Sea-land total ozone differences from TOMS: GHOST effect

Abstract

Mean global TOMS (Total Ozone Mapping Spectrometer) ozone data, averaged in time, reveals persistent year-to-year differences in total ozone between continents and oceans. This feature has been named GHOST (global hidden ozone structures from TOMS). During Northern Hemisphere summer it can be seen within the latitudinal belt between 40°S and 50°N. The most pronounced land-sea difference in total ozone with values of up to 18 Dobson units is observed between latitudes 35°N and 40°N. The gradients associated with the coastlines are stronger in summer, when transient planetary wave activity decreases, but is still observable in the winter period. The Iberian Peninsula has been selected as a case study to examine the effect of each possible contribution quantitatively. It has been found that the truncation of the lower tropospheric column due to the topography explains 26% of the land-sea differences, while permanent differences in tropopause height distribution can account for a further 8%. After these “corrections” other structures remain. Additional contributions due to the TOMS total ozone retrieval algorithm artifact (absorbing aerosol distribution) are also explored. After considering the optical depths and absorbance of aerosols above the Iberian Peninsula, the remaining 66% is compatible with the presence of UV-absorbing aerosols whose effects may not be correctly accounted for in the TOMS retrieval algorithm.