The impacts of 2008 snowstorm in China on the ecological environments in the Northern South China Sea

ABSTRACT At the beginning of 2008, disastrous weather with snowstorms truck South China. Analysis of satellite and in situ data revealed obvious changes in ecological environments in the northern South China Sea after snowstorms. The sea surface temperature (SST) data from NOAA and Moderate Resolution Imaging Spectroradiometer (MODIS) from 2000 to 2013, and the chlorophyll-a (Chla) concentration from MODIS are analysed. In the study area, the multiple-day-averaged SST dropped by 20.01% and Chla concentration increased by 22.42% during and after snowstorm. SST decreased more significantly and Chla increased in the coastal waters, while suspended sediment concentration (SSC) increased more remarkably offshore. The biggest drop of SST reached 6°C and striking elevation in Chla with 52.55% in the Taiwan Strait, and SSC changed significantly near to the Taiwan bank after the snowstorm. Further analysis indicated that the remarkable reductions in SST were caused by large cold water input from Min-Zhe coastal water and the Pearl River Estuary, as a result of strong sea-land-air interaction in the coastal waters during the snowstorm. The increase in Chla may be related to the abundant nutrients from the plenty of cold water, a strong front and vertical mixing in the coastal waters.


Introduction
The South China Sea (SCS) is the largest marginal sea of the Western Pacific. Generally, the northern area of it ranges from the west of the Luzon Strait (LZS), southwest of Taiwan Island (TWI), to the north of 18 N of SCS, which is in the south of the East China sea (ECS). From October to March in the following year, this area is controlled by the winter northwest monsoon with average wind speed about 8-11 m/s , meanwhile the precipitations mainly happen in midsummer, effected by monsoon and tropical hurricane, with the amount of the annual rainfall about 1100-1800 mm (Yan et al. 1993). In the Northern South China Sea (NSCS), the thickness of the mixed layer is usually only 30-50 m; however, it can reach to 75 m in winter while the sea surface temperature (SST) of the coastal waters is about 21 C (Wang et al. 2002). The NSCS has complex circulation and hydrodynamic environment because it is affected by the strong Kuroshio with high temperature and salinity and intrusions of currents from Taiwan Strait (TWS), Fujian-Zhejian (Min-Zhe) coastal waters (MZCW) coming from Yangtze River (YR) and discharge from the Pearl River (PR), as well as alternating monsoons (Liu et al. 2003;Fu et al. 2014Fu et al. , 2016.
A severe weather event with wide-range sustained low-temperature, freezing rain and sleet took place in Southern China from 11th January to 2nd February 2008. This disaster was extremely severe in terms of its spatial extent, duration and damage, affecting Shanghai (SH), Zhejiang (Zhe), Fujian (Min), Anhui (Wan), Guangdong (Yue), Henan (Yu), Jiangxi (Gan), Jiangshu (Shu), Hunan (Xiang), Guizhou (Qian) and Guangxi (Gui), among other areas, and causing substantial mortality of wild fish and 80% loss of caged Mariculture fish (Chang et al. 2009(Chang et al. , 2013. The damaged area of the fishery industry in Guangdong province exceeded 0.17 million hectares and the economic losses to the fishery industry were as high as 6 billion Yuan. At the same time, the disaster caused enormous damage to the environment and greatly influenced people's life. Some scholars thought that the sea temperature of middle-east Pacific Ocean of the equator entered into La Nina status with booming development from August 2007 to January 2008. This strong La Nina event impacted the NSCS and it was one of important reasons that affected the wide-range sustaining low-temperature, freezing rain and sleet disastrous weather in the Southeast of China (Zheng et al. 2008;Gao et al. 2008). The convergence in Southeast China of the cold air from Eurasia and warm/moist air from the Indian Ocean and SCS caused conditions of freezing rain and sleet in early 2008 in Southeast China (Ma 2009;Shi et al. 2010) (A in Figure 1(a)). During this event, much MZCW from the Yangtze River Estuary (YRE) and the discharge form Pearl River Estuary (PRE) was injected into the TWS and NSCS, bringing a great deal of sand and nutrients. The sea-land-air interaction was enhanced with larger-than-normal discharge and colder temperatures (B in Figure 1(a)) (Shang et al. 2012;Liao et al. 2013;Wang et al. 2013). At the same time, the Kuroshio stream entered the NSCS via LZS Shang et al. 2012;Liao et al. 2013;Vigan et al. 2014), and its warm seawater interaction with the cold diluted water impacted the ecological environment (C in Figure 1(a)).
Sea surface temperature (SST) is a comprehensive indicator of the interaction of marine thermodynamic processes. It is not only an important physical parameter for studying humidity and heat exchange across the sea-land-air interface, but also the most intuitive variable for marine studies (Chu et al. 1997;Bao et al. 2002). Chlorophyll-a (Chla) concentration, an index of phytoplankton biomass, is another important indicator of changing conditions in marine ecosystems and climate (Tang et al. 1998(Tang et al. , 2002(Tang et al. , 2004b(Tang et al. , 2004aBabin et al. 2004). Likewise, the suspended sediment concentration (SSC) in the sea is an important water quality parameter that can directly influence optical properties, such as water transparency, turbidity, water colour, etc. (Tassan, S., 2003;Yan and Tang 2009), and its absorption coefficient has significant influences on ocean biogeochemical processes (Sipelgas et al. 2006;Liu et al. 2010Liu et al. , 2012.
In order to understand the impacts of the 2008 snowstorm on ocean ecosystem, especially in the NSCS, it is important to study the spatial and temporal distribution of SST, Chla and SSC in the area influenced by the event, as well as the mechanism of these changes, as done in this paper.

