Download North Atlantic Climate Change as a Response to Indian Ocean

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.
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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.
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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.