The roles of mixing, geothermal heating, and surface buoyancy forcing in ocean meridional overturning dynamics

The dynamics of the oceanic large-scale meridional overturning circulation are investigated through a series of numerical experiments using an idealized single-hemisphere general circulation model. In addition to the system's scaling behavior, the consequences of diapycnal mixing location, the impact of deep buoyancy fluxes, and the importance of the surface restoring timescale are considered. As required by advective-diffusive balance, upwelling across isopycnals in low latitudes occurs where diapycnal mixing is specified. Downward mass transport into the abyss is relatively buoyant; the abyssal heat budget is such that this flow is subsequently cooled through deep convective mixing and re-warmed by diapycnal heat fluxes. Thus, mixing below the thermocline affects the abyssal stratification and upwelling profile, but does not contribute significantly to the zonally averaged circulation through the thermocline or the meridional oceanic heat transport. Boundary mixing is more efficient than interior mixing at driving the meridional overturning circulation; with interior mixing, the planetary vorticity constraint interferes with the communication of interior water mass properties and the eastern boundary. The results are consistent with thermodynamic considerations that suggest the strength of the overturning is a function of the vertical heat fluxes through the thermocline. Accordingly, diapycnal mixing must result in surface heat input to influence the portion of large scale overturning that effects the meridional heat transport. When a buoyancy flux (e.g., geothermal heating) is applied to the ocean floor, a perturbation deep meridional overturning cell on the order of several Sv is produced. The surface flow is also perturbed at high latitudes, allowing the additional heat to be released to the atmosphere. Rising motion is concentrated near the equator. The upward penetration of the deep cell is limited by the thermocline, analogous to the role of the stratosphere in limiting the upward penetration of convective plumes in the atmosphere. The magnitude of the advective response is inversely proportional to the deep stratification; with a weaker meridional overturning circulation and hence a less stratified abyss, the overturning maximum of the deep cell is increased. These results suggest that geothermal heat fluxes, typically ignored in general circulation models, might play a more significant role than thought in the determining the abyssal circulation. For the lowest two decades of changes to diapycnal mixing diffusivity (K), the system's response is largely "self-similar", but experiences a transition to a different regime at very high values of diffusivity. The maximum in overturning circulation obeys an approximate 2/3 power scaling law over both regimes. In contrast, given changes in the imposed equator-to-pole temperature difference AT, the behavior is not self-similar except in the meridional pro.- file of surface heat exchange. Moreover, the power law scaling of overturning with AT is similar to that of K, in contradiction with the 1/3 law predicted by scaling arguments and the Marotzke (1997) theory. The ocean's dynamical behavior is also strongly influenced by the restoring timescale at which the surface temperature is restored; with weaker restoring, the deep sinking region of the ocean becomes more narrow in the zonal mean, and the maximum in meridional heat flux declines even though the maximum in overturning remains nearly constant. These results are interpreted by considering the fundamental thermodynamics of the system.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2000

Author dc:creator

• Scott, Jeffery R

Advisor dc:contributor.advisor

• Jochem Marotzke.

• Earth, Atmospheric, and Planetary Sciences.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/59091

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/59091