Possible influence of AAO on North Korean rainfall in August

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 34: (2014) Published online 8 August 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: /joc.3801 Possible influence of AAO on North Korean rainfall in August Ki-Seon Choi, a Su-Bin Oh, b Do-Woo Kim c and Hi-Ryong Byun b * a National Typhoon Center, Korea Meteorological Administration, Jeju, South Korea b Department of Environmental Atmospheric Sciences, Pukyong National University, Busan, South Korea c Forecast Research Laboratory, National Institute of Meteorological Research, Seoul, South Korea ABSTRACT: This study found a positive correlation between the August rainfall in North Korea and Antarctic Oscillation (AAO) in August. Causes of increasing rainfall in the positive AAO phase are (1) the increasing frequency of tropical cyclones that land in or affect the Korean Peninsula, (2) the reinforcement of Australian high (AH) in the Southern Hemisphere, and (3) atmospheric instability at all levels in August in North Korea. The reinforcement of AH forms an anomalous cross-equatorial flow in the western Pacific and plays a decisive role in the northward development of the subtropical western North Pacific high (SWNPH). Furthermore, large volumes of warm and humid air are supplied to North Korea owing to this development. As a result, atmosphere in North Korea becomes unstable, and it is found that the reinforcement of anomalous warm sea surface temperature (SST) in the middle latitude of East Asia is another cause of the instability. KEY WORDS Antarctic Oscillation; rainfall; tropical cyclone; Australian high Received 20 May 2012; Revised 13 November 2012 and 10 April 2013; Accepted 5 July Introduction August in South Korea is the month that has the largest rainfall in a year. August is very important because the second Changma falls in this month, and if rainfall is insufficient during this month, it may cause drought in the autumn of the same year and even in spring next year (Byun and Lee, 2002). Furthermore, the frequency of tropical cyclones (TCs) landing in or affecting the Korean Peninsula is also the highest in August, and plays a decisive role in the variation of rainfall in August (Choi and Kim, 2007; Ahn et al., 2010). There are a considerable number of studies addressing large-scale atmospheric circulations associated with summer rainfall in South Korea. To summarize them briefly, the years with large summer rainfall have been found to have such characteristics as the development of subtropical western North Pacific high (SWNPH) (Lim, 1997; Yun et al., 2001; Ha et al., 2003), continuous heating of the East Asian continent (Lu, 2002), the development of lower-level anomalous anticyclone in the East Asian continent (Ho et al., 2003), and the reinforcement of lower and upper-level jets (Ha et al., 2003). Another study insisted that when SWNPH is developed, Changma finishes late and the summer rainfall increases (Lu, 2002). On the other hand, the years with small * Correspondence to: H.-R. Byun, Department of Environmental Atmospheric Sciences, Pukyong National University, Daeyeon 3-Dong Nam- Gu Busan , South Korea. hrbyun@pknu.ac.kr summer rainfall have been found to have such characteristics as broad snow cover in the Northern Hemisphere in late autumn (October to November) preceding year (Choi et al., 1998), high genesis frequency of TCs in the western North Pacific (Choi et al., 2008), and the rapid weakening of the East Asian Jet (Lu, 2002). It is expected that North Korea will have similar variations in summer rainfall because of its similar geographical location as South Korea, but not much has been known about the meteorological research results of North Korea including summer rainfall variations. Choi et al. (2010) demonstrated through empirical orthogonal function (EOF) analysis that the summer rainfall variations of South and North Koreas have the out-of-phase relationship which accurately appears with the Military Demarcation Line as the boundary line between them. In particular, they emphasized that the summer rainfall in North Korea has been sharply decreasing from the mid-1990s. This study intends to show that the August rainfall in North Korea is closely related to the Antarctic Oscillation (AAO) in the same month. AAO is a representative annular mode circulation that appears in the Southern Hemisphere during boreal summer and is characterized by large-scale seesaw oscillation in atmospheric circulation between the middle and high latitudes of the Southern Hemisphere (Kidson, 1988; Gong and Wang, 1999; Thompson and Wallace, 2000). It has been found that AAO not only affects the various climate changes in the Southern Hemisphere (Thompson and Wallace, 2000; 2013 Royal Meteorological Society

