**3. Response of UEH to super El Niño events**

#### **3.1 Background**

As mentioned above and in previous studies, the UEH has interannual variability associated with the ENSO events [37]. Comparing with the UEH, the upwelling east of Vietnam coast (UEV) in the western SCS has different response to the ENSO events [8, 43]. Jing et al. [33] analyzed the response of UEH and UEV to a super El Niño event of 1997–1998 and found that in summer 1998, southerly winds were intensified in the northern SCS, so that UEH was strengthened, while easterly winds were abnormally intensified along the Vietnam coast over the southern SCS, so that UEV is weakened. Most of these previous investigations explained the mechanisms of response of coastal upwelling to El Niño events based on wind forcing, and less attentions are paid to the background ocean dynamic processes.

Recently, another super El Niño event occurred in 2015–2016, which lasted for longer than 12 months with sea surface temperature anomaly (SSTA) peaks higher than 2°C for longer than a half year as shown in **Figure 6**. Shen et al. [39] use SST, sea surface height (SSH), wind fields, and heat flux data from 1995 to 2016 to compare different responses of UEH and UEV to the two super El Niño events of 1997–1998 and 2015–2016 and found that ocean mesoscale eddies significantly affected the response of coastal upwelling to El Niño events.

**Figure 6.**

*Niño 3.4 index from 1995 to 2016. Blue, orange, and red color bars represent SSTA lower than 0.4°C, but higher than 0.4 and 2°C, respectively (cited from [39]).*

#### **3.2 SSTA**

The SSTA with respect to climatologic SST is generally used as an indicator to represent response of upwelling to the El Niño events [8, 33, 43, 44].

**Figure 7a**–**b** shows the averaged SSTA fields of UEH and UEV in local summer (June–July–August) of 1998. One can see that the extent of cold water in UEH expanded with totally averaged SST 0.04°C lower than that of climatology, implying that UEH was slightly intensified. On the other hand, the extent of cold water in UEV was remarkably shrunk with greatly increased SSTA. The totally averaged SSTA was 1.15°C. In particular, at the cold center of climatology near 11° 30'N, SSTA was warmed up as high as 1.8°C. This implies that the UEV greatly weakened after the super El Niño event of 1997–1998 ended in spring 1998.

Super El Niño event of 2015–2016 ended in spring 2016 has the longest lifetime and the highest SST anomaly in the equatorial Pacific since 1900. In summer 2016, SSTAs in UEH and UEV are entirely different. As shown in **Figure 7c**–**d**, all SSTs in UEH show warm anomaly with the SSTA amplitudes of 0.4–1.0°C, and the warmest anomaly appeared at about 100 km offshore. Comparing SST distribution (not shown), the cold water area of UEH greatly decreased, with totally averaged SSTA of 0.67°C, implying that UEH weakened. Meanwhile, SST in the UEV decreased about 0.2°C in the area 50 km offshore from 11° 30'N to 12° 30'N, while SST slowly increased from the cold center to the deep water. The totally averaged SSTA was 0.17°C with a maximum warm anomaly of 0.4°C, implying that UEV was also slightly weakened.

#### **3.3 Wind field anomaly**

The variability of local wind field is an important factor that must be considered in response of coastal upwelling to super El Niño events. **Figure 8** shows the wind vector anomaly (arrows) and the wind stress curl anomaly (color) of the UEH and the UEV in summer 1998 and 2016. Forced by abnormal southerly winds and positive wind stress curl anomaly, both favorable conditions for upwelling, UEH was intensified with expanded sea surface cold water area and lower temperature in summer 1998 (**Figure 8a**). This is consistent with SSTA patterns shown in **Figure 7a**. Meanwhile, for the UEV, southerly wind anomaly was remarkable, and wind stress curl decreased with the negative anomaly of −2 × 10<sup>−</sup><sup>7</sup> N m<sup>−</sup><sup>3</sup> . The two factors restrained the upwelling development, so that SST in UEV increased in summer 1998 (**Figure 8b**). This is consistent with SSTA patterns shown in **Figure 7b**.

