Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

Baylor Fox-Kemper

Second Advisor

Jeffrey B. Weiss

Third Advisor

Weiqing Han

Fourth Advisor

Gokhan Danabasoglu

Fifth Advisor

Frank O. Bryan

Abstract

The mechanisms associated with sea surface temperature variability in the North Atlantic are explored using observation-based reconstructions of the historical surface states of the atmosphere and ocean as well as simulations run with the Community Earth System Model, version 1 (CESM1). The relationship between air-sea heat flux and SST between 1948 and 2009 yields evidence of a positive heat flux feedback at work in the subpolar gyre region on quasi-decadal timescales. Warming of the high latitude Atlantic precedes an atmospheric response which resembles a negative NAO state. The historical flux data set is used to estimate temporal variations in North Atlantic deep water formation which suggest that NAO variations drove strong decadal changes in thermohaline circulation strength in the last half century. Model simulations corroborate the observation-based inferences that substantial changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) ensued as a result of NAO-driven water mass perturbations, and that changes in the large-scale ocean circulation played a significant role in modulating North Atlantic SST. Surface forcing perturbation experiments show that the simulated low-frequency AMOC variability is mainly driven by turbulent buoyancy forcing over the Labrador Sea region, and that the decadal ocean variability, in uncoupled experiments, derives from low-frequency variability in the overlying atmospheric state. Surface momentum forcing accounts for most of the interannual variability in AMOC at all latitudes, and also most of the decadal AMOC variability south of the Equator. We show that the latter relates to the trend in wind stress forcing of the Southern Ocean, but that Southern Ocean forcing explains very little of the North Atlantic signal. The sea surface height in the Labrador Sea is identified as a strongly buoyancy-forced observable which supports its use as a monitor of AMOC strength. The dynamics which characterize the model mean overturning and gyre circulations, and which explain the model response to surface momentum and buoyancy forcing perturbations, are investigated in terms of mean and time-varying vorticity balances. The significant effect of bottom vortex stretching, noted in previous studies, is shown here to play a key role in a variety of time-dependent phenomena, such as the covariation of overturning and gyre circulations, the variation of the barotropic streamfunction in the intergyre-gyre region, and changes in AMOC associated with momentum forcing perturbations. We show that latitudinal changes in the AMOC vorticity balance explains the attenuation of buoyancy-forced signals south of Cape Hatteras, and that the dominant frictional balance near the Equator greatly inhibits the propagation of AMOC variability signals from one hemisphere to the other. The long persistence of buoyancy-forced, high-latitude circulation anomalies results in significant predictability of SST in the subpolar gyre. This is demonstrated with an analysis of initialized, fully coupled retrospective predictions of the mid-1990s warming in that region. The atmospheric response is shown to be relatively unimportant on timescales of up to 10 years, but skill for longer lead times is inhibited by an incorrect heat flux feedback in the North Atlantic in the coupled CESM1.

Share

COinS