Bh

The chronology of the latitudinal position of the BH can be estimated from paleoenvironmental records as areas directly under its influence are dominated by subsiding dry air. Arrival of the BH over an area not previously under its influence should result in increased aridity, detectable by a variety of vegetational and isotopic proxies. Palynological studies are especially useful for recording long-term shifts as their relatively low temporal resolution reduces the noise of higher-frequency oscillations and are less dependent upon confounding external factors such as local salinity and hydrological changes.

A large number of studies from the Central American-Caribbean region (Bradbury et al. 1981; Leyden 1985; Hodell et al. 1991; Peterson et al. 1991; Curtis and Hodell 1996; Islebe et al. 1996; Leyden et al. 1996; Curtis et al. 1998, 1999; Higuera-Gundy et al. 1999; Islebe and Sanchez 2002; Nyberg et al. 2001; Rosenmeier et al. 2002) have shown low-frequency environmental changes generally in temporal agreement with the ITCZ movement proposed by Haug et al. (2001). The northwestern edge of the region (in particular the northern Yucatan peninsula) seems to demonstrate a greater variability. This can perhaps be attributed to its location on the edge of the zone of influence, which increases the site's ability to record small latitudinal movements, which leave no detectable signals in locations farther south and east. Complicating these estimations however, are more general hemispheric changes resulting from variability in solar insolation due to orbital influences, especially the processional aspects of the Milankovich cycles (Berger and Loute 1991; Leyden et al. 1994). Additionally, after the mid Holocene the difficulty in separating natural and anthropogenic effects becomes increasingly difficult regionally (Leyden 1987; Leyden et al. 1998). It should be noted that only areas located directly under shifts in BH location are expected to show evidence of such shifts; far southern areas, for example, are not expected to display palynological responses to shifts that occur to the north, as such shifts should not result in aridity changes at their location.

Paleotempestology uses sedimentary evidence to establish long-term proxy hurricane strike records. In such studies, sediment cores are extracted from coastal wetlands and storm-generated layers, identified by a variety of geologic methods, are dated, permitting a chronology of site-specific landfalls (Liu and Fearn 1993, 2000; Liu 2004: Donnelly et al. 2001a,b, 2004; Donnelly 2005; Donnelly and Woodruffe 2007; Scott et al. 2003). Records from several sites can be correlated to develop regional hurricane histories (Liu 2004). Calibrations based on modern analogs indicate that the threshold storm intensity required for depositing recognizable sedimentary signatures is roughly that of major hurricanes (category 3 or greater) (Liu 2004; Donnelly and Webb 2004).

Significantly, the majority of millennial-scale proxy hurricane landfall records display evidence of the temporal clustering of events, often cyclic, indicating periods of hyperactivity. That these intervals are non-synchronous suggests a movement in the zone of maximum TC activity, as opposed to a basin-wide frequency increase. In the United States Liu and Fearn (1993,2000) found evidence for a hyperactive period from 3400-100014C yr BP for the northern Gulf of Mexico, while Scott et al. (2003) emphasize the anti-phase timing of Atlantic and Gulf Coast hyperactivity, based on a site in South Carolina. In the Caribbean, Bertran et al. (2004) found cyclical periods of TC activity on Saint Martin in the French West Indies, with the hyperactivity dating from -4900-2600 BP; McCloskey et al. (2004) found two periods of hyperactivity between 5500-2500 BP for the central coast of Belize, while Donnelly has found evidence for hyperactivity in Puerto Rico for the periods 5400-3600 BP, 2500-1000 BP and 250 BP to the present (Donnelly 2005; Donnelley and Woodruff 2007). If correct, these records indicate increased hurricane landfall from -5500-2500 BP for the northern Caribbean and from - 3500-1000 BP for the northern Gulf Coast, with activity increased for both during the period -3500-2500 BP. The timing of this slow south-to-north migration of hyperactivity roughly parallels that of the ITCZ movement postulated by Haug et al. (2001).

It should be noted that these proxy records, based only on landfall of major hurricanes, represent a minimum record of TC activity. Given the relative scarcity of major hurricanes, which currently comprise -20% of US landfalling TCs, (Landsea 1993) it seems reasonable to assume that stratigraphic intervals providing sedimentary evidence for the increased frequency of landfalling major hurricanes do, in fact, represent extended periods of overall increase in TC activity, once geomorphological and sea level changes are controlled for. We suggest that the spatial/temporal shifts in these intervals result from the migration of the zone of maximum TC frequency.

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