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.

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.

We present a new method for identifying historic tropical cyclone activity utilizing frequencies of intra-annual density fluctuations in longleaf pine in western Florida. In addition, in this work we provide information about the causal factors that determine the formation of intra-annual density fluctuations (IADFs) in longleaf pine latewood. Specifically, we test the viability of using late latewood (L+) IADFs in longleaf pine as a proxy for historic tropical cyclone frequency and precipitation for the period 1950-2017. The stabilized frequency of L+ IADF occurrence is significantly (p < 0.01) associated with the Palmer drought severity index (PDSI) for the months June through October, indicating that high amounts of late growing-season moisture promote the formation of IADFs in latewood. We find the strongest relationships between PDSI and IADF occurrence during September and October, indicating the influence of tropical cyclone (TC)-sourced precipitation on IADF formation. High IADF stabilized frequencies (i.e. >0.50) nearly always (88%) coincide with a TC tracking into the study area, and we find a significant (p < 0.01) relationship between TC-sourced precipitation and the stabilized frequency of L+ IADFs. Via this relationship, reconstruction of historic tropical cyclone frequency and precipitation is probable, which would allow for increased understanding of historic tropical cyclone activity prior to the historic climate record.

The synoptic processes that end droughts are poorly understood, yet have significant climatological implications. Here we examined the spatiotemporal patterns of rapid drought cessation (RDC) in the southeastern United States during the1979–2013 warm season (April–November) for three storm types: Frontal, Tropical, and Air mass. We defined RDC as a 1 month shift in soil moisture sufficient to alleviate an existing drought. We found that 73% of all warm‐season droughts were ended by RDC events and the three storm‐type groups ended droughts over similar spatial areas. Frontal storms were the most frequent mechanism for RDC events, yet their occurrences significantly decreased and were negatively related to increases in Northern Hemisphere air temperatures. Projected future warming in the Northern Hemisphere suggests a continued decline in the frequency and relative contribution of Frontal storms as RDC events, potentially influencing the timing and spatial scale of drought cessation in the southeastern U.S.
Key Points
The majority (73%) of droughts in the southeastern United States were ended rapidly over a month period
Storm types had different spatial patterns of rapid drought with closed lows and atmospheric rivers ending drought over the largest areas
The Frontal storm type ended drought rapidly the most frequently but is significantly decreasing in occurrence over the study period

Background: The Florida Department of Health, Environmental Public Health Tracking Program, in collaboration with the Escambia County Health Department and the University of West Florida, used the Health Impact Assessment Framework to examine adverse health outcomes that may be related to an extreme flood event in Pensacola, Florida (Escambia County) during April 29 to May 3, 2014. In this 2014 flood event, portions of Pensacola received more than 15.5 in of rain in a single day. Infrastructure impacts from this extreme event included destroyed bridges and roads and the failure of many sewage lift stations.
Objective: To determine whether there were associated increases in injury, illness, and death, data on reportable diseases, hospitalizations, emergency department (ED) visits, and deaths that occurred during the impact period in 2014 were compared with a control period in 2008.
Design: We used an ecological design to compare impact and control periods and examined the proportion of hospitalizations, ED visits, and deaths potentially attributable to the extreme flood event.
Results: The results of this comparison were mixed, with some Escambia County zip codes showing increased hospitalizations and ED visits, and some zip codes showing a decrease. However, countywide, there were increases in the proportion of both injury- and respiratory-related hospitalizations and ED visits during the impact period.
Conclusions: It is challenging to characterize human health impacts from natural disasters such as extreme floods. Still, it is believed that specific policy changes could result in fewer health impacts during future flood events. For example, this study recommended raising the electric panels on lift stations above the flood elevation to keep them operational during extreme rainfall events. For more maps and tables, consult the complete project report available at the link given above.

We examined the spatial distribution of monthly, seasonal, and annual changes in comfortable weather hours (CWHs) between 1950 and 2011 and explored the relationship between human wellness and the amount and timing of CWHs. Using a thermohygrometric index based on air temperature and dewpoint temperature recorded every three hours from thirty-five U.S. cities, we determined whether changes in human thermal comfort were coincident with warming and more humid atmospheric conditions. We tested for significant trends in CWHs for every season for each city for nighttime, daytime, and total (i.e., night and day) periods. Although approximately 75 percent of the cities did not experience significant changes in CWHs on an annual basis, total changes in CWHs were marked by increases during spring and decreases in summer conditions, with the largest positive changes in CWHs found during spring nights, spring days, and autumn nights and the largest negative changes during summer nights and days. Spatially, increases in CWHs were principally located west of 117 degrees W and decreases in cities east of 81 degrees W. Significant relationships existed between wellness metrics and seasonal and annual CWHs. Greater CWHs during the summer were positively correlated with happiness and well-being and negatively correlated with obesity. These results suggest that further declines in summer CWHs for cities might affect human wellness, as peak optimal weather conditions shift toward spring and autumn months.

Droughts and landfalling tropical cyclones (TCs; tropical depressions, tropical storms, and hurricanes) are important features of the hydroclimate of the southeastern USA at seasonal, interannual, and interdecadal scales. The societal impacts and climatological aspects of both droughts and Atlantic TCs have been widely addressed in the scientific literature. However, in general, previous research has assessed the two phenomena separately. Recently, the spatiotemporal patterns and hydroclimatic impacts of drought amelioration by landfalling TCs have been analyzed for the southeastern USA, as well as the large-scale dynamic forcing mechanisms that enhance or suppress drought-TC co-occurrence. At multidecadal time scales, both droughts and TCs in this region vary in association with several leading modes of basin-wide and regional climate variability. These climate modes appear to be coherently linked to a background oceanic-atmospheric pattern that either promotes or suppresses the likelihood of both droughts and TC landfalls. The relative frequency of TC landfalls in drought-stricken areas, the importance of these events in the regional moisture budget, and the potential for future changes in the large-scale forcing environment raise fundamental questions about possible changes in the hydroclimate of the southeastern USA, where population growth and rising water demand already place strain on freshwater resources. In this article, we provide a review and synthesis of the recent research on variability in drought, landfalling TCs, the characteristics of the space-time association between these two phenomena in the southeastern USA, and the coherent large-scale oceanic-atmospheric environment that either promotes or suppresses their co-occurrence. Further, we review Global Climate Model projections related to these factors, and we identify avenues for future research on this important topic.