3 Analysis of specific events occurring in 2014

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1 3 Analysis of specific events occurring in Winter 2013/2014 cold waves over North America Introduction In winter 2013/2014 (December 2013 February 2014), extremely cold conditions were frequently observed over central and eastern North America, resulting in an influence on socioeconomic activity there. The results of analysis indicated that both tropospheric and stratospheric circulation may have contributed to cold waves over North America via the reflection of planetary waves in the lower stratosphere. A number of studies have addressed such reflection and other aspects of interaction between the troposphere and the stratosphere. Perlwitz and Harnik (2003) reported that planetary waves can propagate downward from the stratosphere to the troposphere, and that such propagation is related to the vertical structure of the polar-night jet stream. Kodera et al. (2008, 2013) also reported the possibility that planetary waves originating from the troposphere are reflected downward in the lower stratosphere. This section reports on related surface climate characteristics, atmospheric circulation and primary factors contributing to the extreme cold conditions observed in North America Surface climate conditions Seasonal mean temperatures were above normal in Alaska, around California and around the Florida Peninsula, and were below normal in much of the rest of North America, especially from central Canada to the southern USA (Fig ). Ten-day mean 1 temperature anomalies of more than 6 C below normal were observed in western North America in early December 2013 (Fig (1)). Similar values were recorded from northern to eastern Canada from mid- to late December 2013, from central Canada to the Midwest of the USA in early January, from the Midwest to the southern USA in late January, from southwestern Canada to the southern USA in early February, in northwestern North America in the middle of February and from western Canada to the Midwest in late February (Fig (2) (9)). At Great Falls, Montana, daily mean temperatures fell below -30 C (approximately 27 C below normal) in early December and early February. At Minneapolis/St. Paul Int., Minnesota, daily mean temperatures were below normal in early to mid-december, early January, late January to early February and late February, and fell below -25 C (approximately 16 C below normal) in early January. At Chicago/O Hare, Illinois, daily mean temperatures fell below -20 C (more than 16 C below normal) in early and late January. At Montreal/Pierre Elliott Trudeau Int'l A, Quebec, daily mean temperatures fell below -20 C (more than 10 C below normal) in mid-december and in early and late January (Fig ). Cold waves reportedly caused at least 40 fatalities in the USA from mid-december to early January and at least 10 fatalities in Canada around late December (EM-DAT). In addition, storms reportedly caused more than 80 fatalities throughout the USA during winter (EM-DAT) as well as power outages at hundreds of thousands of homes, with transportation influences including flight delays and cancellations (FEMA). 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

2 Fig 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, and The normal values and standard deviations were calculated from statistics. Areas over land without graphical marks are those where observation data are insufficient or where normal data are unavailable Characteristics of atmospheric and oceanic circulation in winter 2013/2014 Sea surface temperatures (SSTs) for winter 2013/2014 were below normal in the central and eastern equatorial Pacific and above normal in the western tropical Pacific (Fig (a)). In association with enhanced convective activity over the area from the Maritime Continent (the Indonesian Archipelago) to the western Pacific (Fig (b)), large-scale divergence anomalies in the upper troposphere were seen there (Fig (c)). Quasi-stationary Rossby wave trains in the middle and upper troposphere were seen over the area from the northwestern Pacific or the eastern part of East Asia to North America and the Atlantic with anticyclonic circulation anomalies (ridges) over western North America and cyclonic circulation anomalies (troughs) over its central and eastern parts (Fig (a) and (b)). These anomalies (ridges/troughs) indicate the enhancement of the corresponding climatological ridges and troughs (not shown). The enhancement of convective activity over the area from the Maritime Continent to the western Pacific may have contributed to the formation of anticyclonic circulation anomalies to the east of the Philippines and the northeastward propagation of Rossby waves downstream from the area (see Subsection for details). In the sea level pressure field, positive and negative anomalies were seen over western and eastern North America, respectively, and cold-air masses frequently moved over central and eastern North America (Fig (c)). In the lower troposphere, northerly wind anomalies and below-normal temperature anomalies contributed to cold-air advection over central and eastern North America, respectively (Fig (d) and Fig ). 48

