Observing ocean conditions

Working Group on Oceanic Hydrography
Published: 24 June 2020

​​​Since the 1970s, regular assessments of the North Atlantic hydrographic conditions have been provided on the basis of repeated in situ measurements at key sites around the North Atlantic, the Nordic Seas, and adjacent shelf seas. The longest time-series span over a century while a majority of observations cover at least the last two to three decades, allowing for analysis of variability and trends on interannual and decadal time scales. 

The annual ICES Report on Ocean Climate (IROC​​​), produced by the Working Group on Oceanic Hydrography (WGOH) since the late-1990s, describes the current status of sea temperature and salinity, as well as atmospheric conditions in the North Atlantic and Nordic Seas. IROC regional analyses, prepared by local experts directly involved in data collection, provide details on the recent and long-term vari­ability of North Atlantic upper, intermediate, and deep waters. They are also used to identify patterns - linking hydrographic changes across the North Atlantic and adjacent seas - and to detect anomalies propagating through the Atlantic domain. A​n interactive webpage allows users to browse and download IROC data. 

The assessment areas included in the IROC cover the main regions of the Atlantic water inflow to the Arctic Ocean as well as major pathways of the Arctic outflow towards the subpolar North Atlantic. Atlantic inflow is the main supply of oceanic heat, salt, nutrients and organisms into the Arctic Ocean while the fresh Arctic outflow may affect deep convection in the subpolar North Atlantic and thus the meridional overturning circulation.

During recent decades, anomalies originating in the Atlantic have been found to propagate around the Arctic Ocean. Long-term observations from the 1990s and 2000s reported in IROC, revealed two warm pulse-like temperature anomalies on the order of 1°C (relative to the long-term mean) propagating through the eastern subpolar North Atlantic and entering the Arctic Ocean through Fram Strait. There is increasing evidence that additional oceanic heat provided by warmer Atlantic water has non-negligible impact on sea ice reduction in the European sector of the Arctic Ocean. Observed warming of the Atlantic inflow also caused a dramatic reduction in the sea ice extent and an increase of the ocean heat content in the Barents Sea, both of significant consequence​ for the regional marine ecosystem.

The Atlantic-origin oceanic warming has slowed slightly in the last decade. However, simultaneously, an accelerated freshening has started to spread in the upper ocean, first observed in the eastern subpolar North Atlantic in the mid-2010s. The most recent low-salinity anomalies have expanded northward into the Nordic seas, influenc­ing the Greenland Sea, the northern Norwegian Sea to Fram Strait, and the southern reaches of the Barents Sea. Freshening is also observed spreading westward into the Irminger Sea. Interestingly, in most key sites in the North Atlantic, the large changes in salinity are decoupled from changes in temperature.

Sustained observations in the North Atlantic are crucial to predict the future fate of the Arctic Ocean under the ongoing warming as well as to decipher a potential impact of melting Arctic sea ice on the global ocean and climate. Within the last two decades, the long-term observations have evolved quickly incorporating technological advances such as autonomous devices and platforms into classical in situ sampling programs. WGOH has embraced such technological developments without diverting focus from ongoing in situ long-term monitoring programs in the North Atlantic.

The Working Group on Oceanic Hydrography (WGOH) closely monitors the ocean conditions in ICES area by updating and reviewing results from standard hydrographic sections and stations.

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Schematic of the general circulation of the upper ocean 
(0–1000 m) in the North Atlantic. Blue arrows: movement of cooler waters of the Subpolar Gyre; red arrows: movement of warmer waters of the Subtropical Gyre. Click to see more. IROC 2018.

Time-series of normalized temperature anomalies in the upper ocean at the selected stations around the North Atlantic (IROC 2018). The anomalies are normalized with respect to the standard deviation (s.d.; e.g. a value of +2 indicates 2 s.d. above normal). Click to enlarge.

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Observing ocean conditions

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