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Satar, M.N.; Akhir, M.F.; Zainol, Z.; Chung, J.X. Upwelling in Marginal Seas. Encyclopedia. Available online: https://encyclopedia.pub/entry/47600 (accessed on 02 June 2024).
Satar MN, Akhir MF, Zainol Z, Chung JX. Upwelling in Marginal Seas. Encyclopedia. Available at: https://encyclopedia.pub/entry/47600. Accessed June 02, 2024.
Satar, Muhammad Naim, Mohd Fadzil Akhir, Zuraini Zainol, Jing Xiang Chung. "Upwelling in Marginal Seas" Encyclopedia, https://encyclopedia.pub/entry/47600 (accessed June 02, 2024).
Satar, M.N., Akhir, M.F., Zainol, Z., & Chung, J.X. (2023, August 03). Upwelling in Marginal Seas. In Encyclopedia. https://encyclopedia.pub/entry/47600
Satar, Muhammad Naim, et al. "Upwelling in Marginal Seas." Encyclopedia. Web. 03 August, 2023.
Upwelling in Marginal Seas
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This entry is adapted from the peer-reviewed paper 10.3390/cli11070151

Upwelling refers to the upward movement of deep nutrient-rich and low-temperature waters to the surface, resulting in colder surface or near-surface waters with low dissolved oxygen, high density, and high salinity. Upwelling is economically and ecologically significant in the coastal marine system, making it high-priority research. Although representing <1% of the total surface area of the ocean, upwelling regions provide approximately 8% of the global marine primary production and more than 20% of the world’s capture fisheries. With an increase in offshore transport, strong upwelling usually transports phytoplankton and zooplankton towards the convergence offshore frontal system rapidly, relative to a weaker upwelling that limits the nutrient enrichment in the photic zone. Apart from boosting primary productivity and fishery production, upwelling is also crucial for the atmosphere-ocean carbon dioxide exchange and carbon recycling processes.

upwelling climate change marginal seas

1. Characteristics of Wind-Driven Upwelling

Wind-driven upwelling is mostly driven by the local wind stress, which generates currents in the frictional Ekman layer [1]. An alongshore wind interacts with Earth’s rotation, causing Ekman transport and Ekman pumping to take place (Figure 1). The Ekman pumping creates surface divergence in the coastal current causing an upward movement of water, which generates an upwelling [2]. In general, the time taken for the water to be uplifted varies from several days to several weeks over a distance of nearly 100 m or more [3].
Figure 1. Schematic diagram on coastal upwelling mechanisms.
In EBUS, which is located in the tropics and sub-tropics, the upwelling is generated by equatorward alongshore trade winds (Figure 2). The trade winds blow equatorward, parallel to the eastern borders of the ocean basins, and induce an Ekman transport from the coast to the open ocean, perpendicular to the wind stress forcing. This creates a transport divergence and thereby leads to an upwelling at the coast [4]. Meanwhile, in the marginal sea, the upwelling is usually seasonal and depends on the monsoonal winds that blow parallel to the coastline (Table 1). For example, in the southwest of Luzon Strait and northwest Sabah, strong northeast monsoonal wind (December to February) generates and strengthens the Ekman transport, which in turn induces the upwelling in the subsurface layer [5][6][7][8][9][10]. Apart from the Ekman transport, the presence of positive wind stress curl also generated Ekman pumping, which contributes to the upwelling formation. Meanwhile, on the east coast of Peninsular Malaysia, the southwesterly wind (June to August) is the responsible factor that creates positive Ekman transport and also Ekman pumping [11][12][13]. A similar mechanism was observed in southern Vietnam, however, with a stronger upwelling intensity [14][15].
Figure 2. All the upwelling locations analyzed in this paper. The red line indicates the upwelling in EBUS areas, and the green line indicates all the other upwelling in marginal seas/small scale upwelling areas.
Specifically, in the South China Sea, earlier findings of upwelling are only based on direct measurements or in-situ data, as in the study by Wrytki [16]. But later, with the rapid development of science and technology, especially in remote sensing and hydrodynamic models, researchers are able to access the image data easily, which allows new qualitative insights and understanding of this phenomenon [17][18].
Table 1. Several locations of upwelling based on seasonality.
Marginal Sea Location of Upwelling Time Occurring Author
South China Sea Taiwan Strait Southwest Monsoon
(June to August)		 [19][20]
Southern Vietnam Southwest Monsoon
(June to August)		 [14][15]
Hainan Island Southwest Monsoon
(June to August)		 [21][22]
East Coast of Peninsular Malaysia Southwest Monsoon
(June to August)		 [11][12][13]
Northwest Luzon Northeast Monsoon
(December to March)		 [5][6][7]
Northwest Sabah Northeast Monsoon
(December to March)		 [8][9][10]
Andaman Sea Andaman Sea Northeast Monsoon
(December to March)		 [23]
Arabian Sea Somali and Oman Southwest Monsoon (June to September) [24][25][26][27]
Southwest India Southwest Monsoon (May to September) [28][29]
Indian Ocean Southern Java Southeast Monsoon (June to October) [30][31][32]
Baltic Sea Gulf of Finland Summer
(June to September)		 [17][33]

