Spatial Relationships Between Circulation Features Short Term

The central node of the NA subtropical high-pressure ridge is referred to as either the BH or the Azores High, which, though displaying some differences in the winter are basically interchangeable during the summer months (Davis et al., 1997). Here we use the term BH. Being a high pressure system the BH rotates clockwise, spinning the northeasterly trade winds off its southern flank and the midlatitude westerlies to the north. Although the BH is a well-recognized atmospheric phenomenon, it remains a rather nebulous entity, defined in a number of ways (Shasamangolou 1990; Davis et al. 1997; Portis et al. 2001). The effect of the BH is most readily quantified by the NAO Index, which is the normalized difference, measured at a number of different locations, between the SLP below the BH and the IL (Hurrell 1995; Jones et al. 1997; Portis et al. 2001). The BH and IL generally behave in a "see-saw" manner, with high BH pressure intensity values correlating with low IL values, which coincides with large/positive NAO Index values and is referred to as a "strong" NAO; while low BH values generally correlate with high IL pressure intensity values, resulting in a "weak" NAO, with small/negative NAO Index values (Elsner et al. 2000). Because a more intense BH generally correlates to a northeastern position and a weaker BH to a southwestern position (Shasmangolou 1990; Machel et al. 1998), a "strong" NAO generally means a northeastern position of the BH, and a "weak" NAO a southwestern one.

Stronger NAO values, driven by the intensification of the BH, and the increased pressure gradient between the BH and IL, generally increase the intensity of the trade winds to the south and the midlatitude westerlies to the north, significantly impacting weather in the circum-NA region (Hurrell 1995; Kapala et al. 1998; Machel et al. 1998; Nyberg et al. 2001; Visbeck et al. 2001). Wind strength exerts an important control over SST, as increased surface winds increase heat transfer from the ocean to the atmosphere, lowering SST in the affected area (Hasanean 2004; Chiang and Bitz, 2005; Chiang, 2006). The strength of the trade winds also affects TC, by the effects on both SST and vertical shear.

Because NAO intensity is such a key feature of the NA circulation system the NAO index is more useful than the record of the central intensity of the BH in understanding the relationships between different components of the system.

That a close positive geographical relationship exists between the BH and TCs is demonstrated by a roughly tandem movement of TC tracks and the BH throughout the hurricane season, as they move first northeastward, before returning to the southwest in the fall.

The BH also operates as an important control over the steering of TC tracks, and consequently, for location of landfall. Elsner et al. (2000) show a direct relationship between NAO values and latitudinal position of landfalls for major hurricanes making landfall along the US coastline since 1865. Using bootstrap analysis they show statistically significant differences between the July NAO values for years with at least one major hurricane strike along the Gulf Coast (south) and Atlantic Coast (north), with lower NAO values for the Gulf Coast strike years (Elsner et al. 2000).

Using the NOAA dataset (Neumann et al. 1999) for the period 1948-2003, we visually selected the 9 extreme years each for the most eastern and western group of TC tracks, and applied a kernel density surface interpolation to display their geographical distribution (Fig. 1). A calculation of the average annual NAO index values shows negative values for the extreme western track years and positive values for the extreme eastern years, for both the NAO and NAO-mobile indices. Annual NAO values were used due to the influence that a strong winter NAO can have on SST during the following TC season.

Using a data set consisting of all 6 hour segments for all TC for the period, we queried out all TC segments by monthly NAO value, forming two groups

1. All segments which occurred during periods of "Extreme High'' NAO (>2.5 a)

2. All segments which occurred during periods of "Extreme Low'' NAO (<—2.5 a)

Kernel density surface interpolations were applied to each group, after which the "Extreme Low'' kernel density values were subtracted from the "Extreme High'' kernel density values and the resulting differences plotted (Fig. 2). In this figure light gray shading represents negative values, indicating areas experiencing more

Fig. 1 Interannual variability in TC track location displayed as kernel density surface interpolation, a technique that generalizes individual point locations or events to an entire area and provides density estimates at any location within the study region. Density estimates were derived by placing a symmetrical surface (195 miles bandwidth), over each event and are displayed according to the resulting kernel density value. (a) all TC tracks for the 9 years visually selected as having the westernmost track location for the period 1948-2003, (b) the same for the 9 easternmost years. The average annual NAO Index value for the 9 westernmost years are -0.11 (data from http://www.cru.uea.ac.uk/cru/data/nao.htm), and -0.18 (data from Portis et al., 2001); for the 9 easternmost years the corresponding values are 0.20 and 0.10

Fig. 1 Interannual variability in TC track location displayed as kernel density surface interpolation, a technique that generalizes individual point locations or events to an entire area and provides density estimates at any location within the study region. Density estimates were derived by placing a symmetrical surface (195 miles bandwidth), over each event and are displayed according to the resulting kernel density value. (a) all TC tracks for the 9 years visually selected as having the westernmost track location for the period 1948-2003, (b) the same for the 9 easternmost years. The average annual NAO Index value for the 9 westernmost years are -0.11 (data from http://www.cru.uea.ac.uk/cru/data/nao.htm), and -0.18 (data from Portis et al., 2001); for the 9 easternmost years the corresponding values are 0.20 and 0.10

