MCCIP ARC 2010-11: Salinity (original) (raw)
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Temperature, salinity and flow variability on the north-west European Shelf west of Scotland are examined. Analyses are derived from a range of marine and atmospheric data sources which together comprise a "proto-observatory" of the European Shelf to the west of the Scottish mainland. The main time series are from a current meter mooring which has been maintained on the northwest European continental shelf since June 1981, and a hydrographic section repeated at least annually across the shelf at~56°N since 1975. With the exception of occasional episodes the water column at the mooring site is well mixed or weakly stratified throughout the year. The mean water temperature was 10.1°C and the dominant mode of variance in the temperature record is the seasonal cycle (SA), with amplitude of 3.2°C. The currents are constrained by a passage and are dominated by the semi-diurnal tidal species. There is also significant energy in SA in the along channel velocity but not in the across channel velocity. The along channel residual has mean value of 10.8 cm s − 1 directed towards the north, a clear manifestation of the coastal current, with very few, short duration periods of flow reversal. The monthly temperature anomaly time series are compared with the NE Atlantic upper layer heat content anomalies. Both anomaly time series show highs in the late 1980s and 1990s and lows in the early 1980s and mid 1990s. The overall trend is of warming at a rate of + 0.57°C per decade, with the timing of maximum annual temperature receding by 12 days per decade through the time series. A simple barotropic dynamic balance for the along channel flow in the passage demonstrates that the near surface wind dominates the variability of the northward flow, and that both the north/south gradient in sea level slope and atmospheric pressure are significant in driving the coastal current northwards. This balance reveals a bias is previous summer-only estimates of the flow rate of the coastal current, and a revised figure approximately double that of previous estimates is suggested. Residual winter flows correlate significantly with the North Atlantic Oscillation index (r = 0.59). In contradiction to previous published analyses, shelf salinities exhibit only weak seasonality. No single determining factor is found for the longitudinal excursion of surface isohalines across the shelf, however prolonged periods of high NAO index do coincide with a raised salinity of shelf waters.
2010
Recent decadal salinity changes in the Greenland-Scotland overflow-derived deep waters are quantified using CTD data from repeated hydrographic sections in the Irminger Sea. The Denmark Strait Overflow Water salinity record shows the absence of any net change over the 1980s-2000s; changes in the Iceland-Scotland Overflow Water (ISOW) and in the deep water column (σ 0 > 27.82), enclosing both overflows, show a distinct freshening reversal in the early 2000s. The observed freshening reversal is a lagged consequence of the persistent ISOW salinification that occurred upstream, in the Iceland Basin, after 1996 in response to salinification of the northeast Atlantic waters entrained into the overflow. The entrainment salinity increase is explained by the earlier documented North Atlantic Oscillation (NAO)-induced contraction of the subpolar gyre and corresponding northwestward advance of subtropical waters that followed the NAO decline in the mid-1990s and continued through the mid-2000s. Remarkably, the ISOW freshening reversal is not associated with changes in the overflow water salinity. This suggests that changes in the NAO-dependent relative contributions of subpolar and subtropical waters to the entrainment south of the Iceland-Scotland Ridge may dominate over changes in the Nordic Seas freshwater balance with respect to their effect on the ISOW salinity.
The ICES Working Group on Oceanic Hydrography (WGOH) maintains, analyses and develops coastal, shelf and deep ocean repeated stations and sections around the subpolar North Atlantic, the Nordic Seas and adjacent shelf seas. The WGOH annually reviews the latest results and research from repeat hydrography sections and stations, and generates a summary of hydrographic conditions in the North Atlantic, the ICES Report on Ocean Climate (IROC). By combining expertise in variability in physical processes and the relationship with ecosystems from around the North Atlantic, the WGOH is an excellent forum for developing new insight into climate variability and the impact on ecosystems. Recent new collaborative research highlights include observations of unusually warm and saline Atlantic Water in the Nordic Seas, a comparison of gridded sea surface temperature products to selected time series, and an isopycnal analysis over 300,000 profiles from the Nordic Seas. The WGOH encourages developme...
Deep Sea Research Part II: Topical Studies in Oceanography, 2011
A serendipitous 48-month time series of hydrographic properties was obtained from the vicinity of the South Orkney Islands, Southern Ocean, by tagging a southern elephant seal (Mirounga leonina) on Signy Island with a Conductivity-Temperature-Depth/Satellite-Relay Data Logger (CTD-SRDL) in March 2007. Such a time series (including data from the austral autumn and winter) would have been extremely difficult to obtain via other means, and it illustrates with unprecedented temporal resolution the seasonal progression of upper-ocean water mass properties and stratification at this location. Sea ice production values of around 0.15-0.4 m month À 1 for April to July were inferred from the progression of salinity, with significant levels still in September (around 0.2 m month À 1). However, these values presume that advective processes have negligible effect on the salinity changes observed locally; this presumption is seen to be inappropriate in this case, and it is argued that the ice production rates inferred are better considered as ''smeared averages'' for the region of the northwestern Weddell Sea upstream from the South Orkneys. The impact of such advective effects is illustrated by contrasting the observed hydrographic series with the output of a one-dimensional model of the upper-ocean forced with local fluxes. It is found that the difference in magnitude between local (modelled) and regional (inferred) ice production is significant, with estimates differing by around a factor of two. A halo of markedly low sea ice concentration around the South Orkneys during the austral winter offers at least a partial explanation for this, since it enabled stronger atmosphere/ocean fluxes to persist and hence stronger ice production to prevail locally compared with the upstream region. The year of data collection was an El Niñ o year, and it is well-established that this phenomenon can impact strongly on the surface ocean and ice field in this sector of the Southern Ocean, thus the possibility of our time series being atypical cannot be ruled out. Longer-term collection of in situ ocean data from this locality would be desirable, to address issues relating to interannual variability and long-term change.
