Atlantic inflow to the Nordic Seas: current structure and volume fluxes from moored current meters,VM-ADCP and SeaSoar-CTD observations, 1995–1999

This study deals with the inflow of warm and saline Atlantic water to the Nordic Seas, an important factor for climate, ecology and biological production in Northern Europe. The investigations are carried out along the Svinøy standard hydrographic section, which cuts through the Atlantic inflow to the Norwegian Sea just to the north of the Faroe–Shetland Channel. In the Svinøy section, we consider the Atlantic inflow as water with salinity above 35.0, corresponding to temperatures above 5°C. Current measurements for the period April 1995 to February 1999, positioned on the continental slope in water depths between 490 and 990 m, are combined with VM-ADCP, SeaSoar-CTD and CTD transects to estimate long-term transports and spatial features of the Atlantic inflow. A well-defined two-branched Norwegian Atlantic Current was revealed with an eastern and a western branch. The eastern branch appears as a narrow, topographically trapped, near barotropic, 30–50 km wide current, with a maximum speed of 117 cm/s. The western branch is also about 30–50 km wide, and appears as an unstable frontal jet about 400 m deep with a maximum speed of 87 cm/s. Between these two prominent branches, the observations show an average eddy field with a recirculation to the southwest. Transport estimates from the current records in the eastern branch show an annual mean inflow of 4.2 Sv (1 Sv=10⁶ m³/s) with variation on a 25 h time scale ranging from −2.2 to 11.8 Sv, and between 2.0 and 8.0 Sv on a monthly time scale. The current record in the core of the eastern branch mirrors the estimated transport on a monthly time scale with a correlation coefficient of 0.86. Except for the year 1995–1996, this nearly four-year current record shows evidence of a systematic annual cycle with summer to winter variations in the proportion of 1 to 2. Comparison between the North Atlantic Oscillation (NAO) index and the current record on a three-month time scale shows a strong connection for most of the period. This reflects the strong coupling between the westerly winds and the inflow. The baroclinic transport west of the eastern branch, including the frontal jet, is inferred from hydrography in combination with VM-ADCP transects, and has a total mean of 3.4 Sv. Thus, investigations to date indicate a yearly mean Atlantic inflow of 7.6 Sv in the Svinøy section.     

Interannual thermohaline changes in the northern North Atlantic 1991–1996

In the period 1991–1996 the WOCE hydrographic section A1E/AR7E between Greenland and Ireland was repeated five times. The observed thermohaline changes altered the baroclinic structure along the eastern margin of the subpolar gyre significantly. Between June 1995 and August 1996 an overall increase of the temperature and thickness and a decrease of the density of the Subpolar Mode Water (SPMW) layer were observed, accompanied by an increase of its salinity east of the Reykjanes Ridge and a decrease of its salinity in the Irminger Sea. The changes were most pronounced in the Iceland Basin, where the Subarctic Front retreated westwards, coinciding with a strong weakening of the Westerlies as determined by the North Atlantic Oscillation. They are related to a local reduction of the Ekman upwelling and the ocean-to-atmosphere heat flux on the one hand and to the advection of anomalies from the subtropics on the other hand.The eastward spreading of the different Labrador Sea Water (LSW) vintages led to a corresponding cooling of the LSW in the Irminger Sea and in the Iceland Basin in the period 1991–1996. The renewal of the LSW in the Rockall Trough occurred more sporadically, indicating that the North Atlantic Current (NAC) impedes the southward spreading of LSW in the eastern Atlantic. The changes in 1996 seem to have also counteracted this spreading.     

Black Carbon in the Atmospheric Boundary Layer Over the North Atlantic and the Russian Arctic Seas in June–September 2017

Oceanology - The article presents the results of a study of black carbon concentrations in the atmospheric boundary layer over the Baltic and North seas, the North Atlantic, and Norwegian, Barents,...     

Cyclical Variations in the Surface Temperature in the Sea of Okhotsk and Adjacent Waters,According to 1998–2018 Satellite Data

Izvestiya, Atmospheric and Oceanic Physics - A series of monthly mean values of the surface temperature in August are created for the Sea of Okhotsk and adjacent waters using 21 years’ worth...     

