Decadal variation of pressure pattern and wind field

present t he distributi on of sea level pressure and surf ace wi nd around Japan for each cat egor y (Fi g. 26) . For t he aver age of sunny li ght -wi nd days, the Pacific Hi gh extend ed to t he south of Japan, with weak surface wi nd fr om south around t he Ka nto regi on. In Categor y A, hi gh pressure anomal y was noted i n the sout heast of Japan, and the Pacific Hi gh ext end ed str ongl y to the south of Japan, wit h the sout hwest wind around t he Kanto r egion bei ng stronger than t he average . In Cat egor y B, though t he extent of the Pacific Hi gh was si milar t o the average of sunny li ght -wind days, the pressure gr adient t o the east of Japan was large and sout hwest wi nd was str o ng. In Categories C and D, t he pressure patter n was si milar to the aver age of sunn y light -wind days , with a small pressure gradi ent around t he Kant o region, and the surface wi nd was weak. However, in Categor y D, a weak l ow -pressure area was present around the Kant o regi on, and the surface wind was from southeast. Categor y E showed a low pr essure anomal y over the ocean southeast of Japan, and northeast wind bl ew into the Kanto regi on.

Fi gur e 27 shows t he distri bution of the average sea level pr essure and surface wi nd during 2 months (Jul y and August) on each 11 -year period.

Duri ng t he peri od 1979–1989, compared wit h t he average of the peri od 1979–2011, pressur e anomal y was seen in easter n Hokkai do, l ow pressure anomal y was obser ved southeast Japan, and t he extent of the Pacifi c Hi gh was biased to t he north. On the other hand, in the peri od 2001–2011 , l ow pressure anomal y was obser ved i n easter n Hokkaido, and hi gh pressure anomal y was obser ved sout heast Japan. A southward extension of t he Pacific High was also

strengt hened, and t he wind dir ection changed to southwest in the Kant o region. Nagata and Mikami ( 2010) poi nted out that the western edge of t he Nort hern Pacific Hi gh tended to shift to southwest i n the period fr om 1 980 to 2000. This is not i n contradicti on with t he str engt heni ng of t he Nort hern Pacific Hi gh in r ecent years. This cha nge in the pressure pattern corresponded t o the characteri stics of the pressur e pattern f or Categor y A, wher e a si gni ficant i ncreasi ng trend was noted i n t he relati ve frequency, indicati ng the changi ng of wi nd system accompanying t he changes in the pressur e pattern.

To exami ne the relationshi p bet ween t he relati ve frequency for each categor y and the decadal changes in t he pressure patter n i n detail , Tables 3 t o 5 show the number of sunny li ght -wind days and t he r elati ve fr equency of each cat egor y during each 11 -year period f or each half month. For the period 1979–1989 (Table 3) , in t he for mer hal f of July, Categor y A was not present at all and t he r elati ve frequenci es of Categories B and E were hi gh. In the latter half of Jul y, the relati ve frequency of Categories B and E decreased, wher eas the relati ve f requency of Cat egori es C and D increased. In August , Categor y A was present at approxi matel y 10%. The r elati ve frequencies of other cat egori es wer e g ener all y close t o t he average. For the period 1990–2000 (Table 4) , in the for mer half of July, the relati ve frequencies of Categories B and E were hi gh, and alt hough they wer e less t han the average, Categor y A was present at 13% . In the latter half of Jul y, Categor ies B and E decreased, whereas Categories C and D increased. Cat egor y A al so showed a

si gnificant incr ease at 24%. In August, Categories B, D, and E showed average r elati ve fr equencies. Categor y A decreased in the f or mer hal f of August, but i ncreased in the l atte r half. Categor y C i ncreased i n the for mer half of August , and showed a tr end to decr ease i n t he l atter half of August.

Duri ng the period 2001 –2011 (Table 5), the relati ve frequency of Categor y A was extremel y hi g h ( 43%) du ring t he for mer half of Jul y, and the relati ve frequencies of Categories B, C, and D were much lower. In t he latter half of July, Cat egor y A decr eased sli ghtl y, and both Cat egori es C and D incr eased.

In August , Categori es D and E decreased during t he for mer half , but increased in the latter half. Mea nwhile, Categor y B increased i n t he for mer half of August, but decreased i n the latt er half . As shown, t he relati ve frequency of each cat egor y had a seasonal transition, and an increase in the relati ve frequency of Cat egori es C and D in the latter half of Jul y was common duri ng all three periods. On the ot her hand, whil e there was al most no appearance of Categor y A in Jul y during t he period 1979 –1989, the relati ve frequency i n t he for mer half of Jul y during the period 2001 –2011 was hi gh, and t here was a change i n the seasonal transiti on.

Fi gur es 28 t o 30 show the sea level pressure and the distribution of surface wind for each half month during each 11 -year peri od . First, the focus was pu t on the seasonal tr ansition during the per iod 1979 –1989 ( Fi g. 28). In the for mer half of Jul y, t he Pacific Hi gh extended around the latit ude of 25° N.

