This study attempts to explain the considerable spatial heterogeneity in the observed linear trends of monthly mean maximum and minimum temperatures (T max and T min ) from station observations in the southeastern (SE) United States (specifically Florida, Alabama, Georgia, South Carolina, and North Carolina). In a majority of these station sites, the warming trends in T min are stronger in urban areas relative to rural areas. The linear trends of T min in urban areas of the SE United States are approximately 78F century 21 compared to about 5.58F century 21 in rural areas. The trends in T max show weaker warming (or stronger cooling) trends with irrigation, while trends in T min show stronger warming trends. This functionality of the temperature trends with land features also shows seasonality, with the boreal summer season showing the most consistent relationship in the trends of both T max and T min . This study reveals that linear trends in T max in the boreal summer season show a cooling trend of about 0.58F century 21 with irrigation, while the same observing stations on an average display warming trends in T min of about 3.58F century 21 . The seasonality and the physical consistency of these relationships with existing theories may suggest that urbanization and irrigation have a nonnegligible influence on the spatial heterogeneity of the surface temperature trends over the SE United States. The study also delineates the caveats and limitations of the conclusions reached herein due to the potential influence of perceived nonclimatic discontinuities (which incidentally could also have a seasonal cycle) that have not been taken into account.
This paper diagnoses the temperature trends in maximum (T max ) and minimum temperatures (T min ) over a selection of 65 stations spread over the southeast United States (SEUS) from three observed datasets. They are the Cooperative Observer network program (COOP), the COOP data corrected for documented shifts in time of observation (COOP1), and the COOP data additionally corrected for documented changes in instrumentation (COOP2). These 65 stations have been isolated for having the three observed datasets for the same time period from 1948 to 2009. The authors' comparisons suggest that COOP2 displays stronger warming (cooling) trends in T max (T min ) compared with COOP1 in all four seasons. This is consistent with the expectation from the bias correction applied for the instrument change. In comparison, the differences between COOP and COOP2 are relatively larger. In the spring, summer, and fall seasons, the median T max trend is warming in COOP2 while it is cooling in COOP. In the winter season, the median trends of T max in the two datasets are positive, but their magnitudes are substantially different. Similarly, in the winter, summer, and fall seasons, the warming trend in T min in COOP is contrary to the cooling trend in COOP2. In the spring season, the median trend in T min is comparable between the two datasets. COOP2 shows the relationship of trends in T min , with the extent of urbanization in these 65 stations, to be statistically significant and to be consistent with expectations from theory in contrast to the COOP data.
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