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

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~5

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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.