2. Annual Summaries of the Climate System in 2011 (Pdf: 5.9MB)

2. Annual summaries of the climate system in 2011 Annual sunshine durations were above normal on the

2.1 Climate in Japan Pacific side of eastern Japan, below normal in western - Above-normal annual precipitation in western Japan and significantly below normal in Japan and on the Sea of Japan side of northern Okinawa/Amami. and eastern Japan - Below-normal annual sunshine durations in 2.1.2 Seasonal features western Japan and Okinawa/Amami (a) Winter (December 2010 – February 2011, Fig. - Below-normal temperatures nationwide in spring, 2.1.4a) and above-normal temperatures nationwide in Intraseasonal temperature variations were very summer and autumn large nationwide. From the end of December 2010 - Significantly earlier onset and end of the rainy to the end of January 2011, temperatures were below season in many regions normal nationwide, and snowfall amounts were above - Record-breaking heavy rainfall in the prefectures normal in all areas on the Sea of Japan side due to of Niigata and Fukushima at the end of July intermittent cold surges. Conversely, in the first half - Record-breaking heavy rainfall due to typhoons of December and the last half of February, Talas and Roke in September temperatures were above normal nationwide due to the weak winter monsoon. 2.1.1 Average surface temperature The annual anomaly of the average surface (b) Spring (March – May 2011, Fig. 2.1.4b) temperature over Japan (i.e., averaged over 17 Temperatures were below normal nationwide. In observatories confirmed as being relatively unaffected the first half of the season, values were significantly by urbanization) for 2011 is estimated to have been below normal in western Japan, while precipitation +0.15°C above the 1981 – 2010 average, making it the amounts were below normal and sunshine durations 17th highest since 1898. On a longer time scale, were above normal on the Pacific side due to the average surface temperatures have risen at a rate of strong winter monsoon and anti-cyclones bringing about +1.15°C per century since 1898 (Fig. 2.1.1). cold air. In the second half of the season, temperatures were below normal in northern Japan 2.1.2 Annual features due to the presence of cold vortexes. Temperatures tended to be below normal nationwide until May due to the effects of cold surges, (c) Summer (June – August 2011, Fig. 2.1.4c) and above normal from June through November. Although seasonal mean temperatures were above Annual mean temperatures were near normal except in normal, intraseasonal temperature variations were Okinawa/Amami. very large nationwide. The onset and end of the Annual precipitation amounts were above normal rainy season were significantly earlier than normal in except in Okinawa/Amami and on the Pacific side of many regions. At the end of July, record-breaking northern and eastern Japan. In particular, values heavy rainfall caused disaster conditions in the were significantly above normal on the Sea of Japan prefectures of Niigata and Fukushima. side of northern Japan, which is subject to the effects of low-pressure areas and fronts. (d) Autumn (September – November 2011, Fig. 2.1.4d)

6 As the westerly jet was shifted northward of its cyclones during the period resulted in above-normal normal position, seasonal mean temperatures were precipitation amounts nationwide. In September, above normal nationwide and significantly above record-breaking heavy rainfall brought by typhoons normal in eastern and western Japan and Talas and Roke caused disaster conditions in many Okinawa/Amami. The formation of typhoons and areas.

Fig. 2.1.1 Long-term change in the annual anomaly of average surface temperature over Japan Anomalies are deviations from the baseline (i.e., the 1981 – 2010 average). The black line indicates the annual anomalies of the average surface temperature for each year. The blue line indicates the five-year running mean, and the red line indicates the long-term linear trend.

Table 2.1.1 Regional average and rank of annual mean temperature anomaly, annual precipitation ratio, and annual sunshine duration ratio for divisions and subdivisions (2011)

Table 2.1.2 Number of observatories reporting record monthly mean temperatures, precipitation amounts and sunshine durations (2011) From 154 surface meteorological stations across Japan. Temperature Precipitation amount Sunshine duration

Highest Lowest Heaviest Lightest Longest Shortest January 2 31 16 7 February 3 3 March 2 7 1 April 1 9 9 May 22 4 June 6 2 July August September 7 7 2 October 1 1 November 19 2 1 1 December 3 2

Table 2.1.3 Onset/end of the Baiu (Japan’s rainy season) for individual subdivisions (2011) Area Average Average date of date of averaged Onset of rainy End of rainy precipitation Subdivisions season* onset of season* end of ratio during rainy season rainy season (1981 – 2010) (1971 – 2000) rainy season (%) Okinawa 30 April 9 May 9 June 23 June 138 Amami 30 April 11 May 22 June 29 June 128 Southern 23 May 31 May 8 July 14 July 135 Kyushu Northern 21 May 5 June 8 July 19 July 119 Kyushu Shikoku 21 May 5 June 8 July 18 July 146 Chugoku 21 May 7 June 8 July 21 July 74 Kinki 22 May 7 June 8 July 21 July 95 Tokai 22 May 8 June 8 July 21 July 103 Kanto- 27 May 8 June 9 July 21 July 83 Koushin Hokuriku 18 June 12 June 9 July 24 July 96 Southern 21 June 12 June 9 July 25 July 110 Tohoku Northern 21 June 14 June 9 July 28 July 70 Tohoku * The onset/end of the rainy season normally has a transitional period of about five days. The dates shown in the table denote the middle day of this period.

8 Fig. 2.1.2 Five-day running mean temperature anomaly for divisions (January – December 2011)

Annual temperature anomaly (°C)

Annual precipitation ratio (%)

Annual sunshine duration ratio (%)

Fig. 2.1.3 Annual climate anomaly/ratio for Japan in 2011

9 (a) Winter (b) Spring

Fig. 2.1.4 Seasonal anomalies/ratios for Japan in 2011 (a) Winter (December2010 to February 2011), (b) spring (March to May), (c) summer (June to August), (d) autumn (September to November).

10 2.2 Climate around the World northern Mexico, and in central Polynesia (Fig.

