A study of the influence of the Saharan Air Layer on tropical cyclones using TOMS data

Recent studies (Dunion and Velden, 2004; Evan et al., 2006; Niscovic et al., 2008) have shown that the Saharan Air Layer (SAL) tends to inhibit tropical cyclone (TC) formation and further development. Although, the thermodynamical and dynamical conditions that characterize the air masses in presence of the SAL (strong temperature inversion, mid-level jet and intense shear) are theoretically inhibiting conditions for TC formation and evolution, the role played by the transported dust, particularly on the cloud microphysics and consequently the TC development, remains still unclear. A quick look at the TC behavior in presence of dust reveals that some of them remain insensitive to the dust presence (either no change in intensity or intensifying), whereas some others decrease in intensity. The main goal of this work was to check the possibly inhibiting role of the SAL and of the contained dust on TC formation and development by using TOMS aerosol index (AI) data corresponding to the 1979-2007 period. The number of pixels containing dust and the corresponding AI values in an Atlantic box (0°-30° N, 15°-60°W) for the time laps between 20 August and 30 September (as in Evan et al. (2006)) were analyzed in relation to the successive positions of the 78 TC (344 cyclonic days) that crossed the box in the same time laps. The mapping of the mean AI values for the 1979-2007 period and the position of all the TCs on the day they were named shows that the TCs are positioned all along the southern and western boundary of the mean AI value of 1.5, thus suggesting an AI cut off value above which cyclogenesis is strongly inhibited. The analysis of the aerosol TOMS map for each day that a TC was named, in relation to its position, reveals that up to 35% of the TC were named on dust (AI<1.5) and cyclogenesis occurs therefore mainly in dust free regions. For the 344 cyclonic days studied, there is an increase in TC intensity on 45% of the days, a decrease for 17% of the days and no change for 38% of the days. In presence of dust, the behavior of the TC development tends to indicate more cases of intensification (26%) than weakening (13%), no change in intensity being the most common behavior (61%). Nevertheless, when taking into account the total number of cyclonic days corresponding either to an increase in intensity or to a weakening, there is roughly the same percentage of days (8%) with intensification and weakening in the presence of dust. Further, the presence of the dust seems to affect differently the development of the TC according to its position relative to the dust. Most commonly, the TCs intensify when the dust is located in the region above the TC daily trajectory (no penetration of the dust into the TC and subsequently no interaction), and decrease or do not change intensity when dust is situated under the TC daily trajectory.

References

Dunion, J.P., and C.S. Velden , 2004 : The impact of the Saharan air layer on Atlantic tropical cyclone activity, BAMS; March 2004. Evan, A.T., J.Dunion, J.A. Foley, A.K. Heidinger, C.S. Velden, 2006 : New evidence for a relationship between Atlantic tropical cyclone activity and African dust outbreaks, Geoph. Res. Letters, 33, L19813. Nickovic, S., C. Perez, O. Jorba, J.M. Baldasano, 2008 : Atlantic tropical cyclones and Saharan dust : a simulation study, Geoph. Res. Abstracts, 10, EGU2008-A-06697, EGU General Assembly, 2008.