Invited reviewExploring records of Saharan dust transport and hurricanes in the western North Atlantic over the Holocene
Introduction
An anti-correlation between Atlantic dust loads and hurricane frequency has been documented over the past 50 years (1955–2005) (Dunion and Velden, 2004; Evan et al., 2006; Wang et al., 2012). However, there is debate as to whether there is any direct causal relationship between dust and hurricane activity or whether the two parameters are simply mutually affected by larger scale atmospheric patterns (Braun, 2010; Shu and Wu, 2009). Nonetheless, atmospheric modeling (Pausata et al., 2017) suggests that there was enhanced hurricane activity during the African Humid Period (roughly 5 to 10 thousand years ago) (Adkins et al., 2006; DeMenocal et al., 2000; McGee et al., 2013) in the mid-Holocene, due, in part, to the reduced African dust transport over the Atlantic during that time. Furthermore, as African dust emissions are predicted to decline in future decades, an associated increase in hurricane activity has been suggested (Evan et al., 2016).
Geographically, the main development region (MDR) of Atlantic hurricanes (10–20°N, 15–85°W) coincides with the region of the longest range transport of Saharan dust to the northwestern tropical Atlantic (Fig. 1). This geographical coincidence results from several atmosphere-ocean dynamical factors. Clearly, the distribution of tropical cyclone tracks shares a southern boundary with the summer North African dust plume that extends out over the Atlantic (Fig. 1). In boreal summer, aerosol dust emissions from North Africa are transported by easterly winds across the Atlantic toward the Caribbean Sea, the Gulf of Mexico and the southeastern United States (Ben-Ami et al., 2012; Prospero, 1999; Prospero et al., 2014; Prospero and Nees, 1986). During winter, the Harmattan winds carry the dust further south (generally south of 15°N) toward the Amazon region. This corridor of dust is generally bound by the intertropical convergence zone (ITCZ) to the south, where rainout and wet deposition of dust occur, and by the mid-latitude westerly winds to the north. In the case of tropical cyclone tracks, the MDR, also bound by the ITCZ to the south, develops in the zone of reduced vertical wind shear and sea surface temperatures (SSTs) in excess of 26.5 °C in summer, properties considered to be the key requirements in cyclone formation (Gray, 1968). With this situation, African dust presents a potential negative feedback on hurricane activity for two reasons. First, dust over the Atlantic cools SSTs (Evan et al., 2008). Second, the warm, dry Saharan Air Layer that carries North African dust over the MDR at about 700 hPa may contribute to an increase in vertical wind shear (Wu, 2007).
To interrogate the accuracy of the predictions involving feedbacks between African dust emissions and hurricane activity, we take a geological approach in this study and explore available records of both properties (dust and storms) in the instrumental period and beyond. Using timescales dictated by the nature of available records, we investigate this problem over 3 periods of increasing temporal scale: the past 200 years, the past 3000 years and the past 12,000 years (the Holocene). In all three timescales, we find evidence for non-stationary, weak or unexpected relationships between African dust emissions and tropical cyclone activity. Past records of both properties are imperfect and the records may potentially be overprinted by other important factors. In some instances, this makes it difficult to directly test the potential negative feedback of African dust on Atlantic tropical cyclone activity. In the Discussion, we suggest future studies which may be able to address this potential feedback with better spatiotemporal resolution than is possible with existing data.
Section snippets
Records of African dust emissions
African dust emission and transport over the Atlantic has been estimated with annual resolution for the period 1853–2009 (Evan et al., 2016). This record is based on a reanalysis of wind-field data (CIRES-20CR) for the same time period which is calibrated to dust optical depth satellite observations and a coral-based record of dust deposition near Cape Verde (16.762°N, 22.890°W) covering the past 60 years (Evan and Mukhopadhyay, 2010; Mukhopadhyay and Kreycik, 2008).
Available records of dust
The past 200 years
We performed a least-squares linear correlation analysis between the Cape Verde Dust Optical Depth record (Evan et al., 2016) and the HURDAT2 record of annual Atlantic named storms, hurricanes, major hurricanes and accumulated cyclone energy (ACE). Using the full time period for which the two records overlap (1853–2009), all 4 of the HURDAT2 cyclone indices have a negative correlation with the dust optical depth dust record (Table 1). Of the annual records (named storms, hurricanes, major
Discussion
While the records of African dust emissions and Atlantic storm activity we have examined have lacked a consistent relationship, there are limitations to this study resulting from inherent differences in the type of proxy records used which deserve discussion in light of the results. The first relates to the geographical representativeness of the proxies and the second relates to sparse data on North Atlantic dust. Both of these limitations lead to clear formulations for future work.
Conclusions
In this study, we have undertaken a focused review to recount observed relationships between African dust emissions and Atlantic tropical storm activity. Over the instrumental period (past 200 years), during a majority of the time (61% of 30-yr periods), there is a significant anti-correlation between Atlantic hurricane counts and dust aerosol optical depth near Cape Verde, confirming other studies that have documented this relationship over the past few decades (Dunion and Velden, 2004; Evan
Acknowledgements
This study received support from the University of Southern Mississippi. We thank Michael Toomey for providing the grain size records from the Grand Bahama Bank study published in 2013.
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