2 1786 K.-S. CHOI et al. Hall and Visbeck, 2002; Kwok and Comiso, 2002; Silvestri and Vera, 2003; Reason and Rouault, 2005; Hendon et al., 2007), but also influences the summer rainfall variations in the East Asia (Gao et al., 2003; Xue et al., 2004; Fan, 2006; Wang and Fan, 2007; Sun et al., 2009) and in other regions (Sun, 2010; Sun et al., 2010) through teleconnection pattern. However, these studies are limited in that they only investigated the rainfall changes in the Yangtze River valley regarding the correlation between AAO and the summer rainfall in East Asia. A common finding of these studies is that Australian high (AH) which is reinforced in the positive AAO phase plays the role of a bridge between AAO in the Southern Hemisphere and the East Asian summer monsoon (EASM). In particular, they mentioned that AH could sufficiently act as a predictor of EASM as AH appears clearly from the preceding spring season. In the case of South Korea, Choi et al. (2010) demonstrated that June rainfall and Changma onset are controlled by AAO and this characteristic can be found from the AAO of preceding March. They stressed that when in the positive AAO, the cold cross-equatorial flow in the direction from the region around Australia to the equator is intensified, which in turn, forces a SWNPH to develop northward; this eventually drives the rain belt (Changma) north. In addition, some studies analysed the relationship between summer rainfall in South Korea and the climate indices indicating climate changes. Kim et al. (2007) showed through the statistical method of multivariate linear regression that the July rainfall in South Korea has a positive correlation with Equatorial Eastern Pacific see level pressure (SLP), whereas Choi et al. (2011) showed that the summer rainfall in South Korea has a negative correlation with North Pacific Oscillation (NPO). This study intends to find out whether the summer rainfall in North Korea is affected by AAO. Section 2 introduces the data and the methods of analysis. Correlation between North Korean rainfall and AAO is examined in Section 3, and in Section 4, the differences between positive AAO and negative AAO phases are investigated. Finally, Section 5 sums up the findings from this study. 2. Data and methodology 2.1. Data Rainfall data This study used the total monthly rainfalls that were observed in the 26 weather observation stations in North Korea during the last 30 years ( ). The number of weather observation stations in North Korea is 12 until 1981 and increased to 26 since 1981 (Table 1). Thus, the data after 1981 was used for reliability of the results. In addition to North Korean rainfall data, the total rainfalls in August which were observed in 258 weather stations (146 in China, 61 in Korea, and 51 in Japan) in East Asia excluding those in the dry region west of 90 E were used Reanalysis data Together with the rainfall data, the reanalysis data set of the National Centers for Environmental Prediction National Center for Atmospheric Research (NCEP-NCAR) (Kalnay et al., 1996; Kistler et al., 2001) for the same period was used. For parameters, geopotential height (gpm), zonal and meridional flow, velocity potential (m 2 s 1 ), air temperature ( C), and relative humidity (%) were used. These data have the resolutions of for latitude and longitude and 17 levels of vertical space and are available from 1948 to the present. Furthermore, the National Oceanic and Atmospheric Administration (NOAA) interpolated outgoing longwave radiation (OLR) data that were retrieved from the NOAA satellite series were used. These data are available from 1974 to the present at the Climate Diagnosis Center (CDC), but has the missing period of March to December For more information on OLR data, please refer to the CDC website ( and the paper by Liebmann and Smith (1996). This study also used the NOAA extended reconstructed monthly sea surface temperature (SST) (Reynolds et al., 2002). These data have the resolution of for latitude and longitude, and are available from 1854 to the present Tropical cyclone (TC) data To investigate the effect of TC on the variations of total rainfall in August in North Korea, this study used the TC best track archives from the Tokyo Typhoon Center of Regional Specialized Meteorological Center (RSMC). These data contain such information as the TC name, latitude and longitude locations, minimum surface central pressures, and maximum sustained wind speeds (MSWSs; 10-min average maximum winds to the nearest 5 kts) in intervals of 6 h. In general, TCs are divided into four grades based on the MSWSs: tropical depression (TD; MSWS < 34 kts), tropical storm (TS; 34 kts MSWS 47 kts), severe tropical storm (STS; 48 kts MSWS 63 kts), and typhoon (TY; MSWS 64 kts). The present used not only these four grades of TCs but also the extratropical cyclones (ETs) that were transformed from TCs for analysis, because ETs also give considerable damages to the middle latitude regions of East Asia AAO index This study used the AAO index produced by NOAA s Climate Prediction Center (CPC) ( cpc.ncep.noaa.gov). The AAO index is obtained through empirical orthogonal function (EOF) analysis using the monthly mean 1000 hpa (or 700 hpa) height anomalies poleward of 20 latitude for Southern Hemisphere. Here, monthly mean height field that removed the seasonal cycle in NCEP-NCAR reanalysis data set was employed at a horizontal resolution of for latitude and

3 POSSIBLE INFLUENCE OF AAO ON NORTH KOREAN RAINFALL IN AUGUST 1787 Station Table 1. Information of 26 weather observation stations in North Korea. Geographic features Name Number Lat ( N) Lon ( E) Observation starting year SENBONG SAMJIYON CHONGJIN CHUNGGANG KANGGYE PUNGSAN KIMCHAEK SUPUNG CHANGJIN SINUIJU KUSONG HUICHON HAMHEUNG SINPO ANJU YANGDOK WONSAN PYONGYANG NAMPO CHANGJON SARIWON SINGYE RYONGYON HAEJU KAESONG longitude for the period from 1979 to present. The time series from EOF analysis are normalized by the standard deviation of monthly index ( ). 