interannual variability of north american summer precipitation in nasa/nsipp and ncar/cam2.0 amip...
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Interannual Variability of North American Summer
Precipitation in NASA/NSIPP and NCAR/CAM2.0 AMIP
SimulationsBy
Alfredo Ruiz-Barradas and Sumant Nigam
Department of Meteorology
University of Maryland September 3, 2003
Goal
• To assess interannual variability of precipitation over North America in AMIP-like runs of CAM2.0 and NSIPP models during summer months (June, July, August).
Data
• Precipitation:– Retrospective US and Mexico analysis.– Hulme (University of East Anglia) data set.– Xie/Arkin precipitation data set.
• SST from Hadley Center.• NCAR/NCEP Reanalysis.• AMIP simulation (ensemble no. 5) from the NSIPP
model.• AMIP simulation (case newsstamip06) from the
CAM model.
Method
• Reanalysis and simulations extrapolated to a 5°2.5 grid on 17 pressure levels.
• Monthly climatology for the 1950-1998 period.• Monthly anomalies wrt 1950-1998 climatology.• JJA is the mean of June, July, August.• Assessment through:
– Standard Deviation– Precipitation Index– Multivariate analysis
~12
yea
rs p
eak
~5
year
s pe
ak~
5 ye
ars
peak
Remarks
• Large precipitation variability in observations and simulations over central US. Although it is shifted in simulations.
• Great Plains precipitation indices from simulations do not correlate with the observed index.
• SST regressions on the Great Plains index suggest linkage with Pacific midlatitude variability. CAM however emphasizes the tropical influence.
Remarks
• Multivariate analysis indicates:– Great Plains precipitation variability is the main
mode of summer variability in observations;– This is however not the case in both model
simulations; – Wet/dry events are cold/warm events in both
observed and simulated summers.
Remarks
• PC regressions on moisture fluxes and geopotential heights indicate:– Observed precipitation variability is linked to a
coherent, barotropic circulation that enhances/diminishes southerly stationary moisture flux from the Gulf of Mexico;
– Model simulated variability does not have such circulation linkages.