¹V.I. Il'ichev Pacific Oceanological Institute

Vladivostok, Primorskii Krai, RUSSIA

ponomarev@poi.dvo.ru

　

²Far Eastern Regional Hydrometeorological Research Institute

Vladivostok, Primorskii Krai, Russia

　

³Faculty of Engineering, Kanazawa University

Kanazawa, Ishikawa, JAPAN

　

　

This paper describes major patterns of centennial/semi-centennial climatic tendencies of surface air temperature and precipitation in the Northeast Asia in 20^th century, as well as, sea surface temperature (SST) in the Northwest Pacific in the second half of the century. Linear trend of monthly mean precipitation and air/water temperature is estimated using two statistical methods. First one is least squares method with Fisher's test for significance level. Second method is nonparametric robust method based on Theil's rank regression and Kendall's test for significance level applicable to dataset with abnormal distribution function typical for precipitation time series. Consistency of tendencies in surface air temperature, precipitation, and SST, their seasonality, differences in continental, marginal and coastal areas, as well as, in subtropic and subarctic zones are revealed and discussed.

　

　

Recent examination of global and hemisphere changes in annual mean surface air temperature, precipitation (Bradley et al., 1986; Vinnikov et al., 1990) and SST (Casey and Cornillon, 2001) in the 20^th century have shown statistically significant global warming (Vinnikov et al., 1990; Folland et al., 2001) and precipitation increase in latitude band 35°-70°N over land areas (Bradley et al., 1986; Vinnikov et al., 1990). It is increasing in late 20^th and dominating in moderate latitudes (40°- 60°/55°N) and subarctic zone (55°/60°-70°N) (Folland et al., 2001; Kondratiev and Demirchan, 2001). Climatic tendency of annual mean and winter warming during 20^th century over Northeast Asia and Far-East found in (Arakawa, 1957; Rankova and Gruza, 1998; Varlamov et al., 1998 and so on) are in agreement with major conclusions on climate change in northern hemisphere mentioned above. At the same time, it was shown that precipitation tendencies over the former Soviet Union and Russian Far-East in the 20^th century are unstable and insignificant (Rankova and Gruza; 1998; Dashko et al., 1997). Statistical significance of precipitation trend in Japan estimated earlier is quite similar. For example, precipitation decrease from 1948 to 1985 (Matsumoto and Yanagimachi, 1991) was not confirmed later by using extended dataset for the next decade (Tase and Nakagawa, 1996). It seams to be due to substantial decadal (Nakamura et al., 1996 and others) and interdecadal (Minobe and Mantua, 1999) oscillations in the extratropic North Pacific and East Asia. Moreover, distribution function of precipitation time series is usually abnormal. In this case it is more accurate to use nonparametric robust method for estimation of trend and its statistical significance (Gan, 1995; Krokhin, 1997, 2001). Our paper is also focused on application of this method for precipitation data set. The aim of this study is to reveal consistency of climatic tendencies in surface air temperature, precipitation and SST, as well as their seasonality and difference in the continental and marginal, subtropic and subarctic zones of the area studied.

　

　

The linear trends of surface air temperature and precipitation in the 20th century and second half of the century are estimated for each month of a year in the wide continental area of the extratropic Asia east of 55°E, from Ural Ridge to the coastal areas of the Northwest Pacific and Alaska Peninsula. Semi-centennial tendency of the monthly mean SST in the Northwest Pacific region extended to the west of 180E is examined for the second half of the 20^th century. Dataset of monthly mean gridded SST also covers East China, Japan, Okhotsk and Bering Seas. Thus, the climate change in the wide latitude band from the North Tropic to the coast of Arctic Ocean is estimated. Monthly mean time series of air temperature and precipitation at the meteorological stations were selected for the area studied from data bases of NOAA Global History Climatic Network (USA), RIHMI-WDC (Russia) and JMA (Japan) for the period of instrumental observations since late 19^th century to 2000. To outline the details of climate change associated with extreme cooling or warming in winter and summer, we also used the daily time series of surface air temperature at some meteorological stations. Two monthly datasets of the Northwest Pacific SST on different grids were selected from: (l)- WMU/COADS World Atlas of Surface Marine Data NOAA/NESDIS/NCDC CD-ROM, 1994 of time series since 1945 till 1989 with horizontal resolution 1°x1°; (2)- JMA data base of time series since 1946 till 2000 with horizontal resolution 2°x 2°for the ocean area 15°- 65°N, 110°- 180°E. Initial time series of air temperature, precipitation and JMA SST have missing data. To use complete datasets missing data of the time series in each months was implemented by the statistical method of incomplete multivariate data analyses (Schafer, 1997) using EM and AM algorithms.