3. Correlation between North Korean rainfall and AAO Table 2 shows the correlations between the North Korean average monthly total rainfalls during the warm season (March to September) and AAO index. The March AAO index and the North Korean average total rainfall in May show the positive correlation of 0.38 at the 95% confidence level. The August AAO index and the North Korean average total rainfall of the same month show the positive correlation of 0.48 at the 99% confidence level. The former correlation is similar to the result of Choi et al. (2010) which showed that AAO of the preceding spring is the main factor of the rainfall variations in early summer in South Korea. In other months except for August, however, no statistically significant correlation between the two variables is found. Therefore, this study focuses on revealing the correlation between North Korean average total rainfall and AAO index that appears in August. Figure 1 shows the time series of the North Korean average total rainfall in August and the AAO index of the same month. Both variables show clear interannual variation, and we can see that these two variables change at similar phases. In more detail, the phase variations between the two variables seem more similar after the middle of 1990 rather than before the middle of Thus, the total period of 30 years can be divided into two periods of (P1) and (P2), and the correlation between the two variables is analysed again in Table 2. Correlation coefficients between North Korean rainfall and AAO. AAO Rainfall Mar Apr May Jun Jul Aug Sep Mar Apr May Jun Jul Aug Sep 0.11 Dark and light boxes are significant at the 99 and 95% confidence levels, respectively.

4 1788 K.-S. CHOI et al. Rainfall (mm) Corr = 0.48 Corr = 0.27 ( ) Rainfall AVG (Rainfall) AAO Corr = 0.61 ( ) AAO index Figure 1. Time series of August total rainfall that averaged over North Korea (solid line with a close circle) and August Antarctic Oscillation (AAO) index (dotted line with an open circle). Dashed line is a trend of August total rainfall that averaged over North Korea. each period. During the P1 period, the two variables show a low correlation of 0.27, which is statistically insignificant. During the P2 period, however, they show a high positive correlation of 0.61 at the 95% confidence level. Thus, we can see that the positive correlation between these two variables is more strengthening recently. Meanwhile, the time series of North Korean total rainfall in August shows almost no changing trend (dashed line in Figure 1). Therefore, even if we analysed again the correlation between North Korean average rainfall in August with removed trend and the AAO index in August, the values were almost equal to the past correlation coefficients. Table 3. Positive and negative AAO phases selected from the time series in Figure 1 and August total rainfall averaged over North Korea in each year of each phase. Positive year Negative year Average Average Climatology Climatology Differences between positive AAO phase and negative AAO phase As shown in the previous section, to understand the possible causes of the positive correlation between the North Korean average rainfall in August and the AAO index in August, we first chose 6 highest AAO years (hereinafter referred to positive AAO phase ) and 6 lowest AAO years (hereinafter referred to as negative AAO phase ) were selected first (Table 3) from the AAO time series in August. Here, because the North Korean average August total rainfall and August AAO index both did not show statistically significant correlation with the August Niño-3.4 index (North Korean average August total rainfall-august Niño-3.4 index: 0.09, August AAO-Niño-3.4 index: 0.17), each 6 years were extracted from all years including El Niño Southern Oscillation (ENSO) years Spatial distribution of rainfall Table 3 shows the each AAO phase selected and the North Korean August total rainfall for the corresponding year. There is no year with 2000 mm or less in the positive AAO phase except for 2 years (1993 and 2001) and every rainfall in the positive AAO phase is greater than the climatological average August total rainfall except for 2 years (1993 and 2001). However, the rainfall in the negative AAO phase is 2000 mm or less in every year except for only one year (1995) and is smaller than the climatological average August total rainfall except for only one year (1995). As a result, the average rainfall in the positive AAO phase is 1.5 times greater than the average rainfall in the negative AAO phase, which is significant at the 90% confidence level. The North Korean average August total rainfalls in the two phases and the differences between the two phases were applied to each weather observation station to examine spatial distributions (Figure 2). In the positive AAO phase, the rainfall generally tends to decrease from the south to the north. In other words, less rainfall is observed in the mountainous regions at the northeast of North Korea. Furthermore, there is no weather observation station with a rainfall under 1500 mm in the positive AAO phase except for four stations (Senbong: 42.3 N, E; Chongjin: 41.8 N, E; Kimchaek: 40.7 N, E; Sinpo: 40.0 N, E). On the other hand, in the negative AAO phase, all weather observation stations except for three stations (Wonsan: 39.2 N, E; Changjon: 38.7 N, E; Singye: 38.5 N, E) show rainfalls under 2000 mm. Owing to these rainfall characteristics in each AAO phase, the differences in August total rainfall between the two AAO phases are

