Planetary Wave Climatology Experiments

Abstract

In order to determine cLimatic evoLution on a seasonaL time scaLe, an understanding of the dynamics of the pLanetary waves is essentiaL. At Exeter research is centered on using G.C.M.'s to investigate the factors infLuencing the phasing and ampLitudes of these waves. There are two Lines of approach in appLying G.C.M.'s to cLimate study. The first is to use a highLy detaiLed modeL in which the effect of anomaLies (e~g. SST anomaLies) as the net resuLt of the compLex feedback mechanisms within the modeL are evaLuated. WhiLe this method is satisfying from the point of view that LittLe is omitted from the modeL which ~muLates the reaL atmosphere to a cLose approximation, it has the drawbacks of being expensive in computer time and it can be difficuLt to perceive the mechanisms behind the resuLtant teLeconnections. The second approach, which is adopted here, is to use a simpLified G.C.M. in which the physics are highLy parameterised and examine how various forcings act both separateLy and together.RecentLy our interest has focussed on the investigation of atmospheric modes. A mode here is taken to mean a cLimatic regime in which the pLanetary waves (wave numbers 1 ~ 4) show persistence to systematic evoLution over an extended period of time. The muLtipLe equiLibria soLutions for barotropic fLow demonstrate that the simpLified atmospheric equations possess mode type soLutions (Charney and Devoren 1979) and for the reaL atmosphere, there is some suggestion that mode type behaviour exists during the winter evoLution; it has been found that the orientation of the 200 mb. October-November circum-polar vortex during the spin up period can be used as a reasonable indicator for the severity of the subsequent winter (Reeve, 1982, Murdoch andDavies, 1983). The orientation condenses, in an empiricaL manner, the information concerning the phasing of the pLanetary waves. Given that Less simpLified systems than the barotropic wiLL aLso possess mode type soLutions and that the reaL atmosphere does, the question arises, what are the determinants of a particuLar mode ?It is possibLe that knowLedge of past cLimate can heLp to answer this question. Different modes have existed on MiLankovitch time scaLes where the soLar insoLation and aLso, presumabLy, mean gLobaL temperature were different. What might be more usefuL to aid the understanding of the present cLimate wouLd be a knowLedge of the gLobaL cLimate within the Last thousand uears when the externaL forcings remain approximateLy constant but the internaL distributions of anomaLies are different from today. Thus the summers and winters of the LittLe Optimum and LittLe Ice Age, for exampLe, are of interest since they may represent extreme soLutions for the present set of cLimatic parameters.In order to investigate the determinants of the soLutions for the present climate parameters, several numerical experiments have been undertaken using a two-LeveL quasi geostrophic, U co-ordinate hemispheric spectraL modeL. The aim of the experiments described beLow has been to examine how the or...