Abstract
Moisture anomaly conditions within humid and subhumid tropics that are associated with different types of El Niño and La Niña phenomena are described and analyzed with a focus on their spatial distribution and seasonal variability. Five dryness indices (Keetch–Byram Drought Index, Weighted Anomaly Standardized Precipitation Index, Standardized Precipitation Index, Palmer Drought Severity Index, and Percent of Normal Precipitation) were derived from ECMWF (European Centre for Medium-Range Weather Forecasts) fifth generation reanalysis (ERA5) reanalysis and University Corporation for Atmospheric Research (UCAR) datasets for the period from 1979 to 2019. Cross-correlation analysis was used to evaluate the relationships between the El Niño Southern Oscillation (ENSO) and selected dryness indices. To describe the seasonal variability of the ENSO–surface moisture relationships, the composite maps of dryness indices in different seasons were analyzed. The results showed a significant heterogeneity of the ENSO-induced moisture anomaly conditions both within and across various geographical regions. Four main areas in humid and subhumid tropics with the maximum effects of El Niño/La Niña events on the surface moisture conditions were found: Southeast Asia and Australia, Eastern and South Africa, Northeastern and Eastern South America, and Central America. It was shown that the effects of La Niña were usually opposite to those of El Niño, while the responses to the two types of El Niño differed mostly in the moisture anomaly intensity and its spatial patterns.
Highlights
The El Niño Southern Oscillation (ENSO) is a major phenomenon of climate variability at the interannual timescale that induces dramatic weather anomalies within and outside the tropics [1].The anomalously large release of heat and moisture from the ocean into the atmosphere leads to a large-scale transformation of atmospheric circulation [2,3] that invokes precipitation and temperature anomalies around the world [4]
In the case of Keetch–Byram Drought Index (KBDI), the driest conditions were observed in northwestern Australia, in the central and northeastern part for WASP, and in the eastern part for Standardized Precipitation Index (SPI)-3
The negative correlations between the El Niño and SPI data with 3-month averages (SPI-3) indices were observed in eastern Australia during Eastern Pacific (EP) events
Summary
The El Niño Southern Oscillation (ENSO) is a major phenomenon of climate variability at the interannual timescale that induces dramatic weather anomalies within and outside the tropics [1].The anomalously large release of heat and moisture from the ocean into the atmosphere leads to a large-scale transformation of atmospheric circulation [2,3] that invokes precipitation and temperature anomalies around the world [4]. The first mechanism involves the propagation of the Kelvin and Rossby waves in the tropical troposphere eastward and westward, respectively, which are forced by El Niño (La Niña) warming (cooling) of the troposphere over the tropical central and eastern Pacific region [5,6]. These tropical tropospheric temperature anomalies affect the temperature and humidity in the atmospheric planetary boundary layer, resulting in anomalies of the sea surface temperature (SST) in the tropical Indian and Atlantic Oceans through changed surface fluxes [7]. During El Niño, the eastward displacement of convective activity occurs over the Pacific region, leading to anomalous
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