WANG Hui,WANG Aimei,LI Wenshan,LUO Jinxin,ZHANG Jianli,ZUO Changsheng

National Marine Data and Information Service,Tianjin 300171,China

Abstract:Based on the observation data from China coastal marine stations,the key indicators of marine climate along China coast were explored,including sea surface t emperature(SST),sea level and sea ice.Results show that:(a)The SST along China coast continues rising and increased by 0.25 °C/decade during 1980-2019,the warming accelerated significantly after 2011 and it has been well above normal for five consecutive years since 2015.In 2019,the average SST along China coast was 1.1 °C higher than normal(with 1981-2010 taken as a reference period),ranking the highest since 1980.Besides,the SST extremes have been explored based on four long-term marine stations for the period 1960-2019.(b)Sea level along China coast continues to rise at an accelerated rate.The mean sea level rise rate along China Coast was about 2.4 mm/yr during 1960-2019,3.4 mm/yr during 1980-2019,and 3.9 mm/yr during 1993-2019,with significant regional differences.The relatively stronger sea level rise trends were observed along the coastal waters of the Bohai Bay,the Laizhou Bay,the Yangtze River Estuary,the Pearl River Estuary,and the Hainan Island,respectively.Besides,the extreme sea levels along China coast showed an obvious upward trend from 1980 to 2019.During this period,the annual rise rate of extreme high water level along China coast was 4.4 mm/yr,and had obvious regional characteristics,with the highest rate of 9.9 mm/yr observed at Yantai of Shandong Province.(c)The annual sea ice period and sea ice cover of the Bohai Sea(BS)decreased substantially during 1963-2019 by 0.7-1.3 days/yr and 45-59 %/yr,respectively,and the decrease rate of ice cover is larger in the north than that in the south.2019 was the year of light icing.

Keywords:marine climate,SST,sea ice,sea level,extreme water levels

1 Introduction

Under the background of global warming,the global ocean has warmed unabated since 1970(Trenberth,et al.,2007;Rhein M,et al.,2013;IPCC,2014;IPCC,2019).Recent ocean changes are widespread,and key ocean indicators are in states unprecedented for centuries to millennia(IPCC,2021).The global mean sea level(GMSL)is rising with accelerati on in recent decades.The global marine heat waves have increased in frequency and intensity.Extreme wave heights have increased in the Southern and North Atlantic Oceans over the period 1985-2018.The ocean warming,sea level rise,reduced sea ice extent,and extreme waves are projected to impact marine ecosystems through direct and indirect effects on habitats,populations,and their viability(Cai R,et al.,2016).Many studies have been done in past years about the change of mean sea surface temperature(SST),mean sea level,and extreme marine events regionally and globally(Menéndez and Woodworth,2010;Mudersbach,et al.,2013;Weisse,et al.,2014).All of these changes are not globally uniform and vary regionally(Wu,et al.,2012;Yeh S W and Kim C H,2010).The international community is increasingly aware of the threat and serious challenge of global climate change to the contemporary and future living space of human beings,as well as the importance and urgency of taking active measures to reduce and prevent climate risks.

China is a maritime power with a continental coastline of more than 18 000 kilometers.The area with an elevation<10 meters along the coast accounts for about 1.3% of the total land area and is the most populous country in the low-altitude areas of the world(NMDlS,2019).Coastal waters,the most developed and urbanized areas in China,are sensitive and vulnerable to global climate change(Cai R,et al.,2016).With the warming climate,the rising of SST,sea level rise,and the intensification of extreme weather and climate events(such as typhoons,huge waves,and storm surges)have a great impact on the marine environment and coastal socio-economic development of China(MNR,2019).

However,since the limitation of observation data along global coastlines,main scientific interest has focused on the variation of the global open oceans,there is limited information on the changes along global coastline,especially along China coast(Smith T M and Reynolds R W,1998;Bao and Ren,2014).Based on the data of China Coastal Ocean Observatory Network,this paper comprehensively analyzed and discussed the changes of key indicators of China's coastal marine climate in recent decades,including SST,sea ice and sea level(Tab.1),and the latest monitoring facts of coastal marine climate change,hoping to provide scientific support for marine disaster prevention and mitigation,coastal zone development and utilization,and natural resource management under the new situation.

