariant. This applies essentially when the freshwater forcing is increased for fixed thermal boundary conditions. For a fixed freshwater forcing, on the other hand, an increase of the equator-to-pole temperature difference yields a weaker asymmetric circulation when the stability-dependent diffusivity is employed, whereas the reverse holds true for the constant diffusivity representation. Further, the numerical experiments show that the hysteresis characteristics of the asymmetric thermohaline circulation may be sensitive the nature of the small-scale vertical mixing.

Document Type: Research article

DOI: http://dx.doi.org/10.1111/j.1600-0870.2006.00194.x

Affiliations: 1: Department of Meteorology, Stockholm University, SE-106 91 Stockholm, Sweden

Publication date: 2006-10-01