Upper ocean variability between Iceland and Newfoundland, 1993–1998 (original) (raw)
Temporal and spatial variability of the sea surface salinity in the Nordic Seas
Journal of Geophysical Research, 2002
In this paper, the temporal and spatial variability of the sea surface salinity (SSS) in the Nordic Seas is investigated. The data include a Russian hydrographical database for the Nordic Seas and daily to weekly observations of salinity at Ocean Weather Station Mike (OWSM) (located at 66°N, 2°E in the Norwegian Sea). In addition, output from a mediumresolution version of the Miami Isopycnic Coordinate Ocean Model (MICOM), forced with daily National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data, is used to complement the analysis of the temporal and spatial fields constructed from the observational data sets. The Nordic Seas show a strong seasonal variability in the vertical density stratification and the mixed layer (ML) depth, with a weak stratification and a several hundred meters deep ML during winter and a well-defined shallow ML confined to the upper few tens of meters during summer. The seasonal variability strongly influences the strength of the high-frequency variability and to what extent subsurface anomalies are isolated from the surface. Highfrequency variability has been investigated in terms of standard deviation of daily SSS, calculated for the different months of the year. From observations at OWSM, typical winter values range from 0.03 to 0.04 psu and summer values range from 0.06 to 0.07 psu. Results from the model simulation show that highest variability is found in frontal areas and in areas with strong stratification and lowest variability in the less stratified areas in the central Norwegian Sea and south of Iceland. Investigation of the interannual variability over the last 50 years shows a marked freshening of the Atlantic Water in the Norwegian and Greenland Seas. Moreover, the strength of the southern sector of the Polar front, as defined by the 34.8-35.0 psu isohalines along the western boundary of the inflowing Atlantic Water, undergoes significant interannual variability with gradient stretching reaching up to 300 km. In comparison, the variability in the strength of the eastern front and northern sector of the Polar front, seemingly controlled by the shelf break off Norway and the ridge between the Norwegian and the Greenland Seas, typically undergoes stretching only between 60 and 80 km. The investigation also demonstrates that the low-frequency variability in the upper ocean density field in the Greenland Sea, a key factor for the deep water convection, is governed by the variability in the sea surface field. Since the early 1960s, there has been a negative trend in the salinity, probably contributing to the observed decrease in the deep water production in that period.
The role of the Atlantic Water in multidecadal ocean variability in the Nordic and Barents Seas
Progress in Oceanography, 2014
The focus of this work is on the temporal and spatial variability of the Atlantic Water (AW). We analyze the existing historic hydrographic data from the World Ocean Database to document the long-term variability of the AW throughflow across the Norwegian Sea to the western Barents Sea. Interannual-to-multidecadal variability of water temperature, salinity and density are analyzed along six composite sections crossing the AW flow and coastal currents at six selected locations. The stations are lined up from southwest to northeast -from the northern North Sea (69°N) throughout the Norwegian Sea to the Kola Section in the Barents Sea (33°30 0 E). The changing volume and characteristics of the AW throughflow dominate the hydrographic variability on decadal and longer time scales in the studied area. We examine the role of fluctuations of the volume of inflow versus the variable local factors, such as the air-sea interaction and mixing with the fresh coastal and cold Arctic waters, in controlling the long-term regional variability. It is shown that the volume of the AW, passing through the area and affecting the position of the outer edge of the warm and saline core, correlates well with temperature and salinity averaged over the central portions of the studied sections. The coastal flow (mostly associated with the Norwegian Coastal Current flowing over the continental shelf) is largely controlled by seasonal local heat and freshwater impacts. Temperature records at all six lines show a warming trend superimposed on a series of relatively warm and cold periods, which in most cases follow, with a delay of four to five years, the periods of relatively low and high North Atlantic Oscillation (NAO), and the periods of relatively high and low Atlantic Multidecadal Oscillation (AMO), respectively. In general, there is a relatively high correlation between the year-to-year changes of the NAO and AMO indices, which is to some extent reflected in the (delayed) AW temperature fluctuations. It takes about two years for freshening and salinification events and a much shorter time (of about a year or less) for cooling and warming episodes to propagate or spread across the region. This significant difference in the propagation rates of salinity and temperature anomalies is explained by the leading role of horizontal advection in the propagation of salinity anomalies, whereas temperature is also controlled by the competing air-sea interaction along the AW throughflow. Therefore, although a water parcel moves within the flow as a whole, the temperature, salinity and density anomalies split and propagate separately, with the temperature and density signals leading relative to the salinity signal. A new hydrographic index, coastal-to-offshore density step, is introduced to capture variability in the strength of the AW volume transport. This index shows the same cycles of variability as observed in temperature, NAO and AMO but without an obvious trend.
