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Troposphere-stratosphere connections associated with geostrophic adjustment and wave propagation. Maarten H. P. Ambaum, University of Reading, U.K. Abstract: The Charney and Drazin (1961) paradigm of vertical Rossby wave propagation is still central to our understanding of the influence of tropospheric circulation on the stratospheric circulation. However in a zonally asymmetric world with jets at different latitudes this picture has to be augmented with some bells and whistles. Furthermore, geostrophic adjustment dictates that the stratosphere also modifies the troposphere nearly instantaneously. Theoretical arguments and diagnostics based on reanalyses are presented to illustrate these interactions. It is found that in the Northern Hemisphere the North Atlantic Oscillation (NAO) is the main pattern through which these interactions occur. Furthermore, the possibility of a positive feedback loop between the stratosphere and the troposphere is highlighted. This may be important for long range predictability properties of the NAO. North Atlantic Climate Change as a Response to Indian Ocean Warming Jim Hurrell, National Center for Atmospheric Research, USA Abstract: Results on the origin for recent decadal change in the North Atlantic climate are presented using atmospheric general circulation model simulations. The observed pattern of change has included a poleward shift of the prevailing tropospheric westerlies over the North Atlantic, having a strong projection on the positive phase of the North Atlantic Oscillation. This has been accompanied by an strengthening of the lower stratospherics polar vortex and a cooling of stratospheric temperatures over the Arctic. Several studies have confirmed, using simulations forced by the observed monthly variability of global sea surface temperatures since 1950, that this pattern of change is consistent with a forcing by global oceans. Results of new experiments will be presented that provide additional evidence that the tropical oceans are the primary forcing agent. It will first be demonstrated that North Atlantic climate change since 1950, both in the troposphere and the stratosphere, can be understood as the response to the linear trend of tropical sea temperatures since 1950. It is further shown that the warming trend of the Indian Ocean is of primary importance. The findings point to a strong dynamical feedback in which the progressive warming of waters over the tropical Indian sector has been been a key causal factor in driving a poleward shift of the North Atlantic westerly jet, a poleward shift of the Atlantic storm track and its influence on regional patterns of Eurasian precipitation and temperature change, and a strengthening of the stratospheric polar vortex. Download the PowerPoint presentation. Climate teleconnections in the Euro-Atlantic region David B. Stephenson, Dept. of Meteorology, University of Reading, UK Abstract: This talk will review the major modes of climate variability that influence the Atlantic and European sectors. In addition to well-known teleconnection patterns such as the North Atlantic Oscillation other less discussed teleconnection patterns will also be presented. Links to well-known oceanic patterns such as the Atlantic tripole and the tropical Atlantic dipole will also be covered. Download the PowerPoint presentation Peter deMenocal´s comments on his own presentation: This lecture focuses mainly on the amplitudes and patterns of late Holocene climate change, and the results of comparisons between the high- and low latitudes. The main reason for this is that the Holocene represents the current warm period when climate was roughly similar to the last ~100 years when we've had instrumental records. So, the main rationale for this work is that we can understand the kinds of natural shifts the climate system is capable of. In the end, we find that these shifts are much larger than the instrumental record, they persisted for centuries, and they involved very large areas (at least the whole surface and deep North Atlantic Ocean, subtropics and subpolar regions). These climatic excursions were large relative to the historical changes - ranging from 1-3°C departures from average conditions. In some cases the shifts were very abrupt, occurring within decades. We have for the first time some new evidence that the changes were not in the same direction for all places - there is clear new evidence that the NW Atlantic was WARM during the Little Ice age (1850-1300 AD) - a result that is reminiscent of the North Atlantic Oscillation SST pattern.