Extreme Sea Level Rise off the Northwest Coast of

J. Ocean Univ. China(Oceanic and Coastal Sea Research)https:///10.1007/s11802-018-3681-9ISSN 1672-5182, 2018 17 (5): 991-999/xbywb/E-mail:xbywb@Extreme Sea Level Rise off the Northwest Coast ofthe South China Sea in 2012LIU Lin1), 2), *, LI Juan1), 2), *, TAN Wei3), WU Yue1), LIU Yanliang1), 2),and WANG Huiwu1), 2)1) Center for Ocean and Climate Research, First Institute of Oceanography, State Oceanic Administration,Qingdao 266061, China2) Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Scienceand Technology, Qingdao 266061, China3) Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, China(Received September 26, 2017; revised October 10, 2017; accepted October 27, 2017)© Ocean University of China, Science Press and Springer-Verlag GmbH Germany 2018Abstract Tide gauge data are used to investigate sea level variability off the northwest coast of the South China Sea (SCS) in 2012, and a significant sea level elevation with a magnitude approaching 79mm is observed. Analysis suggests that an abnormal sea sur-face heat flux and freshwater flux may have contributed to this abnormal rise in sea level, together with the remote influence of an ENSO event. Further investigation shows that the event was dominated by the positive freshwater flux, where large volumes of water entered the ocean, and a maximum is centered to the south of Guangdong province, China. Simultaneously, a positive anomalous heat flux occurred in the northwestern part of the SCS, which is considered to have made a positive contribution to the high local sea level elevation. In addition to the heat flux, the ENSO event also had a significant effect on the event, where the La Niña-induced northwest Pacific cyclone contributed to sea level rise over the northwestern SCS through dynamic and thermodynamic interactions. Key words sea level rise; South China Sea; interannual variability; extreme1 IntroductionThe South China Sea (SCS) is the largest marginal sea in the western Pacific region. It connects with the Sulu and Java Seas in the south through shallow passages and with the Pacific Ocean through the Luzon Strait in the north (Fig.1). The SCS is located in a dominant East Asian monsoon region, and local sea level variability is affected by seasonal monsoon wind forcing, which is northeasterly in winter and southwesterly in summer. Previous studies have used satellite observations to analyze the different time-scale features operating in the SCS (Ho et al., 2000a, b; Hu et al., 2001; Liu et al., 2001; Li et al., 2003; Rong et al., 2007).The rate of sea level rise in the SCS has been faster than the global averaged value, and many studies have been conducted on sea level trend estimations in the SCS using satellite data, gravity data, tide gauge data, and ocean general circulation models (Cheng and Qi, 2007; Rong et al., 2009; Cheng and Qi, 2010; Qiu and Chen, 2012; Cheng et al., 2014). The Pearl River Delta is a highly * Corresponding authors. E-mail: liul@E-mail: twlj0419@ developed region of China located at the north west of the SCS. It is situated adjacent to Hong Kong and Macau, the two important Chinese administrative regions; it has a high gross national product and has been an economic hotspot for decades. As such, it is a key global megacity region, while its geography makes it highly vulnerable to sea level rise. The China Meteorological Administration (2009) had identified the Pearl River Delta as one of the areas in the country most at risk from rising sea levels dueto the low mean land level. Previous studies concur that sea levels in this area are rising and will continue to do soin the foreseeable future (Xia et al., 2015); such an oc-currence will cause heavy waterlogging (Xia et al., 2017) and salt water intrusion (Qiu and Zhu, 2015).On seasonal to interannual time scales, extreme sea level events are usually associated with large-scale ocean dynamics and climate extremes; nevertheless, extreme sea level rise has received little attention to date (Goddard et al., 2015). However, the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) focused on extreme sea level variability under climate change (IPCC, 2013, 2014); extreme sea level rises are predicted to