2:45 PM - 3:00 PM
[AOS14-05] PIV Analysis of an Sea-Ice Eddy observed at the Sea of Okhotsk off the Hokkaido Coast in January 2003
Keywords:Ice-Ocean Eddy, the Sea of Okhotsk, the Soya Warm Current, the East Sakhalin Current
The coastal area of the Sea of Okhotsk in Hokkaido is characterized as complex ocean current system, consisting of the Soya Warm Current and the East Sakhalin Current, and ocean eddies (Wakatsuchi & Ohshima, 1990). It is also the lowest latitude sea ice area in the Northern Hemisphere, and ocean eddies visualized by sea ice are occasionally seen in winter. Many maritime accidents have occurred in this area including the accident of the passenger boat near Shiretoko Peninsula on April 23, 2022. However, the search for missing persons has been delayed in many cases because it is difficult to predict ocean current off the Hokkaido coast. Despite the complexity of this current system, a few data analysis are reported. Therefore, we need to conduct further research in this area to prevent marine accidents.
In this study we focused on an ice-ocean eddy near Shiretoko Peninsula on January 27, 2003, photographed by Japanese Maritime Self-Defense Forces during aircraft reconnaissance. We analyzed, in addition to aerial photographs, Geostationary Meteorological Satellite (GMS-5) visible images, radar images of sea ice provided by Hokkaido University and surface weather observational data in coastal areas by Japan Meteorological Agency. On the day of the ice-ocean eddy formation, the weather condition was fine so that it became a good example enabling us to analyze the ice-ocean eddy was detected by both satellite and radar image in addition to aerial photographs.
In the overlapping area, the radar and satellite images are compared to determine the threshold for visible level to discriminate the presence or absence of sea ice. After the threshold was applied to satellite visible images for further focusing on sea ice alone, motion of sea ice was analyzed with particle image velocimetry software (Ftr PIV 3 <Flowtech Research, Inc.>). Prior to the analysis, motion blurring seen in successive satellite images, which was typically observed in those images obtained by spin-stabilized geostationary orbit satellites such as GMS-5, was removed using motion correction software (Motion Compensator <Flowtech Research, Inc.>).
According to the PIV analysis when the ice-ocean eddy was emerged (11:00-13:00, 27th), the southward current visualized with thick ice, which may originate from the East Sakhalin Current, hit the coastline of Hokkaido and then separated into eastward and westward branches. While the latter branch turned northwestward along the coastline west of Monbetsu against the Soya Warm Current, the former branch flowed northeastward along the Shiretoko Peninsula. Then, after leaving the coastline of Shiretoko Peninsula, it divided again into eastward and westward branches. Especially, the westward one formed a cyclonic curvature in which the ice-ocean eddy was embedded near its center. This eddy was located on the continental slope at a depth of 1000-2000m down to the Kuril Basin. With the collapse of the cyclonic curvature originated from the East Sakhalin Current due to wind effects, the ice-ocean eddy also disappeared. On the other hand, focusing on the Soya Strait area, the southeastward flow with thin ice, thought to be the Soya Warm Current, was obscured near the coastal area of Monbetsu possibly due to the influence of the East Sakhalin Current. In this respect, it was suggested that the factors of the ice-ocean eddy formation in our case were apparently different from those of ice-ocean eddies reported by Wakatsuchi & Ohshima (1990), which were formed on the shear between the Soya Warm Current and offshore cold water originated from the East Sakhalin current. Finally, we would like to note that an anticyclonic curvature visualized by sea ice, which was originated from eastward branch of the East Sakhalin Current, was seen at about 80km north-northwest of our ice-ocean eddy.
In this study we focused on an ice-ocean eddy near Shiretoko Peninsula on January 27, 2003, photographed by Japanese Maritime Self-Defense Forces during aircraft reconnaissance. We analyzed, in addition to aerial photographs, Geostationary Meteorological Satellite (GMS-5) visible images, radar images of sea ice provided by Hokkaido University and surface weather observational data in coastal areas by Japan Meteorological Agency. On the day of the ice-ocean eddy formation, the weather condition was fine so that it became a good example enabling us to analyze the ice-ocean eddy was detected by both satellite and radar image in addition to aerial photographs.
In the overlapping area, the radar and satellite images are compared to determine the threshold for visible level to discriminate the presence or absence of sea ice. After the threshold was applied to satellite visible images for further focusing on sea ice alone, motion of sea ice was analyzed with particle image velocimetry software (Ftr PIV 3 <Flowtech Research, Inc.>). Prior to the analysis, motion blurring seen in successive satellite images, which was typically observed in those images obtained by spin-stabilized geostationary orbit satellites such as GMS-5, was removed using motion correction software (Motion Compensator <Flowtech Research, Inc.>).
According to the PIV analysis when the ice-ocean eddy was emerged (11:00-13:00, 27th), the southward current visualized with thick ice, which may originate from the East Sakhalin Current, hit the coastline of Hokkaido and then separated into eastward and westward branches. While the latter branch turned northwestward along the coastline west of Monbetsu against the Soya Warm Current, the former branch flowed northeastward along the Shiretoko Peninsula. Then, after leaving the coastline of Shiretoko Peninsula, it divided again into eastward and westward branches. Especially, the westward one formed a cyclonic curvature in which the ice-ocean eddy was embedded near its center. This eddy was located on the continental slope at a depth of 1000-2000m down to the Kuril Basin. With the collapse of the cyclonic curvature originated from the East Sakhalin Current due to wind effects, the ice-ocean eddy also disappeared. On the other hand, focusing on the Soya Strait area, the southeastward flow with thin ice, thought to be the Soya Warm Current, was obscured near the coastal area of Monbetsu possibly due to the influence of the East Sakhalin Current. In this respect, it was suggested that the factors of the ice-ocean eddy formation in our case were apparently different from those of ice-ocean eddies reported by Wakatsuchi & Ohshima (1990), which were formed on the shear between the Soya Warm Current and offshore cold water originated from the East Sakhalin current. Finally, we would like to note that an anticyclonic curvature visualized by sea ice, which was originated from eastward branch of the East Sakhalin Current, was seen at about 80km north-northwest of our ice-ocean eddy.