In summer 2016, southerly wind anomaly was still dominant over the UEH; the wind stress curl anomaly was positive and negative over the northern and southern *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 7.** *SSTA in UEH (a, c) and UEV (b, d) in summers 1998 and 2016. Color codes are in °C (cited from [39]).*

UEH, respectively (**Figure 8c**). The wind speeds were higher than the climatologic means. Theoretically, the wind fields were favorable to the upwelling development. However, the comparison with the SSTA patterns shown in **Figure 7c** indicates that the temperature in the UEH cold water area greatly increased and the region of cold water became narrower. Based on this fact, Shen et al. [39] speculate that there are other factors to weaken the UEH. In the UEV, the southerly wind anomaly was dominant, and the wind stress curl anomaly was positive in most cases with the mean positive anomaly as high as 1.5 × 10<sup>−</sup><sup>7</sup> N m<sup>−</sup><sup>3</sup> (**Figure 8d**). The wind speeds were greater than the climatologic values. Such wind field anomaly was greatly favorable to the upwelling development. However, results in the above section indicate that in summer 2016, the mean SST in the UEV was close to the climatologic mean, cold water area was not obviously expanded, and lower SST appeared in partial area only. This implies that there are some factors to counteract forcing of wind field, resulting in unchanged upwelling intensity in the UEV.

**Figure 8.**

*Composite maps of wind vector anomaly (arrows) and wind stress curl anomaly (color codes in N m<sup>−</sup><sup>3</sup> ) of UEH (a, c) and UEV (b, d) in summer 1998 and 2016 (cited from [39]).*

#### **3.4 Surface heat flux**

The sea surface heat flux is an important factor to affect the SSTA. **Figure 9a** shows summer mean net heat flux of the western SCS in 1998 and 2016. One can see that the horizontal distributions are characterized by dipole patterns, i.e., a low-value center, located at the deep basin coexisting with a high-value center at the northwestern shelf. The extent of high net heat flux (NHF) (>7.5 × 106 J m<sup>−</sup><sup>2</sup> ) in summer 2016 (**Figure 9b**) was much larger than that in summer 1998 (**Figure 9a**), suggesting that the heat absorbed by UEH and UEV was higher than that in 1998 summer.

**Figure 9c** shows the difference of spatial mean NHF in summer 2016 from that in summer 1998. For both of the UEH and the UEV, all the heat flux differences are positive, i.e., the heat gain due to shortwave solar radiation in summer 2016 was greater than that in summer 1998, and the heat losses due to the long-wave radiation, latent heat flux, and sensible heat flux were smaller than that in summer 1998. This result reveals that NHF absorbed by the sea water of two upwelling zones in summer 2016 was near twice of that in summer 1998. The increase amplitude of the NHF in UEV was twice of that in UEH. However, comparing that in summer 1998, the mean SST in UEH in summer 2016 increased near 0.71°C, while that in UEV decreased by 0.98°C as shown in **Figure 7**, namely, in the UEV, the NHF increased, but SST decreased. Therefore, the NHF is not a dominant factor to

*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 9.**

*Downward mean net heat flux (NHF) in the western SCS in summer 1998 (a) and 2016 (b) and difference of mean heat fluxes of summer 2016 from 1998 in UEH and UEV (c). Color codes in a–b represent NHF in J m<sup>−</sup><sup>2</sup> (cited from [39]).*

affect SSTA in the upwelling cold water area during the El Niño events, but there must be other dynamic factors to counteract the SST increase in the upwelling cold water areas.

#### **3.5 Effects of ocean dynamic processes on upwelling cold water**

As a component of the ocean circulation, coastal upwelling is affected by the background current. The surface cold water in the upwelling zone is formed by transport of cold water in lower layers to upper layers carried by the vertical motion of sea water. Meanwhile, if horizontal currents transport water masses with different properties from other areas to the upwelling zone, or alter the current fields to affect divergence of upper layer sea water, it is possible to change the intensity and location of upwelling.

Mesoscale eddies are active in the SCS [45, 46]. The horizontal and vertical motions of eddies would modulate background current and mass transport, so

**Figure 10.**

*SSHA distribution in the western SCS in summer 1998 (a) and 2016 (b). Color codes are in m (cited from [39]).*

that they may be the important processes to affect UEH and UEV. Here, the SSH anomaly (SSHA) is used as an index to analyze effects of mesoscale eddies on SSTA in the upwelling zones. **Figure 10** shows SSHA distribution in the western SCS in summer 1998 and 2016. In summer 1998, SSHA in the northern SCS was negative (**Figure 10a**), favorable for UEH development. Meanwhile, there was an ellipseshaped warm eddy in SW-NE direction off the South Vietnam coast. The UEV was located within the extent of warm eddy, so that the UEV was restrained. On the other hand, in summer 2016, the UEH was located in the positive SSHA area, and there were multiple weak warm eddies east of Hainan Island (**Figure 10b**). The SSHA was also positive in the offshore water of Vietnam. Near the Indo-China Peninsula, there were a train of stronger warm eddies centered at 10°, 12°, and 15°N, respectively. This might explain that although the wind field anomaly along the Vietnam coast was quite favorable for upwelling development, the extent of cold water did not expand, and SST also did not decrease with respect to the climatology. It is the warm eddies that may counteract the wind forcing effect.