3 Fig 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 Figures for February are shown on the next page. ( C) 49

4 ( C) Fig 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, Illinois (USA) and Montreal/Pierre Elliott Trudeau Int'l, Quebec (Canada). Figures for December and January are shown on the previous page. Fig Daily mean temperatures ( C) at four stations from 1 December 2013 to 28 February 2014 (Based on SYNOP reports) Red circles denote daily mean temperatures. The black dashed line denotes normal values ( average) for daily mean temperature. 50

5 In the 30-hPa height field, the Aleutian anticyclone developed and the polar vortex was displaced toward Canada (Fig (a)). Upward propagation of planetary waves from the troposphere to the stratosphere was seen over the area from Siberia to the Bering Sea in line with the westward tilting of a trough with height (Fig (b) and (c)). The planetary waves originating from the troposphere partially turned downward around Canada in the lower stratosphere (Fig (b) and (c)), which may have contributed to the enhancement of the trough over central and eastern North America (Fig (b)). The characteristics seen in the vertical propagation of these planetary waves were more distinct than those seen in the climatological normal (not shown), and the characteristics of the planetary-wave reflection observed during winter were similar to those reported in recent studies. Fig 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 m 2 /s. 51

6 Fig 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: m 2 /s 2 ). Fig 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

7 Fig 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. (b) The arrows (shading) indicate the horizontal (vertical) components of wave activity flux. Warm (cold) colored shading denotes upward (downward) propagation. (c) The contours show height anomalies at intervals of 100 m. The arrows indicate wave activity flux, which was calculated with reference to the method of Plumb (1985) (units: m 2 /s 2 (horizontal); Pa m/s 2 (vertical)). Fig Steady response in a linear baroclinic model (LBM) to heating anomalies around the eastern Indian Ocean The shading denotes (a) diabatic heating anomalies for the LBM and the steady response of (b) 200-hPa velocity potential anomalies and (c) 300-hPa stream function anomalies. The anomalies in (b) and (c) represent deviations from the individual basic states representing the climatological normal, and values for (c) are additionally subtracted from the zonal mean. 53

8 Fig Time-series representation of five-day running mean vertical component of wave activity flux (red lines; Plumb 1985) averaged for the area around northern Canada (60 o N 85 o N, 130 o W 80 o W; shown by the red rectangle on the right) and 850-hPa temperature anomalies (blue lines) averaged over central and eastern North America (30 o N 60 o N, 100 o W 70 o W; shown by the blue rectangle on the right) from 1 December 2013 to 28 February 2014 Positive (negative) values of the vertical component of wave activity flux indicate upward (downward) propagation. The blue and red lines indicate 500-hPa and 925-hPa temperature anomalies, respectively. Fig Five-day mean (upper panel) longitude-height cross section of height anomalies from the zonal mean (contours) and wave activity flux (arrows) averaged over 40 N 80 N and (lower panel) 300-hPa stream function anomalies from the zonal mean (shading) and wave activity flux (arrows) for (a) January, (b) January, (c) January and (d) January 2014 The contour intervals in the upper panel are 200 m. Wave activity flux was calculated with reference to the method of Plumb (1985) (units: m 2 /s 2 (horizontal); Pa m/s 2 (vertical)). 54

9 Fig Five-day mean longitude-height cross section of extended refractive indices (shading) and zonal wind (contours) averaged over 40 N 80 N for (a) January, (b) January, (c) January and (d) January 2014 The contour intervals are 5 m/s, and zero lines are omitted. The refractive indices were calculated with reference to the method of Karoly (1983), and are represented as the equivalent zonal wavenumber for latitude circles (i.e., non-dimensional values) Influence of convective activity on tropospheric circulation in the Northern Hemisphere As described in Subsection 3.1.3, convective activity was enhanced over the area from the Maritime Continent to the western Pacific. Steady response to diabatic heating anomalies (Fig (a)) around the area was examined using a linear baroclinic model (LBM; Watanabe and Kimoto 2000). The results showed that large-scale divergence anomalies were seen over the area from the Maritime Continent to the western Pacific and wave trains were seen from the northwestern Pacific to North America in the upper troposphere (Fig (b) and (c)). This echoed the characteristics seen in winter 2013/2014. It can be presumed from this analysis that enhanced convective activity over the area from the Maritime Continent to the western Pacific may have contributed to cold conditions over central and eastern North America through wave trains downstream from the northwestern Pacific Case study on a cold-wave event in the second half of January 2014 Central and eastern North America experienced below-normal temperatures during winter except in the first half of mid-january and the second half of mid-february, and were frequently hit by cold surges (shown by the blue lines in Fig ). Downward propagation of planetary waves was frequently observed around northern Canada in association with their reflection (shown by the red lines in Fig ). The time-series representation shown in Fig indicates that below-normal temperature anomalies over central and eastern North America tended to become distinct a few days after peaks in the downward propagation of planetary waves. This subsection reports on the characteristics of atmospheric circulation associated with a cold-wave event observed over central and eastern North America in the second half of January In the first half of mid-january, anticyclonic circulation anomalies over the northwestern Pacific 55

