The amount of Pacific water in the upper layers of the Arctic Ocean varies in time, affecting the amount of freshwater discharged to the Nordic Seas through Fram Strait. Observations show higher fractions of Pacific water passed though Fram Strait during the 1990s than today, possibly due to changes in the large-scale atmospheric circulation.

 

Why monitor Pacific water?

The Pacific inflow though the Bering Strait is a significant source of heat and freshwater to the Arctic Ocean. The paths Pacific water takes though the Arctic Ocean are thought to be affected by the large-scale atmospheric circulation, which means that its contribution to the Arctic Ocean outflow will vary. Pacific water contains more silicate than other halocline water masses, and can therefore affect silicate-limited diatom blooms, which enhance carbon sequestration in the northern North Atlantic. For example, a diatom species that had been absent from the North Atlantic for 800 000 years was reintroduced in 1997-1998 when strong pulses of Pacific water coincided with a reduced sea ice extent. Variations in the volume of freshwater exported from the Arctic Ocean can also modulate the large-scale overturning circulation by affecting deep water convection.

 

How do you recognise Pacific water?

It is difficult to identify Pacific water in the European Arctic from its temperature and salinity alone. Fortunately, we can detect it by measuring nutrients in the water. The Redfield ratio quantifies the relationship between nitrogen and phosphorus atoms, which is remarkably stable throughout the world’s oceans. However, due to strong denitrification over the Chukchi Shelf, Pacific water entering the Arctic Ocean is depleted in nitrate, and can be identified by deviations from the Redfield ratio. The Fram Centre TRIMODAL project – which uses tracers, atmospheric indices and models to study changes in Arctic Ocean inflow/outflow through Fram Strait – collated 21 new and existing sections of nutrient measurements and used them to investigate the fractions of Pacific water present in Arctic Ocean outflow between 1990 and 2017.

Does Pacific water reach Fram Strait?

Very high Pacific water inventories were observed in the late 1990s, accounting for up to 90% of the upper halocline water found between 25 and 75 m in the core of the East Greenland Current. However, the contribution of Pacific water to the outflow is highly variable and in 2006 there was almost no Pacific water in Fram Strait. Significant pulses of Pacific water were observed in 1992, 1998, 2007 and 2012. The frequency of pulses has not changed with time, but pulses released in the 1990s contained much higher fractions of Pacific water than those released in 2007 and 2012. Pacific water inventories are typically highest in the core of the East Greenland Current and lower over the East Greenland Shelf. Pacific water maxima are slightly sub-surface, due to dilution by melting sea ice in the top 25 m.

 

Pacific water at the North Pole

Nutrient samples were collected at the North Pole Environmental Observatory from 2005 to 2015, providing 10 years of overlap with the Fram Strait Observatory 1200 km downstream (see map). After adjustment for a calculated advection time of about 2.25 years, peaks in Pacific water inventories at the two locations are well aligned. The co-variability at Fram Strait and the North Pole suggests that Pacific water inventories in the Arctic Ocean outflow are modulated by large-scale changes in the Beaufort Gyre and the Transpolar Drift – two major currents in the Arctic Ocean – rather than by local process in Fram Strait.

a) Map showing Pacific water pathways though the Arctic Ocean. b) Samples from the 1998 Fram Strait section show the different nitrate and phosphate relationships in Pacific and Atlantic water. c) Hovmöller diagram of Pacific water inventories in Fram Strait. Imagine looking down at Fram Strait with Greenland on the left and Svalbard on the right. Colours indicate the total amount of Pacific water over time, from the early 1990s at the top to the mid-2010s at the bottom. d) Pacific water inventories at Fram Strait and at the North Pole.

Connection to the atmosphere

The Arctic Oscillation (AO) is a non-seasonal variation in atmospheric pressure north of 20°N. A positive AO index describes abnormally low pressure in the Arctic, paired with abnormally high pressure at about 37-45°N. A negative AO index describes the inverse pressure situation. It has been suggested that transport of Pacific water towards Fram Strait is more efficient when the AO index is positive, whereas a persistently negative AO should reduce the Pacific water contribution to the outflow through the Strait. In the early 1990s the AO exhibited a significant positive phase, but shifted in the late 1990s to a more neutral state that continues to this day. The shift in AO state in the late 1990s corresponds well with the decline of Pacific water inventories observed in Fram Strait and the system seems to have responded to the change in forcing within 3-4 years.

 

Numerical model experiments

Although there is a long time series of nutrient measurements in Fram Strait these observations only allow us to compare the situation in Fram Strait with the prevailing atmospheric conditions over the Arctic in years when nutrient measurements are available. Researchers in the Fram Centre TRIMODAL project have added a Pacific water tracer to two different numerical model simulations. These model tracer experiments can be validated by comparison with the long time series of nutrient measurements in Fram Strait and are currently being used to investigate the pathways Pacific water takes though the Arctic ocean at other times and under different atmospheric conditions.

Maps showing the fraction of Pacific water at 10 m depth predicted by NAOSIM in years when high (1998, 2011) and low (2004) inventories were observed in Fram Strait.
Elina Nystedt and Torgeir Blæsterdalen collecting nutrient samples during the 2017 Fram Strait Cruise. Round-the-clock operations allow an extensive biogeochemical sampling program. Photo: Paul A. Dodd, Norwegian Polar Institute.

 

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