　

Two methods of the linear trend estimation are applied. First one is based on least-squares (LS) method, Pirson's regression and Fisher's test for statistical significance level. Second one is nonparametric robust (NR) method (Holander and Wolfe, 1973; Hettmansperger, 1984), based on Theil's rank regression and Kendall's test for statistical significance level (Bendat and Piersol, 1986). The NR method should be applied to time series with abnormal distribution function typical mainly for precipitation time series. It does not demand the assumption that function of distribution is Gaussian. In this case the rank statistics is used to determine both linear regression and its significance. The NR method was earlier applied to examine trends of precipitation in Canada and northeastern USA (Gan, 1995), as well as, in Russian Far East for a warm season (Krokhin 1997, 2001). To estimate trends of surface air temperature, precipitation and SST we have applied both LS and NR methods to all of time series independently on distribution function of datasets.

　

　

Large-scale areas of warming and cooling in the Northeast Asia, their seasonality are revealed for both whole period of instrumental meteorological observations and second half of the 20th century by using two statistical methods of linear trend estimation. Sign and statistical significance of semi-centennial air temperature trend for the second half of the 20th century are shown in Figure 1 for winter and summer months. The area studied is most covered by observation data for this period.

　

　

A semi-centennial warming of high significance level 99% (Fig.1) in the second half of the 20^th century is clear recorded over subtropic Pacific marginal zone (Korean Peninsula, Japanese Islands) all the year round, over Kamchatka Peninsula in summer, spring, and fall, and at the Pacific side of Alaska Peninsula in most months. Weak semi-centennial warming is also found over Chukchi Peninsula, but only in summer months. Significant semi-centennial cooling in the Northwest Pacific marginal area is found only in southeast subtropic continental area adjacent to the East-China Sea in latitude band 25°-35°N (Figs.1a, c, d). Negative air temperature trend of 95%-99% Significance level occurs in latitude band 25°-35°N in June and July, as well as in 25°-30°N band it occurs in August, September, November, December, March and April. The significant centennial cooling in other months is also typical for this latitude band but mainly in the offshore continental area. Most substantial seasonality of semi-centennial air temperature trends is found in continental area 35°-55°N, 90°-110°E. As shown in Figure 1, seasonality of climatic trend in this large-scale area is characterized by warming in winter and cooling in summer. Positive temperature trend in this area is most significant and expanded in December - March, and negative one expands in June - September with maximal significance in June - July. Correspondently, differences between monthly mean air temperature in June and December, July and January, August and February substantially decreases in this continental area both in 20^th century and second half of the century. The substantial difference of the air temperature tendencies in the offshore continental area and marginal zone of the Northwest Pacific is also manifested. It seems to be due to amplification of ocean impact to the mid-latitude Asian continental areas, as well as, with long-term anomaly of the Asian monsoon system.

　

Statistically significant centennial warming (1°- 3℃/90 years) from the beginning of 20^th century till 1990 or 2000 also occurs over marginal subtropic Northwest Pacific throughout a year, over subarctic coastal area in most months, and over arctic marginal zone only in some months, particularly, in December, January, July and August. Centennial trend in offshore area of mid latitude continental Asia also shows warming in winter and cooling in summer (Ponomarev et al., 2001). So, centennial and semi-centennial trends of surface air temperature are similar and stable. At least, at the meteorological stations where period of instrumental observations is more than 100 years (Japan, Korea, Russia) sign and significance of centennial trend do not substantially depend on a period of time series varied from 73 to 120 years. Similar patterns of linear trends in the area studied were found for datasets of monthly mean air temperature since beginning of instrumental observations in late 19^th century until 1990 or since 1917 until 1990 (Ponomarev et al., 2001). It is also in agreement with tendencies of annual/seasonal mean surface air temperature and other climatic characteristics estimated for Japan and Russia by many authors, particularly by Arakawa (1957), Rankova and Gruza (1998), Varlamov et al. (1998) and others. On the whole, significant warming of both centennial and semi-centennial scale predominates in a cold period of a year in a broad mid-latitude continental zone north of 35°-40°N (Fig.1a).