Study area
The study area is the NSCS and TWS, from 110 E to 122 E, and 19 N to 27 N (blue box in Figure 1 (a)). It receives a large volume of freshwater discharge from the PRE and the YRE (Shang et al. 2012;Liao et al. 2013), is controlled by northeast monsoon in winter Vigan et al. 2014) and is invaded by the Kuroshio through the LZS (Shaw & Chao 1994;Shang et al. 2012;Liao et al. 2013). To analyse the impacts of the snowstorm on the ocean environment, we divide the study area into 16 bins of 2 £2 , with Bins 1-9 (shaded portion in Figure 1(b)) for coastal waters (Fujian, Guangdong and the TWS) and Bins 10-16 for offshore waters far away from landmass.

In situ data
We obtained climatological monthly mean temperature data of Guangdong province in winter (December to February.) for the period of 2007-2013, by averaging temperature of the same month at 86 national meteorological stations in the province. The monthly mean discharge and the surface water temperature data in February from 2006 to 2012 was obtained from Tianhe and Wanshan Stations (the red and blue dots in Figure 1(b)) near the PRE, and in January and February from 2006 to 2012 from Datong Station (blue triangle in Figure 1(a)) near the YRE, respectively.

Remote sensing data
We analysed daily level-3 SST data with 1.1 km resolution obtained from the AVHRR sensor on the National Oceanic and Atmospheric Administration (NOAA) satellites (12-18 series) and Aqua/ Terra Moderate Resolution Imaging Spectroradiometer (MODIS) from 2000 to 2013; and Chla data (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) and SSC data from Aqua/Terra MODIS before and after the snowstorm. All data-sets with a resolution of about 1.1 km were archived at and supplied by the State Key Laboratory of Satellite Ocean Environment Dynamics (Second Institute of Oceanography, State Oceanic Administration (SIO/SOA), Hangzhou, China). Surface wind data with a spatial resolution of 0.25 £ 0.25 and 24 h time interval was obtained from the National Climatic Data Center (NCDC) reanalysis archived by the NOAA(http://www.ncdc.noaa.gov /). In this study, 'before snowstorm' refers to the 17 days period (25th December 2007-10th,January 2008); 'during and after snowstorm' refers to the following 41 days (from 11th January to 20th February, 2008. Because both the duration of the sleet and snowmelt time were about 20 days, respectively, and most of the remote sensing data were lost due to the extended period of bad weather, therefore, 41 days was taken as the effect time by snowstorm).

