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Because of its low topographic relief, unique hydrology, and the large interannual variability of precipitation, Florida is especially vulnerable to climate change. In this paper, we investigate a comprehensive collection of climate metrics to study historical trends in both averages and extremes of precipitation and temperature in the state. The data investigated consist of longterm records (1892-2008) of precipitation and raw (unadjusted) temperature at 32 stations distributed throughout the state. To evaluate trends in climate metrics, we use an iterative pre-whitening method, which aims to separate positive autocorrelation from trend present in time series. Results show a general decrease in wet season precipitation, most evident for the month of May and possibly tied to a delayed onset of the wet season. In contrast, there seems to be an increase in the number of wet days during the dry season, especially during November through January. We found that the number of dog days (above 26.7 C) during the year and during the wet season has increased at many locations. For the post-1950 period, a widespread decrease in the daily temperature range (DTR) is observed mainly because of increased daily minimum temperature (Tmin). Although we did not attempt to formally attribute these trends to natural versus anthropogenic causes, we find that the urban heat island effect is at least partially responsible for the increase in Tmin and its corresponding decrease in DTR at urbanized stations compared with nearby rural stations. In the future, a formal trend attribution study should be conducted for the region.Values marked with ' * ' are statistically significant at the 0.05 level. DTR, daily temperature range. 2242 M. M. IRIZARRY-ORTIZ ET AL.

[1] In previous studies, a zonally symmetric, synchronously coupled biosphereatmosphere model (ZonalBAM), which includes explicit representation of ecosystem dynamics, has been developed and validated based on current conditions over the region of West Africa. Here, we use ZonalBAM to study the response of the coupled biosphereatmosphere system to changes in the Earth's orbital forcing during the Middle Holocene (6K yrs BP) and the relative contribution of vegetation feedbacks. Simulations in which vegetation conditions were fixed to the current distribution, show that an orbitally induced increased seasonality in insolation for the Middle Holocene, by itself, results in a 1.1°n orthward shift in the location of the southern margin of the Sahara as compared to current solar forcings. When vegetation is allowed to be dynamic, a 2.4°northward shift is simulated. However, when dynamic vegetation is initialized to palaeovegetation, a 5.1°n orthward shift is simulated, bringing results more consistent with palaeoevidence. Based on previous studies on the role of the gradient of moist static energy on the dynamics of large-scale tropical circulations, a mechanism for the enhancement of the summer monsoon circulation has been developed. Our results suggest that multiple equilibria could have coexisted over the region of West Africa during the Middle Holocene. Furthermore, based on previous studies on the current climate over the region, we hypothesize that transitions between the different equilibria could have taken place during the Middle Holocene causing the southern desert margin to migrate between 18.1°N and 21.4°N and shaping climate variability.

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