2.2.1 Global average surface temperature 2.2.6). The annual anomaly of the global average Major extreme climatic events and surface temperature for 2011 (i.e., the combined weather-related disasters in 2011 were as average of the near-surface air temperature over follows (Fig. 2.2.2): land and the SST) is estimated to have been 0.07 (1) Light precipitation in southeastern China ± 0.12°C above the 1981 – 2010 average, (January – May) making it the 12th highest since 1891. On a (2) Flooding on the Indochina Peninsula (July – longer time scale, global average surface December) temperatures have risen at a rate of about (3) Tropical storm in the Philippines +0.68°C per century since 1891 (Fig. 2.2.1). (December)

2.2.2 Regional climate (4) Heavy precipitation in southern Pakistan Annual mean temperatures were above (August – September) normal from Siberia to western Europe and from (5) Light precipitation in Europe (March – May, eastern North America to northern Central September – November)

America, while they were below normal from (6) Drought in eastern Africa (January – Mongolia to Central Asia, around the Indochina September) Peninsula, in western North America, and in (7) High temperatures from the Seychelles to northern Australia (Fig. 2.2.3). Extremely high Mauritius (April – December) temperatures were frequently observed around the southern USA, and extremely low (8) Heavy precipitation around the northeastern temperatures were frequently observed in USA (February – May, August – September) northern Australia (Fig. 2.2.5). (9) Tornados in southeastern and central parts of Annual precipitation amounts were above the USA (April – May) normal from the Philippines to the Indochina (10) High temperatures around southern parts of Peninsula, around southern Pakistan, around the the USA (March – September) northeastern USA, in northern South America (11) Light precipitation from the southern USA and in Australia, while they were below normal to northern Mexico (January – November) in southern China, Saudi Arabia and Europe, from the southern USA to northern Mexico, and (12) Torrential rains in southeastern Brazil in central Polynesia (Fig. 2.2.4). Extremely (January) heavy precipitation amounts were frequently (13) Light precipitation in central Polynesia observed around the northeastern USA, while (March – October) extremely light amounts were frequently (14) Low temperatures in northern Australia observed in Europe, from the southern USA to (January – June)

Fig. 2.2.1 Long-term change in the annual anomaly of global average surface temperature Anomalies are deviations from the baseline (i.e., the 1981 – 2010 average). The black line indicates annual anomalies of the global average surface temperature for each year. The error bars indicate 90% confidence intervals. The blue line indicates the five-year running mean, and the red line indicates the long-term linear trend.

Fig. 2.2.2 Extreme events and weather-related disasters in 2011 Schematic representation of major extreme climatic events and weather-related disasters occurring during the year

Fig. 2.2.3 Annual mean temperature anomalies in 2011 Categories are defined by the annual 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 are calculated from 1981 – 2010 statistics. Areas over land without graphical marks are those where observation data are insufficient or where normal data are unavailable.

Fig. 2.2.4 Annual total precipitation amount ratios in 2011 Categories are defined by the annual precipitation ratio to the normal averaged in 5° × 5° grid boxes. The thresholds of each category are 70%, 100% and 120%. Areas over land without graphical marks are those where observation data are insufficient or where normal data are unavailable.

Fig. 2.2.5 Frequencies of extreme high/low temperature in 2011 shown as upper/lower red/blue semicircles The size of each semicircle represents the ratio of extremely high/low temperature based on monthly observation for the year in each 5° × 5° grid box. Since the frequency of extreme high/low temperature is expected to be about 3% on average, occurrence is considered to be above normal for values of 10 – 20% or more.

Fig. 2.2.6 Frequencies of extreme heavy/light precipitation in 2011 As per Fig. 2.2.5, but for monthly values of extremely heavy/light precipitation

14 (a) Winter (December – February) (b) Spring (March – May)

Fig. 2.2.7 Seasonal mean temperature anomalies for (a) winter (December 2010 – February 2011, (b) spring (March – May), (c) summer (June – August), and (d) Autumn (September – November) 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. Areas over land without graphical marks are those where observation data are insufficient or where normal data are unavailable.

(a) Winter (December – February) (b) Spring (March – May)

Fig. 2.2.8 Seasonal total precipitation amount ratios for (a) winter (December 2010 – February 2011, (b) spring (March – May), (c) summer (June – August), and (d) Autumn (September – November) Categories are defined by the seasonal precipitation ratio to the normal averaged in 5° × 5° grid boxes. The thresholds of each category are 70%, 100% and 120%. Areas over land without graphical marks are those where observation data are insufficient or where normal data are unavailable.

15 2.3 Extratropical circulation 2.3.1 Zonal mean temperature anomaly calculated In the first half of winter 2010/2011, the negative from thickness phase of the Arctic Oscillation (AO) was pronounced, The zonal mean temperature calculated from bringing cold air to the mid-latitudes of the Northern thickness (Fig. 2.3.1) in the tropical troposphere (the Hemisphere, while in the second half it turned bottom part of the figure) was below normal in winter positive. From spring to autumn, wave trains were and spring, when a La Niña event occurred. The zonal observed over Eurasia and an area stretching from the mean temperature in the extratropics of the Northern Pacific to North America, indicating significant Hemisphere (the middle part of the figure), which had meandering of westerly winds. Tropospheric air rapidly decreased since summer 2010, turned slightly temperatures in the Northern Hemisphere remained below normal in early 2011. The temperature in the near normal, except in summer when they were Northern Hemisphere was above normal in summer, temporarily above normal. This section briefly and remained near normal in autumn. The zonal mean describes the characteristics of atmospheric temperature anomaly in the global troposphere circulation in the extratropics of the Northern remained around normal except in summer when it Hemisphere during 2011. was higher than normal.

Fig. 2.3.1 Time series of zonal mean temperature anomaly calculated from thickness in the troposphere (January 2002 – December 2011) The thin and thick lines show monthly and five-month running mean values (unit: K).

16 2.3.2 Winter (December 2010 – February 2011) 2010/2011, and turned positive in the second half. In In the sea level pressure field (Fig. 2.3.2), the the 850-hPa temperature field (Fig. 2.3.4), positive Siberian High was stronger than normal around its anomalies were observed over the area from center, and the Aleutian Low was weaker than normal. northeastern Canada to Greenland and the central In January 2011 (Fig. 2.3.6), both the Siberian High North Pacific, and negative anomalies were seen over and the Aleutian Low were significantly enhanced, northern Europe, Mongolia and the eastern USA. In causing a stronger-than-normal winter monsoon the 200-hPa wind field (Fig. 2.3.5), the subtropical jet around Japan. In the 500-hPa height field (Fig. 2.3.3), stream over Eurasia and the jet stream from the the Arctic Oscillation remained in a significantly eastern USA to the Atlantic were stronger than negative phase during the first half of winter normal.

Fig. 2.3.2 Three-month mean sea level Fig. 2.3.3 Three-month mean 500-hPa Fig. 2.3.4 Three-month mean 850-hPa pressure and anomaly in the Northern height and anomaly in the Northern temperature and anomaly in the Hemisphere (December 2010 – Hemisphere (December 2010 – Northern Hemisphere (December February 2011) February 2011) 2010 – February 2011) The contours show sea level pressure at The contours show 500-hPa height at The contours show 850-hPa temperature intervals of 4 hPa. The shading indicates intervals of 60 m. The shading indicates at intervals of 4°C. The shading sea level pressure anomalies. height anomalies. indicates temperature anomalies. The wavy hatch patterns indicate areas with altitudes exceeding 1,600 m.

