Shifting Climate Normals: A moving target for seasonal weather comparisons

1. Shifting Climate Normals: A moving target for seasonal weather comparisons By: Heather Wheeler, Summer 2014 RET - University of New Hampshire Advisor: Mary Stampone, Department of Geography & New Hampshire State Climate Office - University of New Hampshire Abstract The 2013/14 New England winter was considered by many to have been colder than a typical New England Winter. In this study, climate division level temperature and precipitation averaged for the 2013/14 meteorological winter (December, January and February) across New England are compared to the average winter values for the 1981–2010 climate normal period. Results indicate that, despite complaints about the cold, winter 2013/14 for most of New England was statistically normal for both temperature and precipitation. Further comparison of winter 2013/14 to preceding 20th century normal periods illustrates the 2013/14 winter temperatures and precipitation to have been well within the range of normal variability for the region. The common perception of New Englanders that the 2013/14 winter was unusually cold suggests that the lens through which climate is viewed may literally change with the weather. With a rising set of temperature norms over the 20th century, winter 2013/14 may have seemed cold compared to recent warmer winters! Results Compared to recent New England winters (2005-2014), winter 2013/14 was significantly colder than the average for all of New England (Table 1). It was the coldest winter in just over 10 years for most of northern New England (ME, NH, VT), and the coldest winter since 2011 for southern New England (CT, MA, RI). There were no significant differences in total precipitation for winter 2013/14. Discussion It is interesting to note that comparison of the 2013/14 New England winter to historic normal periods dating back to 1950 demonstrates the 2013/14 winter to have been quite normal when the period of comparison predates 1990. Only in the context of comparison to the normal temperatures for the most recent normal period (1980-2010) does the 2013/14 winter seem unusual. The disparity from normal increases drastically when the winter 2013/14 is compared to an even more recent period of comparison, that of the last decade (2005-2014). Further comparison of the 2013/14 New England winter to historic normal periods dating back to the early-20th century illustrates the 2013/14 New England mean winter temperature was well within the range of normal for the climate normal periods from the mid-20th century and was warmer than early-20th century winters. Comparing historical climate normal periods to others illustrates consistently rising temperatures during meteorological winters in successive normal periods. This interrelationship between normal periods illuminates a shift in climate (Figure 5). The winter of 1917/18 is the coldest on record for much of New England with the exception of Rhode Island and Cape Cod. Given the historical comparison, Introduction The 2013/14 New England winter was widely remarked in the region’s television news networks to have been unusually cold. However, it is well understood that weather is often confused with climate; current high and low temperatures, precipitation, wind speeds, etc., cumulatively constitute weather, while long term averages, extremes, and trends in those variables constitute climate. The degree to which a current weather variable conforms to, or differs from, the pattern an individual accepts as ‘normal’ is intrinsically dependent upon the conditions to which it is being compared. Differentiating between weather and climate is the first necessary step in accurately placing current weather experience into a comparative climate context for ‘normal’ weather and is crucial for understanding climate change. Table 1: Seasonally averaged winter mean temperatures for 2013/14 compared the average over the past ten years. Data Source: NOAA-NCDC “Climate Divisional Dataset.” Mean Maximum Winter temperature Compared to the NOAA 1981–2010 climate normal period, the 2013/14 mean winter maximum temperatures were significantly below normal across ME and southern NH and VT. Southern New England as well as northern NH and VT were within the range of normal variability (+/- one standard deviation) and considered near normal. All 2013/14 values were near normal when compared to the three earlier normal periods (Figure 3). A climate normal is quantified as the 30 year average of meteorological conditions, usually air temperature and precipitation, for a specific location (Figure 1; Arguez, 2012). Climate normals have been calculated for 30 year periods beginning every ten years from the earliest meteorological data on record up to the current climate normal period of 1981-2010. Comparing the Figure 5: State-averaged mean winter temperaturefor New England by year (black dashed line) and for all 20th century climate normal period (colored soled lines). Annual variability is shown with respect to the average over the period of record (1895-2014). Data Source: NOAA-NCDC “Climate Divisional Dataset.” the 2013/14 New England winter was more similar to mid-20th century winters than to recent and early-20th century winters in the region. Summary The 2013/14 New England winter has been regarded in the region as unusually cold. Compared to winters in the region of the last decade, it was indeed! Compared to winters from the most recent climate normal period (1981-2010), it was slightly unusual, and compared to historic winters, it was quite normal. The period of time to which the 2013/14 winter is compared is the critical variable in determining the level of typicality of the 2013/14 winter. Given the differences in the periods of comparison, the shift in what constitutes normal becomes apparent: the normal winter climate in New England includes rising winter temperature norms over successive climate normal periods. Figure 3: Mean winter maximum daily temperature for New England climate divisions averaged for late 20th century climate normal period compared to the winter 2013/14 value. Data Source: NOAA-NCDC “Climate Divisional Dataset.” Figure 1: Schematic diagram of a normal temperature distribution. Image Source: IPCC Working Group II - http://ipcc-wg2.gov/SREX/ Mean Minimum Winter temperature winter 2013/14 to normal maximum, minimum, and average temperatures and precipitation across New England offers insight as to how the 2013/14 New England winter compares to the winters of New England’s past. Mean minimum temperatures were significantly below the 1981-2010 normal in only western and coastal CT, with otherwise normal temperatures and precipitation across the region (Figure 4). Data & Methods Spatially averaged, mean winter temperature (minimum, maximum, and mean) and total liquid water equivalent precipitation for New England climate divisions (Figure 2; NCDC, 2014) were used to calculate the climatological average (mean) and variability (one standard deviation) for winter weather over the four most recent climate normal periods. Seasonally averaged winter temperatures for 2013/14 were compared to the normal values for temperature and precipitation over the latter half of the 20th century as well as the 20th century (1901-2000), period of record (1895-2014), and recent (2005–2014) ten year averages. • Literature Cited • Arguez, A. et al., 2012: NOAA’s 1981-2010 U.S. Climate Normals An Overview. Bulletin of the American Meteorology Society, Volume 93, Number11, 1687-1697. • NOAA National Climate Center (NCDC), 2014: nClimDiv Dataset - ftp://ftp.ncdc.noaa.gov/pub/data/cirs/climdiv/ Acknowledgements This research was supported with funding from the National Science Foundation’s grant to NH EPSCoR (IIA-1101245). I would also like to thank Stephen Hale from the UNH Joan and James Leitzel Center for Mathematics, Science, and Engineering Education and NH EPSCoR for assistance with Excel for data analysis. Figure 4: Mean winter minimum daily temperaturefor New England climate divisions averaged for late 20th century climate normal period compared to the winter 2013/14 value. Data Source: NOAA-NCDC “Climate Divisional Dataset.” Comparison of the 2013/14 New England winter to historic normal periods illustrates that last winter was well within the range of normal for the mid-20th century climate normal periods with average winter temperatures and precipitation most similar to the 1951-1980 and 1961-1990 climate normals. Figure 2: NOAA-NCDC Climate Divisions for New England. Image and Data Source: http://www.ncdc.noaa.gov/monitoring-references/maps/us-climate-divisions.php