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Northwest European High Summer Climate Variability, the West African Monsoon and the
Summer North Atlantic Oscillation
Jim Hurrell, NCAR, & Chris Folland, Hadley Centre, Met Office, UK
• Central England temperature and atmospheric circulation
• Atmospheric circulation changes
• Links to the Sahel
• “Optimum Detection” of the summer NAO in several models
• SST forcing v internal variability
• Conclusions
C20C WORKSHOP, TRIESTE, APRIL 19-23 2004
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Early CLIVAR Exchanges ArticleHurrell and Folland, CLIVAR EXCHANGES, Sept 2002, 0052-54
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Central England Temperature (1950-2002)
JA
1.3 ± 0.9°C
1.6 ± 1.1°C
JFM
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Central England Temperature (1950-2002)
JA
JFM
1.6 ± 1.1°C
1.3 ± 0.9°C
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SLP regressed on CET (1950-2002)
JFM
JAhPa
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First observed rotated EOF of extratropical NH sea level Pressure, July-August. First noticed by Barnston and Livezey (1987)
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Pressure at mean sea level in July and August, 30o W-30oE, 40oN-70oN, 1899-2003
Often cyclonic and wet
Often dry and settled
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July and August Rainfall 1967-1998as a Percentage of 1921-1960
Surface Pressure Difference
July and August 1967-1998
relative to 1921-1960. Stars
locally significant at 5% levelWeather over the UK
became more settled anddrier in high summer in
recent decades
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Mean Storm Track (July-August) 1948-2002(m)
Change (1967-2002) — (1948-1966) (m)
300 hPa rmstransient height(2-8 day bp)
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North west Scotland in winter follows the winter NAOSouth east England in summer follows the summer NAO
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North East Atlantic SLP(—) Sahel Rainfall
r = 0.55
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Anomalous Rainfall (July-August) 1967-2000Percentage of 1921-1960 Mean Rainfall
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Sahel Rainfall Interhemispheric SST Contrast
r = 0.59
NH Cold
NH Warm
Updated from Folland et al Nature, 1986
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Interhemispheric contrast is related in part to extratropical Atlantic temperature
and varying heat transport from tropical regions
Extratropical North Atlantic currently warming quite fast.
2003 warmest year on record
Experiments with HadCM3 suggest that the recent
warming is partly related to recent stronger heat flow from the tropics
wthin the Atlantic. Part is global
warming.
From Knight et al, in review
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First optimum rotated SST forced EOFC20C runs, 1950-1999, 6 members
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Time series of SST forced summer NAO, 1950-99
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First noise EOF of C20C runs is also the summer NAO. Thus it is very likely an internal mode
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First observed EOF of extratropical NH sea level pressure, July-August is essentially the first noise and first global SST
forced EOFs of HadAM3
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ECHAM4 (23 member ensemble) 1950-1999
Optimal Detection
Leading Mode of SST-forced MSLP Variability (JA)
hPa
Global SST Forcing
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Total Precipitation regressed on NE Atlantic SLP (ECHAM4)mm day-1JA
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Optimal Detection
Rainfall Associated with Leading Mode (JA)
mm day-1
ECHAM4 (23 member ensemble) 1950-1999
Global SST Forcing
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Sahel Rainfall, 1950-2002, C20C runs, ensemble of 6
HadAM3 with all forcings, including changing land surface.
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ARPEGE(v3) (6 member ensemble) 1950-1999
Optimal Detection
Leading Mode of SST-forced MSLP Variability (JA)
hPa
Atlantic SST Forcing
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CCM(v3) (12 member ensemble) 1950-1994
Optimal Detection
Leading Mode of SST-forced MSLP Variability (JA)
hPa
Tropical Atlantic SST Forcing
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Atlantic forcing at 500hPa from Rodwell and Folland, (2003). Uses lagged maximum covariance analysis 1947-1997 in both observations (left) and HadAM3 (right).
Central time series is cross validated and shows the increasing anticyclonic tendency. SST pattern shown is at least part of that related to the forced summer NAO
500hPa Observed JASSST June
ObservedHindcast
500hPa HadAM3 JASSST June
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Conclusions
Coherent fluctuations of pressure, temperature and precipitation occur in summer in the North Atlantic in both tropics and extratropics Change toward persistently anticyclonic flow over north west Europe in recent decades
Evident in many climate variables
Change due, in part, to SST forcing of probability distribution of an internal mode we call the summer NAO. Some of the SST forcing is in N Atlantic but the full scale remains to be found.
A relatively strong link with Sahel rainfall on decadal time scales may be through SST but internal dynamics may play a role too. Does the thermohaline circulation play some role?