An ENSO-induced aerosol dipole in the west-central Pacific and its potential feedback to ENSO evolution
We examine the impact of El Niño–Southern Oscillation (ENSO) on satellite observed aerosol optical depths (AODs) over the equatorial Pacific. AODs are found to decrease close to the International Dateline and increase in the vicinity of the Maritime Continent during the El Niño developing phase (and vice versa during La Niña) to form an AOD dipole. The positive AOD anomaly over the Maritime Continent is caused by increases in fire-induced fine smoke aerosols in Indonesia and coarse dust particles transported from Australia via the strengthening winds when El Niño develops and affects the Pacific walker circulation. The negative AOD anomaly around the International Dateline is caused by decreases in the emission of marine aerosols, as the developing El Niño is associated with weakening surface wind speeds in the region. This aerosol dipole is found particularly strong during the Central-Pacific type of ENSO, which has most of its sea surface temperature and wind variations around the International Dateline. The Eastern-Pacific type of ENSO produces positive AOD anomalies over the Maritime Continent but weaker negative AOD anomalies near the International Dateline. As a result, the AOD dipole is less obvious during the EP ENSO. The shortwave radiative forcing associated with the aerosol dipole is 29% (21%) as large as the ENSO-induced outgoing longwave radiation in the Maritime Continent (International Dateline) region and represents an important positive feedback to help sustain the ENSO development that may be potentially important in Central Pacific ENSO studies and predictions.
KeywordsAerosol dipole ENSO Aerosol shortwave radiative forcing Outgoing longwave radiation Equatorial Pacific
We thank two anonymous reviewers for their valuable comments. This research was supported by the National Science Foundation’s Climate and Large Scale Dynamics Program under Grant AGS-1505145. L. X. is grateful for the support from the Department of Energy’s Earth System Modeling Program under Grant DE-SC0016362. The MODIS aerosol optical depth data is downloaded from https://ladsweb.modaps.eosdis.nasa.gov, the SeaWiFS aerosol optical depth data from https://measures.gsfc.nasa.gov/data/DeepBlueSeaWiFS_Level3/, the NCEP/NCAR Reanalysis data set, the outgoing longwave radiation data and the Niño 3.4 index from https://www.esrl.noaa.gov/psd/.
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