2 Data and methods

2.1 Data

Hydro-meteorological(SST and tide level)observation data from 34 marine stations along Chin a coast were obtained from the China Coastal Ocean Observatory Network,dating from January 1980 to December 2019.The locations of the stations are given in Fig.1.The sea ice data used were from the Bayuquan,Huludao,Zhimaowan,and Tanggu marine stations,of which the observation period is from January 1963 to April 2020.The sea ice cover data refer to the percentage of sea ice coverage to the whole visible sea surface,the annual sea ice cover is defined as the sum of daily sea ice cover.The data of these stations have been subjected to quality control,interpolation,and homogeneity adjustment(Wang,et al.,2013;Li,et al.,2018).

Fig.1 Long-term marine stations along China coast.BS,YS,ECS and SCS denote the Bohai Sea,Yellow Sea,East China Sea and South China Sea,respectively.

Without specification,in this paper the period 1981-2010 is used as the reference time for calculating anomalies to analyze climate change,and the mean value of each indicator during 1981-2010 is referred to as normal.

2.2 Method

In order to analyze the inter-decadal variations and climate oscillations of China coastal SST and sea level,a 10-year low-pass Gaussian filter is used to derive the low-frequency variation.The method of least-squares is used to calculate the linear trends of the anomalies series for given time periods,and the non-parametric MK test(Kendall,1962;Kendall,1975;Mann,1945;Neeti N and Eastman J R,2011;Khambhammettu,P,2005)is used to examine the significance of the linear trends of the time series.

Two percentile-based extreme SST indices(extreme high SSTs,extreme low SSTs)derived from daily observation data were calculated for extreme SST events.The extreme high SSTs are defined as the threshold of extremum in terms of the 90th(95thand 99th)percentile.The extreme low SSTs are defined as the threshold of extremum in terms of the 10th(5thand 1st)percentile.The extreme sea levels are computed based on minute sea level data from tide gauges(Tab.1).

Tab.1 Definitions of 8 indices used in this study

3 Results and discussion

3.1 Temporal variations in mean and extremes of SST

SST in China's coastal waters is a key marine environmental factor affecting coastal biological resources and ecological service functions.Under the background of global warming,the global upper ocean is undergoing significant warming,and the near-surface sea temperature is warming at the fastest rate,with significant regional differences in the rate and amplitude.The annual mean SST and extreme SSTs were explored to provide a view of SST variations along the China Coast.

From 1980 to 2019,the SST along China coastal waters showed a fluctuating upward trend,with a rise rate of 0.25 °C/decade,and the rise rate was much larger than that of global mean SST(0.09 °C/decade).The warming accelerated significantly after 2011 and it has been well above normal for five consecutive years since 2015,with 2019 ranking the highest since 1980.The rise rates of SST along the coastal water of the Bohai Sea(BS),the Yellow Sea(YS),the East China Sea(ECS),and the South China Sea(SCS)were 0.22 °C/decade,0.28 °C/decade,0.27 °C/decade,and 0.22 °C/decade,respectively.All of the trends were significant at the 95% confidence level.The rise rates along the coast of ECS and YS were higher than that along the SCS and the BS.

In 2019,the average SST along the coast of China was 1.1 °C higher than normal,reaching the highest level since 1980.The SST in the coastal water of the BS,the YS,the ECS,and the SCS were about 1.0 °C,1.1 °C,0.8 °C,and 1.3 °C higher than normal,ranking the second,the first,the third,and the first highest level since 1980,respectively(Fig.2).

Fig.2 Mean Annual SSTA along(a)the China coast and coastal waters of(b)the Bohai Sea,(c)the Yellow Sea,(d)the East China Sea(ECS),and(e)the South China Sea(SCS)from 1980 to 2019.

