Global Characteristics of Tropical Droughts During ENSO Warm Events Bradfield Lyon

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 ~
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?
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
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
• Will consider only the
global scale characteristics
of drought (in the tropics)
Land Rainfall
Land area
Local Correlation: PRCP’ and Surface T’
• Temp-prcp relationships markedly different for ocean/land
Dominant EOF of Tropical Temp Anomalies - Reanalysis
1st
EOF
T’ zonal average
84%
1st EOF
T’ avg. over [lat./long.]
89%
PWAT’ similar
structure (54%)
• Time series of these EOFs highly correlated with ENSO & each other
Tropical heating lags Nino 3.4 SST (and convection)
• Reanalysis consistent with radiosonde and MSU temp obs.
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
How is a drought defined?
Weighted Anomaly Standardized Precipitation
“WASP”
WASP N 
N
1
 WASP

N
i 1
 Pi  Pi Pi

  P
A
 i




• 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.
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. ]
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
• Linear correlation between
top and bottom time series
is 0.86
r = 0.86
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 =
rms[log(area)-log(area)]
Consistent relationship: drought spatial coverage and El Nino –
but lots of scatter, too….
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.
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).
NINO 3.4 and Tropospheric Temperature Anomalies…
• See the ENSO lead and in-phase relationship with trop. heating.
Trends …
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…
Average values of WASP and temp. anomalies for Zimbabwe…
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.