Search Results
Abstract
The advances in our understanding of extratropical atmosphere–ocean interaction over the past decade and a half are examined, focusing on the atmospheric response to sea surface temperature anomalies. The main goal of the paper is to assess what was learned from general circulation model (GCM) experiments over the recent two decades or so. Observational evidence regarding the nature of the interaction and dynamical theory of atmospheric anomalies forced by surface thermal anomalies is reviewed. Three types of GCM experiments used to address this problem are then examined: models with fixed climatological conditions and idealized, stationary SST anomalies; models with seasonally evolving climatology forced with realistic, time-varying SST anomalies; and models coupled to an interactive ocean. From representative recent studies, it is argued that the extratropical atmosphere does respond to changes in underlying SST although the response is small compared to internal (unforced) variability. Two types of interactions govern the response. One is an eddy-mediated process, in which a baroclinic response to thermal forcing induces and combines with changes in the position or strength of the storm tracks. This process can lead to an equivalent barotropic response that feeds back positively on the ocean mixed layer temperature. The other is a linear, thermodynamic interaction in which an equivalent-barotropic low-frequency atmospheric anomaly forces a change in SST and then experiences reduced surface thermal damping due to the SST adjustment. Both processes contribute to an increase in variance and persistence of low-frequency atmospheric anomalies and, in fact, may act together in the natural system.
Abstract
The advances in our understanding of extratropical atmosphere–ocean interaction over the past decade and a half are examined, focusing on the atmospheric response to sea surface temperature anomalies. The main goal of the paper is to assess what was learned from general circulation model (GCM) experiments over the recent two decades or so. Observational evidence regarding the nature of the interaction and dynamical theory of atmospheric anomalies forced by surface thermal anomalies is reviewed. Three types of GCM experiments used to address this problem are then examined: models with fixed climatological conditions and idealized, stationary SST anomalies; models with seasonally evolving climatology forced with realistic, time-varying SST anomalies; and models coupled to an interactive ocean. From representative recent studies, it is argued that the extratropical atmosphere does respond to changes in underlying SST although the response is small compared to internal (unforced) variability. Two types of interactions govern the response. One is an eddy-mediated process, in which a baroclinic response to thermal forcing induces and combines with changes in the position or strength of the storm tracks. This process can lead to an equivalent barotropic response that feeds back positively on the ocean mixed layer temperature. The other is a linear, thermodynamic interaction in which an equivalent-barotropic low-frequency atmospheric anomaly forces a change in SST and then experiences reduced surface thermal damping due to the SST adjustment. Both processes contribute to an increase in variance and persistence of low-frequency atmospheric anomalies and, in fact, may act together in the natural system.
Abstract
Processes in the climate system that can either amplify or dampen the climate response to an external perturbation are referred to as climate feedbacks. Climate sensitivity estimates depend critically on radiative feedbacks associated with water vapor, lapse rate, clouds, snow, and sea ice, and global estimates of these feedbacks differ among general circulation models. By reviewing recent observational, numerical, and theoretical studies, this paper shows that there has been progress since the Third Assessment Report of the Intergovernmental Panel on Climate Change in (i) the understanding of the physical mechanisms involved in these feedbacks, (ii) the interpretation of intermodel differences in global estimates of these feedbacks, and (iii) the development of methodologies of evaluation of these feedbacks (or of some components) using observations. This suggests that continuing developments in climate feedback research will progressively help make it possible to constrain the GCMs’ range of climate feedbacks and climate sensitivity through an ensemble of diagnostics based on physical understanding and observations.
Abstract
Processes in the climate system that can either amplify or dampen the climate response to an external perturbation are referred to as climate feedbacks. Climate sensitivity estimates depend critically on radiative feedbacks associated with water vapor, lapse rate, clouds, snow, and sea ice, and global estimates of these feedbacks differ among general circulation models. By reviewing recent observational, numerical, and theoretical studies, this paper shows that there has been progress since the Third Assessment Report of the Intergovernmental Panel on Climate Change in (i) the understanding of the physical mechanisms involved in these feedbacks, (ii) the interpretation of intermodel differences in global estimates of these feedbacks, and (iii) the development of methodologies of evaluation of these feedbacks (or of some components) using observations. This suggests that continuing developments in climate feedback research will progressively help make it possible to constrain the GCMs’ range of climate feedbacks and climate sensitivity through an ensemble of diagnostics based on physical understanding and observations.
