The Atlantic ITCZ and related atmospheric features: a model-data comparison

Yochanan Kushnir, LDEO, Columbia University

John Chiang, UC Berkeley

Data and Models• Observations:

– SST climatology: Reynolds and Smith V2

– Surface wind stress and SST anomalies: NCEP/NCAR CDAS1(1979-2001)

– Precipitation: GPCP 1979-2001

• CCM3:

– “AMIP” runs: CCM3.10 (T42/L18) forced with HadISST 1.1

• ECHAM:– “AMIP” runs: ECHAM 4.5 (T42/L19) forced with NCEP SST

Climatology of SST & PrecipProjection on SST PC 1 Projection on SST PC 2

Climatology: Observations & CCM3Projection on SST PC 1 Projection SST PC 2

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SC

CM

3

Climatology: Observations & CCM3-OBSProjection on SST PC 1 Projection SST PC 2

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SC

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3-O

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Atlantic ITCZ Rainfall: Mean & VariabilityMiddle Basin 35°-15°W Eastern Basin 15°W-10°E

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3

Climatological annual cycle and 1st EOF of interannual variability (1979-2001)

SWCF-precip relationship, SST > 27C ERBE/CMAP and CCM3

(following C. Bretherton)

ERBE/CMAP CCM3

Total CF-precip relationship, SST > 27CERBE/CMAP and CCM3

ERBE/CMAP CCM3

A SWCF ‘fix’ for CCM3

• Ad-hoc change to CCM3 ice and liquid water scattering particle sizes from 20S-20N

• Resulting SWCF-precip and Total CF-precip relationship resembles ERBE-CMAP

• TOA global radiative balance approximately preserved

Precip vs. SWCF

Precip vs. total CF

CCM3 ‘fix’ - control: projection on the SST mean

Control CCM3 - ObsCCM3 ‘fix’ - control

CCM3‘fix’ - control: projection on the SST first harmonic

Control CCM3 - ObsCCM3 ‘fix’ - control

Climatology: OBS & ECHAM4.5-OBSProjection on SST PC 1 Projection SST PC 2

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SC

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3-E

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AM

Atlantic ITCZ Rainfall: Mean & VariabilityMiddle Basin 35°-15°W Eastern Basin 15°W-10°E

OB

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Climatological annual cycle and 1st EOF of interannual variability (1979-2001)

EC

HA

M

CF-precip relationships for ECHAM 4.5 (SST > 27C)

Pcp vs SWCF Pcp vs LWCF Pcp vs total CF

Summary

• CCM3 exhibits substantial biases in precipitation over the Atlantic ITCZ and neighboring land regions in both the annual mean and seasonal cycle

• An ad-hoc adjustment to the CCM3 SWCF-precipitation relationship to match the observed appears to decrease the precipitation biases

• ECHAM 4.5 biases appear generally smaller than CCM3, and it exhibits better CF-precip relationship

The following are extra slides

SWCF-precip relationship - ERBE/CMAP and CCM3, land regions 20S-20N

ERBE/CMAP CCM3

Total CF-precip relationship - ERBE/CMAP and CCM3, land regions 20S-20N

ERBE/CMAP CCM3