Reversal of the 1960s to 1990s freshening trend in the northeast North Atlantic and Nordic Seas
Geophysical Research Letters, 2008
Hydrographic time series in the northeast North Atlantic and Nordic Seas show that the freshening trend of the 1960s-1990s has completely reversed in the upper ocean. Since the 1990s temperature and salinity have rapidly increased in the Atlantic Inflow from the eastern subpolar gyre to the Fram Strait. In 2003-2006 salinity values reached the previous maximum last observed around 1960, and temperature values exceeded records. The mean properties of the Atlantic Inflow decrease northwards, but variations seen in the eastern subpolar gyre at 57°N persist with the same amplitude and pattern along the pathways to Fram Strait. Time series correlations and extreme events suggest a time lag of 3-4 years over that distance. This estimate allows predictions to be made; the temperature of Atlantic water in the Fram Strait may start to decline in 2007 or 2008, salinity a year later, but both will remain high at least until 2010.
Earth System Science Data Discussions
The datasets described here bring together quality-controlled seawater temperature measurements, from over 130 years of Departmental government-funded marine science investigations in the UK (United Kingdom). Since before the foundation of a Marine Biological Association fisheries laboratory in 1902 and through subsequent evolutions as the Directorate of Fisheries Research and the current Centre for Environment Fisheries & Aquaculture Science, UK Government marine scientists and observers have been collecting seawater temperature data as part of oceanographic, chemical, biological, radiological, and other policy driven research and observation programmes in UK waters. These datasets start with a few tens of records per year, rise to hundreds from the early 1900s, thousands by 1959, hundreds of thousands by the 1980s, peaking with > 1 million for some years from 2000 onwards. The data source systems vary from time series at coastal monitoring stations or offshore platforms ...
The North Sea — A shelf sea in the Anthropocene
Journal of Marine Systems, 2014
Global and regional change clearly affects the structure and functioning of ecosystems in shelf seas. However, complex interactions within the shelf seas hinder the identification and unambiguous attribution of observed changes to drivers. These include variability in the climate system, in ocean dynamics, in biogeochemistry, and in shelf sea resource exploitation in the widest sense by societies. Observational time series are commonly too short, and resolution, integration time, and complexity of models are often insufficient to unravel natural variability from anthropogenic perturbation. The North Sea is a shelf sea of the North Atlantic and is impacted by virtually all global and regional developments. Natural variability (from interannual to multidecadal time scales) as response to forcing in the North Atlantic is overlain by global trends (sea level, temperature, acidification) and alternating phases of direct human impacts and attempts to remedy those. Human intervention started some 1000 years ago (diking and associated loss of wetlands), expanded to near-coastal parts in the industrial revolution of the mid-19th century (river management, waste disposal in rivers), and greatly accelerated in the mid-1950s (eutrophication, pollution, fisheries). The North Sea is now a heavily regulated shelf sea, yet societal goals (good environmental status versus increased uses), demands for benefits and policies diverge increasingly. Likely, the southern North Sea will be re-zoned as riparian countries dedicate increasing sea space for offshore wind energy generationwith uncertain consequences for the system's environmental status. We review available observational and model data (predominantly from the southeastern North Sea region) to identify and describe effects of natural variability, of secular changes, and of human impacts on the North Sea ecosystem, and outline developments in the next decades in response to environmental legislation, and in response to increased use of shelf sea space.
Deep-sea Research Part I-oceanographic Research Papers, 1999
Two standard sections across the deep water channel separating the Faroese Plateau from the Scottish continental shelf have been surveyed regularly since the start of the 20th century. There have been significant changes in the characteristics of surface, intermediate and deep water masses during this period. At intermediate depths, the presence of Norwegian Sea Arctic Intermediate Water (NSAIW) was evident as a salinity minimum during the first decade of the century. During the decades 1960-1980 this salinity minimum disappeared, and only four water types were identified in the Channel. Since 1980 the salinity of the intermediate water has again decreased, due to changes in the atmospheric forcing over the Nordic Seas, and it is again evident on a S curve as a distinct minimum. The salinity of the bottom water in the Channel has also decreased (0.01/decade) linearly since the mid-1970s, although at a slower rate than the intermediate water (0.02/decade). The decline in salinity of the bottom water cannot be accounted for by changes in the salinity of upper Norwegian Sea Deep Water (NSDW), which Faroe Shetland Channel Bottom Water (FSCBW) has traditionally been assumed to be composed of. There is evidence that the upper level of NSDW has become deeper outside the Channel owing to a reduced supply from the Greenland Sea. This has resulted in a change in the composition of FSCBW, from being approximately 60% NSDW during the period 1970-1985 to 40% NSDW since 1990. Thus, the thermohaline circulation of the Nordic Seas has lost its deep water connection. The associated freshening of FSCBW has propagated out through the Channel into the North Atlantic and has resulted in a reduction of the salinity (0.02/decade) and transport (1-7%/decade) of Iceland Scotland Overflow Water (ISOW) into the North Atlantic.