occur more frequently with global warming, and when combined with alterations in high frequency processes, such as eddies and tides, will cause widespreadLIU et al. / J . Ocean Univ . China (Oceanic and Coastal Sea Research) 2018 17: 991-999992 devastation and pose a considerable threat to coastal com-munities.In 2012, an abnormal sea level rise event occurred along the northwest coast of the SCS with a maximum sea level elevation reaching up to 70 mm, which is much higher than any previously recorded in historical records. The event caused considerable problems for local socie-ties and effected human coastal activities. In this article, reanalysis data and observations are used to address the possible mechanisms responsible for this anomalous event. The remainder of the paper is organized as follows: data sets are introduced in Section 2; diagnostic results are presented in Section 3; and the main findings are summa-rized in Section 4.2 DataThis study uses records from 18 tide gauges (data are 70% complete and cover a period up to 2016) obtained from the Permanent Service for Mean Sea Level global database (/data/obtaining) and South China Sea Marine Prediction Center, State Oceanic Ad-ministration, China (Fig.1, Table 1). Altimeter-derived sea level anomalies are produced and distributed by the Co-pernicus Marine and Environment Monitoring Service (http://www.marine.copernicus.eu), and this monthly data set provides a horizontal resolution of 0.25˚×0.25˚ from January 1993 through December 2016. Surface heat fluxes (including latent heat flux, sensible heat flux, shortwaveradiation flux, longwave radiation flux) and freshwater flux are obtained from Japanese 55-yr Reanalysis (JRA-55) with a TL319-L60 resolution (0.5625˚×0.5625˚) dur-ing 1958–2013. The Niño 3.4 index from the Climate Prediction Center of the National Oceanic and Atmos-pheric Administration spanning the years between 1950 and the present day are also employed. All data sets areanalyzed from 1975 to 2015.Fig.1 Locations of 18 tide gauge stations used in this study and topography of SCS.Table 1 Tide gauges along northwest coast of the SCS used in this studyStation name Latitude (˚N) Longitude (˚E) PeriodBei Hai 21.48109.08 1975–2015 Da Nang16.10 108.22 1978–2013 Da Wan Shang 21.93 113.72 1984–2013 Dong Fang 19.10 108.62 1975–2015 Hai Kou 20.02 110.28 1975–2015 Hon Ngu 20.67 106.80 1986–2013 Hon Dau 18.80 105.77 1975–2013 Hui Zhou 22.70 114.53 2006–2013 Quarry Bay 22.29 114.21 1986–2014 Qui Nhon II 13.78 109.25 2008–2014 Shan Wei 22.77 115.37 1975–2015 Shek Pik 22.22 113.89 1998–2014 Tai Miu Wan22.27 114.29 1997–2014 Tai Po Kau, Tolo Harbour 22.44 114.18 1975–2014 Tsim Bei Tsut 22.49 114.01 1993–2014 Waglan Island 22.18 114.30 1987–2015 Zha Po 21.58 111.82 1975–2015 Zhe Lang22.65115.372002–20133 Results3.1 Sea Level Evolution in 2012Fig.2 presents the composited annual sea level among 18 tide gauges from 1975 to 2015. A significant rise in sea level occurred over the 40-yr period, with peak period occurring in 2001 and 2012. In 2012, the combined sea level was 79 mm throughout the whole year, which islarger than that of 63 mm in the year 2000. Between 1975 and 2001, there was a positive trend in the sea level of 3.6 mm yr −1 off the northwest coast of the SCS, and this level reached an initial peak in 2001. However, a larger rising phenomenon with an average rising trend of 5.3 mm yr −1 was then noted from 2002 to 2015 (Fig.2).In addition to the annual sea level variation, monthly sea level anomalies from 2011 to 2012 were also calculated to provide more information about sea level variation offLIU et al. / J . Ocean Univ . China (Oceanic and Coastal Sea Research) 2018 17: 991-999993the northwest coast of the SCS (Fig.3 shows the monthly evolution). There were obvious seasonal variations in sea level during 2011, and maximum (minimum) sea levels occurred in the winter (summer), but the seasonal cycle was not significant during 2012. However, in contrast to the low sea level in the climatological summer of 2011, the sea level was high in the summer of 2012, which con-tributed to the annual maximum for the whole year.The seasonal distribution of sea level anomaly was calculated from 2011 to 2012. In the early spring of 2011, a