1. SAIC

On the Nature of the Recent Southwestern Drought

Philip Pegion1, Siegfried Schubert, Randy Koster, Max Suarez.

NASA-GSFC Global Modeling and Assimilation Office

1. SAIC

Characteristics of recent drought

MotivationWe want to understand the cause of the recent drought, and can we

determine if it has ended?

•Started with La Nina in 1999

•5 years in a row of below normal precipitation

•2002 was 4th driest year on record, the region received only 70% of normal precipitation, and in 2003, the region only received 83%.

•Driest period since the 1950s.

•Persisted past weak El-Nino in 2002

YEAR

Desert SW. region

25-month running mean anomaly (mm/day)

YEAR

Ano

mal

ym

m/m

onth

Observed Precipitation Desert SW region.

YEAR

Ano

mal

ym

m/m

onth

Observed Precipitation Desert SW region.

25-month running mean anomaly (mm/day)

Observed Precipitation: % of normal

HADISST Sea Surface Temperature anomaly (K)

YEAR

Ano

mal

ym

m/m

onth

ENSO

2nd EOF: HADISST

Correlation: 0.51

Observed Precipitation Desert SW region.

25-month running mean anomaly (mm/day)

• NASA Seasonal-to-Interannual Prediction Project (NSIPP-1) AGCM

– 4th Order finite difference dynamical core (Suarez and Takacs, 1995). – Simple K-scheme boundary layer (Louis et al., 1982). – Relaxed Arakawa-Schubert (RAS) convection (Moorthi and Suarez, 1992). – Chou and Suarez (1994, 2000) radiation. – Mosaic land model (Koster and Suarez, 1996).

• AMIP

– 3.75o longitude, 3o latitude, 34 levels– 14 member ensemble (1902-present)– Additional AMIP runs at 2o and 1o degree.

• DSP– 0.625o longitude, 0.5o latitude, 34 levels– Initialized December 1, 2001 with the NCEP Reanalysis.

Model Description

Precipitation Desert SW region.

25-month running mean anomaly (mm/day)

Precipitation Desert SW region (1998-2004)

7-month running mean anomaly (% of normal)

CMAP

Model ensemble mean

October 2001 -March 2002Precipitation (% from normal)

Surface Air Temperature Anomaly (K)

October 2001 -March 2002

200 mb Height Anomaly (m)

October 2001 -March 2002

200 mb Height Anomaly (m)

October 2001 -March 2002

Model La NINA Composite

NCEP La NINA Composite

850 mb v’q’ Anomaly (m s-1 g Kg-1)

October 2001 -March 2002

High Resolution GCM runs

•Initialized 00z1DEC2001 with the NCEP/NCAR reanalysis.

•Run through end of March.

•10 ensemble member run over Reynolds SST

•10 additional ensemble members with MODIS SST.

DJFM 2002: SST anomaly (K)MODIS

MODIS-Reynolds

Reynolds

DJFM 2002: Precipitation Anomaly (mm/day)

0.5deg DSP w/ MODIS SST

CMAP

0.5deg DSP w/ Reynolds SST

MODIS - Reynolds

DJFM 2002: Precipitation (% of normal)

0.5deg DSP w/ MODIS SST

CMAP

0.5deg DSP w/ Reynolds SST

MODIS - Reynolds

DJFM 2002: 200 mb Height Anomaly (m)

0.5deg DSP w/ MODIS SST

NCEP/NCAR Reanalysis

0.5deg DSP w/ Reynolds SST

MODIS - Reynolds

DJFM 2002: Precipitation (% of normal)

NCEP/NCAR Reanalysis

0.5deg DSP w/ Reynolds SST

Ensemble 1 MODIS SST Ensemble 2 MODIS SST

Ensemble 2 Reynolds SST

Has the long term drought ended, or is last winter just a bump in the time series?

Annual Mean SST Anomaly (K)

Annual Mean SST Anomaly (K)

12 month coupled model forecast

Lake PowellConclusions

•Recent drought was of the most severe and longest lasting in recent times.

•Start of drought is linked with changes in Pacific Ocean SSTs

•Most severe part of drought, 2002, lies outside of the envelope of response in the model.

•Some hi-resolution runs do get the correct magnitude of the winter drought.

•Last winter rains provided big relief to the drought, but the southwest hasn’t recovered yet.

Surf

ace

Elev

atio

n

Year

JFM 2002: 200 mb Height Anomaly (m)

DJFM 2002: 200 mb Height Anomaly (m)

DJFM 2002: 200 mb Height Anomaly (m)

Precipitation Desert SW region (1998-2004)

7-month running mean anomaly (mm/day)

CMAP

Model ensemble mean

Precipitation Desert SW region (1998-2004)

7-month running mean anomaly (normalized)

CMAP

Model ensemble mean