Methods
The SSC and Chla were retrieved from the Aqua/MODIS and Terra/MODIS data using the SeaWiFS Data Analysis System (SeaDAS, He et al. 2014). The data fusion of remote sensing data for multi-day averages (MDA) was done using the algorithm by Fu et al. (2009), and the processing of Chla and SSC data was carried out through a log10 transformation. As the duration of freezing rain and snowstorm weather conditions was very long, much of the remote sensing data of SST, Chla and SSC were missing due to low/no visibility, so the MDA-Chla and SSC before the snowstorm in each bin were calculated as where covered by valid data of remote sensing as that of during and after snowstorm.

SST and air temperature drops during and after the snowstorm
The SST in the NSCS decreased significantly during and after the snowstorm in early 2008, especially in the coastal waters (Figure 2(a, b and f)). The MDA-SST in the study area was 21.73 C before the snowstorm, but only 17.73 C during and after the snowstorm, reflecting an average drop of 4 C, and a remarkable temperature front aligned in the southwest-northeast direction formed through TWS (Figure 2(b)). The SST in the coastal waters (Bins 1-9) decreased by 5.34 C, while that in offshore areas (Bins 10-16) dropped by only 2.26 C (Figure 2(f)). What is more remarkable, the average change of SST in Bins 1, 2, 3 and 4 reached 5.99 C, showing as a 'dark-blue water trough' in the TWS. In addition, Bins 6, 7 and 8 near the PRE had the second largest drop with an average temperature drop of 4.95 C. Due to the cloudy weather conditions, which resulted in partial data loss (the blank areas in Figure 2), and the interpolation by the software used for drawing in areas such as 10 and 11, the SST in this area shows as relatively low (dark blue). The bins with the smallest SST reduction were Bins 13 and 14 around the LZS, dropping by 1.17 C and 0.48 C, respectively.
Three MODIS images on 5th January, 21th January and 15th February with the lowest SST data loss are selected (Figure 2(c, d and e)), being the best images with the largest one-day areal coverage of the study area before, during and after the snowstorm. SST values in excess of 24 C in part of the NSCS on 5th January covered a larger area than that on 21th January and on 15th February, and the highest SST values before and after the snowstorm were observed in the LZS. Moreover, there were well-marked cold tongues (dark-blue colour) from the YRE along the Min-Zhe coast, stretching to the PRE of Guangdong province. Both during (Figure 2  The MDA-SST in the coastal waters of the study area from 11th January to 20th February 2008 was the lowest in the 14-year period from 2000 to 2013 ( Figure A1(a-n)). The TWS was a low-temperature zone, while the south end of the LZS was a high-temperature zone every year. The sizes of the low-temperature and high-temperature zones were clearly and visibly different from year to year. The low-temperature zone in 2008 was more prominent in size and degree than that in the other years, indicating a sharper SST decrease than in all the other years. In the coastal waters, the MDA SST over the 14 years period was 16.19 C, compared to only 14.19 C in 2008 with a relatively consistent trend in every Bins ( Figure 3). The average SST in the coastal waters in the six years of 2007-2013 (excluding 2008) was 16.72 C, while it was only 14.19 C in 2008, with a change of ¡2.53 C.
The average temperature in Guangdong province declined greatly from 1st January to 28th February 2008 compared to that in the recent seven years (Figure 4(a)). The average temperature in Guangdong province for the 60-day period from 1st January to 29th February in the seven-year period of 2007-2013 was 14.03 C, while it was only 11.55 C in these two months in 2008 (Table 1)  ). The data were from the Datong hydrometric station. Surface water temperature in the YRE in January and February 2008 was only 6.9 and 5.2 C, respectivelythe lowest in the corresponding period in the recent seven years, or -0.73 and ¡3.08 C, compared to the mean surface water temperature of the YRE (a drop of 37.20%). Although the monthly average discharge in January and February 2008 was not the largest among the seven years, the discharges were large enough, at 11,000 m 3 /s and 12,200 m 3 /s in January and February, respectively, to bring large amounts of colder fresh water with massive suspended sediment and nutrients into the ECS to help form MZCW.