There is a slowdown of warming during 1998-2011 along China coast,especially for the coastal waters of the ECS and SCS.This slowdown of warming has been reported in global and regional scales(Kosaka and Xie,2013;Smith,2013;Bao and Ren,2014)and ascribed to various factors including internal climate variability,reduced solar energy,decreased stratospheric water vapor concentration,negative phase of the Pacific Decadal Oscillation and strong shifts to La Niňa states(Kosaka and Xie,2013;Solomon et al.,2010;FrÖhlich,2012;Held,2013;Trenberth and Fasullo,2013;Guan,et al.,2015).

To further explain the evolution of the SST trend along China Coast,the time series of SSTAs and their linear trends of the China coastal long-term marine stations during 1960-2019 are delineated in Fig.3.The SSTs at Beihuangcheng,Lianyungang,Kanmen,and Zhapo marine stations were characterized by surface cooling during 1960-1969,followed by surface warming during 1969-1998,surface cooling again during 1998-2011,and strong surface warming during 2011-2019.Furthermore,the warming rates were larger during 1980-2018 than those during 1960-2018,with the rise rates increased from 0.07 °C/decade(did not pass 95% significance test),0.20 °C/decade,0.15 °C/decade and 0.18 °C/decade,respectively,to 0.13 °C/decade(did not pass 95% significance test),0.31 °C/decade,0.27 °C/decade and 0.28 °C/decade,respectively.The SST warming rates have been higher than the global average.Bao and Ren(2014)also reported the phenomenon of abnormal warming in the China seas after the mid-1980s.Acceleration of warming during 1980-99 was accompanied by a weakening of the East Asian monsoon(EAM)and a strengthening of the west Pacific subtropical high(WPSH)(Cai et al.,2017).He and Wang(2012)found the East Asian Winter Monsoon(EAWM)intensity decreased from 1986,the WPSH indices of area and intensity have notably increased during 1982-2018(Li Yan et al.,2019).These are all consistent with the SST regime shift and high-temperature states revealed in this study.

Fig.3 Mean annual SSTA of the long-term marine stations along the China coast from 1960 to 2019.

The mean value,however,does not represent all the climate.Besides the extremes often directly impact the ocean ecosystem and human society.The trends of the extreme temperatures were computed based on four long-term representative stations.Fig.4 and Fig.5 illustrate the linear trend in extreme high SSTs and extreme low SSTs during 1960-2019,respectively.The linear trends are listed in Tab.2.At Zhapo station,the high SST extremes ranged between 29 °C and 32 °C showing a significantly increasing trend with rise rate from 0.05 °C/decade(TX99)to 0.09 °C/decade(TX90)during 1960-2019.Increasing trends were apparent in the low SST extremes for all indices of the four representative stations,namely Beihuangcheng(BHC),Lianyungang(LYG),Kanmen(KMN),and Zhapo(ZPO),and the rates are larger than those of mean SST during the same periods.

Fig.4 Annual extreme high SSTs for TX90(black),TX95(blue)and TX99(red).See Tab.2 for corresponding trends.

Fig.5 Annual extreme low SSTs for TN10(black),TN5(blue),and TN1(red).See Tab.2 for corresponding trends.

Tab.2 Each index trends of the four representative stations(unit:°C/decade)

3.2 Temporal variations in mean and extreme sea levels

Sea level rise has become an important issue in global climate change.The China coast is vulnerable to sea level rise and extreme climate events.In this section,we investigate the trends in sea level records,including mean and extreme records.Since 1960,China coastal sea level rise has accelerated.The rise rate is 2.4 mm/yr during 1960-2019,and 3.4 mm/yr during 1980-2019,with obvious regional differences.The sea level rise rate was relatively larger in the Bohai and Laizhou Bay,the Yangtze River Estuary,the Pearl River Estuary,and the Hainan coast,while relatively smaller along the coast of western Liaodong Bay,the southern Shandong Peninsula,and Guangxi Province.From 1993 to 2019,the rise rate of China coastal sea level was 3.9 mm/yr,higher than the global average(3.24 mm/yr).The decadal average sea level along China coast has a significant gradient rising trend,the sea level of 2010-2019 average was 61 mm higher than the previous decade.In 2019,China coastal sea level is 72 mm higher than the 1993-2011 average,which is slightly lower than in 2018,the third-highest since 1980(Fig.6).

