On the ``Center of Actions'' and the Long-Term Variability of African Dust Transport Across the Atlantic

Mineral dust plays a key role in global climate and atmospheric chemistry. Variations in dust concentration affect the Earth's radiative budget by scattering and absorption of solar and infrared radiation and modifies cloud properties and photolysis rates. Dust also provides an important reactive surface for heterogeneous reactions and impacts terrestrial and ocean biogeochemistry upon deposition on the ground or ocean. The North African continent represents a dominant global source of mineral dust. Throughout the year large dust plumes are transported to the Atlantic Ocean and the Mediterranean Sea from the Saharan desert and surrounding regions. This export is subject to a strong interannual variability, which in turn is related to the large-scale circulation. Previous studies have proposed that this interannual variability can partially be explained by the North Atlantic Oscillation (NAO) index. In this study we revisit this relationship and first calculate the correlation of the NAO index with the absorbing aerosol index (AAI) of TOMS/Nimbus7 and TOMS/Earth Probe. The satellite data sets cover the time spans of 1979-1993 (TOMS/Nimbus7) and 1996-2004 (TOMS/Earth Probe). We focus on the winter months and the region located over the eastern part of the northern tropical Atlantic (15-30°N, 30-5°W) where dust is dominant. In a second step we extend our correlation analysis, considering the positions and pressures of the Azores High and the Icelandic Low as independent variables ('Center of Actions' approach). Our results show a correlation between NAO index and AAI is r=0.34 for the time span of 1979-93. This contrasts with earlier studies, which reported a higher correlation coefficient for the same area and time. We attribute this difference to an updated processing algorithm (version 8) for the TOMS data. For the total time span of 1979-2004 this correlation decreases further to r=0.14. Hence, we conclude that the NAO index has a negligible correlation with the AAI in our region of interest. In contrast, if we apply the Center of Actions approach and treat the Azores High and the Icelandic Low separately, we can improve the correlation. We find the strongest relationship for the AAI and the latitude of the Azores High (r=0.46 for 1979-2004) indicating that the north-south movement of the Azores High has a greater impact on dust transport to this region than the pressure fluctuations. Overall, the analysis shows that the Azores High has a greater impact than the Icelandic Low on dust transport.