positive sea level anomaly occupied the whole SCS basin and the maximum amplitude occurred in the coastal region of Vietnam. There were extreme differences be-tween the southern part of the SCS and the area west of the Philippines (the sea level anomaly off the coastal re-gion of Vietnam was about 120 mm and that to the west of Philippines was around −80 mm) (Fig.4). In the following March-April-May (MAM) season, a significant decreas-ing sea level pattern occurred along the coast, ranging from the eastern part of Vietnam to the Pearl River region, with the minimum sea level amplitude reaching about −90 mm. This negative sea level anomaly continued to de-velop, and it reached a maximum in the following June-July-August (JJA) season; the whole northern part of the SCS was occupied by this negative sea level anomaly with a minimum amplitude of 110 mm to the west of Hai- nan Island. In the September-October-November (SON) season of 2011, there was a reduction in the decreasing sea level in the central SCS region and in the northwestcoast of the SCS, where sea levels increased and reachedFig.2 Time evolution of annual mean sea level composite (line) along northwest coast of the SCS. Dots represent total numbers of tide gauge stations, which provideduseful data in each year.Fig.3 Time evolution of monthly averaged sea level com-posite along northwest coast of the SCS from 2011 to 2012.Fig.4 Seasonal sea level anomaly in SCS during 2011. DJF, MAM, JJA, and SON represent different season in 2001 (Units: mm).LIU et al. / J . Ocean Univ . China (Oceanic and Coastal Sea Research) 2018 17: 991-999994 an amplitude of 100 mm. In the following DJF, the north-west coast of the SCS was occupied by a positive sea level anomaly (with an amplitude of more than 130 mm) ranging from the Pearl River to the eastern part of Viet-nam (Fig.5). The increased sea level anomaly in the northwest coast of the SCS continued from DJF to SON, except for an abrupt decrease observed in JJA, which is confirmed by the time evolution diagram in Fig.3. In general, the significant increase in sea level occurred throughout the entire northwest coast of the SCS during 2012. A comparison between 2012 and 2010 shows that a significant sea level increase occurred in the northwest coast of the SCS, while a decreasing sea level occurred in the west to central SCS (Fig.6).Fig.5 As in Fig.4 but for 2012.Fig.6 Sea level changes between 2010 and 2012 from A VI- SO data. 3.2 Surface Flux Variations in 2012 There are many mechanisms involved in sea level variations. The volume of water in the ocean is primarily impacted by two factors: thermal expansion (relating to water expanding after being heated) and volume added from freshwater sources. In this paper, surface heat flux and freshwater flux at the sea surface over the SCS are analyzed to identify the potential impact of surface flux on the anomalous sea level change of the northwest coast of the SCS in 2012.The total heat flux at the sea surface over the SCS re-gion in 2012 is firstly analyzed. Although a generally positive heat flux occurred in most parts of the SCS, it was mainly concentrated in the western SCS, with the core region located in the eastern part of Vietnam. The amplitude of the total heat flux reached 20 W m −2 (Fig.7a). In contrast, there was a negative surface heat flux in the eastern part of the SCS, with a core region extending from an area north of Malaysia to south of the Taiwanislands. In this study, the region (15˚–23˚N, 106˚–117˚E) is selected as the core focus region in which to assess theevolution of surface fluxes, as the northwest part of theSCS experienced the sea level rise in 2012. Fig.7b shows the average total heat flux within the selected region thatLIU et al. / J . Ocean Univ . China (Oceanic and Coastal Sea Research) 2018 17: 991-999995is representative of the northwest SCS. Significant vari-ability on the interannual time scale is well-constructed with the positive anomalous total heat flux in 2012. A positive heat flux can induce sea level changes with re-spect to thermal expansion, and therefore the positive heat flux in this region may partly explain the extreme sea level change in 2012 in the northwest SCS. However, total heat flux includes