The Influence Of Kuroshio Intrusion On Primary Productivity In The South China Sea

The South China Sea（SCS）, most of which is located in the tropics, is a marginal sea covering an area of some 3.5 million km². The SCS connects the north Pacific and Indian Oceans through a series of straits, one of which is the Luzon Strait. With about 380 km in width, 1400 m in average depth and 2500 m in the maximum depth, the Luzon Strait is the only direct channel of intrusion of the Pacific waters into the SCS. As a strong west boundary current in the North Pacific Ocean, the Kuroshio deforms and exchanges the waters with the SCS when it comes across the Luzon Strait. In the recent years, the studies of the Kuroshio intrusion focus on the topics of physical oceanography, while the influence of Kuroshio intrusion on primary productivity in the SCS is rarely reported. The Kuroshio intrusion mainly takes place in the wintertime. The Kuroshio intrusion through the Luzon Strait induces upwelling and meso- and micro-scale eddies due to the topographical effect of shorelines and sea ridges. In addition, the winter monsoon that blows paralleling to the shoreline can lead to the effect of Ekman pumping. These factors combined strengthen the vertical mixing of the water column in and beyond the Luzon Strait and enhance the nutrient supplies to the upper ocean. As a result, the Kuroshio intrusion remarkably increases the productivity of the oligotrophic northeastern SCS. The investigation of the influence of Kuroshio intrusion on primary productivity would provide an important basis for analyzing fish productivity and forecasting fishing grounds and fishing seasons in the open SCS.Using remote sensing observations of primary productivity and sea surface current from 1997 to 2014, this paper analyses the influence of Kuroshio intrusion on primary productivity in the northeastern SCS and the adjacent waters and reveals the temporal and spatial patterns of primary productivity under the Kuroshio intrusion. Based on lag correlations of primary productivity variations between the eastern Luzon Strait, northeastern SCS and the waters east off Taiwan, and the spatial and temporal variations of primary productivity and sea surface current, this analysis determines the major pathways of the Kuroshio intrusion for every winter. The one-way ANOVA is also used to compare the effects of three difference types of Kuroshio intrusions on primary productivity in the northeastern SCS. The findings can be summarized as follows.1. The Kuroshio intrusion remarkably promotes net primary productivity（NPP） in the northern SCS, and the area influenced can cover as large as one half of the open northern SCS. During May to September, high NPP only occurs in the coastal and shelf waters, while NPP is very low in the open waters of >200 m depth. Starting from November, when the winter Kuroshio intrusion takes place, the NPP north off Luzon Island apparently increases and the high NPP area extends westwards over 118°E. The highest values of NPP and the largest area of high NPP are observed in January and February.2. Winter intrusion of the Kuroshio not only influences NPP in the northeastern SCS, but also increases NPP in the waters east off Taiwan Island; the Kuroshio intrusion influences the northeastern SCS first, and then has its effect on the waters east off Taiwan Island. Based on 17-year averages, there are lag relationships in winter NPP variations between the eastern Luzon Strait, northeastern SCS, and the waters east off Taiwan Island. High NPP in the eastern Luzon Strait appears from September 29 to March 13 of the following year, high NPP in the northeastern SCS appears later and lasts from October 7 to March 21, and high NPP in the waters east off Taiwan Island further delays and occurs only from October 23 to March13.3. Over the last 18 years since 1997, the NPP in the northeastern SCS and Luzon Strait exhibits the highest in winter but the lowest in summer and none of the years is abnormal. After October, as the northeasterly monsoon prevails and Kuroshio intrusion enhances, the NPP in the northeastern SCS apparently increases, but the NPP east off Taiwan Island and in the eastern Luzon Strait increases more gently. The NPP in the northeastern SCS and east off Taiwan Island peaks in January and start to decrease in late February, but the NPP in the eastern Luzon Strait reaches its maximum in the middle and late December and lasts through the mid-February. The NPP in the northeastern SCS in 1997 and 1998 was lower than in any other years because of the influence of a strong El Ni?o event, and except these two years, the winter and yearly NPP in the northeastern SCS have shown a significant declining trend since 1999.4. The looping path, leaking path and leaping path of Kuroshio intrusions all ever occurred in the 17 winters since 1997. Kuroshio intruded into the SCS mainly in a looping path in 1999, 2000, 2004 and 2011. During the four winters, the NPP east off Taiwan Island variated with lags of 8 to 16 days behind that of the northeastern SCS, the NPP east off Taiwan Island increased apparently, and NPP in the northeastern SCS and Luzon Strait was mostly >350 mg C·m^-2·d^-1, with areas of NPP>570 mg C·m^-2·d^-1 extending from northern Luzon Island to the northeastern SCS. Kuroshio intruded mainly in leaking path in the winters of 1997, 1998, 2001, 2003, 2005, 2006, 2007, 2009, 2012 and 2013. During these ten winters, the variations of NPP in the northeastern SCS and in the waters east off Taiwan Island were basically synchronous, areas of NPP values >570 mg C·m^-2·d^-1 mainly occurred in the open SCS to the west of 120°E and to the south of 22°N, but the increment of NPP east off Taiwan Island was smaller than those in eastern Luzon Strait and the northeastern SCS. Kuroshio leaped the Luzon Strait without intruding into the SCS in 2002, 2008 and 2010. During the three winters, NPP east off Taiwan Island variated behind the eastern Luzon Strait with lags of 8 to 16 days, high NPP mostly occurred north off Luzon Island and with a smaller northwesterly extension, and the areas of NPP >570 mg C·m^-2·d^-1 were much smaller than those winters when the Kuroshio intruded in a looping path. Summing up the above, from 1997 to 2014, winter intrusion of the Kuroshio mainly took a leaking path, i.e., the pathway of a SCS branch. However, the result of statistical tests shows that the NPP increments in the northeastern SCS do not vary significantly when the Kuroshio intrudes in different pathways in the winter.