Global%20Characteristics%20of%20Tropical%20Droughts

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Global Characteristics of Tropical Droughts
During ENSO Warm Events Bradfield
Lyon International Research Institute for
Climate Prediction LDEO, Columbia University
Climate Diagnostics Workshop, October 2003
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INTRODUCTION

• Tropics, as a whole, warm and become moistened
  
• during ENSO warm events with greater heating
• aloft than near surface.
• Land tends to dry while ocean convection
  increases
• will show evidence for this.
• But ENSO is only one factor in forcing drought
• SO -
• How similar are the life cycles of drought and
  ENSO?
• How linear is the relationship?
• Variations over time?

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Some Specifics . . .
Period of study 1950 - 2002
Domain 30S 30N

• Data
• Gridded monthly rainfall
• UEA 1950-1990, CPC 1979 - 2002
• NCEP/NCAR Reanalysis ( T, PWAT )
• NCDC ERSST (1950-2002)
• CPC CAMS Land Temp. Anomaly

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Tropical Rainfall and Land Area
Rainfall

• Vast majority of tropical
• precipitation falls in the ocean

• Land-only rainfall more
• symmetric about the eq. with
• the domain 30S-30N largely
• capturing monsoon excursions

Land Rainfall
Land area

• Will consider only the
• global scale characteristics
• of drought (in the tropics)

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Local Correlation PRCP and Surface T

• Temp-prcp relationships markedly different for
  ocean/land

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Dominant EOF of Tropical Temp Anomalies -
Reanalysis
1st EOF T avg. over lat./long.
1st EOF T zonal average
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PWAT similar structure (54)

• Time series of these EOFs highly correlated with
  ENSO each other

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Tropical heating lags Nino 3.4 SST (and
convection)

• Reanalysis consistent with radiosonde and MSU
  temp obs.

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El Nino and Teleconnections Keyed on Nino
Indices

• General tendency, for tropical land area, to
  dry during ENSO () events
• Want to examine/quantify the spatial extent of
  droughts 30S-30N
• Results not sensitive to the domain considered

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How is a drought defined?
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Weighted Anomaly Standardized Precipitation WASP

• A simple, single variable index based on
  monthly precipitation.
• Weighting factor accounts for seasonality
  damps extremely high
• monthly values which can occur at start/end
  of rainy seasons.
• Climatologically dry areas (i.e. deserts)
  masked.
• Drought inherently an accumulated moisture
  deficit here will look
• at 1, 6 and 12 month accumulations.

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Droughts emerge as rainfall deficits accumulate

• ENSO signal difficult to observe in monthly
  values of the index.
• Drought typically covering 15 of tropical
  land areas
• Post 1990 (red line), this data shows some
  reduction in variance not
• believed to be real based on comparisons with
  other data.

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ENSO and droughts emerge when accumulated
deficits are considered

• Blue line UEA data, red
• line is for CAMS_OPI
• (CMAP data similar)

• Bottom graph is for more
• severe droughts (WASP
• values less than 2)

• More severe droughts tend
• to be more widespread
• during ENSO (), though
• the relationship is not in
• direct proportion to ENSO
• strength

r 0.86

• Linear correlation between
• top and bottom time series
• is 0.86

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ENSO onset typically leads the onset of the most
widespread droughts

• In a linear sense, ENSO associated with 40
  of variance in area

log (area) log (area)
Std area
rmslog(area)-log(area)
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Consistent relationship drought spatial coverage
and El Nino but lots of scatter, too.
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Tropical temp. anomaly and drought time series
Near zero lag
r 0.51
UEA
CAMS

• Tropospheric heating in phase with drought
  development
• consistent with stabilizing thermodynamic
  effect of heating.
• Correlation slightly less than for the NINO 3.4
  index.

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Composite onset and demise

• Define onset when drought area exceeds 1 std
  deviation (10 events).
• Composite relative to onset and demise (drought
  area falls back
• below 1 std deviation).

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NINO 3.4 and Tropospheric Temperature Anomalies

• See the ENSO lead and in-phase relationship
  with trop. heating.

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Trends
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Land-Sea Temp Contrast and Drought Area
(5-year running average)
r 0.87

• Upward trend in drought area coverage
  associated with increasing
• SST and surface land temp.
• Largest correlation with difference between
  land and ocean temp.
• Suggestive of more widespread drought with inc.
  temp. An enhanced
• hydrological cycle does not mean more tropical
  rainfall everywhere

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Average values of WASP and temp. anomalies for
Zimbabwe
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Conclusions -

• Tropical droughts, for the domain average, lag
  onset
• of warm ENSO events by roughly a season.

• Droughts typically persist beyond the demise of
  El Nino,
• consistent with studies of tropical height
  anomalies.

• While the greatest spatial coverage of drought
  occurs
• during El Nino, magnitudes influenced by
  other factors.

• Interesting variations on longer time scales
  implications?
• Though a preliminary result, suggests more
  widespread
• droughts in the tropics with warmer
  oceans/land temps.