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
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 distributionfrom the surface layer to the middle layer of the ocean is being elucidated by a global salinity (and water temperature) observation
network called the ArgoProject, 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 ArgoProject revealedthe general characteristics of
the salinitydistribution in the global ocean.
However, the Argo floats can only provide information up to 5
m below sea surface．
However, the salinityin this 5 m surface layer changes
dynamicallywith 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 floatsare
distributed only at an average rate of 1 unit per 300 km×300 km (an area
similar to Hokkaido), and a detailed distribution of salinitycannot be obtained.
Salinity observation by satellite
That’s where satellites come
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 concentrationcontained
in phytoplanktoncan 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 measuresthe 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.
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 surfaceare known to have
“sensitivity” tosalinity 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 observationis 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 usingelectromagnetic
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 salinityvalues
were comparable. It was also shown that the satellite salinitydeviates 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.
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
The history of satellite
observations,particularly for salinity, is short atapproximately
10 years (approximately 50 years for water temperature), and the development of
this field is expected to continue in the future.