Fig. 2.3.5 Three-month mean 200-hPa Fig. 2.3.6 Monthly mean sea level wind speed and vectors in the pressure and anomaly in the Northern Hemisphere (December Northern Hemisphere（January 2011） 2010 – February 2011) The contours show sea level pressure at The black lines show wind speed at intervals of 4 hPa. The shading intervals of 20 m/s. The dark-blue indicates sea level pressure anomalies. shading indicates values greater than 40 m/s. The green lines show normal wind speed at intervals of 40 m/s.

17 2.3.3 Spring (March – May 2011) 850-hPa temperature field (Fig. 2.3.9), positive In the sea level pressure field (Fig. 2.3.7), anomalies were observed over Europe, central Siberia negative anomalies were clearly observed around the and the eastern USA, and negative anomalies were Arctic region. The Pacific High was stronger than seen over China, Japan, western Northern America normal in the eastern Pacific. In March (Fig. 2.3.11), and around Greenland. In the 200-hPa wind field (Fig. the Siberian High was enhanced. In the 500-hPa 2.3.10), the jet stream was stronger than normal from height field (Fig. 2.3.8), the polar vortex was stronger areas near Japan to the Pacific, and was split into two than normal and located near Greenland. Wave trains branches from the Atlantic to Europe. were seen from Europe to areas near Japan. In the

Fig. 2.3.7 Three-month mean sea level Fig. 2.3.8 Three-month mean 500-hPa Fig. 2.3.9 Three-month mean 850-hPa pressure and anomaly in the Northern height and anomaly in the Northern temperature and anomaly in the Hemisphere (March – May 2011) Hemisphere (March – May 2011) Northern Hemisphere (March – May As per Fig. 2.3.2, but for March – May As per Fig. 2.3.3, but for March – May 2011) 2011 2011 As per Fig. 2.3.4, but for March – May 2011; contour interval: 3°C

Fig. 2.3.10 Three-month mean 200-hPa Fig. 2.3.11 Monthly mean sea level wind speed and vectors in the Northern pressure and anomaly in the Hemisphere (March – May 2011) Northern Hemisphere (March 2011) The black lines show wind speed at The contours show sea level pressure at intervals of 15 m/s. The dark-blue intervals of 4 hPa. The shading shading indicates values greater than 30 indicates sea level pressure anomalies. m/s. The green lines show normal wind speed at intervals of 30 m/s.

18 2.3.4 Summer (June – August 2011) observed in the Northern Hemisphere, especially over In the sea level pressure field (Fig. 2.3.12), an Eurasia in July (Fig. 2.3.16). In the 850-hPa anticyclone developed over the Arctic Sea and temperature field (Fig. 2.3.14), positive anomalies Greenland throughout the summer. The Pacific High persisted over the Arctic region, and temperatures was generally stronger than normal. High-pressure significantly exceeding the normal were observed systems around Japan were frequently enhanced in over the southern USA (see Section 3.4 for details). In association with wave trains along the Asian jet and the 200-hPa wind field (Fig. 2.3.15), the jet stream from the Philippines to the North Pacific. In the showed significant meandering in the Northern 500-hPa height field (Fig. 2.3.13), positive anomalies Hemisphere. were seen over the Arctic region. Wave trains were

Fig. 2.3.12 Three-month mean sea level Fig. 2.3.13 Three-month mean 500-hPa Fig. 2.3.14 Three-month mean 850-hPa pressure and anomaly in the Northern height and anomaly in the Northern temperature and anomaly in the Northern Hemisphere (June – August) Hemisphere (June – August 2011) Hemisphere (June – August 2011) As per Fig. 2.3.2, but for June – August As per Fig. 2.3.3, but for June – August As per Fig. 2.3.9, but for June – August 2011 2011 2011

Fig. 2.3.15 Three-month mean 200-hPa Fig. 2.3.16 Monthly mean 500-hPa wind speed and vectors in the Northern height and anomaly in the Northern Hemisphere (June – August 2011) Hemisphere (July 2011) The black lines show wind speed at The contours show 500-hPa height at intervals of 10 m/s. The dark-blue intervals of 60 m. The shading indicates shading indicates values greater than 20 height anomalies. m/s. The green lines show normal wind speed at intervals of 20 m/s.

19 2.3.5 Autumn (September – November 2011) warmer-than-normal temperatures nationwide due to a In the sea level pressure field (Fig. 2.3.17), the barotropic high over the country associated with wave Icelandic Low was stronger than normal during the trains propagating from Eurasia. In the 850-hPa season, and the Aleutian Low was stronger than temperature field (Fig. 2.3.19), above-normal normal around Alaska. The Pacific High was also temperatures were observed around Europe and stronger than normal. In the 500-hPa height field (Fig. Canada, while below-normal temperatures were seen 2.3.18), wave trains were observed from the northern around the Caspian Sea. In the 200-hPa wind field North Atlantic to Eurasia, with positive anomalies (Fig. 2.3.20), a pronounced split-flow pattern in the over Europe and East Asia. Wave trains were also jet stream was observed over Europe, and the jet seen from the central Pacific to North America. In stream from the vicinity of Japan to the Pacific was November (Fig. 2.3.21), Japan experienced shifted northward of its normal position.

Fig. 2.3.17 Three-month mean sea level Fig. 2.3.18 Three-month mean 500-hPa Fig. 2.3.19 Three-month mean 850-hPa pressure and anomaly in the Northern height and anomaly in the Northern temperature and anomaly in the Hemisphere (September – November Hemisphere (September – November Northern Hemisphere (September – 2011) 2011) November 2011) As per Fig. 2.3.2, but for September – As per Fig. 2.3.3, but for September – As per Fig. 2.3.4, but for September – November 2011 November 2011 November 2011

Fig. 2.3.20 Three-month mean 200-hPa Fig. 2.3.21 Monthly mean 500-hPa wind speed and vectors in the Northern height and anomaly in the Northern Hemisphere (September – November Hemisphere (November 2011) 2011) The contours show heights of 60 m. As per Fig. 2.3.15, but for September – The shading indicates height anomalies. November 2011

20 2.4 Tropical circulation and convective activity area around the Philippines) remained positive

The La Niña event that appeared in summer 2010 (indicating enhanced convective activity) until July, decayed in spring 2011 (see Section 3.1 for details). and then alternated between positive and negative However, sea surface temperatures (SSTs) in summer values. The OLR-MC (for the area around Indonesia) 2011 remained below normal over central to eastern remained positive except in October and November. areas of the tropical Pacific except in its equatorial The OLR-DL (for the area near the dateline) remained region. In autumn, SST deviations from the sliding negative except in September. average for the latest 30-year period turned negative The U200-CP (for the central Pacific) remained (a condition typical of La Niña events) and persisted mostly positive (indicating westerly anomalies), while until winter. Thus, the characteristics of atmospheric the U850-WP (for the western Pacific) and the circulation throughout 2011 resembled those seen U850-CP remained negative, indicating that during La Niña periods. This section briefly outlines equatorial zonal circulation (known as the Walker the details of tropical atmospheric circulation and Circulation) was stronger than normal. The Southern convection for 2011. Oscillation Index (SOI) had remained positive (indicating stronger-than-normal trade winds) since 2.4.1 Tropical indices April 2010. In particular, the SOI for March, April OLR indices and equatorial zonal wind indices are and December showed large positive values exceeding shown in Table 2.4.1 and Figure 2.4.1 (see Section +2.0. 1.4.3 for related definitions). The OLR-PH (for the

