Climate variability in the North Atlantic on decadal and multi-decadal time scales: A numerical study

The goal of this work is to understand the mechanisms that drive the decadal and multi-decadal climate variability in the North Atlantic. Natural climate variability on these particular time scales occupies a central position in discussions of anthropogenic climate changes, but many aspects related, to this issue are still poorly understood.; The major tool used in this study is a coupled general circulation model consisting of the NCAR CCM3 and the Miami Isopycnic Coordinate Ocean Model. The simulated decadal variability in the North Atlantic is dominated by a tri-pole pattern in sea surface temperature and a North Atlantic Oscillation (NAO) pattern in sea level pressure. The associated oceanic fluctuations are characterized by a delayed subtropical gyre response to anomalou's NAO surface wind stress forcing and advection of SST anomalies originating near the western boundary into the interior ocean. Separate ocean-alone experiments suggest that the SST variability can not be attributed solely to passive response of the ocean to atmospheric thermal forcing.; It is also found that a quasi-oscillatory fluctuation of the thermohaline circulation (THC) in the North Atlantic ocean with an approximate time scale of 30 years is present in the coupled but not in uncoupled simulations. The latter were forced with either Newtonian relaxation boundary conditions (based on a monthly climatology of the atmospheric state variables such as the surface air temperature and surface specific humidity) or with imposed monthly varying heat and fresh water flux conditions.; These results suggest that the variability of the THC in this model is neither an ocean internal phenomenon nor a passive response of the ocean to atmospheric forcing. Rather, it is a coupled process involving both the ocean and the atmosphere. The THC oscillation appears to be driven by surface heat flux forcings while the effects of surface fresh water fluxes are secondary. Two delay effects are crucial for maintaining this oscillation. One is the delay of the North Atlantic overturning strength relative to the deep water formation rate in the deep water production region; the other is the delay of the associated SST and surface heat flux anomalies in the sinking region relative to the overturning amplitude itself.; The sea surface temperature signal associated with the THC oscillation bears some resemblance to an SST interdecadal pattern extracted from observational data (Kushnir, 1993). Accompanying anomalies of northern hemispheric surface air temperature have positive values over the Pacific and Atlantic Oceans and negative values over the Eurasian and North American continents. In addition to anomalies in sea surface temperature and surface air temperature, there are also variations in atmospheric flow pattern over the North Atlantic, namely, an anomalous northerly flow over the Labrador Sea when the THC circulation is strong.