different items, such as long wave radiation, shortwave radiation, latent heat flux, and sensi-ble heat flux (Liu et al., 2011, 2014), and as it was not evident which of these is responsible for the positive total heat flux in 2012 off the northwest coast of the SCS, the decomposition of heat flux was utilized in order to iden-tify the most important part of heat flux.Fig.7 Annual heat flux anomalies in 2012 relative to 1981–2010. (a) Heat flux anomalies (W m −2) over SCS in 2012; (b) time series of mean heat flux within black box in (a) from 1975–2013 (W m −2).Fig.8 shows the decomposition of heat flux as long wave radiation, shortwave radiation, latent heat flux, and sensible heat flux, respectively. Of these heat flux branches, latent heat flux (the positive latent heat flux concentrate in the west SCS with the core region located at the eastern part of Vietnam) presents a similar special pattern to that of total heat flux; the total heat flux amplitude reaches 18 W m −2. In contrast, the eastern part of SCS has a negative surface heat flux, with the core region extending from north of Malaysia to the west of the Philippines. Long-wave radiation also shows a similar distribution to latent heat flux, except for its weak amplitude. The maximum longwave radiation flux occurs in the coastal region of China, with an amplitude reaching 5 W m −2. However, shortwave radiation heat flux and sensible heat flux have an opposite distribution, where the northwest coast of the SCS has a negative heat flux and the southern part of SCS has a positive heat flux. There is a minimum shortwave radiation flux value of −10 W m −2 and the value of sensi-ble heat flux reaches −3 W m −2. Finally, from the distribu-tion of the various heat flux branches, it is determined that latent heat flux plays a dominant role in total heat flux variation.In addition to surface heat flux, surface freshwater flux also plays a positive role in local sea level rise on the northwest coast of the SCS. Fig.9 lists surface freshwater fluxes over the SCS in 2012. Positive freshwater flux, which represents massive amounts of water input into the ocean, is dominant in the northwest SCS, with the maxi-mum freshwater flux focused south of Guangdong prov-ince, China (the maximum amplitude of the freshwater flux is 2.0 mm d −1). The positive freshwater flux region covers an area ranging from the Pearl River region to the west of Hainan Island. The negative freshwater flux (out-going water from the ocean to the atmosphere) simulta-neously occurs over the east of Vietnam and in the south-ern part of the SCS.It is also possible to identify a positive freshwater flux extending from the southern part of Taiwan to southwest of the Philippines, with a maximum value larger than 1.8 mm d −1. In this respect, the area (15˚–23˚N, 106˚–117˚E) is also selected as a core region to assess the evolution of surface freshwater fluxes. Fig.9b presents the time evolu-tion of averaged total freshwater flux over the selected core region. Similar to surface heat flux, freshwater flux also shows significant variability on an interannual time scale. In 2012, the total freshwater flux reached a value that was high compared to values of previous decades. With the exception of the year 1977, the value of fresh-water flux in 2012 was the largest amount of water input into the ocean for the past 40 years.Since total freshwater flux comprises precipitation and evaporation, the different parts of freshwater were further separated into a precipitation part and an evaporation part to assess the relevant importance of these two items. Fig.10 presents the precipitation distribution in 2012 over the SCS region. It is evident that positive precipitation occurs in the northwest coast of the SCS; this extends from the coastal region of China into the area west of the Philippines, with a maximum value of around 2.5 mm d −1 throughout the entire year. In contrast, the western part of the SCS is dominated by negative precipitation with a minimum value lower than −2 mm d −1. However, the con-ditions are totally different for evaporation. NegativeLIU et al. / J . Ocean Univ . China (Oceanic and Coastal Sea Research) 2018 17: 991-999996 evaporation occurs in the west to northwest SCS, with