Predictability of weather and climate

Abstract:

Folland et al.1 have reported that persistently dry and wet periods of several years in the Sahel have been accompanied by global-scale patterns of sea-surface temperature (SST) anomaly. They also demonstrated that the response of a general circulation model (GCM) of the atmosphere to an observed composite SST difference field between a number of such dry and wet periods showed substantial reduction in Sahel rainfall compared with values from a simulation with climatological SSTs. I examine here the same model's response to the individual components of the composite SST difference field in the Atlantic, Pacific and Indian Oceans. It is found that over the western Sahel, the Atlantic and Pacific fields have a comparable effect in reducing rainfall whereas the Indian Ocean field produces a slight enhancement. Results suggest that, over the eastern Sahel, the Indian Ocean has the dominant role in reducing rainfall. © 1986 Nature Publishing Group.

Abstract:

Using the comprehensively quality-controlled Meteorological Office Historical Sea Surface Temperature data set (MOHSST)1,2 we show for the first time that persistently wet and dry periods in the Sahel region of Africa are strongly related to contrasting patterns of sea-surface temperature (SST) anomalies on a near-global scale. The anomalies include relative changes in SST between the hemispheres, on timescales of years to tens of years, which are most pronounced in the Atlantic. Experiments with an 11-level global atmospheric general circulation model (AGCM) support the idea that the worldwide SST anomalies modulate summer Sahel rainfall through changes in tropical atmospheric circulation3-6. El Nino events may also play a part. We do not discount the effects of soil moisture and albedo changes in the Sahel7,8, although Courel et al.9 have questioned the importance of albedo changes, but we do suggest that worldwide SST anomalies may have a more fundamental influence on Sahel rainfall. © 1986 Nature Publishing Group.

Abstract:

From observations and a variety of general circulation modeling evidence, it is suggested that the exceptionally cold weather experienced over much of the US during some recent winter months (eg January 1985, December 1976-February 1977) was associated with enhanced latent heat release over the tropical West Pacific. The mechanism associated with such enhancement may not be unique.- Authors

Abstract:

In this paper the response of an 11‐layer general circulation model (GCM) to an enhanced composite El Niño sea surface temperature (s.s.t.) anomaly is studied. The integrations were run for 540 days in a perpetual January mode, and had either mean or envelope orography. A basic objective of this paper is to study the influence of model climatology on the response to a tropical s.s.t. anomaly. This was done both by comparing our results with those from a lower resolution GCM (the NCAR Community Climate Model) with an identical s.s.t. anomaly, and by performing control and anomaly integrations with both mean and envelope orography. The specification of orography strongly influences the strength of the mean zonal flow, and stationary and transient wave activity in our model. Whereas the runs with envelope orography produced an anomalous Pacific/North American (PNA) teleconnection pattern in the northern hemisphere, the runs with mean orography did not. In the extratropical southern hemisphere. our model produced a statistically significant response which had a substantial zonally symmetric component. By contrast, the NCAR model, with weaker zonal flow, produced a weaker and mainly statistically insignificant response in the southern hemisphere. Finally, in the tropics, over the Central and Eastern Pacific, the anomalous rainfall and vorticity fields correlated strongly with the corresponding fields in the control integration. Overall, therefore, we conclude that a GCM's response is strongly sensitive to the basic model climatology. In trying to understand dynamically the GCM response it was concluded that the PNA pattern may not simply be the result of Rossby wave dispersion from a tropical source. The PNA pattern may also be associated with a combination of two separate influences, the interaction with the anomalous mean flow of enhanced storm track activity over the Pacific, and with orography on the North American continent. The implications of the model's tropical response for simple linear theory were also discussed. In these composite El Niño anomaly experiments enhanced rainfall and upper divergence occurred well to the west of the upper tropical anticyclone pairs, implying that a Sverdrup type balance in the vorticity equation cannot hold. It is suggested that the strength of the anomalous vortex stretching term could be a small residual compared with the horizontal advection terms, and that a simple linear relationship between anomalous latent heating and the position and strength of the anomalous anticyclone pairs is not always valid. An alternative qualitative explanation for the relative position of these anticyclone pairs is given in terms of a reduction in the zonally asymmetric component of upper tropospheric vorticity across the tropical Pacific, whilst the position of the rainfall maximum is located close to the s.s.t. maximum. Copyright © 1986 Royal Meteorological Society

Abstract:

The use of an ensemble of integrations for long-range prediction has been studied with a hemispheric version of the Meteorological Office 5-level general circulation model. Some results, showing the potential of the technique, are described. The method is now being used with the global 11-level model to produce real-time long- range forecasts for the long-range forecasting conference in the Synoptic Climatology Branch of the Meteorological Office. Results from the first of these real-time ensemble forecasts are discussed. -Authors