Table 2.4.1 Tropical atmospheric and oceanographic indices (Dec. 2010 – Dec. 2011) Southern Oscillation OLR Index Equatorial Zonal Wind Index Month SOI DARWIN TAHITI OLR-PH OLR-MC OLR-DL U200-IN U200-CP U850-WP U850-CP U850-EP Dec. 2010 2.5 -2.5 2.9 0.5 0.9 -1.6 -3.4 1.8 -1.6 -1.7 -0.2 Jan. 2011 1.7 -1.6 2.7 2.5 0.6 -1.5 -2.8 1.8 -0.7 -0.7 1.2 Feb. 2011 1.8 -1.7 3.3 1.2 0.5 -1.7 -2.1 1.4 -0.7 -0.7 1.1 Mar. 2011 2.3 -2.0 2.6 3.1 1.6 -1.6 -1.7 2.1 -1.3 -0.8 -0.1 Apr. 2011 2.2 -0.8 2.6 1.3 1.2 -1.4 -1.1 2.3 -1.2 -0.4 1.0 May 2011 0.4 0.7 1.3 1.2 1.9 -0.5 0.7 2.9 -0.6 -0.4 0.9 Jun. 2011 0.5 0.6 1.2 0.6 0.2 -0.3 -0.9 2.8 -1.1 -0.9 0.3 Jul. 2011 1.2 -0.2 1.6 1.3 0.0 -0.2 -1.2 2.4 -1.1 -1.2 1.6 Aug. 2011 0.5 0.1 1.0 -0.7 0.0 -0.4 -0.7 1.2 -0.9 -1.1 0.1 Sep. 2011 1.1 0.3 2.2 0.8 0.5 0.3 -1.5 1.6 -1.6 -1.4 0.2 Oct. 2011 1.0 -0.8 1.0 -1.9 -0.6 -1.2 0.2 0.0 -0.5 -0.1 1.4 Nov. 2011 1.3 -0.5 1.7 0.0 -0.1 -0.9 0.0 0.4 -0.9 -1.1 0.1 Dec. 2011 2.2 -2.4 2.2 0.6 0.9 -1.2 -1.7 2.2 -1.9 -1.0 0.4 Monitoring point TAHITI - 12.5ºS 17.5ºS 20-10ºN 5ºN-5ºS 5ºN-5ºS 5ºN-5ºS 5ºN-5ºS 5ºN-5ºS 5ºN-5ºS 5ºN-5ºS or area DARWIN 130ºE 150ºW 110-140ºE 110-135ºE 170ºE-170ºW 80-100ºE 180-125ºW 160ºE-175ºW 170-135ºW 130-100ºW Sea Surface Temperature and Anomaly(℃) Month IOBW NINO.WEST NINO.4 NINO.3 NINO.1+2 Dec. 2010 27.82 -0.04 29.66 0.64 27.0 -1.5 23.7 -1.5 21.8 -1.1 Jan. 2011 27.64 -0.18 29.48 0.94 26.7 -1.6 24.2 -1.4 24.0 -0.5 Feb. 2011 27.91 -0.12 29.36 1.04 27.0 -1.1 25.6 -0.8 26.1 0.1 Mar. 2011 28.50 -0.09 29.18 0.72 27.4 -0.8 26.4 -0.7 25.9 -0.5 Apr. 2011 28.90 -0.15 29.37 0.45 27.9 -0.6 27.2 -0.3 25.3 -0.2 May 2011 28.83 -0.03 29.59 0.24 28.3 -0.5 26.9 -0.2 24.5 0.2 Jun. 2011 28.14 0.12 29.54 0.05 28.5 -0.3 26.6 0.1 23.6 0.7 Jul. 2011 27.42 0.16 29.22 -0.16 28.4 -0.4 25.7 0.0 22.3 0.5 Aug. 2011 27.18 0.28 29.16 -0.10 28.3 -0.4 24.7 -0.4 21.0 0.1 Sep. 2011 27.25 0.17 29.41 0.04 28.1 -0.6 24.3 -0.6 19.9 -0.7 Oct. 2011 27.87 0.36 29.53 0.07 27.9 -0.8 24 -1 20.6 -0.4 Nov. 2011 28.25 0.46 29.70 0.31 27.8 -0.8 23.9 -1.1 20.9 -0.8 Dec. 2011 28.26 0.40 29.42 0.40 27.4 -1.1 24.3 -0.9 22.1 -0.8 Monitoring point 20ºN-20ºS 15ºN-EQ 5ºN-5ºS 5ºN-5ºS EQ-10ºS or area 40-100ºE 130-150ºE 160ºE-150ºW 150-90ºW 90-80ºW

21 Figure 2.4.1 Time series of tropical atmospheric and oceanographic indices for the period from 2002 to 2011

Fig. 2.4.2 Time-longitude cross section of three-pentad Fig. 2.4.3 Time-longitude cross section of five-day mean mean 200-hPa velocity potential around the equator 850-hPa zonal wind anomalies around the equator averaged between 5˚S and 5˚N (December 2010 – averaged between 5˚S and 5˚N (December 2010 – December 2011) December. 2011) The contour interval is 2 × 106 m2/s. The blue (red) shading The contour interval is 2 m/s. The blue (red) shading shows indicates areas of divergence that are stronger (weaker) easterly (westerly) anomalies. than normal.

22 2.4.2 Winter (December 2010 – February 2011) lower troposphere, westerly anomalies along the The anomaly patterns of tropical circulation and equator were observed over the Indian Ocean to the convection were similar to those seen in past La Niña maritime continent, while easterly anomalies were events. Convective activity was enhanced over Sri seen over western to central areas of the Pacific (Figs. Lanka, the Philippines, south of Indonesia and over 2.4.3 and 2.4.6). In northeastern Australia, convective northern South America, and was suppressed over the activity was enhanced from December 2010 to early central equatorial Pacific (Fig. 2.4.4). In the upper January 2011 (Fig. 2.4.7), which brought heavy troposphere, cyclonic and anticyclonic circulation rainfall to the area. The Madden-Julian Oscillation anomalies were pronounced over central to eastern (MJO) propagated eastward along the equator in areas of the Pacific and from the western Pacific to January 2011, but was not clearly seen in December the Indian Ocean, respectively (Fig. 2.4.5). In the 2010 and February 2011 (Fig. 2.4.2).

