Hypothesis Testing

Evidence for/against our proposed low-frequency latitudinal oscillation of the NA circulation system requires the long-term correlation of the various components.

In order to correlate low-frequency ITCZ and BH movements, it is necessary to establish an accurate proxy for BH location. One possibility is determining the zone of aridity resulting from the subsiding air issuing from the BH, recognizable palynologically by increasingly xeric taxa, or isotopically by the changing 818O ratios of closed basin lakes. However, both vegetational and isotopic proxy records are subject to a variety of local controls; additionally, they are constrained geographically as only fringe areas are directly affected by BH movements.

The correlation of ITCZ and TC zone could be more direct, based upon a basin-wide paleo-strike record developed from paleotempestological studies extending to the mid Holocene. With recognizable sedimentary signatures posited to be deposited only for major hurricanes, longer paleo-strike records seem unlikely, since major hurricanes probably did not occur during the early Holocene due to the lowered surface sea temperatures resulting from the large volume of glacial meltwater entering the NA basin.

However, comprehensive paleo-strike records do not currently exist. For the NA basin multi-millennial scale proxy records are spotty at best, particularly for the Caribbean and the US Atlantic Coast. However, new research may prove fruitful; if the frequency of TC-inhibiting El Nino events has increased throughout the late Holocene as posited (Clement et al. 2000; Trudhope et al. 2001; Haug et al. 2001; Moy et al. 2002; Koutavas and Lynch-Stieglitz 2004), one result could be an overall increase in annual frequency for mid Holocene TCs. If so, such an increase improves the possibility that landfall patterns may be preserved in the sedimentary record.

Based on our model, increased hurricane landfall frequencies can be expected to have occurred along the northern area (US Atlantic Coast) from ^8-4000 BP, and during the Medieval Warm Period, ^1100-600 BP, while decreased landfall frequencies should have occurred ^4000-2400 BP and during the Little Ice Age,

400-200 BP). The southern area (Gulf Coast and the Caribbean) should exhibit the reverse pattern.

Obtaining evidence for/against such temporal/spatial shifts in maximum strike frequency therefore presents a means of testing this model. During periods of extreme northern/southern movement, the fringe areas, which currently experience very low levels of TC activity, may have experienced increased activity, thereby producing relatively easily recognizable sedimentary evidence for frequency changes. Due to the latitudinal smearing of TC landfall resulting from the high-frequency oscillation in BH intensity, the record in central areas will quite possibly exhibit a less distinct signal. The long return intervals of major hurricanes, generally >100 years (Elsner and Kara 1999; Liu and Fearn 1993, 2000; Donnelly et al. 2001a,b, 2004) makes the existence of a clear sedimentary record over short time spans somewhat problematic, thereby reducing the utility of the recent oscillations connected to the Little Ice Age and the Medieval Warm Period. It is therefore suggested that studies focusing on the period from 8000-4000 BP in the extreme north and 4000-2400 BP in the south are the most likely to produce useful information.

Obviously, there are very significant practical difficulties in obtaining proxy strike records for the suggested periods, given the magnitude of sea level raise and geomorphological changes in dynamic coastal areas. Coring submerged kettle holes in the northern area is one possibility (J. P. Donnelly, personal communication), as are study sites in areas of uplift and steep bathymetry, or areas where "keep-up" reefs/mangroves have minimized relative sea level rise.

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