a maximum value of −0.6 mm d −1, but positive evaporation occurs in to the west of the Philippines, with a maximumvalue beyond 0.4 mm d −1. As total freshwater flux is a combination of precipitation and evaporation, both vari-ables have a positive effect on sea level rise in the north-Fig.8 Components of annual heat flux anomalies (W m −2) in 2012 relative to 1981–2010. (a) Latent heat flux anomaly into ocean (W m −2); (b) shortwave heat flux anomaly into ocean; (c) sensible heat flux anomaly into ocean; (d) longwaveheat flux anomaly into ocean.Fig.9 Annual freshwater flux anomalies in 2012 relative to 1981–2010. (a) Freshwater flux anomalies (mm d −1) in 2012; (b) time series of mean heat flux within black box in (a) from 1975–2013 (mm d −1).LIU et al. / J . Ocean Univ . China (Oceanic and Coastal Sea Research) 2018 17: 991-999997Fig.10 Components of annual freshwater flux anomalies (mm d −1) in 2012 relative to 1981–2010.west SCS. With respect to precipitation, the positive value illustrates that water input into the ocean may deepen the surface water flux and elevate the local sea level; with respect to evaporation, the negative value represents less water entering the atmosphere, which may help to retain water in the ocean and ensure that the sea level is main-tained and does not decrease. Finally, the variability in freshwater flux in the northwest SCS explains the local rise in sea level.4 Discussion and SummaryThis paper identifies and analyzes an extreme sea level rise event occurring in 2012, which was larger than any previously recorded over the previous 40 years. The ef-fect of surface flux variation on this event are discussed in this section.The volume of water in the ocean is primarily impacted by two factors; thermal expansion (water expands when heated) and volume added from land-based sources of freshwater. According to the IPCC, thermal expansion accounted for 13%–31% of the observed rate of sea level rise for the period 1961–2003 and 50% for the period 1993–2008. In addition, land based sources of water are increasingly contributing to observed sea level changes. The roles of heat flux and freshwater flux on variations in sea level changes in 2012 are analyzed here. The latent heat flux and sensible heat flux induced total heat flux anomaly created a positive heat flux of the northwest coast of the SCS, and thermal expansion then contributed to the elevation in local sea level. However, there was also a significant positive freshwater flux in the same area in relation to strong positive precipitation, which may also have elevated local sea levels with respect to mass accumulation.In addition to surface flux variation, the remote influ-ence from ENSO was also a potential cause of the ab-normal sea level rise of the northwest coast of the SCS. Fig.11 lists the time evolution of the Niño index (Niño 3.4 index) and the sea level anomaly in this area; these two series correlate well (with a correlation coefficient reach-ing −0.76 (significant at 99% confidence interval)) under the condition that the Niño 3.4 index leads sea level anomalies by three months. Since there was a moderate La Niña event in 2011, which suggests that the La Niña event contributed to the high sea level rising over thenorthwest coast of the SCS.Fig.11 Time evolution of SLA (blue) and Niño 3.4 (red) index.Different mechanisms are involved in the significant relationship between La Niña and the sea level rise off the northwest coast of the SCS. Firstly, during the decay phase of the La Niña event, the abnormal northwest Pa-cific (NWP) anticyclone occupies the western Pacific region and plays an important role in connecting the East Asian climate and ENSO. The NWP circulation anoma-lies develop rapidly in the late fall of a year when ENSO matures (Wang et al., 2000), and these anomalies can persist into the following summer. The center of the NWP cyclone for La Niña is closer to the coast of East Asia than for El Niño, and its extent is smaller (Tao et al., 2016); this is associated with the anomalous easterly wind in the northwest SCS region. Through the Ekman effect, the anomalous easterly wind generates local water accu-mulation into the northwest region of the SCS, which contributes to local sea