Dr. Jason T Ortegren

It is evident that human activity is now large enough to influence earth’s climate. Many locations experienced progressively less cold weather during the last century, leading to generalizations about season length. However, much of the Southeast, specifically Florida, has not been examined for changes in the timing of the seasonal transition from winter temperatures to spring temperatures. We examined five Florida locations: Jacksonville, Miami, Orlando, Pensacola, and Tampa, representing northwest/panhandle, northeast, central, west-central, and southern Florida, respectively. The time series of the latest winter temperature dates are tested for trends using linear regression and the Mann-Kendall rank statistic. Overall, these areas are not consistent in either
positive or negative trends but vary by location. While Tampa and Jacksonville have positive trendlines for linear regression, indicating that winter has increased in length on average, Orlando and Miami have negative trendlines for linear regression, indicating that winter has decreased in length on average. Meanwhile, Pensacola has virtually no change. Miami is the only location with significant results in both Mann-Kendall tests and linear regression. This information narrows the knowledge gap on Florida’s winter-to-spring transition since the mid-20th century and may be valuable for both
tourism and agricultural interests.

The population of the southeastern USA is exposed to frequent extreme summertime high heat and humidity and is thus vulnerable to the resulting human thermal stress. Regional dew point variability in the USA is relatively underexplored in the literature compared to extreme heat. Here, we analyze hourly summer dew point data from 34 cities in the region during the period 1973–2022 (n = 50) to identify annual values of extreme dew point hours (EDH) and extreme dew point days (EDD). Regionally, significant (p ≤ 0.05) positive trends for both EDH (rs = 0.28, R2 = 0.078, +1.53 EDH/year) and EDD (rs = 0.30, R2 = 0.086, +0.05 EDD/year) occurred, although not all stations had increased dew point temperatures. Rather, positive changes are most concentrated among stations located along the upper Piedmont of the southern Appalachian Mountains. Conversely, no significant (i.e., p < 0.05) differences in either aggregate mean values of EDH or EDD occurred when splitting the data into early (1973–1997) and late (1998–2022) periods. High summer values of EDH and EDD are associated with variability in the 500 hPa geopotential height flow over North America. In particular, anomalous high pressure over the Gulf of Alaska is associated with the highest frequencies of summer extreme dew points in the study area, and vice versa. This connection to slow-changing ocean–atmosphere variability could lead to enhanced predictability of periods of extreme high dew point conditions in the Southeast, with implications for human well-being.

We analyzed summertime (June–August) cold-front activity via frequency and duration in the southeastern USA during 1973–2020 to summarize and identify the temporal trends of the annual and total number of hours associated with cold fronts, cold-front days, and multi-day cold-front events. Using data from 34 ASOS Network stations, we defined summertime cold fronts as events that lowered the dew point temperature below 15.56 °C (< 60 °F). Additionally, we examined 500 hPa geopotential height anomalies associated with years with cold front frequency/duration deviations of +/− 1.0 SD. The extent of the cold-front activity exhibited a north–south latitudinal gradient with a more southerly latitudinal expression on the east side of the Appalachian Mountains and was negligible south of the 30°N latitude. The cold-front activity was most prominent during the first half of June. Our results suggest that all three metrics of summertime cold-front activity were stable at a regional scale during the 48-year study period with a few (three–five) stations experiencing significant decreases. A regional-scale stability was coincident with significant increases in minimum, maximum, and average summertime temperatures in the southeastern USA. Years with either above-average or below-average cold-front activity were concurrent with synoptic conditions that supported either troughing or ridging in the southeastern USA. We conclude that the observed weakening in the southeastern USA warming hole is the result of external and/or internal forcings unrelated to reductions in anomalously cool summer weather.

Tropical cyclones present substantial threats to coastal communities around the globe. In the United States alone, tropical cyclones (aka hurricanes) have caused over 1.8 billion in economic loss and over 6500 deaths since 1980 (NOAA. 2021 ), Hurricane winds can cause trees to be uprooted from the ground, damaging homes, powerlines, blocking roads, etc. However. most of the damage caused by hurricanes comes from flooding and storm surge. The area of focus for this study is Escambia County, Florida. On September 16, 2004. a category 3 hurricane, Hurrican Ivan, made landfall near Gulf Shores. Alabama and tore through the area. including Escambia County, Florida. Exactly 16 years later, on September 16. 2020. Hurricane Sally (category 2) made landfall also near Gulf Shores, Alabama, and devastated the region.

The goal of the study is to perform a quantitative, trend analysis on precipitation source proportionality in the month of September for the Gulf Coast region. The initial study area includes portions of Escambia County, Florida and Baldwin County. Alabama. As these areas are processed, grid zones along the coast through Mississippi, Louisiana, and Texas may be included. The study will be performed on a temporal scale of every September between 1948 and 2018. Future work may seek to include multiple TCP datasets.

Latewood ring widths of longleaf pine (Pinus palustrisMill.) are effective recorders of annual variability of tropical cyclone (TC) precipitation (TCP), accounting for approximately half of the explained variance. Based on a regional chronology comprised of data from five sites in coastal North Carolina, we reconstructed TCP during 1750-2015 to examine temporal variability of multidecadal dry and wet TCP regimes, the synoptic controls that contributed to an exceptionally dry phase in 1843-1876, and the effectiveness of using latewood to identify droughts independent of TCP. We found six phases of alternating dry/wet phases occurred during the 250+ years in the reconstruction (duration range = 17-62 years) and the 1843-1876 period of exceptionally narrow latewood widths and low TCP values (i.e., the Great Suppression) was unique during the past quarter millennium. The Great Suppression coincided with a period of anomalously low pressure (relative mean hPa deviation = -60 DAM) over the eastern USA at 500 hPa heights, which strongly affects the steering of TCs. We found that while each dry phase was characterized by a persistence of these steering lows, including the most recent (2006-2016) period absent of major landfalling TCs in the United States, the Great Suppression was unmatched in intensity. Finally, we determined that variability in longleaf pine latewood widths do not reflect overall soil-moisture conditions, as neither narrow nor wide latewood widths are coincident with variations in non-TC-related precipitation. Rather, latewood growth flushes are associated with ephemeral periods of elevated water tables following high-intensity TC-related rainfall events.

Tropical cyclones (TCs) are an important source of precipitation for much of the eastern United States. However, our understanding of the spatiotemporal variability of tropical cyclone precipitation (TCP) and the connections to large-scale atmospheric circulation is limited by irregularly distributed rain gauges and short records of satellite measurements. To address this, we developed a new gridded (0.25 degrees x 0.25 degrees) publicly available dataset of TCP (1948-2015; Tropical Cyclone Precipitation Dataset, or TCPDat) using TC tracks to identify TCP within an existing gridded precipitation dataset. TCPDat was used to characterize total June-November TCP and percentage contribution to total June-November precipitation. TCP totals and contributions had maxima on the Louisiana, North Carolina, and Texas coasts, substantially decreasing farther inland at rates of approximately 6.2-6.7 mm km(-1). Few statistically significant trends were discovered in either TCP totals or percentage contribution. TCP is positively related to an index of the position and strength of the western flank of the North Atlantic subtropical high (NASH), with the strongest correlations concentrated in the southeastern United States. Weaker inverse correlations between TCP and El Nino-Southern Oscillation are seen throughout the study site. Ultimately, spatial variations of TCP are more closely linked to variations in the NASH flank position or strength than to the ENSO index. The TCP dataset developed in this study is an important step in understanding hurricane-climate interactions and the impacts of TCs on communities, water resources, and ecosystems in the eastern United States.