Long range transport of aerosol and its implications for climate
Work in collaboration with Sultan Hameed
Large amounts of biomass burning, mineral dust aerosol, and anthropogenic pollution aerosol are exported from source areas of the continents. The transport patterns show strong inter-annual and intra-annual variations, which are linked to both the variability of the source strength and the large scale circulation. This variability is not well understood, but is a critical element for assessing the impact of aerosol on regional and global climate. In order to improve our understanding of the seasonal and interannual variability of aerosol for different regions of the globe, we perform a new method of satellite data analysis, by analysing the interaction of aerosol properties and large-scale transport in terms of the variation of the atmospheric "Centers of Action".
The Centers of Action
The term "Centers of Actions" refers to the extended semi-permanent pressure systems that can be seen in monthly averaged global sea level pressure fields. Four centers exist in the northern hemisphere (the Azores High, the Icelandic Low, the Hawaiian High, and the Aleutian Low), and three more centers of high pressure in the Southern hemisphere (the Indian Ocean High, the South Pacific High, and the South Atlantic High). The figure below shows position and strength of the pressure systems for the Northern Hemisphere winter averaged over the 1980-2000. Each center dominates the atmospheric circulation over a large domain, which varies seasonally and interannually. For this reason, these pressure systems are key players in the large scale dynamics and hence control the transport of aerosol particles. |
| Fig. 1: DJF Surface level pressure in hPa, 1980-2000 average. |
In contrast to the NAO, the Centers of Actions approach makes use of the information on the position as well as the pressure of the COA which provide additional degrees of freedom for our aim to explain the seasonal and interannual variations of aerosol load around the globe. This means that instead of having one NAO index which describes the relationship between Icelandic Low and Azores High in a very condensed and simplified manner, we consider six Centers-of-Action-indices over the North Atlantic: surface pressure, latitude and longitude for both the Icelandic Low and the Azores High.
Variability of mineral dust transport from the Sahara over the Atlantic
Recently, we applied this approach for the first time to the problem of the inter-annual variability of dust transport over the Atlantic [Riemer et al. 2006]. We used the recently updated version (Version 8) of TOMS/Nimbus-7 and TOMS/Earth Probe monthly mean absorbing aerosol index (AAI). This data set covers the 23 years, 1979 to 1993 and 1997 to 2004. We focussed on the winter season, and on the area off the North-West African coast (15-30oN, 30-5oW).We then correlated the aerosol data of our region of interest with the positions and the surface pressure values of the two Centers of Action in the Atlantic, the Azores High and the Icelandic Low. This approach decouples the two semi-permanent pressure systems, and it reveals that the Icelandic Low does not play a role for the dust concentration in our region of interest. Instead, the position of the Azores High turns out to be the most important factor, with the Azores High Latitude yielding the highest correlation. The Figure below displays a map of the spatially varying correlation coefficient of AAI and Azores High Latitude averaged over all winters. This Figure shows that the maximal correlation of AAI and Azores High Latitude with values over r=0.5 occurs within our area of interest.
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Publications
O. Doherty, N. Riemer and S. Hameed [2008] Saharan mineral dust into the Carribean: Observed trends and controls, Journal of Geophysical Research, 113, D07211, DOI: 10.1029/2007JD009171. (pdf)
N. Riemer, O.M. Doherty, S. Hameed [2006] On the variability of African dust transport across the Atlantic, Geophysical Research Letters 33, L13814, DOI: 10.1029/2006GL026163. (pdf)