2011 - Cooperative Institute for Research in Environmental Sciences ...

Figure 1: Time series of seasonally averaged climate indices representing
(top panel) the tropical Pacific Walker Circulation PWC;
(middle panel) the North Atlantic Oscillation NAO; and (bottom panel)
the Pacific North America pattern PNA. The pink curves are obtained
from the Twentieth Century Reanalysis (20CR) data set, and may be
compared with the curves obtained from other observational data
sets over their periods of overlap. Where the pink curve is not visible, it
is indistinguishable from the curve drawn over it. The grey curve and
grey shading are from an ensemble of 20th century climate model
simulations with indicated uncertainties. The main point of this figure
is that there is no statistically significant trend in these important climate
indices over the full period of record (1871 to 2007) in the 20CR
data set, even though some trends are evident over shorter periods.
From Compo, et al. (2011).
The completion, publication and universal distribution of
the 20CR data set are widely acknowledged major contributions.
It is anticipated that the data set will be a valuable
resource to the climate-research community for model validation
and diagnostic studies. Some surprising features of
the data set are already evident. For instance, the long-term
trends of indices representing the North Atlantic Oscillation,
the tropical Pacific Walker Circulation and the Pacific-North
American pattern are weak or nonexistent over the full
period of record (1871 to the present) in this data set. The
long-term trends of zonally averaged precipitation minus
evaporation also differ in character from those in climate
model simulations of the 20th century. These results have
important implications for how global warming is influencing
atmospheric circulation variability, and to what extent
climate models are correctly representing those influences.
The demonstration of the critical importance of correctly
representing the pattern of tropical oceanic warming in
order to capture regional climate trends around the globe—
and that climate models are significantly deficient in this
regard—is another major contribution of the Center and
has been featured in many invited presentations, as well as
noted in NOAA’s review of ESRL’s Physical Sciences Division
last year.
Figure 2: (Top panel) Zonal mean precipitation rate P (in units of mm/
day) averaged over 1980–2000 from the 20CR data set (black curve),
and from the independent GPCP Version 2 data set (red curve). (Bottom
panel) Zonal Mean precipitation rate P minus the evaporation rate E
averaged over 1980–2000 from the 20CR data set (blue curve), and the
1980–2000 P-E average minus the 1871–1891 P-E average (red curve).
The prevailing theory of the global response of the hydrological cycle to
global warming would predict the red and blue curves to be similar. That
they are not is interesting and important for our understanding of 20th
century climate change. From Compo, et al. (2011).
CIRES Annual Report 2011 65