Mesoscale and High Impact Weather in the South American Monsoon Leila M. V. Carvalho 1 and Maria A. F. Silva Dias 2 1

Transcription

1 Mesoscale and High Impact Weather in the South American Monsoon Leila M. V. Carvalho 1 and Maria A. F. Silva Dias 2 1 University of California, Santa Barbara 2 University of Sao Paulo, Brazil

2 Objectives This talk summarizes some of the most recent observational studies addressing extreme rainfall in areas affected by the South American Monsoon and the South Atlantic Convergence Zone A summary of the main contributions of GoAmazon is also presented.

3 Outline Climatological features associated with the South American Monsoon System (SAMS) Main mechanisms modulating rainfall variability in SAMS on synoptic timescales and implications for extreme rainfall Trends in extreme rainfall over Eastern and Southeastern Brazil (regions influenced by the SACZ) Recent field campaigns designed to improve our understanding of mesoscale convective systems over tropical South America.

4 The South American Monsoon (SAMS) summer climatology Oceanic SACZ Bolivia High

5 Mechanisms modulating rainfall variability in SAMS on synoptic time scales (2-12 days): propagation of wave trains from midlatitudes westerly winds anomalies over central Brazil and eastern Amazon: Precipitation increases over the oceanic SACZ, and is suppressed over The Atlantic ITCZ and west Amazon Easterly winds anomalies: suppressed convection over the SACZ; increased convection over the ITCZ and western Amazon Differences in wind anomalies and precipitation between westerly and easterly regimes

6 Example: the 2011 floods over southeastern Brazil: more than 300mm in coastal areas of SE Brazil in 3 days TRMM and 850hPa winds: Jan11 6:00 UTC TRMM and 850hPa winds: Jan 12 6:00 UTC TRMM and 850hPa winds: Jan 13 6:00 UTC TRMM accumulated: Jan L L L Persistent westerly wind anomalies central Brazil and eastern Amazon accompanied by a slow moving low-pressure system off the southern coast of Brazil, over subtropical Atlantic can create dynamical conditions for the organization of mesoscale systems in the equatorward flank of the SACZ.

7 Observations have shown: Tropical South America has experienced great rate of warming in recent decades (Carvalho and Jones 2013); Extreme precipitation events have increased in geographic locations affected by SAMS (e.g, Silva Dias et al. 2013; Pedron et al. 2016; Zilli et al. 2016; among others), whereas other areas have received deficits of precipitation (e.g. Zilli et al. 2016) Question: what are the relationships between these trends and the SACZ?

8 Limited observation from gridded dataset (interpolated from stations: (Liebmann and Alure 2005) % of missing days Average number of stations per grid point Number of valid years per grid point (until 2012)

9 Trends in gridded precipitation (0.5 o resolution, for wet season (October to March) Zilli et al. 2016, a comprehensive analysis of trends in extreme precipitation over southeastern coast of Brazil. Int. Journal Clim. Zilli et al. 20 Positive: blue Negative: red Total Seasonal Precipitation Decrease in total and daily precipitation : equatorward flank of the SACZ Increase in total and daily precipitation : Poleward flank of the SACZ Average daily precipitation (>1mm). 95 th percentile Increase in the 95 th percentile consistent with the poleward shift of the SACZ

10 GPCP DJF precipitation rate (mm/day): (a) Climatology for (contours each 2mm/day); Difference between the first ( ) and last ( ) decades Weakening of the SPCZ in the recent decade Pattern of rainfall change consistent with the weakening of the SACZ From Zilli et al (Clim. Dyn. in revision)

11 Difference in 850 hpa DJF average winds (vectors) and winds speed (color) between first ( ) and last ( ) periods Northerly winds weaken equatorward of the SACZ Difference in ω500 (in Pa/s), with contours representing the climatology Subsidence increases equatorward of the SACZ

12 Summary in observed changes in the SACZ Positive trends in extreme rainfall has been observed in many areas influenced by the SACZ, particularly South Eastern Brazil - a region densely populated However, daily rainfall has decreased in the equatorward margin of the SACZ Reasons: weakening of the northerly winds in the equatorial margin; intensification of subsidence All together indicate the poleward shift of the SACZ in recent years. Attributions require further investigation. From Zilli et al (Clim. Dyn. in revision)

13 Mesoscale and high weather impact in the Amazon: the GoAmazon and ACRIDICON- CHUVA experiments Designed to advance the scientific understanding of cloud processes over Tropical South America GoAmazon quantified the effect of the polluted plume of a large urban area, the city of Manaus with its 2.5 million inhabitants, on the downwind region and specifically on cloud properties The ACRIDICON-CHUVA campaign flew a high altitude airplane to monitor the outflow of deep convective systems and detect the formation of new ultrafine particles in the upper troposphere involved in the cloud vertical circulation that eventually find their way into the boundary layer replenishing the supply of cloud condensation nuclei. These new results suggest a complex interaction between aerosol and cloud life cycles with implications on rainfall prediction during SAMS. The modeling of such interaction remains as a scientific challenge.

14 Location of GoAmazon (2014/2015) Letters indicate locations of sites of measurements and range rings indicate distances from the SIPAM radar location (near T1). Giangrande, S. E., et al. 2017: Cloud Characteristics, Thermodynamic Controls and Radiative Impacts During the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) Experiment, 2017 Atmos. Chem. Phys. Discuss.,

15 Examples of relevant results from 2 years 2014/15 GoAmazon: All seasons wet season Dry season Giangrande, S. E., et al. 2017

16 CHUVA field campaigns over Brazil and main precipitation regimes. Machado et al. 2014: CHUVA field campaigns over Brazil and main precipitation regimes. BAMS

17 CHUVA field campaigns over Brazil and main precipitation regimes. CHUVA provides a comprehensive dataset characterizing the main precipitation regimes in Brazil. made available high spatial and temporal resolution observations (ground and satellite based) of cloud and precipitation characteristics. Available data sets at:

18 Conclusions Extreme precipitation in Tropical South America depends on complex interactions on multiple spatial-temporal scales Variations and changes in the SACZ on synoptic to decadal timescales impacts the geographic distribution of precipitation and extreme events There is observational evidence that extreme rainfall events have increased over tropical SA in the last decades but with patterns of variability that suggest the poleward shift of the SACZ. These changes have been related to the weakening of the northerly winds and enhancement of subsidence at the equatorial flank of the SACZ. Results from field campaigns such as GoAmazon and CHUVA are invaluable to assess the impacts of anthropogenic climate change on the characteristics of convection and precipitation over South America.