Global view of sea-ice production in polynyas and its linkage to dense/bottom water formation

Kay I Ohshima,Sohey Nihashi,Katsushi Iwamoto

doi:10.1186/s40562-016-0045-4

Abstract

Global overturning circulation is driven by density differences. Saline water rejected during sea-ice formation in polynyas is the main source of dense water, and thus sea-ice production is a key factor in the overturning circulation. Due to difficulties associated with in situ observation, sea-ice production and its interannual variability have not been well understood until recently. Methods to estimate sea-ice production on large scales have been developed using heat flux calculations based on satellite microwave radiometer data. Using these methods, we present the mapping of sea-ice production with the same definition and scale globally, and review the polynya ice production and its relationship with dense/bottom water. The mapping demonstrates that ice production rate is high in Antarctic coastal polynyas, in contrast to Arctic coastal polynyas. This is consistent with the formation of Antarctic Bottom Water (AABW), the densest water mass which occupies the abyssal layer of the global ocean. The Ross Ice Shelf polynya has by far the highest ice production in the Southern Hemisphere. The Cape Darnley polynya (65°E–69°E) is found to be the second highest production area and recent observations revealed that this is the missing (fourth) source of AABW. In the region off the Mertz Glacier Tongue (MGT), the third source of AABW, sea-ice production decreased by as much as 40 %, due to the MGT calving in early 2010, resulting in a significant decrease in AABW production. The Okhotsk Northwestern polynya exhibits the highest ice production in the Northern Hemisphere, and the resultant dense water formation leads to overturning in the North Pacific, extending to the intermediate layer. Estimates of its ice production show a significant decrease over the past 30–50 years, likely causing the weakening of the North Pacific overturning. These regions demonstrate the strong linkage between variabilities of sea-ice production and bottom/intermediate water formation. The mapping has also provided surface boundary conditions and validation data of heat- and salt-flux associated with sea-ice formation/melting for various ocean and coupled models.

Highlights

Global overturning circulation is driven by density differences: water sinks in dense water formation areas and gradually upwells in other areas
We present the mapping of coastal polynya and ice production, using the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) algorithm developed by Nihashi and Ohshima (2015) for the Southern Hemisphere and by Iwamoto et al (2014) for the Northern Hemisphere, with the same definition and scale for both hemispheres
Mapping of coastal polynya and sea-ice production is presented with the same definition and scale globally, based on satellite-derived ice thickness and heat flux calculations by Nihashi and Ohshima (2015) and Iwamoto et al (2014)

Summary

Background

Global overturning circulation is driven by density differences: water sinks in dense water formation areas and gradually upwells in other areas. This northwesterly wind transports very cold air over the sea from the continent upwind, which recorded the lowest air temperature in the Northern Hemisphere and is called as “Cold Pole” (Nihashi et al 2009) This makes the Okhotsk Northwestern polynya the highest sea-ice production polynya in the world (Table 1; Fig. 5), according to the present AMSR algorithm and definition of the polynya area (area with the production rate of >3 m/year). Campagne et al (2015), based on a 250-year long sediment core in this region, suggested that large and abrupt changes in local sea ice and bottom water conditions occur with a 70-year periodicity, associated with the MGT calving and regrowth dynamics These demonstrate the strong linkage between the glacier tongue and AABW production. This first observational evidence of a linkage between the annual ice production and OSIW supports a hypothesis that decreasing ice production in the Okhotsk coastal polynyas, at least in part, has led to weakening of the overturning in the North Pacific

Findings

Discussion

Conclusions