4. Discussion

[16] What is causing this interannual variability of rainfall in the summer season to peak at 16:00 EST over southwestern Florida and northwestern Florida in the RSM-R2 simulation? Our answer to this question lies in the modulation of the sea breeze circulation by the large-scale circulation. To further elaborate on this question, consider the composite mean differences of the 850 hPa winds and mean sea level pressure between the large and the small AWP years (Figure 10). In large AWP years the North Atlantic Subtropical High (NASH) is weaker (Figure 10a) compared to small AWP years (Figure 10b), resulting in a southward flow anomaly (Figure 10c). This is consistent with similar observations made in Wang and Enfield [2003] and Wang et al. [2006]. The anomalous northerlies in large AWP years are associated with the Sverdrup balance [Rodwell and Hoskins, 2001; Hoskins and Wang, 2005] given by

where is the meridional variation of the Coriolis parameter, f, V is the meridional wind and is the vertical velocity on pressure surface. Given the location of the panhandle region in the subtropics and its proximity to NASH, the Sverdrup balance applies quite well to the situation. In accordance with this balance the strong anomalous northerlies at the low level should be compensated by divergence and seen below the maximum anomalous descent. This feature is well captured in the RSM-R2 simulation. Figure 11 shows the vertical cross section along 30.5°N of the composite mean difference of the meridional wind (Figure 11a), divergence (; Figure 11b), and vertical velocity (; Figure 11c). The anomalous sinking motion in Figure 11c is consistent with an overall anomalous lowlevel (upper level) divergence (convergence) (Figure 11b; albeit a noisy field) in the panhandle region. Following the Sverdrup balance (equation 1) and mass continuity there is a large-scale anomalous sinking motion associated with low-level anomalous northerly flow along the panhandle coast of Florida in large AWP years compared to small AWP years. This anomalous northerly flow along the panhandle Florida is from the weakening of the NASH and its eastward movement in large AWP years relative to small AWP years as illustrated in the schematic in Figure 12. Consequently, this modulation of the large-scale flow makes the boundary layer circulation associated with the sea breeze weaker (Figure 10c and Figure 12a) despite stronger land-ocean contrasts in large AWP years (Figure 7c).

Figure 10. (left) The JJA composite 850 hPa winds (m s-1; scale is shown on the right of each panel) and mean sea level pressure in hPa (with contour interval of 0.5 hPa) for (a) large, (b) small, and (c) large-small AWP years. [larger image]	Figure 11. (right) The JJA composite mean difference of (a) meridional wind (m s-1), (b) divergence (X106s-1), and (c) omega (vertical velocity, hPa s-1) between large and small AWP years. The two vertical lines at 88W and 84W depict the boundaries of the panhandle Florida where the Sverdrup balance is best depicted in RSM-R2. [larger image]

Figure 12. Schematic of the anomalous conditions over panhandle Florida generated by the modulation of the North Atlantic Subtropical High (NASH) in the (a) large and (b) small AWP years. The composite mean sea level pressure (hPa) from R2 is contoured for the five large (Figure 12a) and small (Figure 12b) AWP years. [larger image]

[17] We examined the moisture budget at the diurnal peak of 16:00 EST (which also coincides with the peak in interannual variation seen in Figure 8) to further understand the low-frequency variations of the Florida sea breeze along the panhandle coast. The terms of the moisture budget equation follow from

where term 1 is time tendency of precipitable water, term 2 () is moisture flux convergence (if positive) or moisture flux divergence (if negative), E is evaporation (term 3) and P is precipitation (term 4). The precipitable water (Q) and the moisture flux (M) are defined as,

[18] In the composite mean difference of the terms of the moisture budget at the climatological diurnal peak of 16:00 E.S.T. from the RSM-R2 simulation we notice that the moisture flux convergence is slightly stronger in large AWP years compared to the small AWP years (Figure 13a). However, due to the unfavorable large-scale conditions from the displacement and weakening of the North Atlantic Subtropical High during large AWP years, the sea breeze convection is relatively suppressed (Figure 13b). Evaporation (Figure 13c) does not play as significant a role as the other terms of the moisture budget along the panhandle coast of Florida. There is therefore a compensatory decrease of the tendency of the precipitable water (Figure 13d) in large AWP years. This compensatory decrease in the tendency of the precipitable water in large AWP years, in fact, translates to higher precipitable water in the atmospheric column at 16:00 EST (Figure 14).

[19] The modulation of the small-scale diurnal variations of the sea breeze from the large-scale variations of the atmosphere and SST points to the importance of examining high-resolution data sets both from observations and models to understand climate variations along the coastlines of Florida.

Figure 13. (left) The JJA composite mean difference between the five large AWP and the five small AWP years at 16:00 EST of the (a) moisture flux convergence, (b) precipitation, (c) evaporation, and (d) tendency of precipitable water. The units of all variables are in mm d-1. [larger image]	Figure 14. (right) The JJA composite mean 16:00 EST JJA precipitable water computed for (a) large AWP years, (b) small AWP years, and (c) the difference of Figures 14a and 14b. The units are in kg m-2. [larger image]