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. An interactive webpage allows users to browse and download
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
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.
Back to ICES Science Highlights: science and advice in a changing Arctic.
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.