Chla increase during and after the snowstorm in NSCS
The Chla concentration had a certain increase during and after the freezing rain and snowstorm ( Figure 6) in the NSCS. Considering the serious loss of data in Bins 10, 11, 12 and 15 during and after snowstorm, we did not list Bins in Figure 6(c). Except for the blank area, there were more than one hundred thousand valid data points in the study area during and after snowstorm, so the results of MDA-Chla can be well obtained with such a large amount of data. The MDA-Chla in 12 Bins before the snowstorm was 1.45 mg m ¡3 and increased to 1.76 mg m ¡3 after the snowstorm with 24.53% growth rate. The increased rates of Chla were 25.16% and 18.49% in the coastal waters and offshore waters, and a striking increase with 57.70% in Bin 4 and 48.65% in Bin 14, respectively.
The three MODIS Chla images taken on the same dates as the SST images, Figure 2(c), are shown in Figure 6(d-f), and were also the best three single track images in January and February. In the region southwest of the PRE (indicated by a circle), the Chla on 5th January was only 1.28 mg m ¡3 , while it amounted to 1.47 mg m ¡3 on 21st January (Figure 6(d,e)). In the TWS (marked by a purple prismatic shape), the Chla increased from 1.26 mg m ¡3 on 5th January to 2.08 mg m ¡3 on 15th February , an increase of 65.08% ( Figure 6(e,f)). In the LZS region (a blank rectangle), the average Chla concentrations were about 0.37, 0.40 and 0.39 mg m ¡3 on 5th January, 21th January and 15th February, respectively. From these three images of Chla of MODIS from before, during and after the snowstorm, it is apparent that the Chla concentration increased significantly during and after the snowstorm in the NSCS.

Temporal variation of Chla in the study area
The MDA-Chla concentrations from 11th January to 20th February during 2000-2009 in both coastal and offshore waters of the study area were calculated (Figure 7 and Figure A2). The Chla concentration in the coastal water was much higher than that in the offshore water from 2000 to 2009. The Chla concentration differed greatly in distribution and size from one year to the next, and the Chla concentration in the offshore and coastal waters were 0.67 and 2.18 mg m ¡3 in 2008, respectively, while the MDA-Chla concentration in the same period in the offshore and coastal waters over the 10 years were only 0.55 and 1.76 mg m ¡3 , respectively, with the Chla concentration increasing by 21.63% and 23.86% in the offshore and coastal waters in 2008, respectively (Figure 7 and Figure A2).

SSC increase during and after the snowstorm in NSCS
The SSC increased during and after the freezing rain and snowstorm (Figure 8). Although the loss of data in Bins 10-12 was serious during and after the snowstorm due to bad weather, there were plenty of pixels in the whole study area to allow reliable estimates of MDA-SSC. The MDA-SSC in the coastal waters were 48.43 and 83.57 mg/L, while in the offshore waters were 5.60 and 16.50 mg/L before and after the snowstorm, which the increase rates of SSC in the coastal waters and offshore waters were 72.56% and 194.63%, respectively. The striking increases rate with 483.38% in Bin 15  near the southwest of TWI, while other bins changed little on average and even decreased in some bins such as 10-12 during and after the snowstorm maybe caused by the much more data loss.

Diluted water from YRE, PRE and their diffusion in winter
The ECS is one of the most productive parts of the world's oceans. The ECS shelf has complex hydrology, as it is affected by the YR discharge, intrusion of the water with about 9.24£10 11 m 3 discharge every year. Usually, of the runoff from YRE, more than 10 000 m 3 /s flows to the ECS in winter. In 2008, the discharges from diluted water from YRE reached at 11 000 m 3 /s and 12 200 m 3 /s in January and February, respectively, which brought larger amounts of colder fresh water with massive suspended sediment and nutrients into the ECS (see Figures 5(a),and 9) and dropped the water temperature in YRE lowest with only 5.2 C in February from 2006 to 2012(as Figure 5(a)). This colder diluted water turned to the south along the bank of the Zhejiang and formed MZCW by the northeast monsoon as Figure 2(e), which was the same as Pan and Tang (Pan et al. 1997;Tang & Wang 2004). Simultaneously, the discharge from YRE extended offshore and intruded to the southwest (Chang et al. 2009(Chang et al. , 2013Liao et al. 2013;Zhu et al. 2013), mixing with the MZCW and flowing into NSCS as Figure 2(c-e). At the same time, the annual mean discharge of the PRE is about 3.36£10 11 m 3 and is the second largest discharge in China (Pang 2006 Figure 4(c). The water temperature in the PRE reached to 14.5 C during snowstorm, which was the lowest from 2006 to 2012 in February, as given in Figure 4(b). In winter, the diluted water from PRE mainly turns to the southwest by the northeast wind and partly diffuses to the east and the shelf area (Lu et al. 2013; Figure 9(a)), bringing lots of suspended sand and nutrients into the NSCS. Controlled by the colder fresh waters from YRE and PRE, the Kuroshio warm current intrusion into the SCS, and southwestward flow and monsoon, the NSCS has a very complex hydrology and dynamic environment (Pang 2006).