5 POSSIBLE INFLUENCE OF AAO ON NORTH KOREAN RAINFALL IN AUGUST 1789 (a) Positive AAO phase (b) Negative AAO phase (c) Positive minus Negative 0 to to to to to to to to to ~ 0 to to to to to to to to to ~ Figure 2. The spatial distributions of August total rainfall in (a) positive and (b) negative AAO phases and (c) the difference in August total rainfall between positive and negative AAO phases. In (c), small white squares are significant at the 95% confidence level. Unit is mm. large in the southwestern region of North Korea and small in the northeastern region of North Korea. To determine whether the somewhat high positive correlation between August AAO index and August total rainfall was distinct only in North Korea, the correlation between these two variables was analysed for the entire East Asia (Figure 3(a)). While many weather observation stations in North Korea show positive correlations of 0.3 or greater at the 90% confidence level, the positive correlation appears only in the southwestern region in South Korea. Even though positive correlation exists between the South Korean average August total rainfall and the August AAO index, it is not as strong as in North Korea and it is not statistically significant (Table 4). Statistically significant correlations are also observed in some observations in China and Japan; instead, the northern region of China shows a negative correlation. This negative correlation has been revealed by Wang and Fan (2005) based on more than 100-year data. Therefore, we can know that in East Asia, the positive correlation between the two variables is the most distinct in North Korea. Furthermore, the differences in August total rainfall between positive and negative AAO phases were analysed for the entire East Asia (Figure 3(b)). The significant positive values at the 90% confidence level are observed in North Korea and western region of South Korea. Meanwhile, although positive values are observed in south and middle of China and Manchurian region, but most of them are not significant statistically. Therefore, we can conclude that the differences in August total rainfall between the two AAO phases are the most distinct in North Korea Seasonal variation To compare the rainfall variations in the two AAO phases during other seasons than August, the seasonal variations of North Korean rainfall were analysed (Figure 4). This study used 7-d running averaged rainfall for analysis of seasonal variations. Ha et al. (2009) successfully used the 7-d running averaged rainfall data to find out that there is a shift in the latter half of 1960s in the variations of South Korean August rainfalls. Climatically, the rainy season of South Korea is divided into three periods: Spring Rainy Season (early April to mid-may), Changma (late June to late July),

6 1790 K.-S. CHOI et al. Table 4. Correlation coefficients between South Korean rainfall and AAO. AAO Rainfall Mar Apr May Jun Jul Aug Sep Mar Apr May Jun Jul Aug Sep 0.07 Dark and light boxes are significant at the 99 and 95% confidence levels, respectively. (a) Correlation coefficients (b) Difference in August total rainfall Figure 3. Spatial distributions of (a) the correlation coefficient between August total rainfall and August AAO index and (b) the difference in August total rainfall between positive and negative AAO phases. Small black dots are significant at the 90% confidence level. In (b), unit is mm. and second Changma or Autumn Rainy Season (hereinafter called second Changma : mid-august to early September) (Byun and Lee, 2002). North Korea also has a similar structure of rainy seasons as South Korea (Figure 4(a)). Rainfall begins to increase from early April and temporarily decreases in late May. This period is similar to the spring rainy season in South Korea. Then the rainfall sharply increases from the late June and reaches the annual peak in late July. This period coincides with the Changma in South Korea. Afterwards, rainfall decreases and increases again in mid-august, and reaches the second annual peak between late August and early September. This period coincides with the second Changma in South Korea. On the basis of these climatological characteristics for seasonal variations of North Korean rainfalls, the rainy seasons in each AAO phase were compared (Figure 4(b)). First, regarding the spring rainy season, similarly to the climatological characteristics, rainfall begins to increase in early April and decreases in late May in the positive AAO phase. In the negative AAO phase, too, rainfall begins to increase in early April, but it is much smaller than in the positive AAO phase, and the spring rainy season ends in mid-may which is a little earlier than in the positive AAO phase. For Changma, it starts in late June which is same as the climatological Changma onset and continues until early August in the positive AAO phase. The Changma in the negative phase is defined as early June to late July which is longer than the Changma period in the positive AAO phase, but the rainfall is generally less than in the positive AAO phase. The second Changma in the positive AAO phase starts in mid-august and ends in early September similarly to the climatological characteristics, but the peak of the second Changma is nearly same as the peak of Changma. In the negative AAO phase, the second Changma does exist, but rainfall is much less than in the positive AAO phase. Therefore, in North Korea, the variation in rainy seasons between the two AAO phases is the greatest for the period after Changma (Figure 4(c)). Owing to these characteristics in the rainfall variation in two AAO phases, the difference in rainfall between the two AAO phases is the largest from mid-may to early September (embedded figure in Figure 4(c)). In particular, the difference in rainfall between the two AAO phases is the greatest in August owing to the less rainfall in the second Changma in the negative AAO phase. Therefore, it is considered that in the negative AAO phase, North Korea needs to prepare for drought because of insufficient rainfall in the warm season which is an important period for agriculture.

