Departamento de Geofísica y Meteorología Universidad Complutense de Madrid (UCM))

1. Seminario CLIVAR-ES Madrid, 11-13 Febrero 2009 Estudio del acoplamiento estratosfera-troposfera y su influencia en la variabilidad climática de la región Euro-Atlántica Study of the stratosphere-troposphere coupling and its influence on the climate variability of the Euro-Atlantic region Encarna Serrano, Álvaro de la Cámara, Blanca Ayarzagüena, Marta Ábalos y Beatriz González Departamento de Geofísica y Meteorología Universidad Complutense de Madrid (UCM)) CLIVAR-ES (11-13 Feb 2009)

2. Strato-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Outline of the talk • Introduction • 2. Relevant results (REN2005-06600-CO2): • 2.1. Stratosphere-troposphere system and winter precipitation over Europe (in collaboration with Prof. C.R. Mechoso, UCLA) • 2.2. Stratospheric final warmings and tropospheric circulation over the Euro-Atlantic region 3. Conclusions CLIVAR-ES (11-13 Feb 2009)

3. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM 1. Introduction and Backgrounds • In recent times,a growing interest in the improved understanding of the impact of stratospheric anomalies on the tropospheric variability is observed (Baldwin & Dunkerton 2001, .….) • Certain studies have shown that large stratospheric anomalies descend to lower levels and are followed by changes at surface (temperature, geopotential…). The authors used indices based on the polar vortex strength (Baldwin & Dunkerton 2001; Thompson et al. 2002) CLIVAR-ES (11-13 Feb 2009)

4. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Anomalous rainfall (Europe) [data: Univ. Delaware, 0.5ºx0.5º] 1stMCA mode (Maximum Covariance Analysis) Geopotential anomalies at 1000 hPa (Northern Hemisphere) [data: ERA40, 1.125ºx1.125º] Monthly data, DJF pcp_idx (sfc = 45 %) 22 HIGH cases Standardized values 23 LOW cases DJF (1957-1999) 2. Results: Stratosphere-troposphere system and winter precipitation over Europe • We have analized the stratosphere-troposphere coupling in the winter rainfall over Europe using a precipitation index (pcp_idx) connected with North hemisphere geopotential anomalies (A. Cámara et al., EGS2007, WCRP2007, to be submitted J.Climate). CLIVAR-ES (11-13 Feb 2009)

5. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM HIGH vs. LOW pcp_idx WINTER Composites of geopotential anomalies Z 500 hPa 22 cases • Negative anomalies over the Arctic, shifted towards the Atlantic basin. • The positive Euroatlantic mid-latitude centre of action is splitted. • Overall, a wavy structure. H I GH 23 cases • Narrower extension of the Northern centre of action. • More zonally-symmetric pattern. LOW CLIVAR-ES (11-13 Feb 2009)

6. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM HIGH vs. LOW pcp_idx WINTER Composites of anomalies Precipitation (std values) Z 500 hPa H I GH LOW CLIVAR-ES (11-13 Feb 2009)

7. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Z 50 hPa HIGH vs. LOW pcp_idx WINTER Composites of anomalies Z 500 hPa H I GH polar vortex strengthened at upper levels polar vortex something weakened at upper levels LOW CLIVAR-ES (11-13 Feb 2009)

8. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM HIGH vs. LOW pcp_idx WINTER Time-latitude plot of the stratospheric jet (zonal-mean zonal wind at 10 hPa) prior to, during and after extreme cases of pcp_idx. High-minus-Lowcomposite of [u10] (m/s) (Shading: significant difference at α = 0.01, t-test) The westerlies at high latitudes (60ºN-80ºN) are statistically significantly stronger not only during high episodes (central month of the diagram), but also during a 20-days period prior to the extreme pcp_idx episode. It suggests that the intensity of the stratospheric jet at 10 hPa could be used aspredictor of the anomalous monthly rainfall regime over Europe. CLIVAR-ES (11-13 Feb 2009)

9. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM High pcp_idx ↔ strong polar vortex Low pcp_idx ↔ weak polar vortex X HIGH vs. LOW pcp_idx WINTER Criterion of Castanheira & Graf (2003) Strong polar vortex regime (SVR): [u]65ºN, 50hPa > 20 m/s Weak polar vortex regime (WVR): [u]65ºN, 50hPa < 10 m/s Neutral conditions: 10 m/s  [u]65ºN, 50hPa  20 m/s CLIVAR-ES (11-13 Feb 2009)

10. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM High pcp_idx ↔ strong polar vortex Low pcp_idx ↔ weak polar vortex X HIGH vs. LOW pcp_idx WINTER High pcp_idx: troposphere -stratosphere coupling Low pcp_idx : not a clear troposphere -stratosphere coupling Differences between AO and NAO? LOW pcp_idx HIGHpcp_idx Composite of Z500 Composite of Z500  AO-like  NAO-like CLIVAR-ES (11-13 Feb 2009)

11. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM 2.2. Stratospheric final warmings (SFWs) and tropospheric circulation over Euro-Atlantic region (B. Ayarzagüena & E. Serrano, J. Climate, accepted) • The SFWs occur once per year at springtime, and present a large interannual variability. • Our studies deal withthe relationship between the late or early occurrence of the SFW and changes in the tropospheric circulation in spring months over the Euro-Atlantic region CLIVAR-ES (11-13 Feb 2009)

12. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Criterion of Black et al. 2006 [u50] at 70ºN Data: daily zonal wind (1958-02, ERA40) date of SFWs day of year 3th Jun 1 std Mean date 17th April 27th Mar 2.2. Stratospheric final warmings (SFWs) and tropospheric circulation over Euro-Atlantic region Selection of TWO sets of years from the study period (45yrs) in basis of the date on which the SFW occurs: EARLYyears and LATEyears (reflecting a early or late breakup of the polar vortex). 7 “late years” 10 “early years” CLIVAR-ES (11-13 Feb 2009)

13. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Early vs. Late years SPRING • Differences in the stratospheric mean flow (related to differences in the stationary wave activity) APRIL easterlies Zonal-mean zonal wind (m·s-1) [u] < 0 [u] > 0 • In ‘early years’, the polar vortex has already disappeared and the easterlies extend to lower levels and latitudes than the climatology (mean flow has changed from westerlies to easterlies)  anomalous low upward propagation of waves. • In ‘late years’, the polar vortex still persists and stronger mean flow than the climatology about 30ºN northwards more difficulties than normal for waves to propagate upwards up to 20 hPa and less than normal above that level CLIVAR-ES (11-13 Feb 2009)

14. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM  AO--like AO+-like Early vs. Late years SPRING • Differences in 500-hPa geopotential and zonal wind fields APRIL Z500 anomalies (gpm) • In ‘late years’: well-defined Atlantic tripole of Z500 anomalies (negative values over high latitudes and subtropical Atlantic, positive values at west of the British Isles) • In ‘early years’: different pattern to ‘late years’ (← Z500-tripole centers are weaker, shifted and deformed. CLIVAR-ES (11-13 Feb 2009)

15. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM  AO--like AO+-like Fisher-Irwin test Independence between AO phase & late/early year (92% confidence level) Early vs. Late years SPRING • Differences in 500-hPa geopotential and zonal wind fields APRIL Z500 anomalies (gpm) CLIVAR-ES (11-13 Feb 2009)

16. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Early vs. Late years SPRING • Differences in the storm track activity (at 500 hPa) APRIL Units: gpm • Statistically significant differences, between ‘early andlate years, both in the entrance (North America coast) and exit (Scandinavia) regions of the storm tracks in the Atlantic area. CLIVAR-ES (11-13 Feb 2009)

17. Strat-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Early vs. Late years SPRING • Differences in the storm track activity (at 500 hPa) APRIL Units: gpm • In ‘early years’: compared to climatology, a southward shift of ST activity in the east coast of North America, and weakness along the whole Atlantic strip • In ‘late years’, the opposite occurs (maximum of the ST activity northwards up respect to climatology; values > 40 gpm reach Scandinavian Peninsula) CLIVAR-ES (11-13 Feb 2009)

18. Strato-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM New topics under current study (REN2008-06295) • Degree of connection between the strength of the stratospheric polar vortex and the arctic ozone content (end of winter and during spring) • Analysis of how well the UCLA global model reproduces the stratospheric circulation(→ estudios de sensibilidad) CLIVAR-ES (11-13 Feb 2009)

19. Strato-troposphere coupling & Euro-Atlantic climate variability Dept. Geophysics & Meteorology, UCM Conclusions - We have defined a precipitation index (pcp_idx), that is a good descriptor of the stratosphere – European rainfall connection in winter months, in such a way that its high values capture the stratosphere-troposphere coupling, and with potential use in seasonal forecasting. - The late or early occurrence of the yearly SFW (spring months) seems to have some impact on the troposphere over the Euro-Atlantic region, especially in April. The storm track activity inlate years (early years) is such that leads to a higher (lower) number of storms crossing North Europe. CLIVAR-ES (11-13 Feb 2009)