| Local mean weather conditions in many regions of the world crucially depend on the direction of the prevailing winds. In wintertime Europe for instance, westerly winds bring mild winterdays as maritime air is advected over the continent. Severe cold weather is brought by the relatively infrequent winds from easterly directions, advecting cold and dry continental air to the area. The prevailing wind direction is connected to the large-scale structure of the mean winter circulation. In the extratropical Northern hemisphere it is characterised by a belt of westerly winds, the jetstream, with a wavy structure, referred to as the stationary planetary waves. The phase of the stationary waves is such that a ridge over Europe leads to a preference for southwestely winds in most parts of Western Europe. Around this mean circulation, variations take place with distinct spatial structures. A well-known variation of the jetstream is the North Atlantic Oscillation (NAO). The NAO describes changes in the strength and position of the jetstream in the North Atlantic area. In the last decades a trend to stronger positive values of the NAO has contributed significantly to the wintertime warming that has been observed over Europe. A less known variation of the jetstream is a structure of wave number five, spanning the whole Northern hemisphere. It is a Rossby wave that travels along the mean jetstream axis which acts as a waveguide. This variation is therefore referred to as the Circumglobal Waveguide Pattern (CWP). The diagnostics and dynamics ofthe CWP is the subject of this study. Atmospheric disturbances in certain regions can affect the atmospheric circulation in remote places through the CWP. Especially changes in tropical heating associated with changes in tropical precipitation are able to excite a CWP response in the extra-tropics, affecting weather conditions all around the Northern hemisphere. Over the North Atlantic region, the changes in the atmospheric circulation associated with the CWP resemble the NAO. It is not clear at present whether the NAO and CWP are physically linked or whether they just happen to have similar structure in the North Atlantic. In this study we will investigate the role of the CWP in the response of the atmosphere to changes in tropical precipitation, assess the contribution of the CWP to variations in the European climate and investigate the relation between the CWP and the NAO. Diagnostic analyses of climate fluctuations in the recent past will be made using the ERA40 dataset. |