7 POSSIBLE INFLUENCE OF AAO ON NORTH KOREAN RAINFALL IN AUGUST Climatology (a) Rainfall (mm dy 1 ) Positive (Pos) Negative (Neg) (b) Pos minus Neg Pos minus Neg (c) Month Figure 4. Temporal variabilities of the 7-d running averaged rainfall in North Korea for (a) climatology, (b) positive AAO phase (solid line) and negative AAO phase (dotted line), and (c) the difference between positive and negative AAO phases. The embedded figure in Figure 4(c) is the difference in total rainfall for May to September between positive and negative AAO phases. Meanwhile, there are almost no differences in rainfall between the two AAO phases in other months, because this period corresponds to a cold season which is cold and dry in North and South Koreas Large-scale environments To investigate the causes of the aforementioned differences in North Korean total August rainfall between the two AAO phases, the differences in large-scale environments between the two AAO phases were examined. If the differences between the positive and negative AAO phases which are defined in this study do not exhibit typical spatial distribution of the positive AAO phase, the analysis results after this may become meaningless. As shown in Figure 5, however, the difference in the 850 hpa geopotential height between the two AAO phases clearly shows the characteristics of spatial distribution in the positive AAO phase. In other words, a seesaw-like spatial distribution appears in the Southern Hemisphere such as positive anomaly in the high latitude region and negative anomaly in the polar region. To investigate the difference in atmospheric circulations between the two AAO phases, the 850 hpa streamline was analysed (Figure 6). An anomalous anticyclonic circulation is strengthened in southern sea of Australia, which is called Australian high (AH, Xue et al., 2004) (Figure 6(a)). Similarly to this, there is an anomalous anticyclone in the middle latitude region of the

8 1792 K.-S. CHOI et al. (a) Northwest and Southwest Pacific AC AC AA AA AA AA Figure 5. The spatial distribution of difference in 850 hpa geopotential height in August between positive and negative AAO phases. Unit is gpm. (b) Northwest Pacific AC Northern Hemisphere. Such locations of pressure systems in the two hemispheres are similar to the typical spatial distribution of the positive AAO phase which was shown in the studies by Gong and Wang (1999) and Thompson and Wallace (2000). In particular, the anomalous southerly from the anomalous anticyclone located east of New Zealand crosses the equator as an anomalous crossequatorial flow converges with the anomalous easterly (anomalous trade wind) at around 10 N ( E) and heads to southern China. In this converged region, many TCs are formed in the August positive AAO phase (dots in Figure 6(b)). These TCs in low latitudes are another factor that supplies heat and water vapour to the middle latitude regions of East Asia. The converged flow keeps proceeding northward along the west side of the anomalous anticyclone and reaches South and North Koreas. In relation to this anomalous flow in the positive AAO phase, AH causes strong cold outbreaks in Australia during EASM and forms a cross-equatorial flow in the western Pacific. This flow reinforces the SWNPH to the north and moves the rain belt in the low latitude to the middle latitude of East Asia. Thus, AH that is reinforced in the east sea of New Zealand plays a decisive role in the variation of EASM rainfall in the positive AAO phase (Tao et al., 1983; Xue et al., 2004). Meanwhile, anomalous anticyclone that is centred in the southern sea of Japan and expanded to the northwest direction to the Korean Peninsula in the positive AAO phase indicates that SWNPH in this AAO phase has expanded more to the northwest than in the negative AAO phase. To examine this, the development of SWNPH in each AAO phase was analysed for the East Asian regions only (Figure 6(b)). Here, SWNPH is defined as the 5870 gpm contour. We can see that SWNPH (solid line) in the positive AAO phase is extended more to the northwest than that (dashed line) in the negative AAO phase. In particular, SWNPH in the positive AAO phase is extended nearly to South Korea, forming AC AA Figure 6. The difference in 850 hpa streamline in August between positive and negative AAO phases in (a) Northwest and Southwest area and (b) Northwest Pacific area. Solid and dashed lines in (b) are 5870 gpm contours in positive and negative AAO phases, respectively. Dots in (b) denote tropical cyclone (TC) genesis locations in August of positive AAO phase. Shaded areas are significant at the 95% confidence level for 850 hpa geopotential height. Abbreviations of AC and AA represent anomalous cyclone and anomalous anticyclone. a favourable environment to supply large volumes of warm and humid air from the tropical region to North