Publication Abstracts

Li et al. 2011

Li, J., B.E. Carlson, and A.A. Lacis, 2011: El Niño-Southern Oscillation correlated aerosol Ångström Exponent anomaly over the tropical Pacific discovered in satellite measurements. J. Geophys. Res., 116, D20204, doi:10.1029/2011JD015733.

El Niño Southern Oscillation (ENSO) is the dominant mode of interannual variability in the tropical atmosphere. ENSO could potentially impact local and global aerosol properties through atmospheric circulation anomalies and teleconnections. By analyzing aerosol properties including Aerosol Optical Depth (AOD) and Ångström Exponent (AE; often used as a qualitative indicator of aerosol particle size) from the Moderate Resolution Imaging Spectrometer (MODIS), the Multi-angle Imaging SpectroRadiometer (MISR) and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) for the period 2000 to 2011, we find a strong correlation between the AE data and the Multivariate ENSO Index (MEI) over the Tropical Pacific. Over the Western Tropical Pacific (WTP), AE increases during El Niño events and decreases during La Niña events, while the opposite is true over the Eastern Tropical Pacific (ETP). The difference between AE anomalies in the WTP and ETP has a higher correlation coefficient (>0.7) with the MEI than the individual time series, and could be considered as another type of ENSO index. As no significant ENSO-correlation is found in AOD over the same region, the change in AE (and hence aerosol size) is likely to be associated with aerosol composition changes due to anomalous meteorological conditions induced by the ENSO. Several physical parameters or mechanism that might be responsible for the correlation are discussed. Preliminary analysis indicates surface wind anomaly might be the major contributor, as it reduces sea-salt production and aerosol transport during El Niño events. Precipitation and cloud fraction are also found to be correlated with tropical Pacific AE. Possible mechanisms including wet removal and cloud shielding effects are considered. Variations in relative humidity, tropospheric ozone concentration and ocean color during El Niño have been ruled out. Further investigation is needed to fully understand this AE-ENSO covariability and the underlying physical processes responsible for it.