# Ocean Remote Sensing Techniques and Applications: A Review (Part II)
## 2 RS Applications in Ocean
### Sea Surface Temperature (SST)  
SST is one of the most important oceanic variables for the global climate system and has been widely utilized to forecast and monitor long-term climate changes [98, 99, 100]. Moreover, the fluctuating flux of dormant and sensible heat from the ocean affects the atmosphere. Thus, SST is often used as a critical variable to study the atmosphere-ocean interaction at different scales [98,99,101, 102, 103]. Furthermore, SST measurements are widely used in various operational applications, such as civilian and military maritime operations [104], validation of atmospheric models [105], estimation and prediction of coral bleaching [106], human health [107], food security and environmental policy [108], transport and energy [109,110],tourism [111], tracking marine life [112; 113], studding the El Nino and La Nina events [114], and commercial fisheries management [115]. SST measurements are negatively affected by variability and complexity of temperature over ocean surface layers due to various factors, such as air-sea fluxes of heat, moisture and momentum, and ocean turbulence [98]. To address the challenges of variation of sea skin temperature, the Group for High Resolution Sea Surface Temperature (GHRSST), as an open international science team, classified SST into five categories [116; 117; 118; 119]: (1) interface SST(SSTint), which is the temperature at the exact air-sea interface on microscopic scales and cannot be measured using current technologies [119]; (2) skin SST (SSTskin), which is the temperature retrieved by an TIR radiometers (wavelength = 3.7-12 \(\upmu\)m) within the conductive diffusion-dominated sub-layer with the depth of approximately 10-20 \(\upmu\)m; (3) sub-skin SST (SSTsub-skin), which is the temperature at the base of the conductive laminar sub-layer of the ocean surface measured by microwave radiometers (frequency = 6-10 GHz); (4) SST at depth (SSTdepth or SSTZ ), which is defined as the temperature at the bottom of the SSTsub-skin, and is measured by drifters, autonomous/non-autonomous profiling floats, or deep thermistor chains at different depths; (5) foundation temperature (SSTnd), which is the temperature at the first time of the day and is independent from diurnal temperature variability and is only measured by in situ contact thermometry at the depths of approximately 1-5 m.  
Figure 4 demonstrates a schematic diagram of the day and night temperature profiles of the ocean near-surface layer for each SST definition. Based on this figure, SST measurements are affected by the main heat transport processes and time scales [119]. During the day, most of the incoming solar radiation is entered into the near-surface ocean (5 m depth), leading to the formation of thermal stratification (layers of different temperatures) in the ocean. This effect is exacerbated by the light winds (low wind speeds) [98; 120; 121; 122]. On the other hand, the water column gradually cools from the surface during the night [120]. This heating and cooling cycle creates a diurnal cycle in SST, which is very important in improving the ocean-atmosphere models [120; 122]. Figure 4: Near-surface oceanic temperature profiles for different types of SSTs at (**a**) nighttime and (**b**) daytime (adopted from the Group for High Resolution Sea Surface Temperature (GHRSST)). SST can be measured by deploying temperature sensors on different instruments, such as in situ moored and drifting buoys, ships (with a thermometer into a bucket of seawater), and offshore platforms, as well as airborne and spaceborne RS systems [99]. Since 1970, by deploying the Visible and Thermal Infrared Radiometers (VTIR) on geostationary satellites, using SST measurements derived from RS data has become routine [98]. In this section, SST measurements from spaceborne RS systems are only discussed. The satellite-based SST is determined by estimating the thermal emission of electromagnetic radiation from the sea surface using radiometers, which can be expressed by the [PERSON]'s Function (Equation (1)) [123; 124].