Korea. It is considered that North Korea is influenced by the flow resulting from the convergence of the anomalous southerly from this anomalous anticyclone and the anomalous southwesterly from the anomalous cyclone that is centred in Manchurian region. The AH in the Southern Hemisphere and SWNPH in the Northern Hemisphere which play an important role in the increase of North Korean average August total rainfall in the positive AAO phase can be confirmed from the analysis result for the differences in 200 hpa divergent wind and velocity potential between the two AAO phases (Figure 7(a)). Overall, the upper-level convergence region shows a similar spatial pattern as the streamline from AH to North Korea. The strongest upper-level convergence region in the analysis is located in the subtropical western North Pacific. This indicates that divergence is strengthened in the lower-level, meaning that SWNPH is stronger than in the negative AAO phase. Furthermore,

9 POSSIBLE INFLUENCE OF AAO ON NORTH KOREAN RAINFALL IN AUGUST 1793 (a) Divergent wind and velocity potential (a) OLR (b) 200 hpa zonal wind (b) Correlation (August total rainfall and 850 hpa HGT) Figure 7. (a) The 200 hpa divergent wind (vector) and velocity potential (shaded) and (b) spatial distribution of the correlation coefficient between August total rainfall that averaged over North Korea and August 850 hpa geopotential height. In (b), shaded areas are significant at the 95% confidence level. The unit of velocity potential is 10 6 m 2 s 1. the convergence is also strengthened in the upper-level of the lower-level anomalous anticyclone that was in the sea near New Zealand. To examine whether these two anticyclones (SWNPH and AH) are correlated to the North Korean August total rainfall, the correlations between the 850 hpa geopotential height and the North Korean total rainfall in August were analysed (Figure 7(b)). Overall, there is a similarity to the spatial distribution of the differences in the 850 hpa streamline between the two AAO phases. In more detail, there is a positive correlation in the middle latitude regions and a negative correlation in the low latitude regions of the two hemispheres. In particular, positive correlations at the 95% confidence level are located in the southern sea of Japan, south of Australia, and far east sea of New Zealand. As explained above, these locations are generally similar to those of SWNPH and AH, and this indicates that the North Korean August total rainfall may be controlled by these two anticyclones in the two hemispheres. Figure 8. Same as in Figure 6, but for (a) OLR and (b) 200 hpa zonal wind. In (a), dots denote TC genesis locations. In (b), thick solid and dashed lines denote jet streams in positive and negative AAO phases, respectively. Shaded areas are significant at the 95% confidence level. Contour intervals are 5 Wm 2 for OLR and 2 ms 1 for 200 hpa zonal wind. To investigate the actual convective activities in the positive AAO phase, the OLR differences between the two AAO phases were analysed (Figure 8(a)). The strongest convection in the analysis regions appears in the South China Sea and the north sea of the Philippines where the cross-equatorial flow derived from AH and the anomalous easterly converge. There is a particularly interesting fact around the Korean Peninsula. A negative OLR anomaly appears in North Korea and a positive OLR anomaly in South Korea around the Military Demarcation Line. This spatial pattern is similar to the result of Choi et al. (2010) which showed that summer rainfalls in North and South Koreas have out of phase relationship. In general, the upper-level jet facilitates rain formation by reinforcing convergence at lower-level and the upward flow or convection in the entire troposphere, and is used as a basis for judging the degree of SWNPH development to the north (Liang and Wang, 1998; Lau and Nath, 2000). This study also analysed differences in the 200 hpa zonal wind and jet streak between the two AAO phases (Figure 8(b)). First, to examine the differences in 200 hpa zonal wind around the Korean Peninsula, as shown in the OLR analysis as well, a positive anomaly appears

