Mixed Layer Depth Measurement in Coastal Waters Utilizing Atmospheric Muons

Abstract Mixed layer (ML) in oceans is defined as the less dense upper region of the water column where turbulent mixing occurs and water exhibits almost uniform density profile. Mixed layer depth (MLD) is the depth of this region and shows diurnal and seasonal fluctuations as well as spatial variations in different regions of water. The determination of MLD is an important indicator for climate change in bodies of water, and the novel method proposed here would allow for a continuous monitoring of MLD. Atmospheric muon count is proportional to the density of water which can be measured by counting muons at the bottom and comparing with a shore-based or buoy-based muon counter. Combining this measurement with sea surface temperature (SST) and sea surface salinity (SSS) data from Earth-observing satellites, daily mean MLD can be estimated with an accuracy of 3% for down to 60-m depth. To determine MLD from muon count difference between surface and bottom detectors, a new method was introduced to calculate MLD using average density of water column. A Geant4 model was made to show the validity and limitations of the method. Unlike open ocean, the system can measure MLD in shallow coastal waters where muon count at the bottom is enough for daily based measurements. The proposed method can provide continuous in situ measurement of MLD in coastal shallow waters, and these data can be used to provide boundary conditions for hydrodynamic and ecosystem models. Significance Statement In our work, we proposed a scintillator-based underwater muon detection system which can measure average water column density by counting surviving atmospheric muons at the bottom of the shallow coastal waters. Combining this measurement with the sea surface temperature, salinity, and altimetry data from Earth-observing satellites, MLD can be estimated. In accordance with UN-SDG-14: Life Below Water, the determination of MLD is an important indicator for climate change in bodies of water, and the novel method proposed here would allow for a continuous monitoring of MLD.