Fig. 2.4.4 Three-month mean outgoing longwave radiation (OLR) anomaly (December 2010 – February 2011) The contour interval is 10 W/m2. Original data provided by NOAA.

Fig. 2.4.5 Three-month mean 200-hPa stream function and anomaly (December 2010 – February 2011) The contours show the stream function at intervals of 8 × 106 m2/s, and the shading shows stream function anomalies.

Fig. 2.4.6 Three-month mean 850-hPa stream function and anomaly (December 2010 – February 2011) The contours show the stream function at intervals of 4 × 106 m2/s, and the shading shows stream function anomalies.

Fig. 2.4.7 Outgoing longwave radiation (OLR) anomaly in December 2010 The contour interval is 10 W/m2. Original data provided by NOAA.

23 2.4.3 Spring (March – May 2011) circulation anomalies were observed over the central The characteristics of tropical circulation and Pacific and from the eastern Indian Ocean to the convection resembled those generally seen in past La western Pacific, respectively (Fig. 2.4.9). In the lower Niña events. Convective activity was enhanced troposphere, westerly and easterly anomalies around the Philippines, Indonesia and northern dominated along the equator of the eastern Indian Australia, and was suppressed around the dateline Ocean and from western to central areas of the Pacific (Fig. 2.4.8). This pattern was observed throughout the (Figs. 2.4.3, 2.4.10). The MJO propagated eastward season, particularly in March (Fig. 2.4.11). along the equator from the Indian Ocean to the Pacific Convection was active from northern South America in the second half of April to the first half of May (Fig. to the Atlantic and inactive over the Indian Ocean. In 2.4.2). the upper troposphere, cyclonic and anticyclonic

Fig. 2.4.8 Three-month mean outgoing longwave radiation (OLR) anomaly (March – May 2011) As per Fig. 2.4.4, but for March – May 2011

Fig. 2.4.9 Three-month mean 200-hPa stream function and anomaly (March – May 2011) As per Fig. 2.4.5, but for March – May 2011

Fig. 2.4.10 Three-month mean 850-hPa stream function and anomaly (March – May 2011) As per Fig. 2.4.6, but for March – May 2011

Fig. 2.4.11 Outgoing longwave radiation (OLR) anomaly in March 2011 As per Fig. 2.4.7, but for March 2011

24 2.4.4 Summer (June – August 2011) Tibetan High was enhanced over its western part. In Although the La Niña event decayed in spring, a the lower troposphere, the Pacific High was broadly La Niña-like atmospheric circulation pattern remained stronger than normal (Fig. 2.4.14), especially south of over the Pacific. Convective activity was enhanced Japan in June (Fig. 2.4.15). Westerly winds across the over the western Pacific, the Bay of Bengal, the Arabian Sea and India were stronger than normal. eastern Arabian Sea, Central America and the Easterly wind anomalies along the equator persisted Caribbean Sea, and was suppressed over the eastern over western to central areas of the Pacific (Fig. 2.4.3). Indian Ocean and central to eastern areas of the Equatorial intraseasonal oscillations with 20- to Pacific (Fig. 2.4.12). In the upper troposphere, 30-day periods were pronounced throughout boreal remarkable cyclonic circulation anomalies were summer (Fig. 2.4.2). See Section 3.2 for details of the observed over the central Pacific (Fig. 2.4.13). The intraseasonal variability observed.

Fig. 2.4.12 Three-month mean outgoing longwave radiation (OLR) anomaly (June – August 2011) As per Fig. 2.4.4, but for June – August 2011

Fig. 2.4.13 Three-month mean 200-hPa stream function and anomaly (June – August 2011) As per Fig. 2.4.5, but for June – August 2011

Fig. 2.4.14 Three-month mean 850-hPa stream function and anomaly (June – August 2011) As per Fig. 2.4.6, but for June – August 2011

Fig. 2.4.15 850-hPa stream function and anomaly in June 2011 The contours show the stream function at intervals of 5 × 106 m2/s, and the shading shows stream function anomalies.

25 2.4.5 Autumn (September – November 2011) trains were seen along the Asian jet stream with Convective activity was enhanced over the western anticyclonic circulation anomalies around Japan, Indian Ocean including the Arabian Sea, the South especially in November (Fig. 2.4.19). In the lower China Sea, the area to the northeast of the Philippines, troposphere, anticyclonic circulation anomalies were and the Atlantic, and was suppressed from western to observed over western to central areas of the Pacific central areas of the equatorial Pacific and over the (Fig. 2.4.18). Easterly wind anomalies were seen eastern equatorial Indian Ocean (Fig. 2.4.16). In the along the equator from the Indian Ocean to the central upper troposphere, anticyclonic and cyclonic Pacific in October and November (Fig. 2.4.3). A circulation anomalies were observed from Africa to large-amplitude MJO propagated from the Atlantic to the western Indian Ocean and from western to central the Indian Ocean in October and November (Fig. areas of the Pacific, respectively (Fig. 2.4.17). Wave 2.4.2).

Fig. 2.4.16 Three-month mean outgoing longwave radiation (OLR) anomaly (September – November 2011) As per Fig. 2.4.4, but for September – November 2011

Fig. 2.4.17 Three-month mean 200-hPa stream function and anomaly (September – November 2011) As per Fig. 2.4.5, but for September – November 2011

Fig. 2.4.18 Three-month mean 850-hPa stream function and anomaly (September – November 2011) As per Fig. 2.4.6, but for September – November 2011

Fig. 2.4.19 200-hPa stream function and anomaly in November 2011 The contours show the stream function at intervals of 10 × 106 m2/s, and the shading shows stream function anomalies.

26 2.4.6 Tropical cyclones over the western North Table 2.4.2 Tropical cyclones forming over the western North Pacific in 2011 Pacific Maximum Number Date In 2011, the number of tropical cyclones (TCs) Name Category1 wind2) ID (UTC) with maximum wind speeds of 17.2 m/s or higher (knots) forming over the western North Pacific was just 21 T1101 Aere 5/ 7 – 5/11 TS 40 – the fourth-lowest total since 1951. Remarkably, T1102 Songda 5/21 – 5/29 TY 105 only two TCs formed from October to December, T1103 Sarika 6/ 9 – 6/11 TS 40 matching the lowest total for the period since 1951. T1104 Haima 6/21 – 6/24 TS 40 This was associated with suppressed convective T1105 Meari 6/22 – 6/27 STS 60 activity over the sea east of the Philippines. T1106 Ma-on 7/12 – 7/24 TY 95 Nine of these TCs came within 300 km of the T1107 Tokage 7/15 – 7/15 TS 35 Japanese archipelago, and three made landfall on T1108 Nock-ten 7/26 – 7/30 STS 50 Japan. The normals (i.e., the 1981 – 2010 average) T1109 Muifa 7/28 – 8/ 8 TY 95 for TC formations, approaches and landfalls are T1110 Merbok 8/ 3 – 8/ 9 STS 50 25.6, 11.4 and 2.7, respectively. The tracks of T1111 Nanmadol 8/23 – 8/30 TY 100 tropical cyclones generated in 2011 are shown in T1112 Talas 8/25 – 9/ 5 STS 50 Figure 2.4.20. T1113 Noru 9/ 3 – 9/ 6 TS 40