10 and northeastward propagation of planetary waves downstream from these anomalies were seen in the upper troposphere (Fig (a), bottom). From the second half of mid-january to the first half of late January, wave trains were seen over the area from the northwestern Pacific or Siberia/the Bering Sea to the Atlantic in the upper troposphere (Fig (b) and (c), bottom), and vertical propagation of these waves was also seen in the same locations (Fig (b) and (c), top). Planetary waves propagated upward from the troposphere to the stratosphere around the Bering Sea, and turned downward around Canada in the lower stratosphere. These processes may have contributed to the enhancement of a trough over central and eastern North America. The reflection of planetary waves and the trough over central and eastern North America were weaker from the second half of late January onward. The extended refractive indices 2 (Karoly 1983) showed low and high values in the upper and lower layers, respectively, of the lower stratosphere (Fig ), which was consistent with the reflection of planetary waves. Further investigation is needed to clarify the relationship between distribution in these indices and the structure of the polar-night jet stream. The atmospheric characteristics observed in the second half of January were seen in other cold-wave events during winter. These analysis results suggest that interaction between the troposphere and the stratosphere may have contributed to cold waves over central and eastern North America. 2 Extended refractive indices indicate characteristics of horizontal and vertical propagation of stationary waves, and are defined in log-pressure coordinates as follows: where U denotes horizontal wind vectors, f o is the Coliolis parameter, N is buoyancy frequency, H 0 is scale height, Q is quasi-geostrophic potential vorticity and H is the horizontal gradient operator. Wave activity tends to propagate along rays that bend toward areas of large positive indices Summary In winter 2013/2014, extremely cold conditions were observed over central and eastern North America. The related characteristics and primary factors can be summarized as follows: - Quasi-stationary Rossby wave trains were seen from the northwestern Pacific to North America and to the Atlantic, which may have contributed to the formation of ridges and troughs over western North America and central eastern North America, respectively. - It can be presumed that enhanced convective activity over the area from the Maritime Continent to the western tropical Pacific may have contributed to the formation of the wave trains discussed above. - Planetary waves propagated upward from the troposphere to the stratosphere over the area from Siberia to the Bering Sea, and turned downward around Canada in the lower stratosphere. This may have contributed to the cold waves observed over central and eastern North America via the enhancement of troughs there. The relationship between the reflection of planetary waves and the cold conditions over North America discussed in this section is consistent with that reported on the basis of recent studies (e.g., Kodera et al. 2008). However, further investigation is needed to clarify the structure of the polar-night jet stream in association with the reflection of planetary waves. References Karoly, D. J., 1983: Rossby wave propagation in a barotropic atmosphere. Dyn. Atmos. Oceans, 7, Kodera, K., H. Mukougawa, and S. Itoh, 2008: Tropospheric impact of reflected planetary waves from the stratosphere. Geophys. Res. Lett., 35, L1806, doi: /2008gl , and, and A. Fujii, 2013: Influence of the vertical and zonal propagation of stratospheric planetary waves on tropospheric blockings. J. Geophys. Res., 118, Nishii, K., and H. Nakamura, 2004: Lower-stratospheric 56