Large colder Min-Zhe coastal waters
In 2008, surface water temperature in the YRE was the lowest it had been in seven years ( Figure 5 (a)), and lower compared with the climatological mean surface water temperature of the YRE. Because the precipitation anomalies more than 140 mm were mainly concentrated in the south of the Chain along with the YR and PR, such as Anhui, Zhejiang, Fujian, Jiangxi, Hunan, Guangdong and Guangxi in Figure 5(c) during the snowstorm. The amount of the precipitation in Zhejiang and Fujian close to the research area had a sharp enhancement above their climatological averages over 1971-2000 with 149.7% and 165.5%, respectively (Shi et al. 2010). The discharge from YRE in February 2008 was up to 12,200 m 3 /s due to the abundant and persistent rain and heavy snowfall. This colder fresh water from precipitation and dissolved snow converged into the YRE and the coast of the Min-Zhe (Figure 10(a)), just as in our observations in Figure 2: low SST (dark-blue colour) appeared in the northwest of the TWS only before the snowstorm (Figure 2(a)), but after the snowstorm, lower SST occupied the area from YRE to the TWS and stretched southwestward to form a strong oceanic front and temperature gradient (Figure 2(b, d and e)).At the same time, stronger northeastern wind controlled the study area during snowstorm (Figure 9). Combining influence of the cooler discharged water from YRE and the strengthened northeastern winter monsoon in February 2008 promoted a large colder Min-Zhe coastal waters. It was mainly the large colder Min-Zhe coastal waters that drove the SST to drop so remarkably in the coastal water during and after the snowstorm, especially in the TWS. Shi et al. pointed out that in February of 2008, the overall precipitation in Guangdong and Guangxi provinces during the snowstorm increased with respect to the climatological averages over 1971-2000 by 258.6% and 269.4% respectively (Shi et al. 2010). This precipitation and the dissolved water from the ice and snow formed colder and larger fresh water runoff that pooled mainly into the PR and intensified the discharge of the PRE, as can be confirmed by noting that the largest discharge with the lowest temperature from PRE for the period 2006-2012 occurred in February 2008 after the snowstorm (Figure 4(b,c)). The larger and colder PRE diluted water flowed into NSCS, whose intrusion of larger and colder PRE diluted water extended mainly to the southwest and partly to the southeast (Hong et al. 2011a;Lu et al. 2013). This larger and colder PRE diluted water and its extension caused the SST to decline further in the study area, especially in the coastal waters near the PRE (Figure 4(b)).

