## 3 RS Applications in Ocean
### Ocean Surface Current (OSC)  
OSC is the continuous and directional movements of the mass of the seawater, transferring nutrients, energy, heat, pollutants, and chemical substances around the world [112; 113]. Ocean currents affect the global climate, the ocean's ecosystems, and fishing productivity. More importantly, they play a key role in reducing shipping costs, fuel consumption, as well as developing policies for preventing natural disasters [112; 114]. OSC can originate from a wide range of factors, such as wind, Coriolis effect, water density variation, Ocean Tide (OT), as well as SST and OS differences [112; 113; 115; 116]. The seafloor and shoreline topography can also affect OSC and hinder or boost the mixing and passageway of water from different areas [117]. Ocean currents can be generally divided into five categories: (1) geostrophic ocean current, which is balanced under pressure gradient force by the Coriolis effect; (2) tidal ocean current, which is created by the gravitational force of the moon, sun, and Earth; (3) wind-driven Ekman ocean current, which is created by the steady ocean wind; (4) wave-induced Stokes drift, which is characterized by the difference between the average Lagrangian flow velocity of a fluid parcel and the average Eulerian flow velocity of the fluid at a fixed position; and (5) small-scale ocean current, which is created by the small features such as eddies, fronts, and filaments [118]. Ocean currents are also separated into two groups based on their temperature: warm and cold ocean currents (see Figure 3) [3]. For instance, the Gulf stream, Kuroshio, and the Agulhas are warm currents that transport heat from the tropics poleward and significantly affect the global climate [112; 119; 120]. The Humboldt, Benguela, and California are cold currents that preserve highly upwelling waters and carry cold water toward the equator [112; 120]. For example, the Labrador Current has a cooling effect with a low OS and is known for transporting icebergs from Greenland's glaciers into shipping lanes in the North Atlantic [120; 121]. Ocean currents can also be categorized into two groups according to their depth: surface and deep (subsurface) [112, 118, 122]. The surface currents are horizontal water streams that occur on local to global scales, and their effects are primarily restricted to the top 400 m of ocean water [112, 123]. Along the coasts and offshore regions, there are local surface currents, which are typically small and short-lived (e.g., hourly/seasonal), generated by OT, waves, buoyant river plumes, and local-scale winds [112, 123]. These currents control the local flooding, algal bloom, marine pollution, sediment transport, and ship navigation [112, 123]. The global surface currents (e.g., the Gulf Stream) are typically controlled by dominant global winds (e.g., trade winds and the westerlies) together with Coriolis force and the restriction of flow by continental deflections [112, 114, 123]. These currents travel over long distances in the same direction as the wind and at a speed of approximately 3 to 4% of winds' speed [112, 123]. However, the Coriolis force deflects these currents from the equator to the right direction in the Northern Hemisphere and the left in the Southern Hemisphere, which creates the clockwise and counterclockwise circular patterns or gyres, respectively [112, 114, 115, 123, 124]. In contrast, the deep ocean currents are vertical streams under the influence of the thermohaline circulation generated by water density differences and depend on temperature and OS [112, 125]. Deep ocean currents are formed with upwelling and downwelling directions below 400 m of the surface water [126]. Depending on the scale of the ocean currents, they are measured by different methods. Figure 4 illustrates various in situ and RS methods for ocean current estimation. It should be noted that the focus of this section is on the OSC using offshore, shipborne, and space-borne platforms.