Subarctic Intermediate Water

Subarctic Intermediate Water
Name	Subarctic Intermediate Water
Type	Intermediate water mass
Location	North Pacific Ocean, North Atlantic Ocean
Depth range	~200–1000 m
Temperature	~2–8 °C
Salinity	~33–34.5 PSU
Formed from	Subpolar surface waters, polar fronts

Subarctic Intermediate Water Subarctic Intermediate Water (SAIW) is an intermediate water mass found primarily in the northern basins of the Pacific and Atlantic Oceans, occupying depths between surface waters and deep water masses. It links high-latitude processes near Bering Sea, Sea of Okhotsk, and the North Atlantic Drift with mid-latitude gyres such as the Kuroshio and Gulf Stream, and it plays a role in modulating heat, salt, and nutrient transport across basins.

Definition and Characteristics

SAIW is defined by potential temperature, salinity, and potential density ranges that distinguish it from North Pacific Intermediate Water, Antarctic Intermediate Water, and Mediterranean Water. Typical SAIW occupies isopycnal layers with potential temperature around 2–8 °C and salinity near 33–34.5 PSU, producing a recognizable T–S signature in hydrographic surveys conducted by programs like World Ocean Circulation Experiment and Argo. Hydrographers identify SAIW using neutral density surfaces and tracer concentrations measured during cruises by institutions such as Scripps Institution of Oceanography, Woods Hole Oceanographic Institution, and National Oceanic and Atmospheric Administration.

Formation and Source Waters

SAIW forms from the mixing and subduction of subpolar surface waters originating near fronts and marginal seas including the Alaskan Current, Oyashio Current, and outflow regions of the Sea of Japan. Source contributions include wintertime cooling, brine rejection linked to sea-ice processes around the Bering Strait, and lateral advection from polar shelves influenced by events like the Great Salinity Anomaly. Water-mass transformation is modulated by atmospheric forcing from systems such as the Aleutian Low, Icelandic Low, and cyclones tracked in reanalysis products from ECMWF and NCEP/NCAR.

Physical Properties and Distribution

SAIW displays a characteristic subsurface salinity minimum or intermediate salinity relative to surrounding layers, identifiable in sections along transects like the GO-SHIP lines and basin-scale maps from CLIVAR. In the North Pacific, SAIW spreads southward beneath the Kuroshio Extension and into the North Pacific Gyre, whereas in the North Atlantic analogues advect from the Greenland Sea and Irminger Sea into the Labrador Sea and North Atlantic Current. Vertical structure is influenced by mixing processes near the thermocline and mesoscale eddies studied by missions such as SeaWiFS and Jason altimetry, and lateral gradients are shaped by boundary currents like the California Current and East Greenland Current.

Seasonal and Interannual Variability

Seasonal cycles in SAIW reflect surface buoyancy fluxes driven by atmospheric variability associated with the North Pacific Oscillation, Pacific Decadal Oscillation, and the North Atlantic Oscillation, producing annual adjustments in depth and properties observable in time series from moorings maintained by PICES and OSCAR. Interannual changes arise from climate modes including El Niño–Southern Oscillation and the Atlantic Multidecadal Oscillation, which alter wind stress, mixed-layer depth, and subduction rates, with documented impacts during events cataloged by National Centers for Environmental Information and studies from IPCC assessments.

Role in Ocean Circulation and Climate

SAIW contributes to the ventilation of the intermediate ocean and participates in basin-scale circulation pathways linking subpolar and subtropical domains such as the Meridional Overturning Circulation and regional return flows adjacent to the Subpolar Gyre. By transporting heat and freshwater anomalies, SAIW influences sea surface temperature patterns that feed back onto atmospheric systems including the Pacific Storm Track and teleconnections that affect regions like Western Europe and East Asia. Its properties and variability are incorporated into coupled models developed by centers like NOAA GFDL, Met Office Hadley Centre, and CSIRO for climate projection efforts.

Biogeochemical and Ecological Significance

SAIW impacts nutrient distributions, carrying regenerated nutrients and dissolved oxygen into intermediate layers that constrain biological productivity in overlying euphotic zones influenced by currents such as the Oyashio Current and California Current System. Its intermediate oxygen concentrations affect habitat ranges for mesopelagic communities and biogeochemical cycles involving carbon and nitrogen processed by microbial assemblages studied at observatories like Station PAPA and programs such as Global Ocean Ship-based Hydrographic Investigations Program. Exchanges between SAIW and deeper waters also modulate sequestration of anthropogenic carbon traced by transient tracers employed by GEOTRACES.

Observations and Measurement Methods

Characterization of SAIW relies on multidisciplinary observations from shipboard CTD casts, water-sample analyses for salinity and nutrient chemistry, tracer measurements of CFCs and radiocarbon by laboratories at Lamont–Doherty Earth Observatory and Marine Biological Laboratory, and autonomous platforms including Argo, gliders deployed by institutions like University of Washington, and moorings operated by Ocean Networks Canada. Satellite remote sensing of sea surface height, sea ice extent from MODIS, and wind fields from scatterometers complement in situ data, while numerical assimilation in reanalysis frameworks produced by ECCO Project and HYCOM improves mapping of SAIW pathways.

Category:Oceanography Category:Water masses Category:North Pacific Ocean