The Strength of El Niño and theSpatial Extent of Tropical Drought

Bradfield Lyon

International Research Institute for Climate Prediction,The Earth Institute at Columbia University

Climate Diagnostics and Prediction WorkshopMadison, WI

October 18-22, 2004

Lyon, B. 2004, The strength of El Niño and the spatial extent of tropical drought, GRL (in press).

Lyon, B. 2004, ENSO and the Spatial Extent of Interannual Precipitation Extremes in Tropical Land Areas, J. Climate (in review).

Thanks to...

Mark Cane, Tony Barnston, Adam Sobel, Alessandra Giannini,Simon Mason, Chet Ropelewski

El Niño and tropical droughts

• General tendency for tropical land areas to dry during ENSO (+) events • Want to examine / quantify the spatial extent of droughts 30S-30N

Questions...• How large a role does El Niño play in forcing drought (particularly) when viewed from a tropics-wide perspective?

• Does the relationship between drought and El Niño vary depending on the severity of drought considered?

• How does this behavior vary between individual El Niño events?

• Briefly: How symmetric is the behavior of excessively wet conditions across the tropics during La Niña?

• Any ideas on possible forcing mechanisms?

Data

• Monthly rainfall - UEA CRU and CMAP

• Monthly SST - Smith and Reynolds ERSST

• Atmospheric data - NCEP/NCAR Reanalysis

• Domain – 30S to 30N, land areas only

• Period of analysis – 1950 to 2003

• Base period for climatology – 1961 to 1990 (1980 to 2000 for CDAS)

NS

NN

N

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SWASP

PPPP

S

1

Defining drought (and excessively wet conditions)

• 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 we’ll look at (1 and) 12 month accumulations

Much of the variance in soil moisture is associated with the variability in precipitation

TPdKdZ

ZPDSIPDSI

ii

iii

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31897.0 1

• Need to compute values for 4 parameters based on a training period

• The PDSI is highly correlated with other, simpler indices

WASP12 is well correlated with PDSI...

Lyon 2004, J. Climate (in review)

% T

ropi

cal L

and

Are

a

Drought is an integrated moisture deficit

• El Nino: 5-month running average of Nino 3.4 SST anomaly > + 0.4 C• La Nina: “ “ “ “ “ “ “ “ < - 0.4 C

WASP1 < - 0.5WASP12 < - 1.5

Percent of tropical land area in drought of varying severity (1950-2003)

Lyon, 2004 GRL (in press)

WASP12

Note: 5 month lag between max. Nino 3.4 SSTA and extent peaks

Lyon, 2004 GRL (in press)

Drought extentvs.

Nino 3.4 SST anomaly

WASP12 < -1 (moderate)

WASP12 < -2 (severe)

Drought occurrence increases with droughtseverity during El Niño

Lyon 2004, GRL (in press)

Lyon 2004, J. Climate (in review)

Is there a lag between the onset of moderate vs. severe conditions?

What about positive rainfall anomalies?

% tr

opic

al la

nd a

rea

L-

L-

x x

Lyon, 2004 J. Climate (in review)

Nino 3.4 SST Anomaly

Asymmetricbehavior relativeto El Niño anddrought…

ENSO is asymmetric…

Lyon, 2004 J. Climate (in review)

Forcing?

• Why should drought extent be linearly related to Nino 3.4 SST anomalies?

• There are different “flavors” of El Niño, so why should drought extent be captured by such a simple index?

• Failed teleconnections (of drought) appear to be compensated by increased drought occurrence elsewhere – how?

Sobel et al. 2002, J. Climate

TropicalSSTs warmin responseto El Nino...

Convectiontransfers this heat tothe atmosphere...

But convective heating is not a linear function of SST...

CMAP (DJF)

Su and Neelin 2003, J. Climate

...and averaged across the tropics, SST and PRCP are NOT well correlated...

(GPCP,TRMM)

Su and Neelin 2003, J. Climate

...but the net tropical heating is still linearly related to the tropical average SST

Linearity ~holds regardless of the location andconfiguration of SSTanomalies, although someregions show greatersensitivity in response (Su et al. 2003)

Lyon, 2004 J. Climate (in review)

“CIN”

Drought Forcing? Consider the Reanalysis...

Su and Neelin, J. Climate 2003

Rainfall anomalies averaged over the tropical oceans (12-month running average)

CMAP

OLR

• A remarkably robust relationship is found between the spatial extent of drought in tropical land areas and El Niño “strength”, with tropical land area in drought increasing by roughly a factor of 2 between weak and strong events – for all levels of severity

• In many teleconnected tropical land regions, the occurrence of drought (i.e., the total months experiencing drought) is greater during El Niño than for all other times (1950-1998) – and the occurrence increases with increasing drought severity

• There are significant asymmetries when comparing the behavior of La Niña and excessively wet conditions with El Niño and drought – ENSO has many asymmetric characteristics...

• The results indicate the importance of separating land from ocean areas when considering the rainfall response of tropical SST anomalies – a warmer tropics does not necessarily imply a wetter tropics everywhere...

• The behavior of tropical rainfall variations in different GCMs is currently being investigated with diagnostic studies to follow...

Summary