**2. Long-term variability of UEH**

## **2.1 Climatology**

As shown in **Figure 1a**–**c**, the SST reaches above 28°C in the most northern SCS. One can see that the low-temperature area along the eastern coast of Hainan Island formed by upwelling is quite evident with SST decreasing from 27.5°C at 40 km offshore to 26°C along the coast. In July and August, the SST reaches above 29°C in the deep water but is still lower than 27.5°C in the coastal upwelling zone. Influenced by East Asia monsoon, the southwesterly winds prevail in the northern SCS in summer as shown in **Figure 1d**–**f**. One can see that the wind stress in the offshore water east of Hainan Island is lower than that in the deep water. The winds blow northeastward over the offshore water. The non-zero wind stress curl is produced by nonuniform spatial distributions of wind speed and wind direction. From the wind stress curl shown in **Figure 1g**–**i**, one can see that in summer, the wind stress curl east of 112°E in the northern SCS is generally negative, while that over the coast upwelling zone between 110° and 111°E is positive in June–August, and the value and extent are larger in July. Driven by the positive wind stress curl, the Ekman pumping can cause upwelling of lower layer cold water, leading to the UEH with low SST.

## **2.2 Variability of upwelling intensity**

We use the SST difference between the upwelling zone (Zone A in **Figure 1**) and that in the deep water without upwelling (Zone B in **Figure 1**) as the upwelling *Response of Coastal Upwelling East of Hainan Island in the South China Sea to Sudden Impact… DOI: http://dx.doi.org/10.5772/intechopen.88828*

#### **Figure 2.**

*Variation of the averaged SST in summer in the upwelling zone (zone A in Figure 1) (a) and the background zone in the deep water (zone B in Figure 1) (b) and the upwelling index (UI) of UEH (c) from 1982 to 2012. The straight lines represent the linear trends of the variations (cited from [26]).*

index (UI) to denote the upwelling intensity (refer to [26] for details). **Figure 2** shows the variation of SST in zones A and B and the UI of the UEH from 1982 to 2012. One can see that both the SST in the upwelling zone A, i.e., *SSTnear*, and that in the deep water without upwelling, i.e., *SSTfar*, have increasing trends in recent 30 years. The increase rate is 0.03°C/a for zone A and 0.02°C/a for zone B, respectively. The warming in the upwelling zone is faster than that in the open-sea waters. Thus, the UI of the UEH has a decreasing trend with a rate of −0.01°C/a. The UEH weakens as the global climate changes. This is opposite to the intensification in the coastal upwelling zones on the east coast of the ocean basin [22, 24–25, 41].

In order to investigate the interannual variability of the UEH, we decompose the UI time series using the empirical mode decomposition (EMD) method [42]. The first three intrinsic mode functions (IMFs 1–3) are shown in **Figure 3**. One can see that the periods of the IMFs 1–3 are about 3 a, 5 a, and 10 a, respectively. The corresponding variance contributions are 82, 10, and 1%. The IMF1 has the largest amplitude and dominates the interannual variability of the UEH. On the other hand, the 3-year variation is highly related to the ENSO events [33].

#### **2.3 Variability of upwelling center**

Taking the latitude of the maximum UI as the upwelling center, one can investigate the variation of the position of the UEH. As shown in **Figure 4a**, one can see

**Figure 3.**

*First three intrinsic mode functions (IMFs 1–3) of the UI series of the UEH (a) IMF1, the first and largest mode, (b) IMF2, the second mode and (c) IMF3, the third mode (cited from [26]).*

#### **Figure 4.**

*The interannual variation (a) and the statistics (b) of the upwelling center of the UEH in 1982–2012 and three IMFs of the variation (c–e) (cited from [26]).*

that the center of UEH oscillated north-southward along the Hainan coast between 18.9° and 19.3°N in the past 30 years. The center positions of the largest probability are in 19.2°–19.3°N and 18.9°–19°N. For the long-term variation trend, one can see that the center position shifted northward from 1980 to 2012. The IMFs of the center position series also show interannual variability with three intrinsic periods of 3 a, 5 a, and 10 a, of which IMF 1 has the largest amplitude and dominates the variance.

#### **2.4 Effects of wind stress and wind stress curl**

The UEH is generally driven by the alongshore wind stress. However, for the long-term variation, the alongshore wind stress has an increasing trend with a rate of 1.1 × 10<sup>−</sup><sup>5</sup> N/m2 /a as shown in **Figure 5a**. This is opposite to the decreasing trend of the UEH shown in **Figure 2c**. The correlation coefficient of the interannual variability of the UI with that of the alongshore wind stress is only 0.43 (P < 0.02). The alongshore wind stress seems not to be the dominant factor for the long-term variation of the UEH. **Figure 5b** shows the variation of the wind stress curl over the upwelling zone. One can see that the wind stress curl has a decreasing trend with a rate of −2.7 × 10<sup>−</sup>11 N/m3 /a. The correlation coefficient of the wind stress curl and the UI reaches 0.66 (P < 0.001). For the first two IFMs, the correlation coefficients reach 0.75 for IMF1 and 0.54 for IMF2 for the wind stress curl but are 0.52 for IMF1 and 0.46 for IMF2 for the alongshore wind stress. This result suggests that the wind *Response of Coastal Upwelling East of Hainan Island in the South China Sea to Sudden Impact… DOI: http://dx.doi.org/10.5772/intechopen.88828*

#### **Figure 5.**

*Variations of the alongshore wind stress τalongshore (a) and the curl of wind stress curlzτ (b) over the UEH zone in 1982–2012. The straight lines represent the linear trends (cited from [26]).*

stress curl-induced Ekman pumping plays a more important role in the long-term variability of the UEH intensity.

By comparing the latitudes of maximum alongshore wind stress (around 19.1°N) and wind stress curl (around 19.25°N), one can see that the latitude of the maximum wind stress curl is more consistent with latitudes of the maximum UI at 19.2°–19.3°N. This shows the importance of the wind stress curl on the long-term variation of the UEH.
