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Characteristics of Tropical Droughts During ENSO Warm Events

This study examines the global characteristics of drought in the tropics during ENSO warm events. It explores the relationships between drought and ENSO, the variations over time, and the spatial extent of droughts. The study utilizes data on rainfall, temperature, and moisture, and analyzes the correlation between ENSO and drought events. The findings suggest that tropical droughts typically lag the onset of ENSO warm events and persist beyond their demise. While the greatest spatial coverage of drought occurs during El Nino, magnitudes are influenced by other factors.

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Characteristics of Tropical Droughts During ENSO Warm Events

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  1. 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 ~

  2. 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?

  3. 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

  4. 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 • Will consider only the • global scale characteristics • of drought (in the tropics) Land area

  5. Local Correlation: PRCP’ and Surface T’ • Temp-prcp relationships markedly different for ocean/land

  6. Dominant EOF of Tropical Temp Anomalies - Reanalysis 1st EOF T’ avg. over [lat./long.] 1st EOF T’ zonal average 84% 89% PWAT’ similar structure (54%) • Time series of these EOFs highly correlated with ENSO & each other

  7. Tropical heating lags Nino 3.4 SST (and convection) • Reanalysis consistent with radiosonde and MSU temp obs.

  8. 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

  9. How is a drought defined?

  10. 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.

  11. 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. ]

  12. 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

  13. 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)]

  14. Consistent relationship: drought spatial coverage and El Nino – but lots of scatter, too….

  15. 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.

  16. 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).

  17. NINO 3.4 and Tropospheric Temperature Anomalies… • See the ENSO lead and in-phase relationship with trop. heating.

  18. Trends …

  19. 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…

  20. Average values of WASP and temp. anomalies for Zimbabwe…

  21. 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.

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