11 Rossby wave trains in the Southern Hemisphere: A case-study for late winter of Quart. J. Roy. Meteor. Soc., 130, Perlwitz, J., and N. Harnik, 2003: Observational evidence of a stratospheric influence on the troposphere by planetary wave reflection. J. Climate, 16, Plumb, R. A., 1985: On the three-dimensional propagation of stationary waves. J. Atmos. Sci., 42, Watanabe, M. and M. Kimoto, 2000: Atmospheric-ocean thermal coupling in North Atlantic: A positive feedback. Quart. J. Roy. Meteor. Soc., 126,

12 3.2 Unseasonable weather conditions in Japan in August 2014 The period from late July to August 2014 was characterized by exceptionally unseasonable weather conditions across Japan, primarily due to two typhoons approaching or making landfall in rapid succession, combined with sustained inflow of moist air. From 30 July to 26 August, parts of the country experienced extremely heavy precipitation, some of which led to substantial damage. Massive landslides caused by torrential rain hit Hiroshima late during the night of 19 August to daybreak on 20 August, eventually causing dozens of fatalities. On top of the loss of human life, the unseasonable weather conditions adversely affected the national economy by retarding crop growth and damping consumer spending. In consideration of the severity and nature of related impacts, the Japan Meteorological Agency (JMA) recorded the extreme precipitation experienced during the period from the end of July to late August on a list of extreme weather events that caused serious disasters in the country. To investigate the atmospheric circulation causally connected to these unseasonable weather conditions, JMA convened an extraordinary session of the Advisory Panel on Extreme Climatic Events 1. This section covers the primary factors contributing to these conditions based on the results of the Panel s investigation. locations on the island of Shikoku and the Kii Peninsula (both in western Japan) receiving more than 400% of the normal. Precipitation totals in excess of 1,000 mm during the same period were recorded in Kochi, Tokushima, and Shionomisaki. At several stations of the Automated Meteorological Data Acquisition Network (AMeDAS) in Kochi Prefecture, total precipitation exceeded 2,000 mm. The monthly precipitation total for August averaged over the Pacific side of western Japan was the highest on record since 1946 at 301% of the normal. Meanwhile the sunshine duration was shorter than normal almost nationwide except in the Hokkaido area of northern Japan, with the record-lowest area-averaged sunshine duration at 54% of the normal for the Pacific side of western Japan. The monthly temperature for August averaged over western Japan was below normal for the first time since Observed weather conditions Figure shows temperature, precipitation and sunshine duration for Japan averaged over the period from 30 July to 31 August 2014 as deviations from or ratios against the normal (i.e., the average). The most remarkable anomalous precipitation occurred across the Pacific side of western Japan, with some Fig Temperature anomalies, precipitation ratios, and sunshine duration ratios for the period from 30 July to 31 August The Advisory Panel, consisting of prominent experts on climate science from universities and research institutes, was established in June 2007 by JMA to investigate extreme climate events based on up-to-date information and findings. 58

13 Weather conditions and atmospheric circulation from 30 July to early August Typhoons Nakri and Halong, drifting northward over the seas south of Japan in close succession from 30 July to early August, approached or made landfall on western Japan and Okinawa/Amami. Weather charts and hourly precipitation intensity maps (inferred from radar observations) for 00 UTC on 1 August and 12 UTC on 8 August are shown in Fig Typhoon Nakri, which formed east of the Philippines on 29 July, followed a path close to Okinawa from 31 July to 1 August and headed northward over the East China Sea before weakening to become a tropical storm over the Yellow Sea. The subtropical jet stream in the upper troposphere little influenced Typhoon Nakri as a steering force because it shifted northward of its normal position and flowed north of Hokkaido during this period. In association, the typhoon drifted slowly over the seas south of Japan and contributed to sustained moist air inflow over the Pacific side of western Japan and substantial precipitation over an extended period. Although the tropical disturbance that was originally Fig Weather charts for 00 UTC on 1 August (top left) and 12 UTC on 8 August (top right), and hourly precipitation intensity maps (inferred from radar observations) for the corresponding hours (bottom left and bottom right). 59