Fig.6 Annual mean sea level change along China Coast from 1980 to 2019(Relative to 1993-2011 average)

The monitoring results of the long-term representative marine stations along China coast indicated that the sea levels at Huludao,Xiamen,and Zhapo showed increasing trends with significant inter-decadal oscillation in the latest 60 years,and the annual mean sea level after 2011 all reached the high level in the complete observation record.

The sea level rise rate at Huludao Station was 1.8 mm/yr from 1960 to 2019,and the sea level in 2019 was 68 mm higher than the 1993-2011 average,reaching the fifth highest since 1960.The rate of sea level rise rate at Xiamen Station was 2.1 mm/yr from 1958 to 2019,and the sea level in 2019 was 80 mm higher than the 1993-2011 average,reaching the third-highest since 1958.The rate of sea level rise rate at Zhapo Station was 2.4 mm/yr from 1960 to 2019,and the sea level in 2019 was 110 mm higher than the 1993-2011 average,reaching the third-highest since 1959(Fig.7).

Fig.7 Sea level changes of the long-term marine stations along China coast from 1958 to 2019

While the knowledge of mean sea level variability is crucial for coastal planning and the design of mitigation strategies for the long-term,most of the hazardous events are usually related to extreme high water levels(Camille Letetrel,et al.,2010).Minutely tide-gauge data from 16 marine stations are used to evaluate changes in extreme sea levels in the past several decades.

The annual extreme high water level along China coast showed a general upward trend from 1980 to 2019,with the rising rate of 4.4 mm/yr,and obvious regional characteristics.The largest rise rate of extreme high water level at 9.9 mm/yr was seen in Yantai,Shandong Province,followed by 9.2 mm/yr and 8.5 mm/yr at the Hangzhou Bay and Pearl River Estuary,respectively.The annual extreme high water level along China coast in 2019 was generally 8 cm higher than the 1993-2011 average.The extreme high water level anomalies at the coastal waters of Longkou,Shandong Province was the largest,about 50 cm higher than the 1993-2011 average,while the extreme high water level anomalies at the coastal waters of the Tianjin and the northern Hebei Province were second largest,about 46 cm and 32 cm higher than 1993-2011 average,respectively(Fig.8).

Fig.8 Variations of annual extreme high water level of representative stations along China coast during 1980-2019

The annual extreme low water levels along China coast increased by 4.1 mm/yr from 1980 to 2019,with significant spatial variability.The largest rising rate was seen at the Tianjin coast of 12.5 mm/yr,the second-largest seen at the coastal waters of northern Liaodong Bay and northern Shandong Peninsula,the Pearl River Estuary,and northern Hainan,with the values between 7.2-7.7 mm/yr.The annual extreme low water level in 2019 was 7 cm higher than the 1993-2011 average in general,while the extreme high water level anomaly at the coastal area of northwestern Liaodong Bay was up to 25 cm,and about 19 cm-20 cm at the coast of northern Hebei Province,northern Shandong Peninsula and the Pearl River Estuary,about-9 cm--3 cm in the central and southern Fujian Province relative to 1993-2011 average(Fig.9).

Fig.9 Variations of annual extreme low water level of representative stations along China coast during 1980-2019

3.3 Temporal variations in sea ice period and sea ice cover

The cryosphere is of great significance in global change(Polyakov,et al.,2002;Serreze and Francis,2006)and the sea ice is a sensitive indicator of climate change.The sea ice in the Northern Hemisphere has experienced an evident decline in its extent and thickness as the climate warming over the past few decades(Parkinson,et al.,1999;Lindsay and Zhang,2005;Stroeve,2005).In the China Seas,the sea ice mainly occurs in the BS and northern YS,the southernmost region in the Northern Hemisphere,in boreal winter of each year.In recent years,the coastal zone economy and the marine economy around the BS and YS are developing rapidly.Once the BS and YS experience severe sea ice events,the marine transportation,fishery,and oil industry would be affected,causing significant economic losses(Gu W,et al.,2013).