2. Upwelling and Climate/Atmosphere Variability

As upwelling is largely influenced by winds, the amplitude and timing of upwelling-favorable winds are sensitive to climate variability [34]. Large-scale climate phenomena such as El Niño Southern Oscillation (ENSO) will affect the upwelling event depending on the upwelling location. ENSO is a periodic fluctuation in sea surface temperature (SST; El Niño) and the air pressure of the overlying atmosphere (Southern Oscillation) across the equatorial Pacific Ocean. The warm ENSO (El Niño) occurs when the surface in the central and eastern tropical Pacific Ocean is warming or above the average SST, and the convection shifts to the central or eastern Pacific [35]. In contrast, the cool ENSO is called La Niña, which is the opposite phase of El Niño.
Upwelling can be influenced by ENSO as this event impacts the wind intensity. For example, the upwelling in Peru usually weakens during El Niño and strengthens during La Nina. This might be due to the occurrence of the El Niño event weakening the trade winds, thus reducing equatorial upwelling and causing an anomalous increase in the coastal SST and an anomalous deepening of the thermocline off the Peruvian coast as well as a decrease in nutrients [36]. This weakened upwelling event was also documented in California as its equatorward wind decreased due to the expansion of the Aleutian low-/contraction of the North Pacific high-pressure systems [37]. Nevertheless, some areas experienced an intensified upwelling, such as in the north SCS (NSCS) and northwest Sabah [10][38]. This occurred due to the presence of an anticyclonic atmospheric circulation anomaly, which intensifies the monsoonal wind during El Niño.
The Indian Ocean Dipole (IOD) is another climate phenomenon that can alter the upwelling intensity. The IOD is a phenomenon coupled between the atmosphere and the ocean with varied anomaly bi-polar temperature sea surface in the tropical Indian Ocean [39]. The irregular oscillation of SST causes the western Indian Ocean to become alternately cooler while the eastern part of the ocean becomes warmer, which is called the ‘negative IOD phase.’ As for the positive IOD phase, the water in the east of the Indian Ocean is cooler but warmer in the west. During the developing phase of a positive IOD, the upwelling of the subsurface cold water along the coasts of Sumatra and Java expands westward and produces a large zonal SST gradient in the central-eastern tropical Indian Ocean. The resultant atmospheric pressure gradient intensifies southeasterly wind anomalies along the coasts. The wind anomalies further strengthen the SST gradient through the enhancement of the upwelling [32].

3. Climate Change versus Upwelling

One of the major concerns on upwelling nowadays is how the upwelling responds to climate change or global warming. According to Bakun [40], upwelling will intensify under a climate change scenario as the sun radiates more heat, causing the land to be heated faster and stronger. This condition steepens the temperature difference between ocean and land, resulting in a stronger alongshore wind (Figure 3). This hypothesis is supported by Wang et al. [41], who found a robust relationship between the increase of land-sea temperature differences and the upwelling intensity in the twenty-first century. They also added that the changes are also latitude-dependent, where upwelling intensity and duration increased at higher latitudes. However, an alternative hypothesis was proposed by Rykaczewski et al. [42], where changes in the magnitude, timing, or location of upwelling winds could be associated with the poleward migration and intensification of major atmospheric high-pressure cells in response to the increased greenhouse gas concentrations (Figure 4).
Figure 3. Illustration diagram of anticipated climate change impacts on upwelling in EBUS based on Bakun Hypothesis. (i) current state of coastal upwelling zones. (ii) Potential future state of upwelling zones. Continental thermal lows (L) are expected to deepen in the future, thus intensifying upwelling-favourable winds.
Figure 4. Expected change of upwelling from climate change based on the alternative hypothesis. (i) Present condition of upwelling as the difference between high- and low-pressure systems drive upwelling-favourable winds (grey arrows) and cause the upwelling to occur (blue arrows). (ii) Poleward migration of high-pressure systems, leading to enhanced wind and upwelling in the poleward region.
Understanding the trends of upwelling under climate change scenarios is essential. Stronger upwelling may increase the nutrient input and offshore transport, potentially leading to rapid transportation of phytoplankton and zooplankton toward the convergence offshore frontal system [43]. On the other hand, weaker upwelling may potentially decrease the primary production as it limits the nutrient enrichment of the photic zone [44]. Improvement can be made for fisheries management and other marine resources in all upwelling areas by understanding the relationship between the upwelling and the nutrient inputs and how they are likely to change in the future.

References

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Muhammad Naim Satar
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Zuraini Zainol
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Jing Xiang Chung
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