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

Early CLIVAR Exchanges Article Hurrell and Folland, CLIVAR EXCHANGES, Sept 2002,

Central England Temperature ( ) JA 1.3 ± 0.9°C 1.6 ± 1.1°C JFM

Central England Temperature ( ) JA JFM 1.6 ± 1.1°C 1.3 ± 0.9°C

SLP regressed on CET ( ) JFM JA hPa

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, 30 o W-30 o E, 40 o N-70 o N, Often cyclonic and wet Often dry and settled

July and August Rainfall as a Percentage of Surface Pressure Difference July and August relative to Stars locally significant at 5% level Weather over the UK became more settled and drier in high summer in recent decades

Mean Storm Track (July-August) (m) Change ( ) — ( ) (m) 300 hPa rms transient height (2-8 day bp)

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

North East Atlantic SLP (—) Sahel Rainfall r = 0.55

Anomalous Rainfall (July-August) Percentage of 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 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 EOF C20C runs, , 6 members

Time series of SST forced summer NAO,

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) Optimal Detection Leading Mode of SST-forced MSLP Variability (JA) hPa Global SST Forcing

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

Optimal Detection Rainfall Associated with Leading Mode (JA) mm day -1 ECHAM4 (23 member ensemble) Global SST Forcing

Sahel Rainfall, , C20C runs, ensemble of 6 HadAM3 with all forcings, including changing land surface.

ARPEGE(v3) (6 member ensemble) Optimal Detection Leading Mode of SST-forced MSLP Variability (JA) hPa Atlantic SST Forcing

CCM(v3) (12 member ensemble) 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 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 JAS SST June Observed Hindcast 500hPa HadAM3 JAS SST 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?