A New Paper On The Role Of Land Surface Processes On Tropical Cyclone Activity

There is a new paper which documents the close coupling between land surface processes (in this case dust from the Sahara Desert and the Sahel in Africa, and tropical cyclone activity. The paper is

Wu L. (2007), Impact of Saharan air layer on hurricane peak intensity, Geophys. Res. Lett., 34, L09802, doi:10.1029/2007GL029564.

The abstract reads,

“The Saharan air layer (SAL), which is associated with African dust outbreaks, forms as air moves across the Sahara Desert, containing substantial amounts of mineral dust. While the relationships of Sahel rainfall with African dust outbreaks and Atlantic hurricane activity have been documented in previous studies, analyses of various independent datasets show that the Sahel rainfall, SAL activity and hurricane peak intensity in the Atlantic basin are highly correlated. The long-term trend in hurricane peak intensity generally follows the Sahel rainfall and SAL activity. The decreasing trend in hurricane intensity by the mid-1980s was associated with the enhancing SAL activity (drying relative humidity and enhancing vertical shear) and the severe drought in the Sahel, while the recent moderate increasing trend in hurricane intensity is consistent with the weakening SAL activity (wetting relative humidity, weakening vertical shear and decreasing dust load) and the ameliorating Sahel drought. This study suggests that the SAL may act as a link between the summer African monsoon and Atlantic hurricane activity.”

Excerpts from the paper read,

“The recent increasing hurricane activity has been related to the SST warming that occurred in the tropical Atlantic since the 1970s [Emanuel, 2005; Webster et al., 2005] and the Atlantic Multi-decadal Oscillation (AMO) [Goldenberg et al., 2001]. In this study, the mean SST for the hurricane peak season is averaged over 6–18°N, 20–60°W, the same area used by Emanuel [2005]. The SST data are obtained from the Extended Reconstructed SST (ERSST) dataset of the National Oceanic and Atmospheric Administration (NOAA) and the AMO index is obtained from the NOAA Climate Analysis Branch. The mean SST is not statistically correlated with the mean peak intensity (Figure 3c). On the other hand, the AMO index is statistically correlated with the peak intensity. The index has a correlation coefficient of 0.46 with the peak intensity. The hurricane peak intensity was consistent with the decreasing trend in the AMO index by the 1970s, but after the 1980s the intensity has been trending upward at a much slower rate than the SST and AMO index. The combined SAL effect index or the Sahel rainfall can better account for the 58-year trend in the mean peak intensity than the SST or AMO index. However, this is not to say that the SST and AMO have nothing to do with the hurricane intensity trends. Sahel is one of the most climatically sensitive zones in the world [Zeng, 2003]. The variability of the Sahel rainfall is closely associated with the global SST changes, the summer African monsoon, and even anthropogenic influences [Giannini et al., 2003; Held et al., 2005]. The tropical SST and AMO may affect hurricane intensity through the SAL.”

“While the close relationships of the Sahel rainfall with African dust outbreaks and Atlantic hurricane activity have been documented in previous studies, in this study evidence is provided through analyses of various datasets associated with African dust outbreaks, Sahel droughts and Atlantic hurricane peak intensity, suggesting that the SAL may act as a link between the summer African monsoon and Atlantic hurricane activity. The combined SAL effect index or Sahel rainfall can much better account for the trend in the mean peak intensity over the past 58 years than the tropical SST or AMO index. The long-term trend in hurricane intensity generally follows the Sahel rainfall or SAL activity. Since these high correlations of the peak intensity with the SAL activity and Sahel rainfall are derived from the independent datasets, uncertainties involved in datasets may not qualitatively affect the results of this study. In summary, the decreasing trend in hurricane intensity by the mid-1980s was associated with the enhancing SAL activity (drying relative humidity and increasing vertical shear) and the severe drought in the Sahel, while the recent moderate increasing trend in hurricane intensity is accompanied with the weakening SAL activity (wetting relative humidity, decreasing vertical shear and dust load) and the ameliorating Sahel drought.”

As has been emphasized in the literature (e.g. see) and in the 2005 National Research Council report, the climate system, in terms of its response to human- and natural-climate forcings (including land degradation in the Sahel from overgrazing), is much more complex than indicated by the 2007 IPCC Report.

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