T1114 Kulap 9/ 7 – 9/ 8 TS 35 T1115 Roke 9/13 – 9/22 TY 85 T1116 Sonca 9/15 – 9/20 TY 70 T1117 Nesat 9/24 – 9/30 TY 80 T1118 Haitang 9/25 – 9/26 TS 35 T1119 Nalgae 9/27 – 10/ 4 TY 95 T1120 Banyan 10/10 – 10/11 TS 35 T1121 Washi 12/15 – 12/18 STS 50 Note: Based on information from the RSMC Tokyo-Typhoon Center

1) Intensity classification for tropical cyclones Fig. 2.4.20 Tracks of tropical cyclones forming in 2011 The lines indicate the tracks of tropical cyclones with TS: tropical storm maximum wind speeds of 17.2 m/s or higher. The numbers STS: severe tropical storm in circles indicate points where maximum wind speeds exceeded this value, and those in squares indicate points TY: typhoon where they fell below it. 2) Estimated maximum 10-minute mean wind speed

27 2.5 Oceanographic conditions 2.5.1). A La Niña event started in summer 2010 and ended The SST deviation from the reference value (i.e., in spring 2011. In the months after the event ended, the latest sliding 30-year mean SST averaged over the La Niña conditions appeared again in autumn 2011 NINO.3 region) increased from -1.5°C in November and persisted at least until winter 2011/2012. 2010 to +0.1°C in June 2011 before decreasing and In the equatorial Pacific, the negative sea surface remaining at about -1.0°C after October. The temperature (SST) anomalies seen in central and five-month running mean of NINO.3 SST deviations eastern parts during winter 2010/2011 (Fig. 2.5.1 (a)) was above -0.5°C in April 2011, but fell below -0.5°C weakened in spring 2011 (Fig. 2.5.1 (b)). SSTs in September 2011. The Southern Oscillation Index remained near average from eastern to western parts (SOI) remained positive throughout the year (Fig. in summer (Fig. 2.5.1 (c)) and turned below normal 2.5.2). in central and eastern parts in autumn (Fig. 2.5.1 (d)). From late winter 2010/2011 to spring 2011, positive In the Pacific, the positive SST anomaly patterns ocean heat content (OHC) anomalies in the western observed during past La Niña events (extending from equatorial Pacific propagated eastward, and the western equatorial region northeastward and consequently negative SST anomalies in the eastern southeastward to the mid-latitudes in the North and part weakened. In summer after the La Niña event had South Pacific, respectively) were seen throughout ended, positive SST anomalies were seen in eastern almost the whole year. In the tropical Indian Ocean, parts, while negative values persisted in central parts. negative SST anomalies were dominant until spring In autumn 2011, negative SST anomalies intensified 2011, but positive anomalies were dominant after in central and eastern parts, and OHC anomalies were autumn. In the tropical North Atlantic, positive SST positive in western parts and negative in eastern parts anomalies were seen from winter to summer 2011 (Fig. (Fig. 2.5.3).

(a) Winter (b) Spring

Fig. 2.5.1 Seasonal mean sea surface temperature anomalies in (a) winter (December 2010 – February 2011), (b) spring (March – May), (c) summer (June – August), (d) autumn (September – November) The contour interval is 0.5°C. Maximum sea ice coverage areas are shaded in gray.

28 (℃)

Fig. 2.5.2 Monthly values (thin lines) and five-month running means (thick lines) of the El Niño monitoring index (top: NINO.3 SST deviation from the sliding 30-year mean; bottom:the Southern Oscillation Index) The shading indicates El Niño (red) and La Niña (blue) events.

Fig. 2.5.3 Time-longitude cross sections of SST (left) and ocean heat content anomalies (right: vertically averaged temperature in the top 300 m) along the equator in the Indian Ocean and the Pacific Ocean in 2010 and 2011

29 2.6 Stratospheric circulation in boreal winter seen in winter 2009/2010 (Fig. 2.6.2 (b)) and

In winter 2010/2011, the polar vortex was stronger 2008/2009 (Fig. 2.6.2 (c)) when a major SSW event than normal, and lower-than-normal temperatures occurred (Harada et al. 2010). were generally seen at the 30-hPa level over the North In winter 2010/2011, two minor SSW events Pole (Fig. 2.6.1). Two minor stratospheric sudden occurred in the first half of January and in the period warming (SSW) events occurred, but did not reach the from late January to early February. In the first event, criteria for categorization as major events. This an anticyclone developed around Alaska and the polar section reports on the characteristics of stratospheric vortex shifted moderately toward Europe and the circulation seen during this winter. Atlantic, showing a pronounced planetary wave of A stratospheric sudden warming (SSW) is a zonal wavenumber 1 (Fig. 2.6.3 (a)). However, the phenomenon in which a rapid stratospheric anticyclone did not extend broadly over the Arctic temperature increase of several tens of Kelvin is region. In the second event, a planetary wave of zonal observed over a period of a few days in the polar wavenumber 2 became pronounced following that of region in winter, and was first discovered by Richard zonal wavenumber 1 (Fig. 2.6.3 (b)). The two centers Scherhag at the Free University of Berlin in 1952. It is of the polar vortex were located over the Atlantic and known that SSWs are caused by enhanced propagation Russia, but the formation of two clearly split vortices of energy from the troposphere as a result of was not seen. The vertical component of the planetary-scale wave action. According to the World Eliassen-Palm flux (EP flux; after Palmer, 1982) Meteorological Organization (WMO) definition averaged over 30° – 90°N at the 100-hPa level was (WMO 1978), a minor SSW occurs when polar remarkable in the first half of the last 10-day period of temperatures increase by 25 K or more within a week January with planetary waves of zonal wavenumber 1 at any stratospheric level. In addition to this criterion, and in the second half of the same period with if the zonal-mean temperature increases in the planetary waves of zonal wavenumber 2 (Fig. 2.6.4 poleward direction and net zonal-mean zonal winds (c)). Considering that the vertical component of the become easterly north of 60°N at 10 hPa or below, the EP flux indicates the vertical propagation of energy event is classified as a major SSW. by planetary waves, it can be inferred that the planetary wave of zonal wavenumber 2 propagated 2.6.1 Characteristics of stratospheric circulation from the troposphere to the stratosphere following the Temperatures at 30 hPa over the North Pole propagation of the planetary wave of zonal remained below normal during most of the period wavenumber 1. When the two SSW events occurred, from mid-November 2010 to March 2011, and were zonal-mean 30-hPa westerly winds at 60°N weakened, remarkably below normal from mid-February to but did not turn easterly (Fig. 2.6.4 (b)). mid-March in particular. The minimum value was observed in mid-February, but is seen in the period 2.6.2 Final warming from late December to early January in the The increase of the 30-hPa temperature over the climatological normal (Fig. 2.6.1). In the three-month North Pole was significant in late March, and the mean 30-hPa height field from December 2010 to temperature rapidly rose in early April (Figs. 2.6.1 February 2011 (Fig. 2.6.2 (a)), the polar vortex was and 2.6.4 (a)). In this period, an anticyclone formed stronger than normal with negative anomalies over the around Alaska and gradually developed and extended Arctic region, which was the opposite of the patterns over the polar region (Fig. 2.6.3 (c)). In mid-April,