Strong sea-land-air interaction after snowstorm
In January and February 2008, Guangdong coast was dominated by a low temperature anomaly (with respect to the climate norm), while the SST of the coastal waters was close to the average climate temperature (Figure 4(a)). A highly consistent relation can be shown between the average temperature for the 60-day period from 1st January to 29th February of Guangdong and the SST of the coastal waters in the study area (Table 1), with a correlation coefficient of 0.88; these temperatures had no relation to the SST in the offshore waters, however. The average temperature in Shenzhen City in January of a La Nina year (2007/2008 was a typical La Nina year) is 0.4 C lower than that in the corresponding period over the 30 years climatology   . We find both the average temperature from January to February in Guangdong province and the average SST after the snowstorm in the coastal waters in 2008 to be lower than the average temperatures in the corresponding period in the recent 30 years climatology ). At the same time, the average temperature anomaly during the snowstorm in the 13 provinces/cities of China (including Shanghai, Zhejiang, Hunan, Guangxi, Guangdong and so on) was ¡2.83 C compared with the same period from 1971 to 2000 (Shi et al. 2010, Table 2). Nicholas et al. (2015) pointed out that the land points that are closer to the ocean would be more strongly linked to the nearby SST variability. However, the differences in the land/sea contrast may also reflect differences in physical interactions between land and oceans. The cold ground, as well as the melting snow and ice in most parts of Southeast China, changed the evaporation and heat transfer of surface water, affected sea-land interaction (SLI) and kept low temperatures in Southeast China during the melting snow in February 2008. This indicates that the lower land temperatures in the early part of 2008 strengthened the SLI and the dropped SST of the NSCS.
The monthly average SST of coastal and offshore waters in the study area demonstrated evident interannual variability in the recent 11 years ( Figure A3(a,b)). The periodicity of the monthly average temperature difference between offshore and coastal waters is even more remarkable. The temperature difference is larger in winter and smaller in summer, and is largest in February and August. The peak of the temperature difference between offshore and coastal waters in February 2008 was up to 8.72 C ( Figure A3(b)), consistent with in situ measurements (Chang et al. 2009(Chang et al. , 2013. It indicates that the impact of land low temperature on the SST in the study area was stronger in winter, and even stronger in the coastal waters than in the offshore waters due to strong SLI. Analysis of hydrometeorological buoy data also shows that the lowest monthly average air temperatures and SST in the TWS for the years 2007-2012 occurred in February 2008 (Li 2013). Thus, the local sea-air interaction was an important factor of SST decrease in the TWS during the cold air outbreaks.
The snowstorm in Southern China might be caused by the 2007 La Nina event (Zheng et al. 2008), and our present study indicates that the strong sea-land-air interaction caused by the cold land and air during and after the snowstorm was one of the important factors that promoted the SST to drop in the north SCS, especially in the coastal waters (Figure 2(c)).