The pr essur e gradi ent on t he sout h er n shore of Japan was large, and a southwest wind was bl owing in the Kanto regi on. In the latter half of Jul y, t he

strengt h of t he Pacif ic Hi gh i ncreased, and t he pressur e gradient around Japan decreased, leading to weaker wind. In the east of Hokkaido , compared with 33 years mean of this season , a hi gh pr essure anomal y was obser ved. In the Kanto r egion, t here was a w eak sout heast wi nd. In the for mer half of August, around the lat itude of 40° N, a hi gh pressure anomal y was noted, and the ext ent of the Pacif ic Hi gh was biased to the north, wit h southeast wi nd i n the Kant o region. In t he latter half of August, the extent of t he Pacific Hi gh sli ghtl y weakened , while low pressure anomal y was noted in a wi de area around Japan. The wi nd d irection in t he Kanto regi on changed to south. In the period 1990 –2000 (Fig. 29) , tendencies i n the f or mer half of Jul y were si milar t o the previ ous 11 -year period, but in the l atter half of July, low pressur e anomal y was not ed in the area west of the l ongit ude of 130° E and the north of t he l atitude of 40° N. The extent of t he Pacif i c Hi gh was weaker than the aver age of this season , and t he ridge of the Pacific Hi gh was biased to the south. Therefor e, the south wi nd around the Kant o region had sli ghtl y a westerl y component . In the for mer hal f of August , l ow pressure anomal y was noted in the east of Japan, and the strengt h of the Pacific Hi gh was weak, with south wi nd around t he Kanto regi on. In the latter half of August, hi gh pressur e anomal y was noted ar ound Japan, and the exten sion of the Pacifi c Hi gh to t he north was sli ghtl y strong, with south east wind ar ound the Kanto region. In the period 2001 –2011 ( Fi g. 30) , for t he for mer half of Jul y, t he Pacific Hi gh ext ended to t he latitude 30° N, wit h a large pressure gr adient on the sout hern shore of Japan, and t he southwest wind in the Kanto regi on was

relati vel y st rong. In t he l atter half of Jul y, the str engt h of the Pacific Hi gh increased, whil e t he pressur e gradient became small . The ext ension of t he Pacific Hi gh t o 30° N was continuousl y str ong; thus, a south wind bl ew with westerl y component around the Kanto regi on. In the for mer hal f of August, the strength of the Pacific Hi gh was strong, a hi gh pressure anomal y was present at 30° N, and the south wind with westerl y component was present in the Kant o r egion. In t he latt er half of August, t he ri dge of the Pacific Hi gh became biased t oward the north , and sout h wind with easterl y component was obser ved around the Kanto region.

As characteristics of the pr essur e pattern i n summer in r ecent years, the ridge of the Pacifi c Hi gh was bi ased t o the sout h , and a trend for wi nd direction to switch to southwest around t he Kanto r egi on has been obser ved . However, when focusing on the seasonal transition, such changes mostl y appear ed i n the latter half of Jul y and the for mer half of August. In t he for mer half of Jul y, the changes in t he pressure patt ern were unclear.

Moreover, in t he latter half of Jul y, r elati ve frequencies of both Categories C and D incr eased. Thi s is li kel y because the pressure gr adient became small with the extens i on of the Pacifi c Hi gh, an d this period was j ust after the end of the rai ny season. Because the ridge of the Pacific Hi gh is bi ased toward the south of Japan af ter the rai ny season, there is li kel y a tr end in recent years, i n which t he wind directi on was f rom southwest around t he Kanto region. It is possible t hat thi s rel ate d to the increase in the rel ati ve freque ncy of Categor y A. However, i n t he for mer hal f of Jul y, t he relati ve frequency of

Categor y A i ncreased si gnificantl y; t hus, other fact ors may be considerable , such as the changes i n the pressure patter n before the end of the rainy season . Nagata and Mi kami (2012) suggested t hat the correlati on between the north –sout h variation of the wester n edge of Pacific Hi gh and the summer temperatur es in Japan inte nsified because of the i ncreased appear ance of t he Pacific -J apan (PJ) pattern after 1951. The PJ patter n is known t o affect t he north –sout h variati on of the North Pacific High si gnifi cantl y ( Nitt a，1987). In regards to t he meridi onal and zonal shifts of the Nort h Pacific Hi gh , several studies have suggested tha t associated with the sea surface temp erature and convecti ve acti vities (e.g., Ueda et al., 1995). These changes may be relat ed to changes in the rel at ive fr equency of each categor y. It is considered t hat the relationship wit h t he pressure pattern ar ound Japan and t he local wind systems and temperat ure di stributi on over t he Kant o Plain changed.

5. Relationship between wind systems and temperature distribution