30 zonal-mean 30-hPa winds at 60°N turned from and T. Hirooka, 2010: A major stratospheric sudden warming event in January 2009. J. Atmos. Sci., 67, 2052 westerlies to easterlies (Fig. 2.6.4 (b)), and an – 2069. doi: 10.1175/2009JAS3320.1. anticyclonic circulation (a normal formation in the Palmer, T. N., 1982: Properties of the Eliassen-Palm flux stratosphere during the summer season) appeared over for planetary scale motions. J. Atmos. Sci., 39, 992 – 997. the polar region. WMO, 1978: Abridged Report of the Commission for Atmospheric Sciences seventh session item 9.4, WMO References Rep., 509, 35 – 36. Harada, Y., A. Goto, H. Hasegawa, N. Fujikawa, H. Naoe

Fig. 2.6.1 Time series of temperatures at the 30-hPa level over the North Pole (September 2010 – April 2011) The black line shows daily temperatures, and the gray line indicates the climatological mean.

(a) (b) (c)

Fig. 2.6.2 Three-month mean 30-hPa height and anomaly in the Northern Hemisphere for (a) December 2010 – February 2011, (b) December 2009 – February 2010, and (c) December 2008 – February 2009 The contours show 30-hPa height at intervals of 120 m, and the shading indicates its anomaly.

31 (a) (b) (c)

Fig. 2.6.3 Five-day mean 30-hPa height and anomaly in the Northern Hemisphere for (a) 11 – 15 January, (b) 31 January – 4 February, and (c) 11 – 15 April, 2011 The contours show 30-hPa height at intervals of 120 m, and the shading indicates its anomaly.

(a)

(b)

(c)

Fig. 2.6.4 (a) Time-height cross section for seven-day variations of zonal mean temperature averaged over 75° – 90°N, (b) time-height cross section of zonal mean zonal wind at 60°N, and (c) time series of vertical components of EP flux averaged over 30° – 90°N at the 100-hPa level (October 2010 – April 2011) The red bars in (c) denote the vertical component of EP flux for whole zonal wave numbers. The purple, light-blue and light-green lines denote the vertical components of EP flux for zonal wavenumbers 1, 2 and 3, respectively. The broken line in (c) denotes the climatological mean for the vertical component of EP flux for whole zonal wavenumbers. The unit for the vertical component of EP flux in (c) is m2/s2.

32 2.7 Summary of the Asian summer monsoon 2.7.2 Tropical cyclones Asian summer monsoon monitoring is very During the monsoon season, 17 tropical important because related fluctuations in convective cyclones(TCs) of tropical storm (TS) intensity or activity and atmospheric circulation can influence the higher formed over the western North Pacific (Table summer climate in Asia, including that of Japan. This 2.4.2) against the 1981 – 2010 average of 16.0. Seven section summarizes the characteristics of the Asian of these passed over the South China Sea and summer monsoon in 2011. approached or hit southern China or Vietnam. Three TCs hit the main islands of Japan. 2.7.1 Temperature and precipitation Severe Tropical Storm Nock-ten and Typhoon Four-month mean temperatures based on CLIMAT Nesat caused more than 70 and more than 80 fatalities reports during the monsoon season (June – in the Philippines, respectively, while Severe Tropical September) were higher than normal from Pakistan to Storm Talas and Typhoon Roke caused 78 and 18 northern China, around southern China and around fatalities in Japan, respectively. Japan, and were lower than normal around northern India, over most parts of the Indochina Peninsula and in eastern China (Fig. 2.7.1). Four-month total precipitation amounts for the same period were above 200% of the normal around southern Pakistan and below 60% of the normal around Java Island (Fig. 2.7.2). The values were mostly consistent with the distribution of OLR anomalies (Fig. 2.7.3). It was reported that heavy rains caused more than 170 fatalities in southern China in June and more than Fig. 2.7.1 Four-month mean temperature anomaly (˚C) for June – September 2011 70 fatalities in Korea from 26 July to 29 July. See 1.3.2 for the data source. Flooding also reportedly caused more than 480 fatalities in the Sindh region of Pakistan from August to September. Precipitation over the Indochina Peninsula remained above normal from June to September, causing floods over a wide area in the basins of the Chao Phraya River and the Mekong River. This caused serious damage over the Indochina Peninsula, especially in Thailand. More than 700 fatalities were reported in Thailand, 240 in Cambodia and 40 in Fig. 2.7.2 Four-month total precipitation ratio (%) for June – September 2011 Vietnam. For more details of the heavy rainfall event See 1.3.2 for the data source. over the Indochina Peninsula, see Section 3.3.

33 2.7.3 Convective activity and atmospheric circulation Convective activity (inferred from outgoing longwave radiation (OLR)) was enhanced over southern Pakistan, the eastern Arabian Sea, the Bay of Bengal, the Indochina Peninsula, the Philippines and the western tropical Pacific, and was suppressed over the eastern Indian Ocean and Indonesia (Fig. 2.7.3). According to the OLR indices (Table 2.7.1), Fig. 2.7.3 Four-month mean outgoing longwave radiation (OLR) and its anomaly for June – September convective activity averaged over the Bay of Bengal 2011 and in the vicinity of the Philippines (both core areas The contours indicate OLR (W/m2) at intervals of 10 W/m2, and the colored shading indicates its anomalies from the of monsoon-related active convection) was enhanced normal. Negative (cold color) and positive (warm color) from May to July and in September. The active OLR anomalies show enhanced and suppressed convection, convection area was located east of its normal respectively, compared to the normal. position in the first half of the monsoon season and Table 2.7.1 Summer Asian Monsoon OLR Index west of it in the second. (SAMOI) from May to October 2011 In the upper troposphere, the Tibetan High was SAMOI is described in 1.4.3. pronounced over a wide area (Fig. 2.7.4 (a)). In the lower troposphere, a prominent monsoon trough was observed stretching from northern India to the Philippines, and westerly/southwesterly winds were stronger than normal over a large area from the Arabian Sea to the Philippines (Fig. 2.7.4 (b)). From May to August, equatorial intraseasonal oscillations propagated eastward with a period of less than 30 days (Fig. 3.2.7). The areas of active convection originally enhanced by these oscillations propagated northward around India and east of the

Philippines (Fig. 2.7.5). In the western North Pacific, off-equatorial intraseasonal oscillations propagating westward/northwestward with a period of two to three weeks prevailed and contributed to convective activity fluctuations around the Philippines and Pacific highs near Japan (see Section 3.2 for details).