Wind changes modestly in 2008
From before to after snowstorm in 2008, as well as in the corresponding period in 2007, the study area and its adjacent area are controlled by the East Asian monsoons, with prevailing northeasterly wind in winter (shown in Figure 9(a-f)). From before to after snowstorm, direction of the wind changed only slightly, while the max wind speed increased from 12 to 17 m/s, which was larger than that in 2007, especially in the TWS. The SST in the study area was much higher than that in 2008 (see Figure A1(h,i)). The article (Price et al. 1986) mentioned that the process of diurnal cycle plays an important role in shaping the long-term response of the upper ocean to atmospheric forcing. The trapping depth of the thermal and velocity response is proportional to wind speed (t), and inversely proportional to the surface heat flux (Q 1/2 ). Meanwhile, changes in the mixed layer depth were directly related to atmospheric forcing, and then the upper ocean temperature gradient was forced to change (Cisewski et al. 2005). Then, the strong wind in winter can cause greater vertical mixing, lower SST and larger productivity in the ocean (Tang et al. 2004b;Hong et al. 2011b;Shang et al. 2012). With the lower temperature and stronger wind during snowstorm especially in the TWS (as in Figures 2, 5(c), 9(d-f) and A1), the vertical mixing become greater and trapping depth become deeper, which promoted larger productivity in the ocean.
According to the above observations and analysis, the mechanisms for the significant SST decline after the 2008 snowstorm in the study area, especially in the coastal waters, include mainly the following (Figure 10). On the one hand, the large colder Min-Zhe coastal waters and larger and colder PRE diluted water from the large infusion of cold fresh water after the snowstorm caused the remarkable SST drops in the coastal water, especially the sharp drops in the TWS. On the other hand, stronger monsoon and sea-land-air interaction in winter, especially during and after the snowstorm in the coastal waters, caused SST to decline further. Suspended sediment is an important parameter of water quality. Scattering and absorption of light by the suspended particles reduce the penetration into the water of photosynthetically available radiation (PAR) for benthic plants (Davies, R J et al. 1992). Fine sediment increased water turbidity, limiting light intensity. The chemisorption of the sand particle impacts the marine biogeochemistry process of the coastal waters, thus indirectly affecting primary productivity. There are about 4.175 £ 10 8 t/a sediment loads from YRE , whereby large amounts of terrestrial detritus, suspended sediments, nutrients and pollutants are carried to the continental ECS with intrusion and diffusion of MZCW (Milliman et al. 1985;Wang et al. 2003;Yuan et al. 2008;Bian 2012;Fu et al. 2014). Lee and Chough (1989) and Jian (2006) pointed out that more than half of the sediment discharge of YRE was carried to the Min-Zhe coastal waters. There was about 2.4 £ 10 8 t/a sediment deposition in the Min-Zhe coastal area . In winter, the diffusion and intrusion of the sediments from YRE are carried out by the monsoon, even to the NSCS through TWS and to open sea areas (Yuan et al. 2008;Bian 2012), which was the same as our results as in Figure 8(a,b). The SSC were much higher in Bin 1 located in the Min-Zhe coastal water than in other bins both before and after the snowstorm (Figure 8(c)). With the increase of the runoff from the precipitation and melted snow during and after the snowstorm (as in Figures 4-5), large amounts of sediments were carried into the study area from the YRE and PRE, significantly increasing the SSC during and after the snowstorm (as in Figure 10). In the offshore waters, there was a much higher increase rate of MDA-SSC than in the coastal waters during and after the snowstorm. Although the increase rate was up to 194% in SSC in the offshore waters, the absolute increment of SSC was only 11.73 mg/L, only about 30% with that in the coastal waters.
First, the discharge from YRE extended offshore and intruded to the southwest (Chang et al. 2009(Chang et al. , 2013Liao et al. 2013;Zhu et al. 2013), mixing with the MZCW and flowing into NSCS, as given in Figure 2(c-e). From before to after snowstorm, the larger runoff from YRE and PRE caused by the heavy rainfall and melting snow and ice (as in Figure 5(c)) and stronger northeast monsoon ( Figure 9) would lead to much diffusion of the cold water with high SSC in the coastal waters to the offshore waters, while much stronger diffusion and advection would carry large amount of sand from the coast to the offshore, which promoted the increase of SSC in the offshore waters.
Second, the increase of SSC in the offshore waters caused by snowstorm is mostly in Bins 13-15, especially in Bin 15, where following the snowstorm, the SSC increased to 5.83 times its pre-storm value (Figure 8(c)). The unique location of Bin 15 at the TW bank (southwest of TW) with only about 30 m depth and where the cold water from the west and warm water from the southeast converge (Hu & Muller-Karger 2007;Fang et al. 2010) should be accounted for the increase of the SSC. Two possible mechanisms may induce the high increased SSC in Bin 15.On the one hand, this bin is relatively near the mainland, from before to after snowstorm, the much greater monsoon mainly concentrated the TWS (as in Figure 9(d-f)) would enhance southward flow from MZCW carrying suspended sediment from mainland China (Bian 2012;Zhu et al. 2013;Wang et al. 2012). On the other hand, strong vertical mixing (SVM) and entrainment in this area caused by stronger northwest monsoon during snowstorm can bring bottom water to the surface, and thereby could cause a re-suspension of the sediment in this bin (Tian et al. 2015), so a significant growth of SSC in Bin 15 would be found in our study. However, in Bins 10-12, there was a little decrease in SSC, maybe due to error from the significant data loss during and after snowstorm. 