Triassic–Liassic Western Moroccan synrift basins in relation to the Central Atlantic opening

The Upper Triassic–Lower Liassic sequence of the Atlantic margin of Morocco is primarily composed of fine-grained detrital sedimentary rocks containing basaltic intercalations. Its age is Carnian to Hettangian. Well data, outcrop observations and seismic sequence analysis suggest that rifting started earlier (Carnian) in the eastern and southern parts of the margin, and later proceeded towards the west and the north, resulting in the formation of the Central Atlantic rift. On the Moroccan margin, the rift was composed of: (i) eastern basins, relatively narrow and bounded by E-dipping faults, which are reactivated Hercynian structures; (ii) a central horst; and (iii) western basins bounded by W-dipping faults. The 020° trending basins are separated by W-dipping faults. The 020° trending basins are separated by 070° striking transfer faults.Estimations of the amount of lithospheric extension along the Moroccan external rift system show a thinning ratio decreasing northward and probably associated to a necking of the lithosphere. Rift evolution on the part of the Moroccan margin shows good correlation to the conjugate North American margin.     

Phytoplankton production characteristics in the Eastern Atlantic and Atlantic sector of the Southern Ocean in October–November 2004

Production parameters of surface phytoplankton were measured along three transects: La Manche-Cape Town (I); Cape Town-54°S (II); 0°-49°W (along 54°S) (III). The Canary upwelling waters were most productive along transect I, where the surface chlorophyll a (Chl ₀) and the surface primary production (PP ₀) were as high as 4.3 mg/m³ and 173 mg C/m³ per day, respectively. Mosaic patterns in the distribution of these parameters were recorded in the northeastern regions of the South Subtropical Anticyclonic Gyre (Chl ₀ = 0.03–0.35 mg/m³; PP ₀ = 1.6–12.6 mg C/m³ per day). Along transect II, the average twofold southward increase in Chl ₀ (from 0.2 to 0.4 mg/m³) and the concurrent decline of the phytoplankton assimilation activity ( AN ₀) resulted in deviations from typical latitudinal changes inPP ₀. At most sites, PP ₀ values varied between 6 and 15 mg C/m³ per day. Negligible changes in Chl ₀ (0.36–0.85 mg/m³), PP ₀ (8–19 mg C/m³ per day), and AN ₀ (0.7–1.6 mg C/mg chl a per hour) were registered for the oceanic waters along transect III. Along all the transects, PP ₀ depended on Chl ₀ to a greater extent than AN ₀. The values of the latter parameter were largely determined by the water temperature and showed a slight correlation with the insolation. Along transect II, the integrated primary production (PP _int) and the layer-integrated chlorophyll a in the upper 200 m (Chl _0–200) generally varied from 180 to 360 mg C/m² per day and from 30 to 70 mg/m², respectively. In the Polar Front region, an increase in Chl _0–200, PP _int, Chl ₀, and PP ₀ up to respective values of 190 mg/m², 520 mg C/m² per day, 1.2 mg/m³, and 32 mg C/m³ per day was observed. A comparison of the water column (0–100 m) stability with the vertical distribution of the primary production and chlorophyll content along transect II implies that the thick (>100 m) upper mixed layer (UML) formed in response to the strong water cooling and wind forcing was largely responsible for the limited primary production in the Subantarctic and Antarctic regions. The large UML thickness resulted in an intense removal of plant cells from the photosynthetic layer and light starvation of a significant (up to 60%) part of the phytoplankton community.     

Interannual variability in the wintertime air–sea flux of carbon dioxide in the northern North Atlantic, 1981–2001

Gridded fields of sea surface temperature (SST), sea level pressure (SLP), and wind speed were used in combination with data for the atmospheric mole fraction of CO₂ and an empirical relationship between measured values of the fugacity of carbon dioxide in surface water and SST, to calculate the air–sea CO₂ flux in the northern North Atlantic. The flux was calculated for each of the months October–March, in the time period 1981 until 2001, allowing for an assessment of the interannual variations in the region. Locally and on a monthly time scale, the interannual variability of the flux could be as high as ±100% in regions seasonally covered by sea ice. However, in open-ocean areas the variability was normally between ±20% and ±40%. The interannual variability was found to be approximately halved when fluxes averaged over each winter season were compared. Summarised over the whole northern North Atlantic, the air to sea carbon flux over winter totalled 0.08 Gton, with an interannual variability of about ±7%. On a monthly basis the interannual variations were slightly higher, about ±8% to ±13%. Changes in wind speed and atmospheric fCO₂ (the latter directly related to SLP variations) accounted for most of the interannual variations of the computed air–sea CO₂ fluxes. A tendency for increasing CO₂ flux into the ocean with increasing values of the NAO index was identified.     