34 (a) (a)

(b)

Fig. 2.7.5 Latitude-time cross section of five-day mean outgoing longwave radiation (OLR) from May – October 2011 in (a) India (65°E – 85°E mean) and (b) east of the Philippines (125°E – 145°E mean) The shading indicates the OLR for 2011. The black lines indicate the climatological mean OLR at intervals of 20 Fig. 2.7.4 Four-month mean stream function and its W/m2 for values of 240 W/m2 and below. anomaly for June – September 2011 (a) The contours indicate the 200-hPa stream function at intervals of 10 × 106 m2/s, and the colored shading indicates 200-hPa stream function anomalies from the normal. (b) The contours indicate the 850-hPa stream function at intervals of 4 × 106 m2/s, and the colored shading indicates 850-hPa stream function anomalies from the normal. In the Northern (Southern) Hemisphere, warm (cold) shading denotes anticyclonic (cyclonic) circulation anomalies.

35 2.8 Arctic sea ice conditions an increase in the sea ice extent (Fig. 2.8.4). Recently, the sea ice extent in the Arctic Ocean has shown a decreasing tendency that has been particularly marked in terms of the annual minimum extent. The monitoring of Arctic sea ice conditions has become more significant because of this phenomenon’s possible influence on the climate as a result of related changes in the radiation budget and heat exchange between the Arctic Ocean and the Fig. 2.8.1 Time series showing annual minimum values atmosphere. This section outlines the characteristics of the Arctic sea ice extent from 1979 to 2011 of the Arctic sea ice extent in 2011 along with those of atmospheric circulation.

2.8.1 Progress of sea ice in the Arctic in 2011 On 9th March, the Arctic sea ice extent reached its second-lowest annual maximum since 1979 after 2006 (Fig. 2.8.2). After June, it showed a rapid decrease from the normal and reached its lowest July level since 1979. The rate of decrease abated from July to Fig. 2.8.2 Time series of the sea ice extent over the early August, and the extent exceeded that seen at the Arctic region in 2011 same time in 2007 (the lowest on record). On 9 The sea ice extent is defined as the area where sea ice concentration is 15 – 100%. The black line indicates the September, it reached its second-lowest annual normal (i.e., the 1981 – 2010 average), and the gray minimum (Fig. 2.8.3) since 1979 after 2007 (Fig. shading denotes the range of the standard deviation 2.8.1). calculated for the time period of the normal.

2.8.2 Arctic atmospheric circulation in terms of dissolution In June – August, a high-pressure system (i.e., an anticyclone) covered the Arctic Ocean (Fig. 2.8.4), creating conditions suitable for a reduction in the sea ice extent. Cyclonic circulation anomalies seen over western and central Siberia also enhanced conditions for a reduction around the East Siberian Sea.

Temperatures at 925 hPa were higher than normal around the Arctic region during the same season, creating conditions suitable for the melting of sea ice. In September, these characteristics of atmospheric Fig. 2.8.3 Sea ice concentration on 9 September 2011 circulation were reversed. A low-pressure system was (left) and sea ice extent climatology (i.e., the 1981 – 2010 average) on 10 September (right) seen over the Arctic region and a high-pressure The blue scale shows deciles of sea ice concentration. system over Siberia, creating conditions suitable for

Fig. 2.8.4 Monthly mean sea level pressure (top) and temperature anomaly at 925 hPa (bottom) over the Arctic region from June (left) to September (right) 2011 Top panels: The contours show sea level pressure at intervals of 4 hPa. The shading indicates sea level pressure anomalies. Bottom panels: The contours show 925-hPa temperature at intervals of 2°C. The shading indicates temperature anomalies.

37 2.9 Snow cover in the Northern Hemisphere 2.9.1 Related characteristics in 2011 The albedo of snow-covered ground (i.e., the ratio of In winter 2010/2011 (December – February), there solar radiation reflected by the surface) is higher than that were more days of snow cover than normal in the USA of snow-free ground. The variability of snow cover has and eastern Europe throughout the season and fewer than an impact on the earth’s surface energy budget and normal around the Caspian Sea in December and January radiation balance, and therefore on the climate. In (Fig. 2.9.1 (a)). In spring 2011 (March – May), there addition, snow absorbs heat from its surroundings and were more days of snow cover than normal in North melts, thereby providing soil moisture and related effects America throughout the season and fewer in western and on the climate system. The variability of atmospheric central Siberia in April and May (Fig. 2.9.1 (b)). In circulation and oceanographic conditions affects the November 2011, there were fewer days of snow cover amount of snow cover, which exhibits a close and mutual than normal around western Siberia and Central Asia (Fig. association with climatic conditions. 2.9.1 (c)).

(a) February 2011 (b) May 2011 (c) November 2011

Fig. 2.9.1 Number of snow-cover days (top) and anomaly (bottom) for February 2011 (left; (a)), May 2011 (center; (b)) and November 2011 (right; (c)) Statistics on the number of snow-cover days are derived using data from the Special Sensor Microwave Imager (SSM/I) and the Special Sensor Microwave Imager Sounder (SSMIS) on board the US Defense Meteorological Satellite Program (DMSP) satellites based on an algorithm developed by the Japan Meteorological Agency. The base period for the normal is 1989 – 2010.

38 2.9.2 Interannual variability and trends trend with a 95% confidence level for May, October, Figure 2.9.2 shows the interannual variation in the November and December, while no trend is seen for the total area of monthly snow cover in the Northern period from January to April. In Eurasia, there is a Hemisphere and Eurasia over the 24-year period from decreasing trend for April, May, October, November and 1988 to 2011. December, while no trend is seen for the period from In the Northern Hemisphere, there is a decreasing January to March.

Fig. 2.9.2 Interannual variation in the total area of monthly snow cover (km2) in the Northern Hemisphere (north of 30˚N; left) and Eurasia (30˚ – 80˚N, 0 – 180˚E; right) for February ((a) and (d)), May ((b) and (e)), and November ((c) and (f)) from 1988 to 2011. The blue lines indicate the total snow cover area for each year, and the black lines show linear trends (95% confidence level).