4.3. The cause for Chla increase during and after the snowstorm in the study area, especially in coastal waters Nutrients abundance, especially N, P and Si, is one of the factors controlling the growth of phytoplankton in the upper layer of the ocean (Syrett 1953;Bongersch 1956;Foggge 1959;Lewinjc 1962;Brzezinskim 1990). Other factors include the light conditions (Gabric & Parslow 1989) and the water temperature (Eppley 1972;Raven & Geider 1988;Tang et al. 2006;Hao et al. 2007;Li 2013). These factors influence Chla content in water by changing phytoplankton growth, metabolism and other physiological activities.  pointed out that among the factors controlling phytoplankton growth, the most important is nutrients, second is the light and third is the temperature. In winter in the MZCW, nutrient concentrations usually are high, sometimes nitrate concentration can reach up to 35 mmol/L in the MZCW (Wang et al. 2012), and the stratification is weak due to the SVM by the northwest monsoon (Figure 9) during snowstorm, which can bring the nutrients to the upper layer from deep layers and even from the bottom (Tang et al. 1998;Li et al. 2000;Liao et al. 2013). During and after the snowstorm in 2008, we observed oceanic fronts with large temperature gradients in the TWS and LZS caused by the large colder Min-Zhe coastal waters, larger and colder PRE diluted water and sea-land-air interaction, respectively (Figures 2(b, d, and e) and 4(b,c)). With the strong extension of the fresh water, front and SVM, higher SSC in the study area during and after snowstorm indicated that abundant nutrients were carried into the upper layer of the ocean, especially in the coastal waters. These nutrients promoted the growth of phytoplankton and enhanced the concentration of the Chla (Figure 10). Light is an important factor for phytoplankton growth, but phytoplankton can also adapt to the intensity of the light by changing intracellular pigment or photosynthetic enzyme (Odum & Wilson 1962). Maucha pointed out that the light intensity is enough for the phytoplankton to assimilate on the surface of the water at any point on Earth (Maucha et al. 1948). Although SSC and turbidity increased, while the transparency decreased during and after the snowstorm, the light was not a limiting factor for phytoplankton growth in the study area.
In the offshore waters, comparing with before or 10-year average, Chla during and after the snowstorm increased a little less than that in coastal waters. The increase mainly happened in the 13-14 and 16 Bins (as Figure 6(b,c)), while other bins in the offshore waters changed uncertainly may be caused by the missing data. The growth mechanism maybe different with that in the coastal waters (for the serious data loss, the discussed did not include bins of 10-12 and 15). On the one hand, with the larger colder Min-Zhe coastal water and larger and colder PRE diluted water (as in Figure 2(b) and 6(b)), as well as higher levels of diffusion, the entrainment and advection by the stronger northwest monsoon (as in Figure 9) during snowstorm, the terrestrial debris (sediment) from the coastal waters would bring the higher SSC and nutrients to the offshore waters, which would been seen from higher increase rate in SSC more than 400% in Bins 13, 14 and 15 (as in Figure 8). The abundant nutrients carried from SSC would promote the growth of phytoplankton and enhance the concentration of the Chla further (as in Figure 10(b)). At the same time, the higher Chla concentration could spread and transport outward and southward with the diffusion of the MZCW and southwest flow of TWS, which could directly raise the concentration of Chla during and after snowstorm. On the other hand, the higher nutrients and Chla in the subsurface would be brought to the surface under the strong mixing caused by the stronger northwest monsoon during snowstorm (as in Figure 9), which was beneficial to the rising of Chla in the offshore waters .
Temperature higher than 15 C is suitable for the growth of many kinds of common phytoplankton, while the optimal temperature for growth is about 20-25 C (Behrenfeld et al. 1997;Tang et al. 2006;Chai et al. 2009). SST did not decrease much in Bins 13, 14 and 16 because they are located near the LZS, where were influenced by the Kuroshio warm water westward flow (red arrows in Figure 10) to this sea area (about 119 E, 20 N) in winter (Metzger & Hurlburt 1996;Liang et al. 2008, Hong et al. 2011aChang et al. 2009Chang et al. , 2013Liao et al. 2013). Mainly due to the contribution of the Kuroshio intrusion water (KIW) (as Figure 10), suitable temperature with average value of 22.42 C in the offshore waters after the snowstorm provided favourable environment for the growth of phytoplankton, which would be beneficial for the further elevation of Chla.

Summary
A severe weather event with wide-range sustained low-temperature, freezing rain and sleet took place in Southern China in 2008 had profound effects on the marine environment in the NSCS and TWS: (1) During and after the 2008 snowstorm, the SST in the NSCS and TWS sharply dropped, especially in the coastal waters, while the SSC and Chla concentrations increased remarkably, compared to their values before the snowstorm and to climatology, especially in the coastal waters and TWS.
(2) The great drop in SST in the coastal waters was likely influenced by the large infusion of colder Min-Zhe coastal waters and diluted water from the PRE, as well as by the strong sealand-air interaction during and after the snowstorm. (3) The large increment in SSC in NSCS mainly came from transport and entrainment from large colder Min-Zhe coastal waters, larger and colder PRE diluted water and their diffusion to the southeast by the monsoon, while the increase of Chla concentration is related to abundant nutrients supply from YRE and PRE, mixing and front in the coastal waters, while in the offshore waters, a strong transport and extension of MZCW with high SSC, nutrient and Chla, as well as suitable temperature, which promoted the increase of Chla.