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Jim Hurrell, NCAR, & Chris Folland, Hadley Centre, Met Office, UK

C20C WORKSHOP, TRIESTE, APRIL 19-23 2004. 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

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Jim Hurrell, NCAR, & Chris Folland, Hadley Centre, Met Office, UK

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  1. C20C WORKSHOP, TRIESTE, APRIL 19-23 2004 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

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

  3. JFM 1.6 ± 1.1°C Central England Temperature (1950-2002) JA 1.3 ± 0.9°C

  4. Central England Temperature (1950-2002) JFM 1.6 ± 1.1°C JA 1.3 ± 0.9°C

  5. JFM JA SLP regressed on CET (1950-2002) hPa

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

  7. Pressure at mean sea level in July and August, 30o W-30oE, 40oN-70oN, 1899-2003 Often cyclonic and wet Often dry and settled

  8. July and August Rainfall 1967-1998 as a Percentage of 1921-1960 Surface Pressure Difference July and August 1967-1998 relative to 1921-1960. Stars locally significant at 5% level Weather over the UK became more settled and drier in high summer in recent decades

  9. Mean Storm Track (July-August) 1948-2002 Change (1967-2002) — (1948-1966) (m) (m) 300 hPa rms transient height (2-8 day bp)

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

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

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

  13. Interhemispheric SST Contrast r = 0.59 NH Warm NH Cold Sahel Rainfall Updated from Folland et al Nature, 1986

  14. Interhemispheric contrast is related in part to extratropical Atlantic temperatureand varying heat transport from tropical regions 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. Extratropical North Atlantic currently warming quite fast. 2003 warmest year on record From Knight et al, in review

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

  16. Time series of SST forced summer NAO, 1950-99

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

  18. First observed EOF of extratropical NH sea level pressure, July-August is essentially the first noise and first global SST forced EOFs of HadAM3

  19. Optimal Detection ECHAM4 (23 member ensemble) 1950-1999 Leading Mode of SST-forced MSLP Variability (JA) Global SST Forcing hPa

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

  21. Optimal Detection ECHAM4 (23 member ensemble) 1950-1999 Global SST Forcing Rainfall Associated with Leading Mode (JA) mm day-1

  22. Sahel Rainfall, 1950-2002, C20C runs, ensemble of 6 HadAM3 with all forcings, including changing land surface.

  23. hPa Optimal Detection ARPEGE(v3) (6 member ensemble) 1950-1999 Leading Mode of SST-forced MSLP Variability (JA) Atlantic SST Forcing

  24. Optimal Detection CCM(v3) (12 member ensemble) 1950-1994 Leading Mode of SST-forced MSLP Variability (JA) Tropical Atlantic SST Forcing hPa

  25. 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 HadAM3 JAS SST June 500hPa Observed JAS SST June Observed Hindcast

  26. 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?

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