Phytoplankton production characteristics in the southern Atlantic and the Atlantic sector of the Southern Ocean in the austral summer of 2009–2010

In December–January of 2010 the spatial distribution of the phytoplankton production characteristics was studied along transects in the vicinity of the Greenwich meridian (I) and in the Drake Passage (II). On transect I, the surface chlorophyll a concentration and primary production varied from 0.11 up to 3.57 mg/m³ and from 4.38 up to 37.47 mgC/m³ per day, respectively. The chlorophyll a in the photosynthetic layer and the integrated primary production varied from 10.7 up to 66.1 mg/m² and from 83 to 646 mgC/m² per day, respectively. On transect II in the surface layer, the chlorophyll a concentration changed within the range of 0.09–1.02 mg/m³ and the primary production ranged from 2.08 to 9.49 mgC/m³. The integrated values ranged from 6.32 to 38.29 mg/m² and from 41 to 221 mgC/m² per day, respectively. The moderate means of themaximum quantum yield (F _v/F _m) on transects I and II (0.41 and 0.35, respectively) testify to the low activity of the phytoplankton’s photosynthetic apparatus. The studied water areas in the Southern Ocean differed both in the phytoplankton biomass expressed in the chlorophyll a concentration values and in the conditions of the primary production formation.     

Upper-ocean hydrography of the Nordic Seas during the International Polar Year (2007–2008) as observed by instrumented seals and Argo floats

Following indications of recent warming trends in the Nordic Seas, we have studied the hydrography of these marginal seas from the summer of 2007 until the fall of 2008, using observations gathered by instrumented seals and Argo floats. The combined dataset shows that the upper ocean was indeed both warmer and saltier over much of the Nordic Seas in 2007–2008 compared to the average ocean state for the period 1956–2006 (based on the World Ocean Atlas 2009). There are also indications that the surface Polar Waters of the East Greenland Current were colder and fresher than the climatology, though the quality of the climatology is questionable for this region given the low number of historical observations. Dynamic height calculations suggest that the observed hydrographic changes were associated with enhanced northward upper-ocean thermal wind transport in the east and possibly also enhanced southward transport in the west.     

The 1996 IOC contaminant baseline survey in the Atlantic Ocean from 33°S to 10°N: introduction,sampling protocols,and hydrographic data

The third in a series of cruises designed to establish the present-day concentrations of trace elements and synthetic organic compounds in major water masses of the ocean, the 1996 Intergovernmental Oceanographic Commission Contaminant Baseline Survey occupied six vertical profile stations in the subtropical and tropical Atlantic. Underway surface samples also were acquired in the transects between these stations. This paper uses the temperature, salinity, oxygen, nutrient, and chlorophyll results from the cruise to set the hydrographic background for the other papers in this special volume. Major features sampled during the surface transect include the Brazil Current, the South Equatorial Current, and the offshore Amazon Plume. Utilizing the above parameters to identify water masses, we observed Antarctic Bottom Water (AABW) that ranged from a relatively undiluted form at 33°S (Station 10) to a highly attenuated form at 8°N (Station 6). Similarly, North Atlantic Deep Water (NADW) was obtained in various mixing stages along its flow path, and samples of NADW and AABW exchanging through the Romanche Fracture Zone to the eastern Atlantic basins were also taken. In addition to these deep water masses, representative samples of Antarctic Intermediate Water and Circumpolar Deep Water were acquired. Besides standard hydrography, these data also were used to verify the sampling integrity of the trace metal-clean, Go Flo bottles deployed on a Kevlar hydrographic cable.     

A coupled ice–ocean model for the Greenland,Iceland and Norwegian Seas

Simulations from a coupled ice–ocean model that highlight the importance of synoptic forcing on sea-ice dynamics are described. The ocean model is a non-hydrostatic primitive equation model coupled to a dynamic thermodynamic sea ice model. The ice modelling sensitivity study presented here is part of an ongoing research programme to define the role played by sea ice in the energy balance of the Greenland Sea. The different categories of sea ice found in the subpolar regions are simulated through the use of equations for thin ice, thick ice and the Marginal Ice Zone. A basin scale numerical model of the Greenland, Iceland and Norwegian Seas has a horizontal resolution of 20 km and a vertical grid spacing of 50 m. This resolution is adequate for resolving the mesoscale topographic structures known to control the circulation in this region. The spin-up reproduces the main features of the circulation, including the cyclonic gyres in the Norwegian and Greenland Basins and Iceland Plateau. Topographic steering of the flow is evident. The baroclinic Rossby radius of deformation is between 5 and 10 km so that the model is not eddy-resolving. The coupled ice–ocean model was run for a period of two weeks. The influence of horizontal resolution of the atmospheric model was tested by comparing simulations using six hourly wind fields from the ECMWF with those generated using six hourly fields from a HIRLAM, with horizontal resolutions of 1° and 0.18° respectively. The simulations show reasonable agreement with satellite ice compactness data and data of ice transports across sections at 79°N, 75°N and Denmark Strait.     

