August-September 2012

Slides for Met Office Seasonal Forecast Discussion

20 September 2012

Mike Blackburn

National Centre for Atmospheric Science, University of Reading

Met Office Seasonal Forecast Discussion, 20 September 2012

Recent circulation

• Circulation change at beginning of September (N.Hem, inc. Arctic)

• Atlantic jet moved north – dry here, but not the forecast blocking signal?

• Moderate MJO activity but strong S. American descent anomaly in September (expected lack of OLR signal for descent)

• c.f. Autumn 2000 - possible origin of N. Atlantic wavetrain

Met Office Seasonal Forecast Discussion, 20 September 2012

Asian monsoon 2012

IITM, Monsoon Online

All India rainfall

• Indian monsoon recovery since mid-August

Asian monsoon 2007-2012

NOAA ESRL + SOEST, Hawaii

Streamfn, σ=0.210 JJA 2007-2012

• Weak monsoon anticyclone in recent summers

• Impact of La Niña bias?

MJO propagation - moderate amplitude continues

CAWCR & CPC

Last 90 days ECMWF forecast

Tropical convection – recent S. American descent

NOAA ESRL & CPC

1-15 September 2012

16-31 August 2012

Descent over S. America + Caribbean

NOAA ESRL (NCEP/NCAR Reanalysis)

Velocity potential anomaly, σ=0.210

NH circulation change, Aug-Sept

NOAA CPC & ESRL

Aug

July

Z250, 45°-60°N

Sep

t

Streamfn. σ=0.210 16-31 Aug

Streamfn. σ=0.210 1-15 Sept

NH circulation change, Aug-Sept

NOAA CPC & ESRL

Aug

July

Z250, 45°-60°N

Sep

t

Streamfn. σ=0.210 16-31 Aug

Streamfn. σ=0.210 1-15 Sept

Jun-Aug

Northern hemisphere jet stream 250hPa

NOAA ESRL

Climatology: 1981-2010

Sept Oct-Nov

Jun to Aug: Climatology Oct to Nov: ClimatologySep: Climatology

Jun-Aug

Northern hemisphere jet stream 250hPa

NOAA ESRL

2007-2011

Sept Oct-Nov

2007-2012 2007-2011

Southward shift Northward shiftNorthward shift

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