S. Davis, K. Rosenlof, and P. Young
Tropical widening in reanalyses
Poleward migration of the latitudinal edge of the
tropics of ~0.25 – 3 deg./decade has been reported
in several recent studies based on satellite,
radiosonde, and reanalysis data covering the
past ~30 years. Disagreements between models and
observations have been noted, and to date, it has
been unclear to what extent this large range of
trends can be explained by the use of different data
sources, time periods, and edge definitions.
In this presentation, we address these issues by
applying a suite of tropical edge latitude diagnostics
based on tropopause height, winds,
precipitation/evaporation, and outgoing longwave
radiation (OLR) to six reanalyses and four satellite
data sets. These diagnostics include both previously
used definitions and new definitions designed for more
robust detection. The wide range of widening trends
is shown to be primarily due to the use of different
data sets and edge definitions, and only secondarily
due to varying start/end dates. We also show that
the large trends (> ~1 deg./decade) previously reported
in tropopause and OLR diagnostics are partially due
to the use of subjective definitions based on
absolute thresholds. Statistically significant Hadley
cell expansion based on the mean meridional
streamfunction of ~1.0 deg./decade is present in
all but one reanalysis, whereas other diagnostics
yield trends of -0.5 - 0.8 deg./decade that are mostly
insignificant. These results are compared to coupled
model trends calculated over both the 20th and 21st
centuries.
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