South American Climate

Transcription

1 South American Climate René Garreaud

2 Global Historical Climate Network (GHCN) Precipitation Mean Temperature All stations (anytime, any length) Century-long stations (Ti<1905, Tf>1995, missdata<20%)

3 Geographical setting Llanos Guinas highlands 10 N 0 N Equatorial Andes Amazon basin Altiplano Brazilian highlands 20 S Chaco Subtropical Andes Pampas La Plata basin 40 S Patagonia 60 S

4 Because of its long meridional extent, South America exhibits tropical, subtropical and extratropical climatic regimes

5 The big picture 0 S Continental Low Level Jet S. Atlantic Anticyclone 30 S SE Pacific Anticyclone Midlat. Precip. Tropical rainfall SCu & Cold SST Midlatitude Storm track 60 S 120 W 90 W 60 W 30 W

6 Zonal wind just upstream of the Andes (over the SE Pacific)

7 Oceanic boundary conditions Warm Pool Cold tongue

8 The low-level air temperature field

9 DJF Seasonal long-term mean 2-m air temperature [ C] U. Delaware MAM JJA SON

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11 Precipitation features 1. ITCZ 2. Continental convection 3. Altiplano convection 4. SACZ 5. Pampas convection 6. Midlatitude storm track 7. Orographic precipitation 8. Coastal desert 9. NE Brazil semiarid 10. Patagonia dry zone 11. Ocean desert Circulation features 1. ITCZ 2. Trade winds 3. Subtropical high 4. Midlatitude westerlies 5. Low level jet 6. SACZ 7. Bolivian high 8. NE Brazil trough 9. Tropical easterlies 10. Midlatitude westerlies 11. Jet stream

12 DJF Seasonal long-term mean Precipitation [mm/day] U. Delaware MAM JJA SON

13 Another Perspective of the Precipitation Field Annual Mean / Zonal Mean Annual Mean / Zonal asymmetry January - July Mean

14 Southeast South America Precipitation (warm season) Vera et al (BAMS) Diaz & Aceituno 2003

15 Northerly low level jet interrupted by cold air incursions that reach as far as the Amazon basin

16 Convective rainfall also exhibits a pronounced diurnal cycle

17 Altiplano Precipitation: annual cycle a. Zonal wind speed Reanalysis data at 17.5 S 70 W Pressure [hpa] W E W/m 2 g/kg b. Near surface mixing ratio and OLR over the Altiplano Precipitación

18 Altiplano Precipitation: intraseasonal variability a. Zonal wind speed Pressure [hpa] W E W/m 2 g/kg b. Near surface mixing ratio and OLR

19 Altiplano Precipitation: conceptual model (summer) Anomalies of wind aloft forced by extratropical disturbances Wind aloft controls the transport of moisture towards the Altiplano

20 Precipitación en latitudes medias Las perturbaciones de latitudes medias (ver clase anterior) también transportan calor hacia latitudes altas, continuando el proceso de transferencia de calor que realiza la atmósfera. Cálido Frio B 500 hpa A

21 Precipitación Orografíca (latitudes medias) El mecanismo más simple es la saturación de vapor de agua, y posterior formación de partículas de precipitación, en flujos estables ladera arriba al barlovento de la topografía.

22 De todas las variables probados, es el flujo de vapor de agua en la dirección zonal (hacia la Cordillera) a 2500 m (un poco arriba del nivel promedio de congelamiento) que tiene la mejor correlación (50%) con precipitación. Falvey and Garreaud 2007

23 Precipitación en latitudes medias Garreaud 2007

24 Precipitacion en Latitudes medias Modest annual cycle in the extratropics

25 Interannual Precipitation Variability (UdW data) Which regions exhibit large year-to-year variability?

26 Patterns / modes of large-scale circulation be aware Quadreli and Wallace, J. Climate, 2002

27 How to make a circulation mode? Choose a variable/level from reanalysis You may want to pre-filter the data and select a sub-domain Use your favorite stat-software (Matlab, Maple, etc) Select a complicated tool (e.g., complex-rotated-extended-multidimensional EOF) Get your spatial pattern and loading factors (time series) Unfortunately, it is very likely that you get a mode that is very similar to something already known. Quadreli and Wallace J. Climate, 2002

28 Leadings modes of interannual (and longer) of atmospheric variability: ENSO PDO(?) - AAO

29 Leadings modes of interannual (and longer) of atmospheric variability: ENSO PDO(?) - AAO

30 Annual mean Precip/SAT regressed upon index of large-scale modes (50 years of data)

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33 ENSO. Atmosphere/ocean (normal) Atmosphere/ocean (ENSO) Atmosphere/ocean (La Niña)

34 Interannual variability - Major ENSO impacts Wetter-JFMA Stronger upper- Level ST Jet Weakened ST High Blocking High H H Warmer - DJF EN years Storm Track Climo. Storm Track Drier/Warmer DJF Wetter-OND Wetter-JJA Drier-DJF Colder

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36 Seasonal correlation between Precip/SAT and Multivariate ENSO Index (50 years of data)

37 Every EN/LN is different and it is superimposed on a slightly different background state. That produce changes in the teleconnection patterns even in current climate

38 Karumuri Ashok & Toshio Yamagata Nature 461, (24 September 2009)

39 Interannual Variability

40 Interannual Variability

41 The "Pacific Decadal Oscillation" (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. While the two climate oscillations have similar spatial climate fingerprints, they have very different behavior in time. Causes for the PDO are not currently known. Two main characteristics distinguish PDO from ENSO: 1. 20th century PDO "events" persisted for 20-to-30 years, while typical ENSO events persisted for 6 to 18 months 2. The climatic fingerprints of the PDO are most visible in the North Pacific/North American sector, while secondary signatures exist in the tropics and the SH - the opposite is true for ENSO. Several independent studies find evidence for just two full PDO cycles in the past century: "cool" PDO regimes prevailed from and again from , while "warm" PDO regimes dominated from and from 1977 through (at least) the mid-1990's.

42 PDO Basics

43 The Southern Hemisphere Annular mode (SAM), or Antarctic Oscillation, is the leading mode of monthly and longer variability of the tropospheric circulation poleward of 20ºS. SAM is tropospheric deep, highly symmetric mode, involving mass exchange between high and mid latitudes. What causes SAM is not well known, likely eddy mean flow interaction The SAM has shown a trend toward decreases pressure over Antarctica (positive polarity; faster polar vortex), partially attributed to decrease in stratospheric O3. Thompson and Wallace 1999! AAOI regressed upon SLP (upper panel) and zonal average of zonal wind (lower panel)

44 Annual mean Precip/SAT regressed upon index of large-scale modes (50 years of data)