3. Analysis of Specific Events in 2014 (Pdf: 4.9MB)
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3 Analysis of specific events occurring in 2014 than 6°C below normal were observed in western North America in early December 2013 (Fig. 3.1.2 (1)). 3.1 Winter 2013/2014 cold waves over North Similar values were recorded from northern to eastern America Canada from mid- to late December 2013, from 3.1.1 Introduction central Canada to the Midwest of the USA in early In winter 2013/2014 (December 2013 – February January, from the Midwest to the southern USA in late 2014), extremely cold conditions were frequently January, from southwestern Canada to the southern observed over central and eastern North America, USA in early February, in northwestern North America resulting in an influence on socioeconomic activity in the middle of February and from western Canada to there. the Midwest in late February (Fig. 3.1.2 (2) – (9)). The results of analysis indicated that both At Great Falls, Montana, daily mean temperatures tropospheric and stratospheric circulation may have fell below -30°C (approximately 27°C below normal) contributed to cold waves over North America via the in early December and early February. At reflection of planetary waves in the lower stratosphere. Minneapolis/St. Paul Int., Minnesota, daily mean A number of studies have addressed such reflection temperatures were below normal in early to and other aspects of interaction between the mid-December, early January, late January to early troposphere and the stratosphere. Perlwitz and Harnik February and late February, and fell below -25°C (2003) reported that planetary waves can propagate (approximately 16°C below normal) in early January. downward from the stratosphere to the troposphere, At Chicago/O’Hare, Illinois, daily mean temperatures and that such propagation is related to the vertical fell below -20°C (more than 16°C below normal) in structure of the polar-night jet stream. Kodera et al. early and late January. At Montreal/Pierre Elliott (2008, 2013) also reported the possibility that Trudeau Int'l A, Quebec, daily mean temperatures fell planetary waves originating from the troposphere are below -20°C (more than 10°C below normal) in reflected downward in the lower stratosphere. mid-December and in early and late January (Fig. This section reports on related surface climate 3.1.3). characteristics, atmospheric circulation and primary Cold waves reportedly caused at least 40 fatalities factors contributing to the extreme cold conditions in the USA from mid-December to early January and observed in North America. at least 10 fatalities in Canada around late December (EM-DAT). In addition, storms reportedly caused 3.1.2 Surface climate conditions more than 80 fatalities throughout the USA during Seasonal mean temperatures were above normal in winter (EM-DAT) as well as power outages at Alaska, around California and around the Florida hundreds of thousands of homes, with transportation Peninsula, and were below normal in much of the rest influences including flight delays and cancellations of North America, especially from central Canada to (FEMA). the southern USA (Fig. 3.1.1). Ten-day mean 1 temperature anomalies of more 1 There are three 10-day periods in each month, making a total of thirty-six in a year. The third nominal 10-day period of each month may not in fact have only 10 days (e.g., the third 10-day period of December actually covers 11 days from the 21st to the 31st). 47 Fig. 3.1.1 Winter 2013/2014 mean temperature anomalies Categories are defined by the seasonal mean temperature anomaly against the normal divided by its standard deviation and averaged in 5° × 5° grid boxes. The thresholds of each category are -1.28, -0.44, 0, +0.44 and +1.28. The normal values and standard deviations were calculated from 1981 – 2010 statistics. Areas over land without graphical marks are those where observation data are insufficient or where normal data are unavailable. 3.1.3 Characteristics of atmospheric and oceanic (Fig. 3.1.5 (a) and (b)). These anomalies circulation in winter 2013/2014 (ridges/troughs) indicate the enhancement of the Sea surface temperatures (SSTs) for winter corresponding climatological ridges and troughs (not 2013/2014 were below normal in the central and shown). The enhancement of convective activity over eastern equatorial Pacific and above normal in the the area from the Maritime Continent to the western western tropical Pacific (Fig. 3.1.4 (a)). In association Pacific may have contributed to the formation of with enhanced convective activity over the area from anticyclonic circulation anomalies to the east of the the Maritime Continent (the Indonesian Archipelago) Philippines and the northeastward propagation of to the western Pacific (Fig. 3.1.4 (b)), large-scale Rossby waves downstream from the area (see divergence