The influence of sea surface temperature anomalies on low-frequency variability of the North Atlantic Oscillation

The influence of sea surface temperature (SST) anomalies on multi-year persistence of the North Atlantic Oscillation (NAO) during the second half of the 20th century is investigated using the Center for Ocean-Land-Atmosphere Studies (COLA) Atmospheric GCM (AGCM) with an emphasis on isolating the geographic location of the SST anomalies that produce this influence. In contrast to many studies on this subject, modeling work in this thesis is focused on calculating atmospheric response to the SST anomalies averaged over 1988--1995 (1961--1968) corresponding to the empirically derived time period of strong persistence of the positive (negative) phase of the decadal NAO. Atmospheric response to this forcing is estimated by averaging the modeled atmospheric behavior over the record of simulation and comparing it to the control simulation.; Response of the COLA AGCM to the global 1988--1995 SST anomalies shows statistically significant large-scale pattern in the North Atlantic characteristic of the positive phase of the NAO, though weaker in magnitude compared to observations. However, forcing with global 1961--1968 SST anomalies generates a negative NAO, barely significant and with less realistic structure compared to the 1988--1995 forcing. Additional analysis and linearity test reveal that over the centers of the NAO, non-linear component of the forced response is quite important and acts to enhance the positive NAO signal.; Forcing of the COLA AGCM with 1988--1995 SST anomalies restricted to the Tropical Indo-Pacific region produces a positive NAO quite similar to the model response under the global 1988--1995 forcing. Analysis of multiple fields strongly suggests that anomalous boundary forcing leading to the persistence of the NAO in its positive polarity during 1988--1995 comes predominantly from the Tropics of the Indian and Pacific Oceans. 200-hPa meridional wind response in these two experiments shows positive NAO pattern embedded in Rossby wave trains propagating from the western and eastern tropical Pacific.; SST forcing confined to the North Atlantic region does not generate a response statistically different from the control simulation. Results from the North Atlantic experiment suggest that SST forcing is confined primarily to the equatorial region, and is not strong enough to significantly affect the phase of the NAO.