14 Typhoon Nakri ceased to exist on 4 August, the remaining moist air contributed to heavy precipitation in some parts of Hokkaido, where daily rainfall for 5 August exceeded 100 mm. Meanwhile, Typhoon Halong, which formed over the sea east of Guam on 29 July, moved westward and subsequently headed northward over the seas south of Japan from 4 to 9 August before making landfall on the island of Shikoku on 10 August. The slow-moving Halong induced a lasting inflow of moist air over Japan in the same way as Typhoon Nakri. This brought heavy rainfall nationwide in combination with a lingering (a) almost-stationary frontal system over the area from the Sea of Japan to northern Japan. In anticipation of extremely heavy rain from Halong, JMA issued an emergency warning for Mie Prefecture on 9 August. Halong weakened to an extratropical cyclone over the northern part of the Sea of Japan on 11 August, but caused intense precipitation that set new hourly rainfall records in some parts of Hokkaido. In summary, large areas of Japan experienced record-breaking precipitation from late July to early August due to two slow-moving typhoons coming in rapid succession and a quasi-stationary front line. (b) (c) (W/m 2 ) (d) (hpa) (kg/kg) (m/s) Fig Ten-day mean atmospheric circulation for 11 to 20 August 2014 (a) 850-hPa stream function anomalies (contours at intervals of m 2 /s) and OLR anomalies (shading), (b) sea level pressure (contours at intervals of 1 hpa) and anomalies (shading), (c) 925-hPa water vapor flux (black arrows) and specific humidity anomalies (shading; unit: kg/kg), (d) 200-hPa zonal wind (contours at intervals of 5 m/s) and anomalies (shading). The green and grey dashed lines in (d) indicate the average position of the subtropical jet stream for the same period and its normal position, respectively. 60

15 3.2.3 Weather conditions and atmospheric circulation in mid-august In the immediate wake of the two typhoons described in Subsection 3.2.2, convective activity around the Philippines plunged into a suppressed phase and distinctive anticyclonic circulation anomalies developed in the lower troposphere (Fig (a)). Coincidentally, the North Pacific Subtropical High (NPSH) receded from Japan, while it intensified to the southeast of the country (Fig (b)). These NPSH characteristics forced southwesterly moist-air inflow around western Japan in the lower troposphere (Fig (c)). Meanwhile, the subtropical jet stream in the upper troposphere meandered and flowed south of its normal position around the area from eastern China to the Sea of Japan. Extratropical cyclones developed repeatedly along a stationary front that formed over the Yellow Sea and the Sea of Japan to the east of an upper-level trough associated with the southward meandering of the jet stream. Under the influences of such factors (including southwesterly moist-air inflow, the nearly-stationary front and extratropical cyclones), a wide area of Japan had another 10 days of conditions favorable for heavy precipitation. Under these wet weather conditions, Hiroshima experienced torrential rain during the hours from nighttime on 19 August to around daybreak on 20 August. During this time, one-hour and three-hour rainfall totals exceeded 100 and 200 mm, respectively. These torrential rains associated with fully developed cumulonimbus clouds triggered massive landslides in residential neighborhoods of Hiroshima, causing as many as 74 fatalities according to the Fire and Disaster Management Agency Weather conditions and atmospheric circulation in late August In the first half of late August, the meandering of the subtropical jet stream weakened. It continued to flow south of its normal position around eastern China with a trough over the area from the Yellow Sea to the Sea of Japan and a ridge southeast of Japan. In association, the front line remained in the vicinity of Japan. An extratropical cyclone, in combination with penetration of upper-level cold air, caused torrential rain with daily precipitation in excess of 160 mm on Hokkaido s Rebun Island. This triggered a landslide disaster that caused a house to collapse and resulted in two fatalities according to the Fire and Disaster Management Agency. In the second half of late August, the atmospheric circulation conditions favorable for heavy rainfall finally dissipated. The front line was displaced southward to sea areas far off the coast of Japan, while the Okhotsk High developed north of Japan (figure not shown) Global atmospheric circulation and sea surface temperature Figure shows anomalies of sea surface temperature (SST), outgoing longwave radiation (OLR) and velocity potential at 200 hpa, all averaged for August Above- normal SSTs were dominant globally from spring 2014 onward, with remarkably positive anomalies over a wide area of the North Pacific (Fig (a)). Below-normal SST anomalies were observed in the eastern part of the equatorial Pacific in early 2014, resembling a pattern often seen during past La Niña events. A westerly burst observed from January to February triggered eastward expansion of subsurface warm water anomalies, and positive SST anomalies became dominant in most of the equatorial Pacific in May, giving rise to an El Niño event in summer. Meanwhile, SSTs in the Indian Ocean remained above normal except in its western part. In association with these SST anomaly patterns, enhanced convective activity and divergence anomalies in the upper troposphere were seen over the eastern part of the 61

