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Hirano D.,National Institute of Polar ResearchTachikawa Japan | Fukamachi Y.,Institute of Low Temperature Science | Watanabe E.,Japan Agency for Marine - Earth Science and Technology | Ohshima K.I.,Institute of Low Temperature Science | And 4 more authors.
Journal of Geophysical Research: Oceans | Year: 2016

The nature of the Barrow Coastal Polynya (BCP), which forms episodically off the Alaska coast in winter, is examined using mooring data, atmospheric reanalysis data, and satellite-derived sea-ice concentration and production data. We focus on oceanographic conditions such as water mass distribution and ocean current structure beneath the BCP. Two moorings were deployed off Barrow, Alaska in the northeastern Chukchi Sea from August 2009 to July 2010. For sea-ice season from December to May, a characteristic sequence of five events associated with the BCP has been identified; (1) dominant northeasterly wind parallel to the Barrow Canyon, with an offshore component off Barrow, (2) high sea-ice production, (3) upwelling of warm and saline Atlantic Water beneath the BCP, (4) strong up-canyon shear flow associated with displaced density surfaces due to the upwelling, and (5) sudden suppression of ice growth. A baroclinic current structure, established after the upwelling, caused enhanced vertical shear and corresponding vertical mixing. The mixing event and open water formation occurred simultaneously, once sea-ice production had stopped. Thus, mixing events accompanied by ocean heat flux from the upwelled warm water into the surface layer played an important role in formation/maintenance of the open water area (i.e., sensible heat polynya). The transition from a latent to a sensible heat polynya is well reproduced by a high-resolution pan-Arctic ice-ocean model. We propose that the BCP, previously considered to be a latent heat polynya, is a wind-driven hybrid latent and sensible heat polynya, with both features caused by the same northeasterly wind. © 2016. American Geophysical Union.

Ohshima K.I.,Institute of Low Temperature Science | Kimura N.,National Institute of Polar ResearchTachikawa Japan | Tamura T.,Japan National Institute of Polar Research
Journal of Geophysical Research C: Oceans | Year: 2015

We examined to what degree a simplified polynya model can explain a real polynya based on satellite-derived sea-ice data. In the model, the polynya area, defined as the frazil ice production region, is determined by a balance between the offshore consolidated ice drift and frazil ice production. We used daily polynya area, ice production, and ice drift data derived from AMSR-E. The study area is the Ross Ice Shelf Polynya (RISP), which has the highest sea-ice production in the Southern Ocean. As a modification of the original model to apply the available satellite data set, we introduced the lag time by which produced frazil ice is transported and accumulated at the polynya edge. The model represents a half (48-60%) of the polynya variability when using a lag time of 1.5 days. The frazil ice collection depth at the polynya edge, a key parameter in the model, is estimated to be ∼16 cm. The expansion of the RISP is achieved by ice divergence, and the contraction is achieved mostly by ice production. Both the wind and the remaining components (mainly regarded as the ocean current component) in the ice divergence are larger in the western part of the RISP, which explains the larger extent there. In the one-dimensional frame, assuming that the frazil ice produced within the RISP transforms into consolidated ice with a thickness of 16 cm, the frazil ice production (∼1.7 × 103 m2 d-1) within the RISP approximately balances the export (∼1.6 × 103 m2 d-1) of consolidated thin ice from the RISP edge. © 2015. American Geophysical Union.

Nishino S.,Research and Development Center for Global ChangeJapan Agency for Marine Earth Science and TechnologyYokosuka Japan | Kawaguchi Y.,Applied Physics LaboratoryUniversity of WashingtonSeattle | Inoue J.,Japan National Institute of Polar Research | Fujiwara A.,National Institute of Polar ResearchTachikawa Japan | And 2 more authors.
Journal of Geophysical Research C: Oceans | Year: 2015

A fixed-point observation station was set up in the northern Chukchi Sea during autumn 2013, and for about 2 weeks conductivity-temperature-depth (CTD)/water samplings (6 h) and microstructure turbulence measurements (2 to 3 times a day) were performed. This enabled us to estimate vertical nutrient fluxes and the impact of different types of turbulent mixing on biological activity. There have been no such fixed-point observations in this region, where incoming low-salinity water from the Pacific Ocean, river water, and sea-ice meltwater promote a strong pycnocline (halocline) that stabilizes the water column. Previous studies have suggested that because of the strong pycnocline, wind-induced ocean mixing could not change the stratification to impact biological activity. However, the present study indicates that a combined effect of an uplifted pycnocline accompanied by wind-induced inertial motion and turbulent mixing caused by intense gale-force winds (>10 m s-1) did result in increases in upward nutrient fluxes, primary productivity, and phytoplankton biomass, particularly large phytoplankton such as diatoms. Convective mixing associated with internal waves around the pycnocline also increased the upward nutrient fluxes and might have an impact on biological activity there. For diatom production at the fixed-point observation station, it was essential that silicate was supplied from a subsurface silicate maximum, a new feature that we identified during autumn in the northern Chukchi Sea. Water mass distributions obtained from wide-area observations suggest that the subsurface silicate maximum water was possibly derived from the ventilated halocline in the Canada Basin. © 2015. American Geophysical Union.

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