GRL - Evidence Of Possible Impacts Of Saharan Dry Air And Dust On Atlantic Tropical Cyclone Activity

[hatrack]

Donglian Sun

Department of Earth Systems and GeoInformation Sciences, College of Science, George Mason University,
Fairfax, Virginia, USA

Center for Earth Observing and Space Research, College of Science, George Mason University,
Fairfax, Virginia, USA

K. M. Lau

Laboratory for Atmospheres, NASA Goddard Space Flight Center,
Greenbelt, Maryland, USA

Menas Kafatos

Center for Earth Observing and Space Research, College of Science, George Mason University,
Fairfax, Virginia, USA

Abstract

<1> In this study, we provide preliminary evidence of possible modulation by Saharan dust of hurricane genesis and intensification, by contrasting the 2007 and 2005 hurricane seasons. It is found that dust aerosol loadings over the Atlantic Ocean are much higher in 2007 than in 2005. The temperature difference between 2007 and 2005 shows warming in the low-middle troposphere (900–700 hPa) in the dusty region in the eastern North Atlantic, and cooling in the Main Development Region (MDR). The humidity (wind) differences between 2007 and 2005 indicate significant drying (subsidence) in the Western North Atlantic (WNA) in 2007. The drier air in the WNA in 2007 is found to be associated with the further westward transport of the Saharan air layer (SAL). To quantify wind pattern favorable for transport of SAL over the WNA, we define a zonal wind stretch index which shows significant long-term correlation with the mid-level humidity in the WNA. Analyses of the stretch index and related environmental controls suggest that the westward expansion of the Saharan dry air and dust layer can be an important factor in contributing to the difference between the relatively quiescent hurricane season in 2007 and the very active season of 2005.

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<15> As compared to 2005, dust aerosols in 2007 increased over the eastern Atlantic and MDR (Table 1). Coincident with dust increase, there was SST cooling in these regions (Table 1). Nevertheless, SST changes may also be affected by other factors, such as surface winds , clouds, and the depth of ocean mixed layer, etc. Lau and Kim <2007b> showed that about 30% SST cooling in the month of June 2006 compared to the same month in 2005 can be attributed to the radiative cooling effect of dust aerosols. The 2007 SST conditions were very similar to 2006. The difference in Relative Humidity (RH) at low-middle level (e.g., 700 hPa) shows significant drying in the western region, including the southern region of WNA, but no significant change in the GOM (see Figure S1). This provides evidence that the GOM receives relatively less dust loading than the MDR and is not likely affected by dust aerosols, while the MDR and southern region of WNA can be partially affected by dust aerosols.

<16> The SST's for both MDR and WNA in 2007 were lower than in 2005 but still above average values (Table 1). Evans <1993> indicated that SST alone is an inadequate factor, while Emanuel et al. <2004> demonstrated that the TC intensity changes are very sensitive to humidity in the lower troposphere. Hoyos et al. <2006> showed that the 850 hPa specific humidity (the ratio of water vapor to air) over 5–25°N, and 90°W–20°E, may be linked to the frequency of intense hurricanes (category 4 and 5). Wu <2007> indicated that the 850 hPa humidity averaged over 10–20°N, 30–80°W, which is similar to the MDR, is correlated with the mean peak hurricane intensity.

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<24> In 2007, dust aerosol loading during the early summer, the most active period for African dust outbreaks, were found to be much stronger over the Atlantic Ocean compared to 2005. As coincident with the increasing dust aerosol concentration in 2007, the U-W wind difference vectors provide evidence of induced anomalous over-turning circulation as compared to the normal circulation in each year. The temperature difference between 2007 and 2005 shows warming in the middle atmosphere in the east dusty region, and cooling (>0.5°C) in the MDR. The humidity and wind differences between 2007 and 2005 suggest an increasing moisture (from increased convection most likely) in the east and significant drying (subsidence) in the west in 2007. This can partially explain why very few TCs were found to form and intensify over the WNA in 2007. To quantify the zonal advection of Saharan dry air and dust into the WNA, a zonal wind stretch index was proposed. The index shows significant long-term correlation with the WNA area averaged mid-level humidity (r = 0.69). Comparing the basic controls of SST , relative humidity and zonal wind index, we find that 2006 and 2007 were similar to each other and to the climatology, with slightly higher SSTs in the MDR and the WNA, and westward penetration of the Saharan dry air and dust. The hurricane season of 2005 was highly anomalous with significantly higher SST and higher relative humidity in the MDR and WNA. Most strikingly, the wind stretch index showed a large reversal from climatology, indicating highly suppressed advection of Saharan dry air and dust into the WNA. Based on the results presented, we conclude that the difference in westward penetration of the Saharan dry air and dust can be an important factor in contributing to the difference between the near- normal 2007 and 2006, and the very active 2005 hurricane seasons. Because the hurricane peak season in the Atlantic basin usually occurs during August–September–October (ASO), the moisture field anomaly associated with peak dust aerosol outbreak during early summer before the peak hurricane season may have some practical implication for the prediction of Atlantic hurricane activities.

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[Betsy Ross]

1. Wow!

Facinating read. Thanks for posting. And the dust aerosol early this summer? Something is stirring up the Atlantic, day after day, storm after storm.