Over the ocean, the solar dimming phenomenon could not be analyzed during the pre-satellite period due to the lack of long-term regular radiation measurements. Changes in atmospheric aerosols could be usefully obtained by satellite aerosol retrievals. An analysis of meridional distribution of aerosol optical thickness, its seasonal variations and tendencies is of importance to our understanding of the solar dimming - brightening phenomenon over the ocean within different latitudinal zones.

We have used the eight-year MISR and MODIS-Terra data sets, from March 2000 to February 2008, as well as the five-year MODIS-Aqua data set, from July 2002 to June 2007, in order to analyze meridional distribution of aerosol optical thickness and its tendencies over the ocean. Obtained meridional distributions of aerosol optical thickness were verified by comparing the three sensors: similar results obtained by the three different datasets support each other. Our analysis has led us to the following conclusions:

a) During the eight-year period under consideration, 2000 – 2008, solar brightening, detected over the land, was not observed in cloudless conditions over the ocean at mid-latitudes (30N - 60N) in the Northern hemisphere. This suggests that the solar dimming-brightening phenomenon, associated with anthropogenic aerosols, is rather a local (regional) one, restricted to highly-populated and/or industrial zones.

b) Over the ocean there is pronounced meridional aerosol asymmetry. It was found that there were seasonal variations in the meridional aerosol asymmetry: it was pronounced during the half-year period, from March to August (the most pronounced asymmetry was observed from April to July). There was no noticeable meridional hemispheric asymmetry during the season from September to December.

c) The Northern hemisphere bimodal maximum was observed in the meridional distribution of AOT over the ocean. An explanation for this includes two contributory factors. The first factor is associated with two different circulation regimes in the Northern hemisphere: predominant westerlies towards the north-east across the Pacific Ocean and predominant easterlies to the south-west across the Atlantic Ocean. Another contributory factor to the bimodal maximum is significant AOT over the Indian Ocean, from the equator up to 20N.

References

Kishcha, P., Starobinets, B., Kalashnikova, O., Long, C., P. Alpert (2009), Variations in meridional aerosol distribution and solar dimming. Journal of Geophysical Research, 114, D00D14, doi:10.1029/2008JD010975.