The evolution process of ice conditions can be divided into three stages:the freezing period,the severe period,and the breakup period.And sea ice appears every winter in the BS,which generally starts from late November to early December,from north to south,and ends in mid-March of the following year.The severe ice period usually occurs in January or February,during which the sea ice cover is the largest.

From 1963 to 2019,the annual sea ice duration and ice cover of the BS showed a downward trend.The average reduction rate was 1.3 days/yr and 59%/yr respectively in the Bayuquan Station,1.0 days/yr,and 59%/yr in the Huludao Station,0.7 days/yr and 59%/yr in the Zhimaowan Station,and 1.0 days/yr and 45%/yr in the Tanggu Station,respectively.There was only a little floating ice in Tanggu Station during 2013-2019.The declining trend of sea ice severity has been linked to variations of the large-scale atmospheric circulation system,such as the El Ni~no/Southern Oscillation(ENSO),the positive upward trend in AO？and the associated changes in the Siberian sea level pressure,East Asian Trough(Liu et al.,2004;Li et al.,2005;Gong et al.,2007;Shi and Wang,2012),and in agreement with the rising trend of SST in BS(Fig.10).

Fig.10 Variations of sea ice condition at marine stations along the Bohai Sea coast from 1963 to 2019(relative to 1981-2010)

Historically,the BS experienced its severe sea ice event during 1968/1969 winter,1976/1977 winter,1985/1986 winter,1996/1997 winter,2000/2001 winter,2009/2010 winter,and 2012/2013 winter,causing significant economic losses,affecting marine transportation and fishery,and impacting the entire marine ecosystem(Yang 2000).During 2010-2017,the direct economic losses caused by sea ice disasters had reached¥7.7 billion.

In 2019/2020 winter,the ice severity in the BS was lighter than normal,which mainly appeared in the Liaodong Bay.And there was only a small amount of floating ice in the Bohai Bay and Laizhou Bay.Floating ice first appeared in early December 2019,and all melted in late February 2020.

4 Conclusions

Since the 20thcentury,under the background of global warming,the climate indicators have undergone significant changes in recent decades and the physical footprint of climate change has become increasingly prominent.The climate variability and extremes have had great impacts on coastal society and economic development and natural ecological environment.The China coast is one of the most vulnerable areas under climate change.ln this paper,we explored the key indicators of marine climates along China coast,including SST,sea level,and sea ice based on the observation data from China coastal marine stations.The mean SST,extreme SSTs mean sea level,extreme water levels,sea ice period,and sea ice coverage have been examined in terms of their trends.

Monitoring and analysis results show that the SST along China coast exhibited an increasing trend,the warming accelerated significantly after 2011 and it has been well above normal for five consecutive years since 2015.ln 2019,the average SST along the coast of China was 1.1 °C higher than normal,ranking the highest since 1980.According to four long-term observation of marine stations,increasing trends were apparent in the extreme low SSTs for all indices,and the rates are larger than those of mean SST during the same periods.Sea level rise has accelerated,and China coastal sea level rise rate was 3.9 mm/yr during 1993-2019,higher than the global average(3.24 mm/yr).ln 2019,the sea level along China coast was 72 mm higher than the 1993-2011 average,reaching the third-highest level since 1980.Both the annual extreme high water level and the extreme low water level along China coast showed significant upward trends during 1980-2019.From 1963 to 2019,both the annual sea ice period and sea ice coverage of BS stations showed fluctuating downward trends.ln the winter of 2019/2020,the ice condition in the BS was lighter than normal,and the sea ice mainly appears in Liaodong Bay,with only a small amount of floating ice in Bohai Bay and Laizhou Bay.These changes have profoundly affected China's natural resource pattern.

Acknowledgements

This research was supported by the National Key Research and Development Program of China(NO.2016YFC1402610).We would like to thank all the people who worked to generate all the data used in this research.