Abstract
We have discussed the physical mechanisms through which changes in the boundary forcings of SST, soil moisture, albedo, sea ice, and snow influence the atmospheric circulation. The slowly changing boundary forcings can increase the predictability of monthly means because their effects on quasistationary flow patterns and statistics of synoptic scale disturbances appears to be potentially predictable. Changes in the boundary forcings produce changes in the moisture sources and diabatic heat sources which in turn change the atmospheric circulation. The magnitude and the structure of the atmospheric response due to changes in any boundary forcing depends upon the existence of a suitable large scale flow which can transform the boundary forcing into a three dimensional heat source, which in turn.can change the large scale flow and its stability properties. The structure of the large scale flow also affects the propagation characteristics of the influence which determines whether the effect is local or away from the source.
We have presented results of numerical experiments conducted with the GLAS climate model to determine the sensitivity of the model atmosphere to changes in boundary conditions of SST, soil moisture, and albedo over limited regions. It is found that changes in SST and soil moisture in the tropics produce large changes in the atmospheric circulation and rainfall over the tropics as well as over mid-latitudes. Although the area occupied by the land surfaces is small compared to the ocean surfaces, the fluctuations of soil moisture can be very important because the diabatic heat sources have their maxima over the land, and therefore, even small fluctuations of soil moisture can produce large changes in the total diabatic heating field. The natural variability due to day to day weather fluctuations is very large in the middle latitudes, and therefore, changes in the mid-latitude atmospheric circulation due to changes in the boundary forcings at middle and high latitudes have to be quite large to be significant. It is suggested that large scale persistent anomalies of SST, snow and sea ice, under favorable conditions of large scale flow, can produce significant changes in the mid-latitude atmospheric circulation. It is also likely that time averaged mid-latitude circulation can have additional predictability due to the influence of tropical boundary forcings.
We have also presented observational evidence to show that interannual variability of atmospheric fluctuations is significantly different from the intra-annual variability, and therefore, we conclude that part of the inter-annual variability is due to the influence of boundary forcings. Since the tropical spectra is found to be redder compared to the mid-latitude spectra, the tropical flows may be potentially more predictable.
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Shukla, J. (1984). Predictability of Time Averages: Part II: The Influence of the Boundary Forcings. In: Burridge, D.M., Källén, E. (eds) Problems and Prospects in Long and Medium Range Weather Forecasting. Topics in Atmospheric and Oceanographic Sciences. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-82132-5_6
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DOI: https://doi.org/10.1007/978-3-642-82132-5_6
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