# Presentation: 375935

## VALIDATION OF THE SIMULATION OF THE SOUTH AMERICAN MONSOON BY THE ECMWF NATURE RUN

- Juan Carlos Jusem
- NCEP 12 April 2007

## ACKNOWLEDGEMENT

- The author has been benefited by the scientific material kindly provided by Dr. Hugo Berbery.

## MAIN FEATURES OF THE SOUTH AMERICAN SUMMER MONSOON (SASM):  ‘A’: Upper Bolivian Anticyclone; ‘B’: Chaco Low; LLJ: Low Level Jet; SACZ: South Atlantic Convergence Zone. Also: The Northeastern Brazilian Trough(This slide has been inspired by Fig.1 in Nogues Paegle and Coauthors (2002))

## MAIN DRIVER OF SASM

- The South American Summer Monsoon  is driven by the diabatic heat source represented by the precipitation over the Amazonian Basin.(Silva Dias et al., 1983)
- Both the upper Bolivian anticyclone and the northeastern Brazilian trough are part of a Rossby wave type response to the forcing by the heat source just mentioned.

## PRECIPITATION

- The monthly precipitation rate simulated by the EC-nature run has been compared with the long term (27 years: 1979-2005) monthly precipitation rate given by the NCEP/NCAR Reanalysis (Kalnay and Coauthors, 1995),
- In the tropical sector of the region under study, there is a precipitation deficit over land in general, and an excess over oceans and mountain slopes.
- In the extratropics, the EC-NR monthly precipitation compares much better with the Reanalysis.
- The simulated SACZ in summer also compared well with the Reanalysis.

## Seasonal Precipitation: Winter

## Seasonal Precipitation: Spring

## Seasonal Precipitation: Summer

## Seasonal Precipitation: Autumn

## THE UPPER TROPOSPHERIC WIND FIELD

- The most important features are the anticyclone  and the northeastern trough. Both form in spring and acquire their maturity in summer.
- There is a systematic shift of the simulated features southward (spring and summer) and westward in relation to the Reanalysis.

## Wind Field at 200 hPa: Winter

## Wind Field at 200 hPa: Spring

## Wind Field at 200 hPa: Summer

## Wind Field at 200 hPa: Autumn

## THE LOWER TROPOSPHERIC WIND FIELD

- The most important feature is the northerly low level jet (LLJ) over the eastern slopes of the Bolivian Andes.
- It transports most of the moisture that comes to South America from the Atlantic trade wind zone (Vera and Coauthors, 2006)
- The intensity of the LLJ is not well represented by the mean vector wind (see next four slides) due to frequent southerly wind episodes.

## Wind Field at 850 hPa: Winter

## Wind Field at 850 hPa: Spring

## Wind Field at 850 hPa: Summer

## Wind Field at 850 hPa: Autumn

## THE LOWER TROPOSPERIC WIND FIELD (Cont.)

- The EC-nature run reproduces very well two outstanding characteristics of the LLJ:
-    (a) Its permanence during the whole year,
-    (b) Its peak intensity during night hours.
- The following slide represents two time series of the meridional wind component at 850 hPa at the grid point (18S/63W). The “nighttime” series (blue curve) is formed by the 365 “observations” taken at 02 hr local time, while the “daytime” time series (orange curve) represents the 14hr local time. Observe the predominance of blue at the bottom (northerly component) of the figure.
- The proof of the night maximum is completed with the table following the next slide.

## Time series showing the night intensification of the LLJ at the lee of the Andes in the simulation.Also, notice the permanence of the LLJ all the year round.Gridpoint at 18 S / 63 W

## Occurrences of values of  v850 < -15 m/s at different local times in the simulation

-                      Point at 18S/63W

| Local Time | Frequency | Percentage |
| --- | --- | --- |
| 02 hr | 83 times | 21.0 |
| 05 | 84 | 21.3 |
| 08 | 78  | 19.7 |
| 11 | 50  | 12.6 |
| 14 | 37  |   9.4 |
| 17  | 39  |   9.9 |
| 20 | 73 | 18.5 |
| 23 | 90  | 22.8 |

## CONCLUSIONS

- The simulation of precipitation by the EC-NR shows a significant deficit of rainfall over most of the tropical part of South America, and excess over mountains slopes and oceans.
- The SACZ is well simulated by  the EC-NR.
- The representation of the upper Bolivian anticyclone and the northeastern trough are good, with a slight shift to the south and to the west with respect to the NCEP/NCAR Reanalysis.
- Two prominent features of the of the LLJ are very well simulated by EC-NR: (a) its persistence all year round and (b) its peak intensity at night.

## REFERENCES

- Kalnay,E. and Coauthors, 1996: The NCEP/NCAR 40-year reanalysis Project, *Bull. Amer. Meteor. Soc., ***77***, 437-471.*
- Nogues-Paegle, J. and Coauthors, 2002: Progress in Pan American CLIVAR research: Understanding the South American Monsoon.* Meteorologica,*** 27**, 3-30.
- Silva Dias,P.L., W.H. Schubert, and M. DeMaria, 1983: Large-scale response of tropical atmosphere to transient convection. *J. Atmos. Sci., ***40**, 2689-2707.
- Vera, C. and Coauthors, 2006: The South American low-level jet experiment. *Bull. Amer. Meteor. Soc., ***87**, 63-77.