anomalies in the upper troposphere were Subsection 3.1.4 for details). In the sea level pressure seen there (Fig. 3.1.4 (c)). field, positive and negative anomalies were seen over Quasi-stationary Rossby wave trains in the middle western and eastern North America, respectively, and and upper troposphere were seen over the area from cold-air masses frequently moved over central and the northwestern Pacific or the eastern part of East eastern North America (Fig. 3.1.5 (c)). In the lower Asia to North America and the Atlantic with troposphere, northerly wind anomalies and anticyclonic circulation anomalies (ridges) over below-normal temperature anomalies contributed to western North America and cyclonic circulation cold-air advection over central and eastern North anomalies (troughs) over its central and eastern parts America, respectively (Fig. 3.1.5 (d) and Fig. 3.1.6). 48 (°C) Fig. 3.1.2 Ten-day mean temperature anomalies for North America from December 2013 to February 2014 (unit: °C) (Based on SYNOP reports) The average period is given in each figure (most of the figures show 10-day mean temperature anomalies). The white circles in each figure denote the locations of (a) Great Falls, Montana (USA), (b) Minneapolis/St. Paul Int., Minnesota (USA), (c) Chicago/O’Hare, Illinois (USA) and (d) Montreal/Pierre Elliott Trudeau Int'l, Quebec (Canada). Daily temperature data for these four cities are shown in Fig. 3.1.3. Figures for February are shown on the next page. 49 (°C) Fig. 3.1.2 Ten-day mean temperature anomalies for North America from December 2013 to February 2014 (unit: °C) (Based on SYNOP reports) The average period is given in each figure (most of the figures show 10-day mean temperature anomalies). The white circles in each figure denote the locations of Great Falls, Montana (USA), Minneapolis/St. Paul Int., Minnesota (USA), Chicago/O’Hare, Fig. 3.1.3 Daily mean temperatures (°C) at four Illinois (USA) and Montreal/Pierre Elliott Trudeau Int'l, Quebec stations from 1 December 2013 to 28 February 2014 (Canada). Figures for December and January are shown on the (Based on SYNOP reports) previous page. Red circles denote daily mean temperatures. The black dashed line denotes normal values (1981 – 2000 average) for daily mean temperature. 50 In the 30-hPa height field, the Aleutian anticyclone stratosphere (Fig. 3.1.7 (b) and (c)), which may have developed and the polar vortex was displaced toward contributed to the enhancement of the trough over Canada (Fig. 3.1.7 (a)). Upward propagation of central and eastern North America (Fig. 3.1.5 (b)). The planetary waves from the troposphere to the characteristics seen in the vertical propagation of these stratosphere was seen over the area from Siberia to the planetary waves were more distinct than those seen in Bering Sea in line with the westward tilting of a trough the climatological normal (not shown), and the with height (Fig. 3.1.7 (b) and (c)). The planetary characteristics of the planetary-wave reflection waves originating from the troposphere partially observed during winter were similar to those reported turned downward around Canada in the lower in recent studies. Fig. 3.1.4 Three-month mean oceanic conditions, convective activity and atmospheric circulation for December 2013 – February 2014 (a) Sea surface temperature anomalies, (b) outgoing long-wave radiation (OLR) anomalies, (c) 200-hPa velocity potential anomalies (red lines: positive; green lines: negative) and divergent wind anomalies (vectors; unit: m/s) and OLR anomalies (shading). The contour interval in (c) is 0.5 × 106 m2/s. 51 Fig. 3.1.5 Three-month mean (a) 300-hPa stream function anomalies (shading) from the zonal mean and wave activity flux (arrows), (b) 500-hPa height, (c) sea level pressure and (d) 850-hPa temperature for December 2013 – February 2014 The contour intervals are (b) 60 m, (c) 4 hPa and (d) 4°C, and the shading indicates anomalies. The wave activity flux in (a) was calculated with reference to the method of Plumb (1985) (units: m2/s2). Fig. 3.1.6 Three-month mean anomalies of 850-hPa horizontal temperature advection (K/day) for December 2013 – February 2014 (a) The shading indicates advection of normal temperature by horizontal wind anomalies, the arrows show wind vector anomalies (m/s) and the green lines show normal temperature (K). (b) The shading indicates advection of temperature anomalies by horizontal normal wind and the arrows show normal wind vectors (m/s) at the 850-hPa level. 52 Fig. 3.1.7 Three-month mean (a) 30-hPa height and anomaly in the Northern Hemisphere, (b) 100-hPa wave activity flux, and (c) longitude-height cross section of height anomalies from the zonal mean and wave activity flux averaged over 40°N – 80°N for December 2013 – February 2014 (a) The contours show 30-hPa height at intervals of 120 m, and the shading indicates height anomalies.