The Bering Strait and the Southern Ocean Winds' Grip on the World Climate

• Other Authors: De Boer, Agatha M. (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Oceanography
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-0336

The Bering Strait's Grip On The World Climate: The Holocene interglacial period of the last 10,000 years and the penultimate interglacial ~125,000 years ago have been characterized by distinctly stable climates. During the intervening glacial period, climate records are marked by rapid large-amplitude oscillations, general known as Dansgaard-Oeschger events. These millennial-scale cycles are generally believed to be a result of freshwater anomalies in the North-Atlantic, followed by a reorganization of the thermohaline circulation. Here, we propose that such long lasting instabilities in the thermohaline circulation are only possible during glacial periods when the Bering Strait (BS) is closed. A semi-global analytical ocean model (which includes both wind and thermohaline processes) is used to show that, during interglacial periods (when the BS is open) perturbations in North Atlantic Deep Water (NADW) formation are rapidly damped out because of a novel BS freshwater feedback mechanism. This new feedback mechanism is due to the strong winds in the Southern Ocean (SO) which, with an open BS, quickly [O(10)years)] flush any low salinity anomalies out of the Atlantic and into the Pacific Ocean. During glacial periods, the stabilizing feedback is prevented by the closure of the BS which traps the anomalies within the Atlantic, causing long lasting perturbations. The sensitivity of the mean stable state to steady changes in the external forcing, namely the wind or the precipitation field, is also tested. A relevant example is a prolonged increase in precipitation due to anthropogenic warming, (predicted by global circulation models). We find that both stronger winds (especially the SO Winds) and a decrease in precipitation over the North Atlantic (NA) will lead to a new (stable) enhanced overturning. Conversely, weaker winds or increased precipitation will reduce the overturning to a slower stable state. The Island Wind-Bouyancy Paradox: In reent years, a variety of studies have suggested that the meridional overturning circulation is at least partially controlled by the Southern Ocean winds. The paraadoxical implication is that a link exists between the surface bouyancy flux to the ocean (which is needed for the density transfor4matio between surface and deep water) and the wind. These forcings have traditionally been viewed as independent drivers of the ocean circulation. Here, the paradox is formally stated in the framework of a gigantic island that lies between latitude bands free of continents (such as the land mass of the Americas). The choice of such an island on a sphere was made because it enables one to obtain analytical solutions and it circumvents the need to calculate the torque exerted on zonal sills adjacent to the island tips (e.g., the Bering Strait). The torque calculation is notoriously difficult and is avoided here by the clockwise integration which goes twice through the western boundary of the island (in opposite directions) eliminating any unknown pressure torques. The derived wind-driven overturning is shown to be consistent with Godfrey's Island Rule when the rule is extended to include the sinking or upwelling adjacent to the island. In addition, the consideration of vertical exchange in the Island Rule eliminates the need to make the level-of-no-motion assumption. The paradox is resolved quanlitatively, using salinity and temperature mixed dynamical-box models and a temperature slab model, and quantitatively, employing a numerical model. We show that in all cases the ocean stratification and thermocline depth adjust themselves to allow the overturning imposed by the wind. The salinity and temperature box model suggests that stronger southern winds will tend to weaken the virtical and horizontal salinity stratfication so that it is esier for the conversion of deep to surface water (and vice versa) to take place. A temperature slab model (i.e., y-dependent)offers a more detailed picture;stronger southern winds flatten the meridional temperature profile and shift it northwards (so that it lags the atmospheric temperature). The (process orientated) numericl model is adapted to include a thermodynamic parameterization for the surface heat and freshwater fluxes. In response to stronger southern winds, the thermocline thickens in the north, releases more heat to the atmosphere and, therefore, converts more surface to deep water. === A Dissertation submitted to the Department of Oceanography in partial fulfillment of the degree of Doctor of Philosophy. === Degree Awarded: Spring Semester, 2003. === Date of Defense: January 7, 2003. === Southern Ocean Winds', World Climate === Includes bibliographical references. === Doron Nof, Professor Directing Dissertation; Christopher Hunter, Committee Member; William Burnett, Committee Member; Alan J. Clarke, Committee Member; Kevin Speer, Committee Member; Georges L. Weatherly, Committee Member.