Two Modes of Atmosphere–Ocean Interaction in the Atlantic Sector of the Arctic Basin

Oceanology - The paper examines the influence of the main mode of interannual variability of the North Atlantic climate system—the North Atlantic Oscillation (NAO)—on the hydrophysical...     

Interhemispheric exchanges of mass and heat in the Atlantic Ocean in January–March 1993

Hydrographic, geochemical, and direct velocity measurements along two zonal (7.5°N and 4.5°S) and two meridional (35°W and 4°W) lines occupied in January–March, 1993 in the Atlantic are combined in an inverse model to estimate the circulation. At 4.5°S, the Warm Water (potential temperature θ>4.5°C) originating from the South Atlantic enters the equatorial Atlantic, principally at the western boundary, in the thermocline-intensified North Brazil Undercurrent (33±2.7×10⁶ m³ s⁻¹ northward) and in the surface-intensified South Equatorial Current (8×10⁶ m³ s⁻¹ northward) located to the east of the North Brazil Undercurrent. The Ekman transport at 4.5°S is southward (10.7±1.5×10⁶ m³ s⁻¹). At 7.5°N, the Western Boundary Current (WBC) (17.9±2×10⁶ m³ s⁻¹) is weaker than at 4.5°S, and the northward flow of Warm Water in the WBC is complemented by the basin-wide Ekman flow (12.3±1.0×10⁶ m³ s⁻¹), the net contribution of the geostrophic interior flow of Warm Water being southward. The equatorial Ekman divergence drives a conversion of Thermocline Water (24.58⩽σ₀<26.75) into Surface Water (σ₀<24.58) of 7.5±0.5×10⁶ m³ s⁻¹, mostly occurring west of 35°W. The Deep Water of northern origin flows southward at 7.5°N in an energetic (48±3×10⁶ m³ s⁻¹) Deep Western Boundary Current (DWBC), whose transport is in part compensated by a northward recirculation (21±4.5×10⁶ m³ s⁻¹) in the Guiana Basin. At 4.5°S, the DWBC is much less energetic (27±7×10⁶ m³ s⁻¹ southward) than at 7.5°N. It is in part balanced by a deep northward recirculation east of which alternate circulation patterns suggest the existence of an anticyclonic gyre in the central Brazil Basin and a cyclonic gyre further east. The deep equatorial Atlantic is characterized by a convergence of Lower Deep Water (45.90⩽σ₄<45.83), which creates an upward diapycnal transport of 11.0×10⁶ m³ s⁻¹ across σ₄=45.83. The amplitude of this diapycnal transport is quite sensitive to the a priori hypotheses made in the inverse model. The amplitude of the meridional overturning cell is estimated to be 22×10⁶ m³ s⁻¹ at 7.5°N and 24×10⁶ m³ s⁻¹ at 4.5°S. Northward heat transports are in the range 1.26–1.50 PW at 7.5°N and 0.97–1.29 PW at 4.5°S with best estimates of 1.35 and 1.09 PW.     

Climate Change in the Hydrometeorological Parameters of the Black and Azov Seas (1980–2020)

Oceanology - To study the nature of climate change in the hydrometeorological parameters of the Black and Azov Seas—surface air temperature (SAT), sea surface temperature (SST), ice cover,...     

Interannual Sea Level Variations in the South China Sea Over 1950–2009

Spatial patterns of interannual sea level variations in the South China Sea (SCS) are investigated by analyzing an EOF-based 2-dimensional past sea level reconstruction from 1950 to 2009 and satellite altimetry data from 1993 to 2009. Long-term tide gauge records from 14 selected stations in this region are also used to assess the quality of reconstructed sea levels and determine the rate of sea level along the coastal area. We found that the rising rate of sea levels derived from merged satellite altimetry data during 1993–2009 and past sea level reconstruction over 1950–2009 is about 3.9 ± 0.6 mm/yr and 1.7 ± 0.1 mm/yr, respectively. For the longer period, this rate is not significantly different from the global mean rate (of 1.8 ± 0.3 mm/yr). The interannual mean sea level of the SCS region appears highly correlated with Niño 4 indices (a proxy of El Niño-Southern Oscillation/ENSO), suggesting that the interannual sea level variations over the SCS region is driven by ENSO events. Interpolation of the reconstructed sea level data for 1950–2009 at sites where tide gauge records are of poor quality (either short or gapped) show that sea level along the Chinese coastal area is rising faster than the global mean rate of 1.8 mm/yr. At some sites, the rate is up to 2.5 mm/yr.     

