Satellite versus surface estimates
of air temperature since 1979.
Hurrell, J. and K. E. Trenberth, 1996
J. Climate, 9, 2222-2232.
A comparison of near-global monthly mean surface temperature anomalies to those
of global Microwave Sounding Unit (MSU) 2R temperatures for 1979-95 reveals
differences in global annual mean trends that are shown to be largely
attributable to important physical differences in the quantities that are
measured. Maps of standard deviations of the monthly mean anomalies, which can
be viewed as mostly measuring the size of the climate signal, reveal pronounced
differences regionally in each dataset. At the surface, the variability of
temperatures is relatively small over the oceans but large over land, whereas
in the MSU record the signal is much more zonally symmetric. The largest
differences are found over the North Pacific and North Atlantic Oceans where
the monthly standard deviations of the MSU temperatures are larger by more
than a factor of 2. Locally over land, the variance of the surface recored is
larger than that of the MSU. In addition to differential responses to
forcings from the El Nino-Southern Oscillation phenomenon and volcanic
eruptions, these characteristics are indicative of differences of the response
to physical processes arising from the relative importance of advection versus
surface interactions and the different heat capacities of land and ocean. The
result is that the regions contributing to hemispheric or global mean
anomalies differ substantially between the two temperature datasets. This
helps to account for the observed differences in decadal trends where the
surface record shows a warming trend since 1979 of 0.8 deg C per decade,
relative to the MSU record. While a common perception from this result is
that the MSU and surface measurements of global temperature change are
inconsistent, the issue should not be about which record is better, but rather
that both give a different perspective on the same events.
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Hongjun Zhang:
zhangho@ucar.edu