Spaceborne microwave sensors, such as microwave scatterometers, radar altimeters, and microwave radiometers, can observe the sea surface day and night and are not affected by clouds. Except for the synthetic aperture radar, their spatial resolution is in the order of 10 km. While visible and infrared radiometers provide high-resolution images of chlorophyll a concentration and sea surface temperature, microwave sensors focus on global observations of physical parameters of the sea surface. To achieve accurate observation, it is indispensable to develop algorithms that are based on the physical processes at the air-sea boundary, evaluate observation accuracy, comprehend the error characteristics, and provide feedback for algorithm improvement. In this paper, I would like to summarize my research on ocean observations using three different types of microwave sensors and their applications as well as describe future prospects.

This article summarizes results of the study conducted on the cycling of carbon and nutrients in Tokyo Bay. Extensive surveys on the partial pressure of CO₂ in the bay showed that the bay acts as a strong net sink for atmospheric CO₂, as a result of active photosynthesis driven by nutrient loading from the land, which overwhelms the decomposition of terrestrial organic matter. This is because sewage treatment plants in the watershed remove most of the labile organic carbon and provide mainly recalcitrant organic carbon to the bay. Furthermore, nutrient concentrations in the bay significantly decreased from January 1989 to December 2015. These reflect the reduced loading of nutrients into the bay due to the implementation of advanced wastewater treatment techniques.