09:15 〜 09:30
[MIS15-02] Processes of Dense Shelf Water variability and Its Mixing with offshore warm water in the Cape Darnley Polynya toward the formation of Antarctic Bottom Water
キーワード：南極底層水、高密度陸棚水、沿岸捕捉波、渦、周極深層水、ケープダンレーポリニヤ
In the Antarctic coastal polynya, brine rejection during sea ice formation creates cold, saline, very dense water known as Dense Shelf Water (DSW). A portion of this water sinks while being transformed into Antarctic Bottom Water (AABW), and occupies the abyssal layer of the global ocean, driving global overturning circulation. The Cape Darnley polynya (CDP) has the second highest ice production around Antarctica (Tamura et al. 2008). The mooring observations off the CDP have revealed that AABW is formed in this region (Ohshima et al. 2013). In the CDP, underwater frazil ice formation actively occurs due to the strong offshore winds and a mixture of frazil ice streaks and open water is maintained at the surface without the accumulation of heat-insulating thick ice. This creates an efficient ice production system in the polynya. However, it remains unclear how DSW generated by high ice production mixes with modified Circumpolar Deep Water (mCDW) to transform into AABW.
In this study, we examine ocean and sea ice variations in the CDP using a comprehensive dataset: the mooring data taken for two years from February 2013 to February 2015 under the Japanese Antarctic Research Expedition, and the satellite sea ice products. The dataset includes velocity and backscatter strength data from throughout the water column derived from acoustic Doppler current profilers, temperature-salinity data obtained from the moorings at three sites. As satellite sea ice products, we used the sea ice production and frazil ice detection data derived from the microwave radiometer AMSR by Nakata et al. (2021), and the landfast sea ice data derived from MODIS data by Fraser et al. (2020).
Results from the moorings on the shelf of the CDP indicate that fluctuations with periods of 3-5 days were predominant from August to November, leading to frequent inflows of warm water originating from mCDW. Additionally, this inflow of warm water was shown to suppress frazil ice formation in the ocean. Furthermore, satellite data indicate that mainly in October, the polynya area became an open water in response to the timing of warm water inflow. This suggests that the polynya may exhibit characteristics of a sensible heat polynya (a polynya formed by heat from the ocean), which was demonstrated for the first time through in-situ observations for this polynya.
To clarify the factors behind the velocity fluctuations associated with the warm water inflow, spectral analysis and complex demodulation were conducted. These analyses suggest that a series of eddies propagated from east to west at a speed of approximately 20 km/day with a period of about four days. This propagation speed closely matched the phase velocity (21 km/day) of the third mode of coastal trapped waves with a 4-day period, calculated from bathymetric data and stratification (buoyancy frequency). Based on the wavelength (84 km) and the offshore structure of this mode, it was suggested that eddy-like fluctuations with a diameter of approximately 40 km were propagating. The propagation of such eddy series was consistent in both scale and propagation speed with the results of the ocean-sea ice coupled model by Mensah et al. (2021). Coastal trapped waves are thought to be generated as a result of column stretching due to the outflow of DSW. These coastal trapped waves, induced in the continental shelf region, are considered to play a crucial role by facilitating the inflow of mCDW from offshore through active velocity fluctuations, as well as by promoting the mixing of DSW and mCDW, ultimately contributing to the AABW formation.
This study is the first report to demonstrate, through field observations, theoretical analysis, and modeling, the generation of coastal trapped waves by DSW outflow, the subsequent inflow of mCDW-origin warm water from offshore, and the mixing of DSW and mCDW leading to AABW formation, which had previously been suggested only by models.
In this study, we examine ocean and sea ice variations in the CDP using a comprehensive dataset: the mooring data taken for two years from February 2013 to February 2015 under the Japanese Antarctic Research Expedition, and the satellite sea ice products. The dataset includes velocity and backscatter strength data from throughout the water column derived from acoustic Doppler current profilers, temperature-salinity data obtained from the moorings at three sites. As satellite sea ice products, we used the sea ice production and frazil ice detection data derived from the microwave radiometer AMSR by Nakata et al. (2021), and the landfast sea ice data derived from MODIS data by Fraser et al. (2020).
Results from the moorings on the shelf of the CDP indicate that fluctuations with periods of 3-5 days were predominant from August to November, leading to frequent inflows of warm water originating from mCDW. Additionally, this inflow of warm water was shown to suppress frazil ice formation in the ocean. Furthermore, satellite data indicate that mainly in October, the polynya area became an open water in response to the timing of warm water inflow. This suggests that the polynya may exhibit characteristics of a sensible heat polynya (a polynya formed by heat from the ocean), which was demonstrated for the first time through in-situ observations for this polynya.
To clarify the factors behind the velocity fluctuations associated with the warm water inflow, spectral analysis and complex demodulation were conducted. These analyses suggest that a series of eddies propagated from east to west at a speed of approximately 20 km/day with a period of about four days. This propagation speed closely matched the phase velocity (21 km/day) of the third mode of coastal trapped waves with a 4-day period, calculated from bathymetric data and stratification (buoyancy frequency). Based on the wavelength (84 km) and the offshore structure of this mode, it was suggested that eddy-like fluctuations with a diameter of approximately 40 km were propagating. The propagation of such eddy series was consistent in both scale and propagation speed with the results of the ocean-sea ice coupled model by Mensah et al. (2021). Coastal trapped waves are thought to be generated as a result of column stretching due to the outflow of DSW. These coastal trapped waves, induced in the continental shelf region, are considered to play a crucial role by facilitating the inflow of mCDW from offshore through active velocity fluctuations, as well as by promoting the mixing of DSW and mCDW, ultimately contributing to the AABW formation.
This study is the first report to demonstrate, through field observations, theoretical analysis, and modeling, the generation of coastal trapped waves by DSW outflow, the subsequent inflow of mCDW-origin warm water from offshore, and the mixing of DSW and mCDW leading to AABW formation, which had previously been suggested only by models.