Tim Palmer

Abstract:

Results from four pairs of 50‐day wintertime integrations of the Meteorological Office's 5‐level general circulation model, with warm and cold sea surface temperature (s.s.t.) anomalies of about 3 K in the northwest Atlantic, are described. Difference fields between the warm and cold integrations are statistically significant at the 1% level with positive geopotential height over the central north Atlantic, and weaker negative height over Europe. the storm track over the Atlantic is displaced from its normal position. Results from four further pairs of integrations with halved s.s.t. anomalies are also described. the response is approximately linear, with systematic differences in 500 mb geopotential height over the Atlantic, parts of which are just significant at the 10% level with half the full s.s.t. anomaly. Overall, however, the model's response is weaker than could be obtained with tropical s.s.t. anomalies of the same magnitude. Results from the model integrations are compared with results from an observational study of the relationship between wintertime s.s.t. in the north‐west Atlantic, and mean sea level pressure and 500mb height. Two independent 30‐year periods were chosen for study, thus minimizing the influence of long‐term trends in s.s.t. Over the Atlantic and Europe the model results compared well with the observations. With s.s.t. data lagging the atmosphere by one month, the observational study appears to show that the s.s.t. anomalies are initially forced by perturbations in the atmospheric circulation. With s.s.t. data leading the atmosphere by one month results show that atmospheric and s.s.t. anomalies are most persistent in the period October to December. Throughout the winter these lagged relationships are much weaker and not statistically significant. Diagnostics of E‐vector divergence from the GCM experiments are used to suggest that anomalous baroclinic wave activity over the Atlantic is important as a momentum forcing for the anomalous time‐mean flow pattern. On the other hand, the role of thermal forcing, provided by anomalous diabatic heating and transient eddy heat flux convergence, may be important. to substantiate this statement, a simple linear steadystate two‐layer model of the response to extratropical thermal forcing is described. With a suitable basic state flow, and a mid tropospheric heat source (given mainly by the transient eddy heat flux convergence), the response is shown to be equivalent barotropic with a downstream ridge and ascent over the thermal source. Conversely, results from an ocean mixed layer model are discussed which suggest that warm s.s.t. anomalies could be maintained by a positive surface pressure perturbation positioned downstream of the anomaly, through anomalous northward advection of warm ocean water by Ekman drift currents. This northward advection would balance the sensible and latent heat loss into the atmosphere over the s.s.t. anomaly. Hence it is possible that some positive ocean‐atmosphere feedback may account for the persistence of such atmospheric and oceanic anomalies. Copyright © 1985 Royal Meteorological Society

Abstract:

A number of tropical Pacific SST anomaly experiments, run on the UK Meteorological Office 11-level general circulation model in perpetual January mode, are described. It is found that the model's extratropical response can be statistically significant as far downstream as the European continent depending on the anomaly used, but is sensitive to the specification of orography in the model. A realistic response to a composite El Niño SST anomaly is obtained in the extratropics provided envelope orography replaces the standard orographic specification. A negative East Pacific anomaly run revealed that aspects of the tropical response are not readily explained by linear theory and some possible reasons for this are proposed. The extratropical response appears in some experiments to be qualitatively consistent with a downstream Rossby wavetrain and this has been tested using a baro-tropic model. In other experiments, and in the Southern Hemisphere, this is less clear. The extent to which the extratropical response is maintained by cyclogenesis in mid-latitudes remains to be firmly established though preliminary results suggest it is important. The extratropical response is sensitive to relatively weak SST anomalies in the tropical West Pacific. © 1985, Elsevier B.V. All rights reserved.

Abstract:

We present several atmospheric general circulation experiments which model the atmospheric response to SST anomalies in the tropical East and West Pacific. We have found that the extratropical response to a relatively small West Pacific anomaly can be stronger than, and qualitatively different from the response to a much larger East Pacific anomaly. These experiments suggest a possible explanation for the difference in mid-latitude response during the 1976-77 El Nino winter and the El Nino winters of 1972-73 and 1982-83.-from Authors