Low Frequency Variability in the Winter NAO and its reproduction in GCMs.

1. Low Frequency Variability in the Winter NAO and its reproduction in GCMs. Adam Scaife, Jeff Knight and Chris Folland. (Hadley Centre, Met Office, Exeter, UK.) 20th April 2004 C20C workshop, ICTP.

2. I. The Problem The NAO/AO is responsible for: • strength of westerly winds in the Atlantic storm track. • amount of advection of warm oceanic air over Europe. • frequency of heavy precipitation over Europe. • ~50% of the JFM warming over Eurasia from 1960s-1990s. Increasing NAO index has been suggested as a sign of observed climate change*. * e.g. Gillett et al., in “The North Atlantic Oscillation”, Ed.: Hurrell, Kushnir, Ottersen and Visbeck, AGU press, 2003.

3. Observed NAO

4. Autocorrelations

5. Coupled model simulations

6. Prescribed SST simulations(normalised)

7. Prescribed SST simulations (not normalised)

8. Surface Pressure Change1960s-1990s

9. 2. Prescribed SSTs? It has been suggested that prescribing SSTs suppresses atmospheric variability by a factor of 2 or so on inter-annual and longer timescales* *Barsugli and Battisti, 1998, J. Atm. Sci., vol. 55, p477-492. *Bretherton and Battisti, 2000, Geophys. Res. Let., vol.27, p767-780.

10. Prescribed vs Coupled SST

11. Prescribed vs Coupled SSTPower Spectra

12. 3. Weak Ocean-Atmosphere Interaction? It has been suggested that weak ocean-atmosphere coupling could reduce the reproducible part of NAO variability from given sea-surface temperatures * *Rodwell and Folland, 2002, Q. J. R.Met. Soc., vol. 128, p1413.

13. Sensitivity of NAO variability to O-A coupling.

14. 4. Upper boundary conditions? It has been shown that the stratospheric climate can affect the NAO.* Could upper boundary conditions in the stratosphere have contributed to the observed NAO trend? *Boville, 1984, J. Atm. Sci., vol. 41, p1132. *Baldwin and Dunkerton, 1999, J. Geophys. Res., vol. 104, p30937-30946. *Norton, 2003, Geophys. Res. Let., vol. 30, p1627

15. Observed Winds in the Lower Stratosphere.

16. Upper level winds in C20C simulations.

17. Observed upper level winds and the NAO.

18. Model links between the NAO and the stratosphere. Norton, Geophys. Res. Let., 2003: DU50 ~ 7ms-1 DNAO ~ s.d./2 => ~ 1.4 hPa/ms-1 Data provided by A.Charlton, Reading University, UK.

19. A stratospheric perturbation experiment. Relax zonal wind above 17km (~75hPa) to zero with an exponential timescale of: • = 2.5 (Z-Zo)/(Z-Zref) (1995-year)/(1995-1965) days Where Zo = 17km and Zref = 53km NET RESULT IN WINTER: trend in stratospheric jet strength

20. Effect on lower stratospheric winds.

21. Effect on the winter mid-latitude jet.

22. Effect on surface P trend.

23. Mechanisms • Induction of tropospheric anomalies from stratospheric PV anomaly. • Downward control through the meridional circulation. • Upper boundary effect on baroclinic eddy structure and hence tropospheric westerlies.

24. Further work • Repeat upper boundary experiment with a weaker and more realistic stratospheric wind trend. • Clarify relative contribution of ocean surface temperature and stratospheric winds to observed NAO increases since 1960s. • Establish a mechanism for the downward effect (PhD student). • Investigate eastward trend in location of NAO

25. Conclusions • The large increase in the NAO index since the 1960s is underestimated in current GCMs, even with specified SSTs. We investigated several possible explanations: • Prescribing SST does not drastically suppress atmospheric variability. • Strengthened coupling of boundary layer fluxes with the ocean did not increase the modelled NAO trend. • Upper boundary conditions can induce significant perturbations in the surface NAO. The observed trend in upper level winds is likely to have made a large contribution to the increasing NAO index since the 1960s.