NOV 2018– APR 2019 BROUGHT TO YOU BY THE ALASKA CENTER FOR CLIMATE ASSESSMENT AND POLICY IN PARTNERSHIP WITH THE ALASKA CLIMATE RESEARCH CENTER, SEARCH SEA ICE OUTLOOK, NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION, AND THE NATIONAL WEATHER SERVICE DEFINING THE SEASONS By Brian Brettschneider, International Arctic Research Center (IARC), UAF What is a season? If you look at a calendar, the seasons start on December 21, March 20, June 21, and September 22. These are more properly called astronomical seasons. Climatologists traditionally use whole calendar months to describe the seasons—December to February, March to May, June to August, and September to November. These are called climatological seasons. Most natural scientists use these month-based season definitions. The groupings are based solely on the artificial construct of calendars developed many years ago. How useful are these distinctions today? Is there a better way to split the year into fall, winter, spring, and summer seasons? Surprisingly little research has been done using climatological data to fine tune the definition of the seasons at the local scale. When I looked into it a few years ago, I only found a single paper in the peer-reviewed literature that specifically looked at reshuffling the dates where the seasons begin—and it only attempted to assess whether the 90–92 days seasons actually line up with the calendar months. They showed that they were actually not that far off. More recently, Boustead et al. (2015) used an elaborate rubric to define the winter season using temperature, snowfall, and snow depth—the Accumulated Winter Season Severity Index (AWSSI), which we discussed in the Winter 2015-16 issue of the Alaska Climate Dispatch. The AWSSI definition of winter is expansive, to capture the maximum length of wintry conditions. THE DATA The Global Historical Climatology Network version 4 (GHCNv4) database of monthly climate temperatures has the data required to evaluate seasonality. Here comes an important point. Climate normals are based on monthly temperatures. This seems counterintuitive. Why not use daily data if we have it? The answer is that daily data are chaotic and messy. In addition, many stations collect data at irregular schedules that make daily estimates difficult but provide good information at the monthly level. Up through the 1971–2000 climate period, the National Centers for Environmental Information (NCEI), formerly the National Climate Data Center (NCDC), used a cubic spline fit of monthly average temperature to interpolate For many Alaskans, winter can be defined as ski season. daily climate normals. In the 1981–2010 climate period, they modified the Photo courtesy Tina Buxbaum. procedure somewhat using a methodology that is explained in some detail but in a way that is not replicable. For this analysis, we used 1981–2010 monthly temperature data and applied the cubic spline technique that NCEI used in prior In this issue climate normal periods. The daily values produced are not very different than the non-replicable NCEI published values. With that caveat, we used these data Defining the Seasons .................................... 1–5 to explore two approaches to identifying the seasons in the U.S., with a focus on Climate and Weather Summary ...............6–8 Alaska. Extreme March ..................................................9 Dangerous Ice ............................................10 –11 Sea Ice .......................................................... 12–14 ACCAP News ...................................................14 IMPROVING THE ABILITY OF ALASKANS TO RESPOND TO A CHANGING CLIMATE ALASKA CLIMATE DISPATCH • NOV 2018-APR 2019 DEFINING THE SEASONS 2 APPROACH 1: USE 90-DAY COLD SEASON AND 92-DAY WARM SEASON BUT ADJUST START/STOP DATES The December 1 to February 28 time period is 90 days long (ignoring Leap Days). Is this really the coldest 90 days of the year? Correspondingly, the June 1 to August 31 time period is 92 days long. Is this really the warmest 92 days of the year? To answer the question, daily normal temperatures were computed for every U.S./Canada station in the GHCN v4 data set (7,636 stations). For each of those stations, a 365-daily normal time series using the cubic spline technique was generated. Then the coldest 90 days and the warmest 92 days of the year were flagged. The seasons are therefore defined as: • Winter: coldest 90 days • Summer: warmest 92 days • Spring: time period between winter and summer Figure 1. Coldest 90-day period of the year using daily climate normals. (undetermined length) • Fall: time period between summer and winter (undetermined length) The length of winter and summer are fixed, and the combined length of spring and fall are fixed—but the length of spring and fall as individual seasons vary from place to place. Figures 1–4 show the