level elevation due to the in-creased water mass. Secondly, the anomalous NWP cy-clone, which persists until the following summer season, decreases the climatological SCS summer monsoon thatLIU et al. / J . Ocean Univ . China (Oceanic and Coastal Sea Research) 2018 17: 991-999998 flows from the southwest region into the northeast region of the SCS. The decreasing SCS summer monsoon con-tributes to the negative latent heat flux into the ocean over the northwest SCS (Fig.8). Under the joint influence of dynamic and thermodynamic mechanisms relating to NWP atmospheric circulation, a La Niña decay phase event plays an important role in maintaining sea level rise over the northwest SCS region.In addition to the influence of the ENSO event, long term variability may also have contributed to abnormal sea level rise off the northwest coast of the SCS in 2012. Previous studies have determined significant decadal variability in SCS sea levels; decadal sea level variability in the SCS follows the western tropical Pacific, and large variance occurs west of Luzon Island. A negative contri-bution to decadal variability in the central SCS is caused by local atmospheric forcing, and Rossby waves radiating from the eastern boundary also appear to be important (Cheng et al., 2016). Decadal variability associated with the PDO account for most of the sea level trends in the SCS over the past two decades (Cheng et al., 2016). However, there are limitations associated with tide gauge data, and it is thus necessary to conduct future studies using numerical modeling to accurately ascertain this effect. In summary, this study aims to reveal the mechanism responsible for the extreme sea level rise off the north-west coast of the SCS in 2012, and the increasing rise in sea level trend occurring over recent decades. In this re-spect, sea level variability off the northwest coast of the SCS is investigated using tide gauge data. Results show a significant elevation in the northwest SCS with a magni-tude approaching 79 mm, which is higher than that of previous years (at 63.8 mm). Our analysis suggests the abnormal sea surface heat flux and the freshwater flux may have contributed to the abnormal rise in sea level. The abnormal heat flux in 2012 was positive in the northwest-ern part of the SCS, and it may have had a positive effect on local sea level elevation. Further analysis illustrates that latent heat flux plays an important role in the heat flux effect. In addition to the heat flux, freshwater flux also contributes to this sea level rise, and the La Niña event also has a significant associated effect. Through dynamic and thermodynamic factors, the La Niña event induced NWP cyclone promotes sea level rise over the northwest SCS. However, it is considered that future work is required to verify this mechanism, in particular numerical experiments related to further analysis of the distinctive parts of dynamic and thermodynamic branches.AcknowledgementsThis research is jointly funded by the National Key R&D Program of China (No. 2017YFC1405100), the Basic Scientific Fund for National Public Research Insti-tutes of China (No. 2017S02), the Global Change and Air-Sea Interaction Program (Nos. GASI-IPOV AI-03, GASI-IPOV AI-02, GASI-02-IND-STSaut, and GASI-02-IND-STSwin), the China Postdoctoral Science Foundation (No. 2017M612166), the China Ocean Mineral Resources R&DAssociation Project (No. DY135-E2-4-02), and the NSFC-Shandong Joint Fund for Marine Science Research Cen-ters (No. U1406404).ReferencesCheng, X. H., and Qi, Y. Q., 2007. Trends of sea level varia-tions in the South China Sea from merged altimetry data. Global and Planetary Change , 57: 371-382.Cheng, X. H., and Qi, Y. Q., 2010. On steric and mass-induced contributions to the annual sea-level variations in the South China Sea. Global and Planetary Change , 72 (3): 227-233. Cheng, X. H., Xie, S. P., Du, Y., Wang, J., Chen, X., and Wang, J., 2016. Interannual-to-decadal variability and trends of sea level in the South China Sea. Climate Dynamics , 46: 3113-3126.Cheng, Y., Xu, Q., and Andersen, O. B., 2014. Sea-level trend in the South China Sea observed from 20 years of along-track satellite altimetric data. 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