Research on Sea Surface Salinity × SDGs
Many of you are probably aware that global warming is causing seawater temperatures to rise on a global scale. As sea temperature rises, the supply of water vapor from ocean to atmosphere increases, causing high salinization of surface water, and in some places, low salinization occurs through increased precipitation and river water and melting of sea ice. Salinity is an indispensable element in understanding the global water cycle and other aspects of the marine environment. I would like to contribute to SDG 2 "LIFE BELOW WATER" through research that contributes to the establishment of technology to measure sea surface salinity from satellites.
The United Nations has designated the decade starting in 2021 as the "Decade of Ocean Science," with the aim of contributing to SDGs.
Salinity: the ocean is the earth’s huge water-storage tank
The ocean can be seen as the earth’s huge water-storage tank, accounting for 97% of the earth’s water (rivers, lakes, atmosphere, glaciers, and oceans combined).
Because the ocean plays an extremely important role in global water circulation, it is necessary to know how much water evaporates from the ocean and how much freshwater flows into it.
To capture the state of evaporation and the inflow of freshwater, it is necessary to accurately know the spatiotemporal distribution of the salinity concentration (hereinafter abbreviated as “salinity”), which is an index of the mixing ratio of freshwater and salt water.
Conventional salinity observation
· The salinity distribution from the surface layer to the middle layer of the ocean is being elucidated by a global salinity (and water temperature) observation network called the Argo Project, which started in the 2000s. This is an international project to elucidate the global ocean structure by putting many Argo floats into the ocean, which are drifting observation instruments that go up and down by their own buoyancy adjustment function and acquire data from the surface layer to the middle layer of the ocean.
Image courtesy of Japan Agency for Marine-Earth Science and Technology
(The following are the links to their homepages)
Conventional salinity observation ～problems～
The Argo Project revealed the general characteristics of the salinity distribution in the global ocean.
However, the Argo floats can only provide information up to 5 m below sea surface．
However, the salinity in this 5 m surface layer changes dynamically with the effects of evaporation and precipitation between the atmosphere and the ocean, river runoff, and the freezing and melting of sea ice.
In addition, the Argo floats are distributed only at an average rate of 1 unit per 300 km×300 km (an area similar to Hokkaido), and a detailed distribution of salinity cannot be obtained.
Salinity observation by satellite
That’s where satellites come in.
Satellites use electromagnetic waves to remotely acquire various information on the sea level．
For example, southern water, such as the Kuroshio, has a clear blue color, while northern water has a greenish color with low transparency. By observing the difference in the sea color in the visible range with satellites, the distribution of the chlorophyll a concentration contained in phytoplankton can be obtained.
The infrared range, which has a longer wavelength than visible light, is used for water temperature observations. For example, thermography installed at an airport measures the intensity of electromagnetic waves radiating from the body surface of a person passing through a gate, converting it into body temperature. Based on the same principle, the sea surface water temperature is measured by satellites.
Microwaves, which have long wavelengths, are used for the observation of sea surface salinity．
Microwaves are widely used in the radio waves of mobile phones, microwave ovens, and radar, as well as in the field of communication.
Electromagnetic waves radiated from the sea surface are known to have “sensitivity” to salinity in the low-frequency micro band (sensitivity means that the intensity of the electromagnetic waves changes corresponding to the changes in salinity). Therefore, the sea surface salinity can be back-estimated based on the change in the intensity of the electromagnetic waves from the sea surface received by satellites．
However, due to its extremely weak signal, the technical level required for salinity observation is extremely high and could not be achieved until recently.
Sea surface salinity measurement by satellites ～research on accuracy evaluation～
Sea surface salinity measured by satellites is the value estimated remotely and indirectly using electromagnetic waves．The estimated value must be validated by comparing it with observation data from the field.
The comparison and validation with the observation data by the Argo floats showed that the satellite and field salinity values were comparable. It was also shown that the satellite salinity deviates greatly from the field salinity in high-latitude regions where the water temperature is low．
This result suggests that the salinity sensitivity of microwaves varies depending on the sea area.
Research on sea surface salinity ～applications～
Aquarius sea surface salinity data with a high spatial resolution has revealed interesting phenomena, one after another.
Examples: equatorial instability waves (Lee et al., 2012)，and freshwater runoff of continental rivers(Gierach et al., 2013)
I focused on mesoscale ocean vortices with a size of several hundred kilometers for the analysis.
I succeeded in capturing how the vortex stirs low-chloride seawater due to precipitation.
Although oceanographic observations using satellites are limited to sea surface information, many variables, not just salinity, can be obtained.
(Example: sea surface water temperature, sea surface salinity, ocean surface wind, and chlorophyll a concentration）
By combining these satellite variables, as well as field observation data in some cases, we can gain knowledge that contributes to improving our understanding of the marine environment and fishery resources.
The history of satellite observations, particularly for salinity, is short at approximately 10 years (approximately 50 years for water temperature), and the development of this field is expected to continue in the future.