dates of winter and summer, and the lengths of spring and fall using this approach. Figure 2. Warmest 92-day period of the year using daily climate normals. Another way to define summer is boating season. National Park Service photo by W. Hill, Lake Clark National Park and Preserve. ALASKA CLIMATE DISPATCH • NOV 2018-APR 2019 DEFINING THE SEASONS 3 APPROACH 2: IDENTIFY THE COLDEST AND WARMEST QUARTERS OF THE ANNUAL TEMPERATURE RANGE AND CALL THOSE WINTER AND SUMMER Instead of retaining the length of climatological seasons to mimic the length of calendar months, why not define some temperature thresholds for the warm and cold seasons and let the chips fall where they may? This is an idea the Rick Thoman with the Alaska Center for Climate Assessment and Policy (ACCAP) came up with. He suggested that if you took the annual range of normal temperatures (highest minus lowest), those days that are in the top 1/4 of that range are summer days, those days in the lowest 1/4 of that range are winter days, and the intervening days are either spring or fall days. Figure 5 shows an example using Fairbanks’ 1981–2010 Figure 3. Length of spring as defined by the period between the coldest 90-day period and the normal daily temperature. warmest 92-day period using daily climate normals. Figure 5. Diagram showing the top and bottom quarters of the annual temperature distribution. This produces a much different distribution of season lengths. Most places "hover" near the warmest and coldest portions of the year for more than 90 or 92 days. For example, the annual maximum normal for Fairbanks is 63.7°F. The Figure 4. Length of fall as defined by the period between the warmest 92-day period and the coldest 90-day period using daily climate normals. annual minimum normal is -8.7°F. The range is therefore 72.4°F. Any days warmer than 45.6°F are considered summer, and any days below 9.4°F are considered winter. Remember, these are climate normal days—not actual temperature days. The transitions between those periods is considered spring and fall. For Fairbanks, this approach yields a winter length of 131 days, a summer length of (coincidentally) 131 days, a spring ALASKA CLIMATE DISPATCH • NOV 2018-APR 2019 DEFINING THE SEASONS 4 length of 54 days, and a fall length of 49 days. Note the dramatically longer lengths of summer and winter compared to the 90- and 92-day lengths from the previous section. Because both the start date and the lengths of summer and winter vary using this methodology, we cannot have a single figure (like Figures 1 & 2) that gives all the necessary information about those seasons; therefore, only the start dates and lengths of summer and winter are shown in Figures 6–9. ANALYSIS There are many factors that determine the climate of a region. Latitude and proximity to water are dominant factors that determine annual temperature regime. Looking beyond temperature, the transition from winter to summer, and summer back to winter have more nuanced factors that determine when those changes take place. SUMMER Figure 6. Start date of the warmest quarter of the annual temperature range. Nowhere else in North America does summer start earlier than the eastern interior of Alaska. The gray areas in Figure 2 and 6 include the region from Fairbanks eastward to the Yukon Territory. Why is this? Several factors work together to make summer early in interior Alaska. First, by mid-May, there are 24 hours of daylight + twilight north of the Alaska Range. This allows for near continuous solar radiation. April and May are also the driest, sunniest months of the year across most of Alaska. Finally, we need to think of the warmest 92 days as not only dependent on the start date, but on the end date as well. By mid-August, southwesterly flow across the interior brings clouds and rain, causing a rather quick drop in temperatures during a time of year when most other places are still near their seasonal peak. The August coolness forces the peak summer temperatures to be earlier in the season. Even as the temperatures drop in August, it is still in the top ¼ of the annual temperature distribution. This has the effect of keeping interior Alaska in the summer category when seasons are defined by the annual temperature range. Figure 7 shows that most of Alaska has a summer length of Figure 7. Number of days in the warmest quarter of the annual temperature range. 118 to 135 days using this quartile definition—rather longer than most of the western Lower 48! In northern and western Alaska, snow cover usually lasts until well into May. This keeps temperatures from rising much early in the season. In southern Alaska, the proximity to water also delays the seasonal warm up when compared to the interior. By the time the clouds and rains of August and September arrive, the summer season in Alaska ends nearly everywhere at about the same time.