Variability of heat and salinity content in the North Atlantic in the last decade
Ocean Science, 2010
The analysis of the heat and salinity contents has been made for the Northern Atlantic for the decade between January 1999 and December 2008. This analysis is based on the Argo profiling data for the upper 2000 m. Basin-averaged values of heat content deviation (HCD) and salinity content deviation (SCD) are robust and stable. The HCD and SCD demonstrate positive trends in the last decade in the upper 2000 m of the North Atlantic. The linear trend of HCD and SCD are (11.14±3.17)×10 20 J/yr, and (2.80±1.17)×10 13 kg/yr, respectively. Both trends are significant at 95% level of significance. Polyakov et al. (2005) demonstrate substantial changes in temperature and salinity over the North Atlantic in the last Century. They used oceanographic measurements from three Published by Copernicus Publications on behalf of the European Geosciences Union.
Warming of Atlantic Water in two west Spitsbergen fjords over the last century (1912–2009)
Polar Research, 2013
The recently observed warming of west Spitsbergen fjords has led to anomalous sea-ice conditions and has implications for the marine ecosystem. We investigated long-term trends of maximum temperature of Atlantic Water (AW) in two west Spitsbergen fjords. The data set is composed of more than 400 oceanographic stations for Isfjorden and Grønfjorden (78.18N), spanning from 1876 to 2009. Trends throughout the last century (1912Á2009) indicate an increase of 1.98C and 2.18C in the maximum temperature during autumn for Isfjorden and Grønfjorden, respectively. A recent warming event in the beginning of the 21st century is found to be more than 18C higher than the second warmest period in the time series. Mean sea-level pressure (MSLP) data from ERA-40 and ERA-Interim data sets produced by the European Centre for Medium-Range Weather Forecasts and mean temperature in the core of the West Spitsbergen Current (WSC) at the Sørkapp Section along 76.38N were used to explain the variability of the maximum temperature. A correlation analysis confirmed previous findings, showing that variability in the oceanography of the fjords can be explained mainly by two external factors: AW temperature variability in the WSC and regional patterns of the wind stress field. To take both processes into consideration, a multiple regression model accounting for temperature in the WSC core and MSLP over the area was developed. The predicted time series shows a reasonable agreement with observed maxima temperature in Isfjorden for the period 1977Á2009 (N 024), with a statistically significant multiple correlation coefficient of 0.60 (R 2 00.36) at PB0.05.
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.
