South Eastern Australian Rainfall in relation to the Mean Meridional Circulation

Transcription

1 South Eastern Australian Rainfall in relation to the Mean Meridional Circulation Bertrand Timbal, Hanh Nguyen, Robert Fawcett, Wasyl Drosdowsky and Chris Lucas CAWCR / Bureau of Meteorology

2 Long-term SEA rainfall deficit: 30 year climatology April to July Annual mean Last 30 years a different climate baseline? Australia becoming wetter Southern edge of the continent becoming drier Drying across southern Australia a late autumn-early winter phenomenon Wetting across Australia a warm season phenomenon November - March

3 Mean climate state driven by the Mean Meridional Circulation MMC transfer excess heat from the Tropics to higher latitude How will it respond to increase trapped heat? SEA

4 SEA: a Mediterranean climate Kottek et al.,, Met Zeit, 2006

5 Pressure in hpa Degree (in Celsius) SEACI-1 key result Intensity of the STR and Global Warming (11-years running means) Central Year STR Intensity annual mean Global Annual Temperature

6 STR relationship with SEA rainfall Relationship stronger with intensity than position of the STR Relationships strongest where rainfall deficit is observed STR-P STR-I Timbal and Drosdowsky, Int. J. of Clim, 2012

7 Monthly rainfall anomalies (mm STR relationship with SEA rainfall Relationship is significant from April to November Match the continuum of observed rainfall deficit (except early autumn) STR related rainfall deficit resemble the observed one (linear reconstruction) Using linear statistics: no additional effect due to shift in STR position IOD Nino4 Tri-pole STR-I Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Observed rainfall anomalies Derived from STR-I anomalies Derived from STR-P anomalies Derived from STR-I and P anomalies Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Timbal and Drosdowsky, Int. J. of Clim, 2012

8 Non linear interactions between intensity and position of the STR Particularly important in autumn: transition season Identified using non linear analysis technique (CART) Winter-like and Summerlike wet autumn cases Winter-like not seen since 1995 Relatively high value of the STR-I can lead to wet autumn in SEA (autumn 2007 and in March 2010, 2011 and 2012). Whan et al., Int. J. of Clim, accepted

9 Hadley Cell is broadening Expansion of the HC in all reanalyses ~0.5 o /decade in the SH ~0.6 o /decade in the NH. Smaller trend than previously found Consistent results Nguyen et al., J. of Clim, submitted

10 Seasonal trends of the widening

11 HC relationship with the STR Correlation between HC and STR monthly anomalies. Expansion of the HC is associated with polewards shift of the STR Intensification of the STR also related to HC expansion much less to the intensity of the HC

12 The Edge of the Tropics each dot is one observation of the tropopause, bin size = 1 km, centred Lucas et al., JGR, accepted

13 Analysis Method Count number of days with tropopause >14.5 km & normalize to June-May Sample bias Random uncertainty Form composite time series in zonal bands 3-5 degrees width. 2-8 stations per bands. 10 bands ANZ, 9 bands SA, 8 bands for AFR Contour ndgt15 in time-latitude space Slope of contours gives trend Lucas et al., JGR, accepted

14 Regional Comparison Global similarity between regions 300 contour different Relation with ENSO Expansion during La Nina, ~3-4 o between extremes Strongest on 100, 50 contours (r ~ ) Strongest over ANZ contour ANZ trend Influence of Volcanoes El Chichón, Mt. Pinatubo Larger in SA Trends stronger in SA

15 Shift of instabilities from STJ to PFJ % Change May Rainfall (1975 to 1994) (1949 to 1968) Storm track :300hPa Streamfunction Amplitude (1997 to 2006) (1975 to 1994) Frederiksen et al.,, Man. Clim. Ch., 2010 Since the ( ) period, the preferred region for growth has shifted from the subtropical jet to the polar front jet (also about 10% reduction in growth rate)

16 Conclusions SEA is affected by a long-term rainfall deficiency The very wet ended the Millennium Drought but not the longterm signal Large-scale modes of variability are important (year to year, e.g. 2010/11) but not convincing to explain long-term deficit SEA mean climate is a result of the Mean Meridional Circulation The MMC is changing HC broadening, intensity is unclear Storm-track, tropopause height confirm this Drive in turn the STR: SEA local controller (other modes drive it as well) Autumn is a transition season where the expansion of the HC is very large and drive both intensification and shift in position (non-linearity)