10 1794 K.-S. CHOI et al. in North Korea and a negative anomaly in South Korea around the Military Demarcation Line. This indicates that the upper-level westerly in North Korea is stronger than that in South Korea. To verify this characteristic, the status of upper tropospheric jet in the two AAO phases was analysed. Jet streak here is defined as an area that shows 25 m s 1 or higher zonal wind speed. The jet streak in the positive AAO phase is extended more to the east and covers North Korea. Also, the southern boundary of the jet streak is very similar to the location of the Military Demarcation Line. Therefore, in this AAO phase, SWNPH is located more to the north and the upward flow that facilitates rain formation is likely to be stronger in North Korea. As explained above, the atmospheric environment that is favourable for rain formation in North Korea in the positive AAO phase can be verified through the differences in vertical atmospheric circulations and atmospheric parameters between the two AAO phases that averaged for E which is the longitude range of North Korea (Figure 9). First, in the meridional vertical atmospheric circulation, the anomalous upward flow is strengthened in the tropical regions (5 S 25 N) of the two hemispheres. Furthermore, the lower-level (850 hpa or lower) anomalous flow that is significant at the 95% confidence level crosses the equator from 10 S. As explained above, this indicates the anomalous cross-equatorial flow that derived from the anomalous anticyclone in the east sea of New Zealand. This anomalous cross-equatorial flow goes north to the latitude range of North Korea (35 45 N) and forms an anomalous upward flow. This anomalous upward flow is strengthened in every level of the troposphere. As the anomalous upward flow that is strengthened in North Korea in the positive AAO phase is located in the same latitude range as the anomalous warm and humid air, the atmospheric environment is favourable for rain formation (Figure 9(b) and (c)). Meanwhile, the anomalous downward flow in the south at 30 S is associated with AH in the southern sea of Australia, and anomalous cold and dry air is reinforced in this latitude range. To find out whether moisture is reinforced in North Korea in the positive AAO phase, the differences in moisture convergence between the two AAO phases were analysed around East Asia (Figure 10(a)). Overall, most East Asian regions showed positive moisture convergence. The positive value was most distinctive in central China and Mongolia, followed by North Korea. What is interesting is that just as the analysis of OLR and jet streak found, the positive moisture convergence (50 g kg 1 s 1 contour line) also appears only in North Korea above the Military Demarcation Line. Difference in moisture flux between the two phases was also analysed (Figure 10(b)). The cross-equatorial flow that crossed the equator at E carried strong moisture, thus affecting South and North Korea, China, and parts of Japan. Therefore, the analysis results show that the North Korean average August total rainfall is affected by AH of the positive AAO phase that is (a) Meridional circulation (b) Air temperature (c) Relative humidity Figure 9. Composite differences of latitude-pressure cross-section of (a) meridional circulations (vectors) and vertical velocity (contours), (b) air temperature, and (c) relative humidity averaged along E between positive and negative AAO phases in August. The values of vertical velocity are multiplied by 100. Thick arrows and shaded areas are significant at the 95% confidence level. Contour intervals are 0.5 hpa s 1 for vertical velocity, 0.3 C for air temperature, and 2% for relative humidity. fortified in the Southern Hemisphere, both directly and indirectly. The SST is another major factor that can cause atmospheric instability by supplying water vapour to the lower-level atmosphere. Thus, the differences in SST between the two AAO phases were analysed (Figure 11). As there is no relationship between AAO and ENSO,

11 POSSIBLE INFLUENCE OF AAO ON NORTH KOREAN RAINFALL IN AUGUST 1795 (a) 850 hpa Moisture convergence Figure 11. Same as in Figure 6, but for SST. Shaded areas denote negative values. Contour interval is 0.2 C. (b) 850 hpa Moisture flux Peninsula in the positive AAO phase, but no TC in the negative AAO phase. Furthermore, in the positive AAO phase, TCs more frequently move to the South China Sea or pass through the East China Sea and the region near Korea, but in the negative AAO phase, more TCs move to Japan and the east sea of Japan. In other words, the TCs in the positive AAO phase more frequently move in the west and affect Korea than TCs in the negative AAO phase. As TCs tend to move along the western periphery of SWNPH, the frequency of TCs landing in or affecting Korean Peninsula is higher in the positive AAO phase when SWNPH is more strengthened to the Korean Peninsula. Therefore, the high frequency of TCs landing in or affecting the Korean Peninsula in the positive AAO phase can be one cause of the increased August rainfall in North Korea. Figure 10. Same as in Figure 6, but for (a) 850 hpa moisture convergence and (b) 850 hpa moisture flux. Shaded areas are significant at the 95% confidence level. Contour interval is 50 g kg 1 s 1. no particular spatial pattern is observed. However, in East Asia, anomalous warm SST is strengthened in the northern sea of around 25 N. This SST condition in the middle latitude seas in East Asia can accelerate atmospheric instability to form an