16 tropical North Pacific and in central to eastern parts of the equatorial Indian Ocean in August. In contrast, convective activity was suppressed and convergence anomalies in the upper troposphere were seen over the area from the South China Sea to the western Pacific, despite positive SST anomalies (Fig (b)). (a) Figure (a) shows diabatic heating anomalies in the tropics for August Positive anomalies are seen in the eastern tropical Pacific and in central to eastern parts of the tropical Indian Ocean, and negative anomalies are seen from the South China Sea to the western tropical Pacific. Figure (b) shows the steady response of 200-hPa velocity potential anomalies in an LBM to the heating anomalies. The anomaly patterns of large-scale divergence and convergence in the upper troposphere from the Indian Ocean to the Pacific are consistent with those seen in August Figure shows the steady response of stream function anomalies at 200 hpa and 850 hpa and those (b) (a) Fig Monthly mean (a) sea surface temperature anomalies and (b) 200-hPa velocity potential anomalies and OLR anomalies for August 2014 (b) The contours indicate 200-hPa velocity potential anomalies at intervals of m 2 /s (thick) and m 2 /s (thin), and the shading indicates OLR anomalies. C and D represent the centers of large-scale convergence and divergence anomalies, respectively. (b) Response to heating anomalies in the tropics This subsection outlines results from a numerical experiment involving the use of a linear baroclinic model (LBM; Watanabe and Kimoto 2000) for comparison of the steady response of atmospheric circulation to diabatic heating anomalies in the tropics with atmospheric circulation seen in August Fig Steady response in a linear baroclinic model (LBM) to heating anomalies in the tropics (30 o S 30 o N) for August 2014 (a) The shading denotes diabatic heating anomalies for LBM and (b) the steady response of 200-hPa velocity potential anomalies, which are deviations from the basic state representing the climatological normal. C and D represent the centers of large-scale convergence and divergence anomalies, respectively. 62

17 seen in August The results from the LBM experiment showed anticyclonic circulation anomalies around southern China and cyclonic circulation anomalies around the northern part of the Maritime Continent and the eastern part of East Asia in the upper troposphere, which were consistent with those seen in August. The strikingly shallower-than-normal trough in the upper troposphere over the central North Pacific also resembled that seen in August. These results show anticyclonic circulation anomalies from the South China Sea to the northern part of the Philippine Sea, cyclonic circulation anomalies around the Sea of Japan, and a weaker-than-normal NPSH (except in its westernmost extension) in the lower troposphere, which were consistent with the conditions seen in August. These model experiment results indicate that tropical convection anomalies may have contributed to atmospheric circulation anomalies around Japan in August Asian Summer Monsoon and the subtropical jet stream Figures (a) and (b) show changes in SAMOI-A (an index defined to describe the overall activity of the Asian Summer Monsoon (ASM); see Subsection for details), and changes in OLR averaged over the vicinity of the Philippines, respectively. The ASM s activity featured a pronounced change in early August when it rapidly shifted from the enhanced phase seen in July to a suppressed phase that lasted (a) (b) (c) (d) Fig Steady response in an LBM to heating anomalies in the tropics juxtaposed with atmospheric circulation anomalies for August 2014 Heating anomalies are shown in Fig (a). The shading denotes the steady response of stream function anomalies at (a) 200 hpa and (c) 850 hpa. These anomalies represent deviations from the individual basic states representing the climatological normal, and are additionally subtracted from the zonal mean. The contours indicate (b) 200-hPa stream function anomalies at intervals of m 2 /s (thick) and m 2 /s (thin) and (d) 850-hPa stream function anomalies at intervals of m 2 /s (thick) and m 2 /s (thin). H and L represent anticyclonic and cyclonic circulation anomalies, respectively. 63

