Interannual Variability of the South American Monsoon in Large - - PowerPoint PPT Presentation

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Interannual Variability of the South American Monsoon in Large Ensemble simulations Ana Claudia Thome Sena Gudrun Magnusdottir Department of Earth System Sciences University of California, Irvine January 2019 Ana Sena (UCI) South American

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Interannual Variability of the South American Monsoon in Large Ensemble simulations

Ana Claudia Thome Sena Gudrun Magnusdottir

Department of Earth System Sciences University of California, Irvine

January 2019

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South America Precipitation

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Methodology

Onset: First day when:

◮ Rainfall greater or equal to 4mm/day

ver Amazon

◮ Zonal Wind at 850 hPa is from East

ver central Brazil

◮ Conditions remain for at least 40 days ◮ Both rainfall and wind were smoothed

using 5-day moving averages

Retreat: When those conditions are no longer met.

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Objectives

Determine the differences between the large-scale circulation associated with early and late onset/retreat of the South American Monsoon System in a large ensemble of simulation

◮ Are the South American Monsoon features reproduced by the CESM Large Ensemble

Simulations (LENS)?

◮ What are the differences between early and late onset in LENS? ◮ What are the differences between early and late retreat in LENS? Ana Sena (UCI) South American Monsoon January 2019 4 / 18

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Why use Large Ensemble Simulations?

Reanalysis models: Model Period ERA interim 1979 - present MERRA-2 1983 - present NCEP/NCAR 1948 - present JRA-55 1958 - present ERA20c 1901 - 2010 Large Ensemble Simulations: Community Earth System Model version 1 (CESM1) 40 ensemble members 1o spatial resolution Daily 1920 - 2005 (historical) 2006 - 2100 (RCP8.5) Period analyzed: 1920 - 2016

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SAMS Onset/Retreat Dates in Observations

Figure: Onset and retreat date of the SAMS - CHIRPS precipitation and ERA interim wind

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SAMS Onset/Retreat Dates in LENS

Figure: SAMS Onset (a) and Retreat (b) dates - LENS

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SAMS Onset/Retreat Dates

Observations

◮ Early Onset: 13-Sep (10 cases) ◮ Late Onset: 30-Sep (10 cases) ◮ Early Retreat: 8-May (10 cases) ◮ Late Retreat: 24-May (10 cases)

LENS

◮ Early Onset: 05-Oct (1053 cases) ◮ Late Onset: 20-Oct (1037 cases) ◮ Early Retreat: 20-Apr (1046 cases) ◮ Late Retreat: 30-Apr (991 cases) Ana Sena (UCI) South American Monsoon January 2019 8 / 18

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SAMS variability in LENS and observations

Figure: Value of one standard deviation of onset date, retreat date and length of the SAMS, for each dataset considered.

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Early and Late Onset - LENS

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Early and Late Onset - LENS

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Early and Late Onset - LENS

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Early and Late Onset - LENS

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Early and Late Retreat - LENS

Figure: 850 hPa Wind Anomalies and Climatology

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Early and Late Retreat - LENS

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Early and Late Retreat - LENS

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Conclusions

General wet season dates and features are reproduced in LENS

◮ Later onset and earlier retreat in LENS ◮ Wet season interannual variability is greater in observations

Onset

◮ Early onset is related to early development of the Bolivian High ◮ Rainfall dipole between Northern and Southern Brazil.

Retreat

◮ ITCZ southward position is related to a later SAMS retreat ◮ Stronger (weaker) trade winds in late (early) Retreat ◮ South Atlantic Low Level Jet resists longer during late retreat ◮ Moisture incursion from South Atlantic Ocean in Southeastern Brazil during early retreat Ana Sena (UCI) South American Monsoon January 2019 17 / 18

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Future Work

Determine the mechanisms for interannual variation of SAMS wet season length in LENS Local and remote influences Modes of Climate variability

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