atmospheric environment that is favourable for increasing rainfall in North Korea Tropical cyclone activity TCs also play an important role in the variations of August rainfall because, as explained above, August is the month that receives the greatest effect of TCs in North Korea as well as in South Korea (Choi and Kim, 2007; Ahn et al., 2010). Therefore, the TC activity in August was compared between the two AAO phases (Figure 12). There were 34 TCs in August in the positive AAO phase and 37 TCs in the negative AAO phase, showing no difference in the TC genesis frequency. Although the difference in the TC genesis frequency between the two AAO phases is not large, the TC frequency that lands in the Korean Peninsula shows a large difference. A total of four TCs land in the Korean 5. Summary and conclusion This study found that North Korean average August total rainfall has a high positive correlation with the August AAO index. This correlation was particularly strong after middle 1990s and most distinct in North Korea among the East Asian regions. To investigate the cause of this positive correlation between the two variables, 6 positive AAO years (positive AAO phase) and 6 negative AAO years (negative AAO phase) were selected with the inclusion of the ENSO years and then the differences in averages between the two periods were analysed. In the positive AAO phase, positive rainfall anomaly was observed in all the 26 weather observation stations of North Korea, and the rainfall differences were greater in all regions except for the mountainous regions. The analysis of seasonal variations showed that rainfall was greater in the positive AAO phase from May to August when the water resource acquisition is critical in agriculture, and the difference in August rainfall was the greatest owing to less rainfall in the second Changma in the negative AAO phase. Therefore, it must be noted that when weakening of the second Changma in the negative AAO phase is detected, there is high likelihood that drought will occur in autumn until the next spring.

12 1796 K.-S. CHOI et al. (a) Positive AAO phase (4 TCs) (b) Negative AAO phase (0 TCs) Figure 13. Schematic illustration of anomalous atmospheric circulation patterns for the positive rainfall in August in North Korea. Abbreviation AA represents anomalous anticyclone. Figure 12. TC tracks in August during (a) the positive AAO phase and (b) the negative AAO phase. Numbers in the low right corner denote average TC genesis locations in each AAO phase. The analysis of 850 hpa streamline in August found that AH was strengthened in southern sea of Australia and SWNPH was more strengthened in the western North Pacific. In particular, the AH is known to cause strong cold outbreaks in Australia during the EASM onset period, thus strengthening the cross-equatorial flow in the western Pacific, which may develop SWNPH in the north, and moving the rain belt northward. This characteristic was found in the positive AAO phase of current study and thus AH influences the rainfall variations in not only South Korea in early summer but also North Korea in peak summer. The characteristics of large-scale atmospheric circulation on the cause of increase in August total rainfall averaged over North Korea are summarized in Figure 13. In addition to such atmospheric environment, the anomalous warm SST was also reinforced in the middle latitude seas of East Asia, which may cause atmospheric instability and contributed to the increased August rainfall in North Korea. Moreover, the frequency of TCs affecting the Korean Peninsula was also high in the positive AAO phase, which meant the atmospheric and ocean environment conditions that are favourable to the increase of North Korean average August total rainfall were formed. This study leaves room for next studies as follows: (1) the lately reinforced relationship between August AAO and North Korean average August rainfall and (2) In most areas other than some parts of East Asia, positive correlation was observed between August total rainfall and AAO, and the reason that the trend is especially strong in South and North Korea. Furthermore, as mentioned above, few studies on North Korean meteorology have been known. Therefore, more diverse and more detailed analyses on North Korean rainfall characteristics in other seasons as well as in summer will be necessary. Acknowledgements This work was funded by the Korea Meteorological Administration Research and Development Program under Grant CATER References Ahn SH, Kim BJ, Park SY, Park GU The climatological characteristics of the landfall typhoons on North Korea. Atmosphere 20: (In Korean). Byun HR, Lee DK Defining three rainy seasons and hydrological summer monsoon in Korea using available water resources index. J Meteor Soc Japan 80:

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