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The long-term spatio-temporal variability of sea surface 1 temperature in the Northwest Pacific and the Near China Sea 2

The variability of the sea surface temperature (SST) in the Northwest Pacific has been studied 13 on seasonal, annual and interannual scales based on the monthly datasets of ERSST 3b (1854-2017, 164 14 years) and OISST V2 (1988-2017, 30 years). The overall trends, spatial-temporal distribution 15 characteristics, regional differences in seasonal trends, and seasonal differences of SST in the Northwest 16 Pacific have been calculated over the past 164 years based on these datasets. In the past 164 years, the 17 SST in the Northwest Pacific has been increasing linearly year by year with a trend of 0.033 °C/10 yr. 18 The SST during the period from 1870 to 1910 is slow decreasing and staying in the range between 25.2 °C 19 to 26.0 °C. During the period of 1910-1930, the SST as whole maintained a low value, which is at the 20 minimum over the 164 years. After 1930, SST has continued to increase until now. The increasing trend 21 in the past 30 years has reached 0.132 °C/10 yr and the increasing trend in the past 10 years is 0.306 °C/10 22 yr, which is around ten times in the past 164 years. The SST in most regions of the Northwest Pacific 23 showed a linear increasing trend year by year, and the increasing trend in the offshore region was stronger 24 than that in the ocean and deep-sea region. The change in trend of the SST in the Northwest Pacific shows 25 a large seasonal difference, and the increasing trend in autumn and winter is larger than that in spring 26 and summer. There are some correlations between the SST and some climate indexes and atmospheric 27 parameters, the correlation between the SST and some atmospheric parameters have been discussed, such 28 as NAO, PDO, SOI anomaly, TCW, Nino 3.4, SLP, Precipitation, T2 and wind speed. The lowest SST 29 in the Near China Sea basically occurred in February and the highest in August. The SST fluctuation in 30 the Bohai Sea and Yellow Sea (BYS) is the largest with a range from 5 °C to 22 °C, the SST in the East 31 China Sea (ECS) is from 18 °C to 27 °C, the smallest fluctuations occurs in the South China Sea (SCS) 32 maintained at range of 26 °C to 29 °C. There are large differences between the mean and standard 33 deviation in different sea regions. 34


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Previous scholars have made great contributions to the study of global warming, but most of them 97 are the overall changes in the regional average SST, and they tend to ignore the characteristics of changes 98 in certain key sea areas. There are great differences in the trends of SST in different sea areas. The long-99 term trend of the SST changes in the Northwest Pacific (0° N-60° N, 100° E-180° E) over the past 164 100 years (1854-2017) have been calculated based on the monthly datasets of ERSST 3b in this study. The 101 temporal and spatial distribution characteristics of SST, the overall long-term sequence variation trend, 102 the regional variation of the seasonal trend, and the seasonal differences were analyzed.

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The Northwest Pacific is the northwest region of the Pacific, is defined as the offshore region of 0°N-117 60°N and 100°E -180°E in this study (Fig.1). There are more tropical cyclones over the Northwest Pacific 118 than any other sea area in the world, with an average annual average of 35. About 80% of these tropical

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The seasonal mean data are obtained by averaging the monthly average SST after the above-141 mentioned processing. The spring is March, April and May (MAM), the summer is June, July and August 142 (JJA), the autumn is September, October and November (SON), and the winter is December of the 143 previous year and January and February (DJF).

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The SST anomaly is the deviation from the long-term SST average of the observations of the SST 145 describing a particular area and time. The year anomaly represents the deviation of the average of the SST 146 for a given year from the mean of the multi-year SST. The month anomaly represents the deviation of the 147 average of the SST for a particular month from the average of the SST for that particular month for many 148 years. In this paper, the mean value from 1854 to 2017 is taken as the climate mean state, and the sea 149 surface temperature anomaly is subtracted from the SST field to obtain the SSTA field.
The correlation coefficient between time ti and xi is:   Table 1. It shows the increasing trend of SST at different time scales. It can be seen that the data 207 shows that the SST in the different region has shown a significant warming trend as a whole. It can be 208 seen from Table 1 that from 1854 to 2017, the SST trend of Northwest Pacific, North Hemisphere and 209 global ocean has increased by 0.033 °C to 0.035 °C per 10 years. In the past 50 years, the increasing rate 210 of SST has reached 0.10 °C/10 yr or more, and the increasing rate in the last 10 years has reached 0.30°C.

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It can be seen that the warming trend of SST in the Northwest Pacific is very significant.

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There exist decadal to multi-decadal variations in the SST and SST anomalies series, with a general 218 cool period from the 1880s to 1910s, a weak warm period from 1920s to 1940s, a weak cool period from 219 1970s to 1980s, and a recent warm period from 1990s to present.   which is also reflected in Fig. 4(c).

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In the low-latitude region, SST is more evenly distributed along the latitudes in January to April and 294 November to December, and are higher in the south and lower in the north. From May to October, the 295 distribution of SST along the latitude is tilted, showing the distribution characteristics of higher in the 296 southwest and lower in the northeast, which is affected by the ocean circulation. In addition, as can also 297 be seen in Fig. 6, in the low-latitude region, the SST range of change in different months is relatively small, 298 between 27 °C to 33 °C, the change range of 5 °C to 6 °C. In the high-latitude region, the SST can be less 299 than 3 °C at the lowest, and greater than 15°C at the highest, with a relatively large variation of more than northeast, which is affected by the ocean circulation. In addition, as can also be seen in Fig. 6, in the low-309 latitude region, the SST range of change in different months is relatively small, between 27 °C to 33 °C, 310 the change range of 5 °C to 6 °C. In the high-latitude region, the SST can be less than 3 °C at the lowest, 311 and greater than 15°C at the highest, with a relatively large variation of more than 12 °C.

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As can be seen from the figure, the spatial distribution of average SST in each season and annual is similar, 316 and similar to the monthly results (Fig. 6). In the low-latitude region, the SST is higher, but in the high 317 latitudes. SST is relatively low. Annual mean SST decreases with increasing latitude, with high 318 temperature ranging from 26°C to 28°C in the south and low temperature ranging from 3°C to 6°C in the 319 north, which is closely related to the solar radiation distribution in the deep-sea region. The isotherm is 320 northeast-southwest oriented and the SST gradient increases as getting closer to the mainland coastal line.

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It is obvious that the landmass effect in the winter time has contributed to the tilting of the isotherms, in the east. From the previous analysis, we found that this extremum is highly coincident with El Niño 336 (Fig. 5). Therefore, it is likely that this phenomenon has been caused by the temperature difference and  ENSO. There is a significant negative correlation between SST and PDO can be seen from Fig. 9(b). The 364 Niño-3.4 index is usually used to indicate the intensity of the El Niño/La Niña event. So there is a 365 significant negative correlation between SST and the atmospheric parameters Nino 3.4 in Fig. 9(d).

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The Northwest Pacific sea surface variability is affected by a combination of oceanic and atmospheric 432 processes and displays significant regional and seasonal behavior. Monthly SST datasets based on ERSST From the perspective of spatial distribution, the annual mean SST decreases with increasing latitude 445 in conclusion, with high temperature ranging from 27°C to 33°C in the south and low temperature ranging 446 from 3°C to 15°C in the north. The SST is higher in the low-latitude (near equator) region and lower in 447 the high-latitude region. In the low-latitude region, SST is more evenly distributed along the latitudes in the distribution characteristics of higher in the southwest and lower in the northeast, which is affected by 450 the ocean circulation.

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There are many correlations between the SST and some climate indexes and atmospheric parameters,