Phytoplankton pigments and functional types in the Atlantic Ocean: A decadal assessment, 1995–2005

The phytoplankton pigment composition (chlorophylls and carotenoids) from 17 Atlantic Meridional Transect (AMT) cruises over the period 1995–2005 was analysed to determine the distributions of pigments and plankton in the Atlantic Ocean between 50°N and 50°S. Data were quality assured by statistical methods, including regression of total chlorophyll a (TChla) versus accessory pigments (AP) and comparison of the AMT-TChla with contemporary SeaWiFS-TChla (cruises AMT-05 to -17). Comparisons of province-mean TChla (±SD) for in situ and satellite data showed good agreement for each cruise. ‘Taxa-specific’ pigments were used to define phytoplankton functional types (PFTs) for each of the biogeochemical provinces along the AMT. Pigment ratios (e.g. TChla/AP) were analysed for each cruise and for each province as indices (characteristic properties) of particular PFTs. Mostly robust positive correlations were observed between TChla and pigment ratios for different PFTs, for some provinces and most cruises. These were consistent with previous observations. Generally there were no significant trends of mean TChla or pigment ratios within provinces over the period 1995–2005, although the previously reported perturbation due to the 1997–1998 ENSO was evident.     

Evolution of provenance in the NE Atlantic rift: The Early–Middle Jurassic succession in the Heidrun Field,Halten Terrace,offshore Mid-Norway

An integrated provenance study using provenance-sensitive heavy mineral ratios, mineral chemistry and U/Pb dating of detrital zircons has revealed significant changes in sediment provenance during deposition of the Early to Middle Jurassic succession in the Heidrun Field, offshore Mid-Norway. The variations result from the interplay of two source regions, one of which was located on the Norwegian landmass and the other on the conjugate East Greenland margin. Sediment sourced from central East Greenland is distinguished by high garnet:zircon, high rutile:zircon, low chrome spinel:zircon, garnet assemblages rich in low-Ca, high-Mg varieties, and zircon populations that include an Archean group, a diverse range of Early–Middle Proterozoic grains, and an Early Paleozoic group. These features indicate derivation from a high-grade (granulite facies) metasedimentary terrain together with Archean basement and Early Paleozoic granitoids. Norwegian-sourced sandstones differ by having lower garnet:zircon and rutile:zircon, variable chrome spinel:zircon, garnet assemblages scarce in low-Ca, high-Mg varieties, and zircon populations that lack an Archean group. Derivation from the Caledonian Nappe Domain, comprising metasediments (predominantly at amphibolite facies), ophiolites and Early Paleozoic granitoids, is indicated. Initially, during deposition of the non-marine lower part of the Åre Formation (Hettangian–Sinemurian), sediment was fed from the west, but in the upper (tidally influenced) part of the Åre Formation (Sinemurian–Pliensbachian), Norwegian-sourced material appears. Greenland-derived material disappears in the subsequent Tilje Formation (Pliensbachian), with the Tilje and subsequent Ror and Ile Formations (Toarcian–Aalenian) being predominantly sourced from the east. The regional regression at the base of the Garn Formation (Bajocian) was accompanied by a switch in provenance, with Greenland-derived material replacing sediment sourced from Norway. Variations in mineralogy offer a framework for correlation on both local and sub-regional scales.     

Variations in the atmospheric aerosol optical depth from the data obtained at the Russian actinometric network in 1976–2006

The results of an analysis of variations in the optical depth of a vertical atmospheric column on the basis of a 30-year (1976–2006) series of observations obtained by the Russian actinometric network are generalized. This analysis is based on the Atmosphere Transparency special-purpose database created at the Voeikov Main Geophysical Observatory on the basis of observational data obtained at the actinometric stations of the Russian Hydrometeorological Research Center. The general regularities of spatial variations in the atmospheric optical depth (AOD) over Russia are revealed: there is a monotonic decrease from the southwest to the northeast, with localized areas having different aerosol loads due to the global and regional factors of their formation. A spatiotemporal structure of the anomalies of AOD annual values within the time interval under consideration, including the El Chichon (1982) and Pinatubo (1991) eruptions, is studied.