Quaternary Science Reviews, 2000
Seasonal sea-ice develops along the eastern continental margins in the northern North Atlantic, where freshwater and/or meltwater out#ow are responsible for relatively low salinity in surface waters and very pronounced water mass strati"cation. Sea-ice constitutes a major parameter in the marine ecosystem since the duration and extent of its seasonal spreading constrain the plankton distribution and the related microfossil assemblages on the sea #oor. Organic-walled dino#agellate cysts that are highly resistant to dissolution were recovered from surface sediments of the northern North Atlantic, and used to develop transfer functions (best analogue method) for the reconstruction of the seasonal spreading and duration of sea-ice cover, in addition to salinity and temperature of the warmest month of the year. Application of the best analogue approach to cores from the Labrador Sea reveals large variations in sea-ice cover and sea-surface conditions throughout the last glacial stage and during the early Holocene. Isotopic analyses in epipelagic and mesopelagic planktonic foraminifers also suggest important changes in salinity and temperature gradients between the surface and sub-surface water masses. Speci"c study of the last glacial maximum LGM time slice (16}20 ka on a C time scale) in the northwestern North Atlantic shows much more extensive sea-ice than at present, with perennial sea-ice lying along the continental margins of eastern Canada. Seasonal spreading of the cover of sea-ice o!shore was accompanied by large seasonal contrasts in temperature, with very cold winters but relatively warm summers, a pattern linked to strong strati"cation between a buoyant low saline surface layer having a low thermal inertia, and the underlying intermediate oceanic waters.
2012
A 2800-yr-long August sea surface temperature (aSST) record based on fossil diatom assemblages is generated from a marine sediment core from the northern subpolar North Atlantic. The record is compared with the aSST record from the Norwegian Sea to explore the variability of the aSST gradient between these areas during the late Holocene. The aSST records demonstrate the opposite climate tendencies toward a persistent warming in the core site in the subpolar North Atlantic and cooling in the Norwegian Sea. At the multicentennial scale of aSST variability of 600-900 yr, the records are nearly in antiphase with warmer (colder) periods in the subpolar North Atlantic corresponding to the colder (warmer) periods in the Norwegian Sea. At the shorter time scale of 200-450 yr, the records display a phase-locked behavior with a tendency for the positive aSST anomalies in the Norwegian Sea to lead, by ;30 yr, the negative aSST anomalies in the subpolar North Atlantic. This apparent aSST seesaw might have an effect on two major anomalies of the European climate of the past Millennium: Medieval Warm Period (MWP) and the Little Ice Age (LIA). During the MWP warming of the sea surface in the Norwegian Sea occurred in parallel with cooling in the northern subpolar North Atlantic, whereas the opposite pattern emerged during the LIA. The results suggest that the observed aSST seesaw between the subpolar North Atlantic and the Norwegian Sea could be a surface expression of the variability of the eastern and western branches of the Atlantic meridional overturning circulation (AMOC) with a possible amplification through atmospheric feedback.
Temperature and salinity variability on the eastern Newfoundland shelf: The residual field
Atmosphere-Ocean, 1992
Time series of temperature, salinity and ice area from the Newfoundland and Labrador Shelves during 1951-86 are examined to determine the temporal and spatial scales of variability outside of the annual band. Significant interannual (periods of 1.8-18 a) variability of temperature (root-mean-square amplitude 0.5°C) and salinity ( 0.1) is found at all depths and, for depths greater than about 100 m, is exceeded or matched only by the annual component. Vertical scales of the low-frequency variability for the depth range 0-75 m are estimated to be twice as large for salinity (60 m) as for temperature (30 m); whereas, for the depth range of 75-175 m, they are the same (90 m) and exceed the scales for the 0-75 m interval. The dominance of low-frequency energy and the size of the vertical scales at depths 100 m suggest that the irregular-temporal and widely spaced vertical sampling of the hydrographic time series would not affect a study of interannual variability in this region. The analysis also suggests that the subsurface low-frequency variability is correlated over horizontal distances of about 1000 km, though the time series are too short to establish statistical confidence. RÉSUMÉ On examine des séries chronologiques de données de température, de salinité et d'englacement pour les plates-formes de Terre-Neuve et du Labrador, portant sur la période 1951-1986, dans le but de déterminer les échelles temporelles et spatiales de variabilité hors de la bande annuelle. Une importante variabilité interannuelle (périodes de 1,8 à 18 a) des températures (amplitude moyenne quadratique 0,5°C) et de la salinité ( 0,1) est