Chapter 2. Improvement of SMAP Sea Surface Salinity in river-dominated oceans using machine
2.4 Results and Discussions
2.4.4. Spatial and temporal distribution of SSS
Figure 2.12 shows the 2018 monthly maps of the SSS improved by RF in the EA, BoB, GoM, ARP, and GoG regions. The seasonality of each region produced varied SSS seasonal changes. In the EA region, the discharge of freshwater from the Yangtze River flows into the East China Sea to form low salinity seawater and travels to the Korean Peninsula by winds and currents (Kim et al., 2015; Kim et al., 2020). Heavy precipitation is concentrated in the summer due to the monsoon climate, when freshwater discharge increases rapidly and spreads eastward (Zhang et al., 2007; from June to September in Figure 2.12(a)). This high SST and low SSS seawater drifts off the sea surface without being well mixed (Chen et al., 2016), affecting the marine environment and the strength of typhoons (Kim et al., 2020). From January to April, SMAP data were not available in the Okhotsk Sea (around 48°N and 145°E; Figure 2.12(a)) due to sea ice from the Arctic region. The Arctic sea ice extent is generally the greatest in March and the least in September by seasonal cycle (Onarheim et al., 2018).
Thus, SSS in the Okhotsk Sea and Northwest Pacific tends to be lower in summer than other seasons by melting of large volumes of sea ice from the Arctic region through the Liman Current (Seki et al., 2005; from July to September in Figure 2.12(a)). If the correction model was developed only in the EA region, low salinity seawater would not have been well trained due to the absence of low salinity samples (Figure 2.13).
Low salinity seawater is always present in the northeastern BoB due to the discharge of the Ganges and Irrawaddy rivers (Figure 2.12(b)). As the monsoon begins in June, precipitation and discharge increase, leading to an increase of low salinity seawater in the Northern BoB (Sengupta et al., 2016).
From August to October, this low salinity seawater flows into the high salinity seawater along the East Indian Coast under the influence of the Southwest Monsoon Current and wind and the southward East Indian Coast Current (Subrahmanyam et al., 2018; Akhil et al., 2020). The low salinity seawater flowing southward forms “river in the sea” along the East Indian Coast (Chaitanya et al., 2014), which is clearly seen from August to October in Figure 2.12(b). The Indian Ocean Dipole (IOD), also known as the Indian Niño, affects the southward expansion of low salinity seawater in the BoB region. The low salinity water in this region, similar to the EA region, affects the marine biological productivity and tropical cyclones intensity (Akhil et al., 2020). Low salinity seawater is mixed with high salinity seawater during winter and returns to the state of pre-monsoon from spring (Akhil et al., 2020).
The northern GoM region receives a freshwater discharge from many rivers, of which the Mississippi River system drains 41% of the United States (Figure 2.12(c)); Milliman and Meade, 1983). Low salinity water formed by the inflow of freshwater from many rivers moves eastward by southerly/southeasterly winds in spring and summer when the discharge is high (Fournier et al., 2016).
Our result clearly showed that low salinity seawater was formed from April and gradually spreads, and
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then spread to the southernmost part in July and August (Figure 2.12(c)).
Freshwater flows into the northwestern tropical Atlantic Ocean from the Amazon and Orinoco rivers, which are the largest rivers on the Earth, account for 20% of the global freshwater runoff (Figure 2.12(d);
Fournier et al., 2015). The discharge is maximum in May and minimum in November (Dai and Trenberth, 2002). The SSS seasonal changes are evident in this region under the influence of the tropical Atlantic surface circulation and the Atmospheric ITCZ (Gouveia et al., 2019). The low salinity seawater formed by freshwater runoff moves to the Caribbean Sea northwestward along the Brazilian coast through the North Brazil Current and the Guyana Current in the dry season (winter and spring) (Fournier et al., 2015; Gomes et al., 2018; Gouveia et al., 2019). In the rainy season (summer and fall), the ARP seawater heads up to the east through the strong North Equatorial Counter Current. Our result for the ARP region agreed that the low salinity seawater gradually spreads to the north from March and then moves strongly to the east from June to November. SSS in the ARP region expands most between May and August as shown in Figure 2.12(d).
A large amount of freshwater from the Congo and Niger rivers and heavy precipitation along the coast of Cameroon flow into the GoG to form low salinity seawater (Figure 2.12(e); Kim et al., 2015).
The SSS in this region are distinctly seasonal with the highest in August and September and lowest in January and February, which is clearly seen in Figure 2.12(e) (Dossa et al., 2019). As ITCZ moves southward along the warm SST of the GoG in April, precipitation increases and as it moves northward in dry season (summer), precipitation becomes the lowest and river discharge is minimum (Berger et al., 2014). As the ITCZ moves southward again in the fall, precipitation increases again. Freshwater inflows continue in the rainy season (fall and winter) as precipitation over the continent takes up to 5 months to flow into the GoG through rivers. Figure 2.12(e) shows that the low salinity seawater gradually expands from October and exists until May.
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Figure 2.12. Monthly maps of the newly corrected SSS by the RF model for 2018 in (a) the East Asian Sea region, (b) the Bay of Bengal, (c) the Gulf of Mexico, (d) the Amazon River Plum region, and (e) the Gulf of Guinea. The location of the main river systems of each region is marked with a pink circle.
34 Figure 2.12. Continuous.
35 Figure 2.12. Continuous.
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Figure 2.13. July maps of (a) the SSS improved by the proposed model in this study and (b) the SSS estimated using only EA samples for 2018. Low salinity seawater in the East China Sea were not well trained when using only EA samples which have no low salinity samples. Note that low salinity seawater was observed in the East China Sea in the summer 2018.