18 throughout August. Convective activity around the Philippines also featured a suppressed phase in August in contrast to the enhanced phase seen in July. See Section 2.7 for an overview of the ASM in summer 2014, see Section 2.7. In July and August 2014, the subtropical jet stream tended to flow northward of its normal position during the enhanced phase of the ASM and southward during the suppressed phase (Fig ). In accordance with these characteristics, statistical analysis indicates that the subtropical jet stream around East Asia tends to meander and flow southward of its normal position in association with suppressed phases of the ASM (Fig ). The above discussion indicates that the suppressed phase of the ASM may have been related to the southward shift and meandering of the subtropical jet stream over the area from eastern China to the Sea of Japan in August Boreal Summer Intraseasonal Oscillation Convective activity around the Philippines shifted from an enhanced phase in July to a suppressed phase in August. In contrast, suppressed convective activity over the equatorial Indian Ocean in July was replaced by (a) enhanced convective activity in August. This change is clearly noticeable in the time-longitude section for OLR anomalies over the area between the equator and 20 o N (Fig , top). Concurrently, the time-latitude section for OLR anomalies averaged over the area between 115 o E and 135 o E in the vicinity of the Philippines (Fig , bottom) indicates that a suppressed convection phase, which started to propagate northward in the second half of July, eventually reached latitudes around the Philippines in August. These characteristics of variability in convective activity resemble those seen in the eight phases of the mode known as Boreal Summer Intraseasonal Oscillation (BSISO). Figure shows these phases (composed of the first and second principal components of the empirical orthogonal functions for OLR and zonal wind velocity at 850 hpa; Lee et al. 2013) and a BSISO phase diagram. Anomaly patterns with enhanced convection around the Philippines and suppressed convection over the Indian Ocean observed in July correspond to those seen in phases 5 to 7, while anomaly patterns with suppressed convection from the South China Sea to the area around the Philippines seen in mid- to late August are consistent with those seen in phases 2 to 3. (b) Fig Time-series representation of (a) SAMOI-A and (b) OLR averaged over the vicinity of the Philippines (10 o N 20 o N, 115 o E 140 o E) from April to October 2014 Thin (thick) green and blue lines represent the daily mean (seven-day running mean) in (a) and (b), respectively. The black line in (b) represents the daily normal overlaid with the range of the standard deviation (grey shading). The blue and red shading areas indicate July and August, respectively. 64

19 In summary, the suppression of convective activity around the Philippines observed in August 2014 may have been partly associated with the marked BSISO. Fig Time-series representation of changes in SAMOI-A (top) and time-latitude section of seven-day running mean zonal wind at 200 hpa averaged over 60 o E 150 o E (bottom) from May to August 2014 Subsequent to the enhanced (suppressed) activity of the Asian monsoon, the subtropical jet stream tended to be displaced northward (southward). The thin and thick lines indicate daily mean and seven-day running mean values, respectively (top). The light-green line represents the normal (bottom). Fig Zonal wind at 200 hpa regressed on SAMOI-A for August The base period for the statistical analysis is Warm (cool) color contours indicate that westerly (easterly) wind anomalies tend to appear during periods of suppressed Asian monsoon. The contours indicate anomalies at intervals of Long-term trend of extremely intense hourly precipitation In August 2014, several extremely intense precipitation events, including torrential rainfall exceeding 100 mm/hour in Hiroshima, gave rise to hydrological disasters in Japan. Annual occurrences of intense precipitation exceeding 50 and 80 mm/hour, based on AMeDAS observation data, are extremely likely to have increased since records began (Fig ). Meanwhile, changes in water vapor abundance in the troposphere based on radiosonde observations show an upward trend, which is consistent with that expected from the observed warming caused by increased atmospheric concentration of carbon dioxide and other greenhouse gases. According to the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC), extreme precipitation events over most of the mid-latitude land masses will very likely become more intense and more frequent by the end of the 21st century, and the average amount of water vapor in the atmosphere could increase by 5 to 25% as the global mean surface temperature increases (Collins et al. 2013). Thus, global warming may be a contributing factor to the upward trend in intense hourly precipitation occurrences observed in Japan, although the current statistics spanning a period of about 40 years is remain insufficient for comprehensive judgement. Further accumulation of data is needed to allow a more robust causal relationship to be established. 0.5 m/s. The grey shading indicates statistical significance at a confidence level of 95%. The red dashed line represents the normal position of the subtropical jet stream. 65