TCs during periods of extreme low than extreme high NAO values, while dark gray areas indicate the reverse. Perhaps the most noticeable feature of this figure is the semicircular dark gray pattern over the western Atlantic, indicating the severe recurvature of TC paths connected with high NAO values. In contrast, the light gray areas form a more horizontal band, indicating a reduced tendency to curve northeastward during periods of low NAO values. A slight northward shift of TC tracks during high NAO values is also noticeable, with the horizontal light gray band covering extreme northern South America and southern Central America indicating that the most southern tracks occur mainly during periods of low NAO values.

Figure 3 maps the geographical distribution of the TC segments occurring during extreme NAO conditions. This figure, a three dimensional surface representation based on the kernel density values of TC incidence, facilitates the visualization of the transformation of the BH from a weak horizontal trough during periods of extreme low NAO (<—2.5 a) (Fig. 3a) to a strong circular depression translated to the northeast during periods of extreme high NAO (>2.5 a) (Fig. 3b). Note the similarity in shape and spatial placement between the extreme low/extreme high surface figures here and the extreme west/extreme east tracks in Fig. 1.

Kernel density analysis on all three storm groupings (TC, hurricanes, and major hurricanes) for the period 1948-2003 (Fig. 4) records a similar pattern in all three cases; namely a westward path occurs across the Atlantic from approximately the Cape Verde islands. Northeast of the Greater Antilles the pathway bifurcates, resulting in two distinct track paths recurving to the northeast. A separate cluster

Fig. 2 Plot of kernel density values of TC tracks occurring during extreme low (<—2.5 a) NAO months subtracted from kernel density values of TC tracks occurring during extreme high (>2.5 a) NAO months for the period 1948-2003. Interpolation is by a 100 km bandwidth kernel density. Light gray shadings represent negative values, dark gray shadings represent positive values

Fig. 2 Plot of kernel density values of TC tracks occurring during extreme low (<—2.5 a) NAO months subtracted from kernel density values of TC tracks occurring during extreme high (>2.5 a) NAO months for the period 1948-2003. Interpolation is by a 100 km bandwidth kernel density. Light gray shadings represent negative values, dark gray shadings represent positive values

Fig. 3 Three dimensional surface representation based on the kernel density values of occurrence incidence of all six hour storm tracks for the period 1948-2000 that occurred during periods of (a) extreme low (<—2.5 a), (b) extreme high (>+2.5 a) NAO. Note that the center hollow, presumably representing the BH which the TC travel around, and not through, changes from a weak horizontal trough during extreme low NAO Index values to a strong circular depression during extreme high value, while moving to the northeast

Fig. 3 Three dimensional surface representation based on the kernel density values of occurrence incidence of all six hour storm tracks for the period 1948-2000 that occurred during periods of (a) extreme low (<—2.5 a), (b) extreme high (>+2.5 a) NAO. Note that the center hollow, presumably representing the BH which the TC travel around, and not through, changes from a weak horizontal trough during extreme low NAO Index values to a strong circular depression during extreme high value, while moving to the northeast

Fig. 4 Kernel density surface interpolation of all (a) TC, (b) Hurricanes, (c) Major Hurricane track sections for the period 1948-2003

of tracks is found in the Western Caribbean and Gulf of Mexico. The bifurcation of the Cape Verde hurricane tracks supports the idea of the bimodal influence the BH exerts over TC paths, related to NAO values. Presumably the distinct western cluster records the Western Caribbean hurricanes.

This preliminary analysis indicates that on an inter-annual basis the strength/ position of the BH (as proxied by NAO values) exerts significant control over the location of TC tracks as well as the location of hurricane landfall, as suggested by Eisner et al. (2000). Low NAO values (southwestern BH positions) correspond to less recurved, southern tracks, and high NAO values (northeastern BH positions) correspond to increased recurvature and more northern tracks.

The relationship between TCs and the NA circulation system extends beyond interactions with the BH. Bell and Chelliah (2006) show that for the period 19502004, hurricane activity is tightly linked to the general NA circulation system through a coherent set of interrelated atmospheric and oceanic features, including SST, West African rainfall, and overall climate variability in the tropics. An enhanced NAO results in strengthened trade winds (Nyberg et al. 2001), which correlates to reduced NA SST between 45-65°N (Black et al. 1999) and enhanced vertical wind shear in the MDR (Nyberg et al. 2007). In turn, a statistically significant correlation exists between these features and hurricane activity (Nyberg et al. 2007). This indicates that TCs are an integral and interconnected component of the larger NA circulation system.

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