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

Early CLIVAR Exchanges ArticleHurrell and Folland, CLIVAR EXCHANGES, Sept 2002, 0052-54

Central England Temperature (1950-2002)

JA

1.3 ± 0.9°C

1.6 ± 1.1°C

JFM

Central England Temperature (1950-2002)

JA

JFM

1.6 ± 1.1°C

1.3 ± 0.9°C

SLP regressed on CET (1950-2002)

JFM

JAhPa

First observed rotated EOF of extratropical NH sea level Pressure, July-August. First noticed by Barnston and Livezey (1987)

Pressure at mean sea level in July and August, 30o W-30oE, 40oN-70oN, 1899-2003

Often cyclonic and wet

Often dry and settled

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

Mean Storm Track (July-August) 1948-2002(m)

Change (1967-2002) — (1948-1966) (m)

300 hPa rmstransient height(2-8 day bp)

North west Scotland in winter follows the winter NAOSouth east England in summer follows the summer NAO

North East Atlantic SLP(—) Sahel Rainfall

r = 0.55

Anomalous Rainfall (July-August) 1967-2000Percentage of 1921-1960 Mean Rainfall

Sahel Rainfall Interhemispheric SST Contrast

r = 0.59

NH Cold

NH Warm

Updated from Folland et al Nature, 1986

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

First optimum rotated SST forced EOFC20C runs, 1950-1999, 6 members

Time series of SST forced summer NAO, 1950-99

First noise EOF of C20C runs is also the summer NAO. Thus it is very likely an internal mode

First observed EOF of extratropical NH sea level pressure, July-August is essentially the first noise and first global SST

forced EOFs of HadAM3

ECHAM4 (23 member ensemble) 1950-1999

Optimal Detection

Leading Mode of SST-forced MSLP Variability (JA)

hPa

Global SST Forcing

Total Precipitation regressed on NE Atlantic SLP (ECHAM4)mm day-1JA

Optimal Detection

Rainfall Associated with Leading Mode (JA)

mm day-1

ECHAM4 (23 member ensemble) 1950-1999

Global SST Forcing

Sahel Rainfall, 1950-2002, C20C runs, ensemble of 6

HadAM3 with all forcings, including changing land surface.

ARPEGE(v3) (6 member ensemble) 1950-1999

Optimal Detection

Leading Mode of SST-forced MSLP Variability (JA)

hPa

Atlantic SST Forcing

CCM(v3) (12 member ensemble) 1950-1994

Optimal Detection

Leading Mode of SST-forced MSLP Variability (JA)

hPa

Tropical Atlantic SST Forcing

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

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?