20 (W/m 2 ) (m/s) Fig Time-longitude section for OLR anomalies averaged over the area from the equator to 20 o N for 1 June to 31 August 2014 (top) and time-latitude section for seven-day running mean OLR anomalies averaged over 115 o 135 o E for 1 March to 31 August 2014 (bottom) The warm (cool) color shading represents suppressed (enhanced) convective activity. Convective activity was suppressed over the Indian Ocean and enhanced around the Philippines in July, and vice versa in August. The yellow arrow at the bottom indicates northward propagation of suppressed convection toward the vicinity of the Philippines from late July to August. Fig Life cycle composite of OLR (shading) and 850-hPa wind (vectors) anomalies in the eight defined phases of Boreal Summer Intraseasonal Oscillation (BSISO) (top) and a BSISO phase diagram (bottom) The top part illustrates composites reconstructed based on the first and second principal components (PCs) of the empirical orthogonal functions for OLR and zonal wind velocity at 850 hpa. The black arrows indicate wind anomalies at 850 hpa, and the colored shading indicates OLR anomalies. The bottom part indicates PC1 and PC2 phase space points from late July to the end of August. 66

21 Fig Annual occurrences of intense precipitation exceeding 50 mm/hour (top) and 80 mm/hour (bottom), based on observations from the Automated Meteorological Data Acquisition System (AMeDAS) from 1976 to 2014 (per 1,000 stations). The blue line indicates the five-year running mean, and the straight red line indicates the long-term linear trend Summary The factors contributing to the unseasonable weather conditions in Japan in August 2014 are summarized here. In early August, two typhoons approaching and making landfall on Japan in quick succession, in combination with a nearly-stationary front, caused exceptionally wet conditions. The slow movement of the typhoons was associated with the subtropical jet stream flowing north of its normal position, thereby prolonging their influence over western and other parts of Japan. From mid-august onward, sustained southerly/southwesterly moist air flow, combined with the front lingering over Japan, gave rise to heavy precipitation. The front formed and remained in association with the southward shift and meandering of the subtropical jet stream, which in turn was probably related to the weaker-than-normal Asian summer monsoon. The sustained moist air inflow was linked to enhanced anticyclonic circulation anomalies to the southeast of Japan and southwesterly wind in the lower atmosphere associated with anticyclonic circulation anomalies around the Philippines. The inactive phase of convection associated with intraseasonal variability, combined with enhanced convection over the eastern tropical Pacific and the eastern tropical Indian Ocean in association with above-normal SSTs, contributed to these anticyclonic circulation anomalies around the Philippines. Global warming may be a contributing factor to the long-term upward trends seen in intense hourly precipitation and in water vapor abundance in the troposphere observed over Japan. The primary factors discussed in this section are summarized in Fig Fig Time-series representation of change in water vapor abundance in the lower troposphere over Japan for summer (June to August) from 1981 to 2014 The black line with dots indicates 850-hPa specific humidity ratios (mass of water vapor divided by mass of total air) to the normal averaged over 13 radiosonde stations across Japan. The thick blue line represents the five-year running mean, and the straight red line represents the long-term linear trend. The red triangles indicate replacement of observation instruments. 67

22 Fig Primary factors contributing to the unseasonable weather conditions seen in Japan in August 2014 References Collins, M., R. Knutti, J. Arblaster, J.-L. Dufresne, T. Fichefet, P. Friedlingstein, X. Gao, W.J. Gutowski, T. Johns, G. Krinner, M. Shongwe, C. Tebaldi, A.J. Weaver and M. Wehner, 2013: Long-term Climate Change: Projections, Commitments and Irreversibility. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA Lee, J., B. Wang, M. C. Wheeler, X. Fu, D. E. Waliser, and I. Kang, 2013: Real-time multivariate indices for the boreal summer intraseasonal oscillation over the Asian summer monsoon region, Climate Dynamics, 40, Watanabe, M. and M. Kimoto, 2000: Atmospheric-ocean thermal coupling in North Atlantic: A positive feedback, Quart. J. Roy. Meteor. Soc., 126,

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