Horizontal Currents Research Articles

THE SEAFLOOR MARINE DEBRIS ON THE NORTH AND THE CENTRAL PART OF THE MOROCCAN ATLANTIC WATERS FROM TANGIER (35° N) TO SIDI IFNI (29° N): COMPOSITION, ABUNDANCE, SPATIAL DISTRIBUTION, SOURCES AND MOVEMENT

Abstract. The accumulation of human debris in the marine ecosystem is one of the main hidden negative impacts of the economic and social development that humanity has witnessed after industrialization era which comes accompanied with the evolution of productions, the consumerism, as well as the emergence of the throw-away culture. In this paper, we studied the marine debris pollution on the seafloor of the North and Central Moroccan Atlantic waters from Tangier (35° N) to Sidi Ifni(29° N). The data were collected during two scientific trawling surveys conducted aboard the Charif Elidrissi Vessel in February–March 2015 and March 2018. The prospection operations allowed to collect a total of 329 kg of Seafloor Marine Debris (SMD) with an average abundance ranging between 16.84 ± 20 kg/km2 to 30.15 ± 43 kg/km2. The plastic materials represented the higher percentage of collected items with more than 40% followed by textile debris which mainly composed of knotted fishing ropes used by the longliners in the area. The analysis of SMD composition and their Spatio-temporal pattern combined with the environmental and socio-economic parameters of the prospected area using GIS tools showed that the majority of collected items comes from fishing activities, and the rest comes from the coastal anthropogenic activities. Then the analysis of hydrodynamic properties of the area using AVISO data showed that the horizontal current in area that controls the exchange of water from the coastal area to the ocean interior plays a major role in scattering debris in this area far from their origins to the open ocean.

Numerical investigation of the effects of causeway opening configurations on horizontal currents of Lake Urmia

Unlike Lake Urmia, its two neighbor lakes with similar climate and size have experienced stabilized condition over the past decades. Actually, greater impact of anthropological activities, compared to climatological parameters, on hydrological and ecological systems of Lake Urmia has been stated in the literature. In this paper, the impact of a recently constructed causeway on this lake is studied using environmental impact assessment approach. This study evaluates different alternatives for future re-development plans and clearly proves the importance of preliminary environmental studies during the design development phase of infrastructure projects. The depth-averaged flow module of NASIR finite volume solver was utilized to simulate the horizontal flow patterns in Lake Urmia. Considering hydrological and meteorological parameters, different scenarios regarding location and number of causeway openings (current opening location, opening in the middle of causeway, and 2 openings) were compared to the condition before causeway construction. Discharge values of 21 streams, average monthly evaporation of 5.93 mm/day and average monthly precipitation of 0.4 m/day were considered as input data of the developed model. The simulation results showed that horizontal flow circulations occur mostly on the southern part of the lake and the causeway opening variations generally affect the form of these circulations. In the scenario with causeway and only one opening in the middle, flow circulations present the most comparability to the lake’s flow circulation pattern before causeway construction, whereas the scenario with 2 openings has the least similarities.

Effects of wave-induced vertical Reynolds stress on upper-ocean momentum transfer over the Scotian Shelf during extreme weather events

The effects of wave-induced vertical Reynolds stress (WIVRS) on the upper-ocean momentum transfer over the Scotian Shelf (ScS) are examined using a coupled wave-circulation modeling system during two extreme storm events: Hurricane Earl and Winter Storm Echo. The effects of WIVRS are specified in the circulation model using the wave pressure-slope stress ( τP) suggested by Mellor (2013). The inclusion of τP enhances the downward momentum transfer within the surface layer when the storm passes overhead associated with strong vertical gradients of τP, leading to relatively weak vertical shears of horizontal currents and thus weak turbulent Reynolds stress (τT) in the upper-ocean layer. This in turn weakens the vertical momentum transfer to deeper water through τT just after the storm passage. The reduced downward momentum transfer after the storm due to the inclusion of τP is found to be in better agreements with both the ADCP observations and the HF-radar observations. The ADCP observations show a relatively slow decay of upper-ocean currents, and the HF-radar observations show relatively strong clockwise-rotating near-inertial surface currents over the offshore area of the ScS just after the storm passage. The model results also demonstrate that the effects of τP are more pronounced during Hurricane Earl due to deeper penetration depths of τP associated with stronger winds (up to 33 m/s) than those during Winter Storm Echo (wind speeds up to 25 m/s).

Array tilt effect induced by tidal currents in the northeastern East China Sea

The relation between the tidal current and the acoustic signal measured at vertical line arrays (VLAs) is investigated from data of the shallow-water acoustic variability experiment 2015 performed in the northeastern East China Sea. From the channel impulse response, the arrival structure measured on the VLA was observed to vary with semidiurnal tidal period. To explain this phenomenon, the direction of the array tilt is assumed to be consistent with the current flow direction. The modeling results based on ray tracing prove the relation between the variation of the arrival structure and array tilt angle in a time-varying form, yielding an estimation of tidal currents in the region. We compared the projected time series of horizontal currents to the acoustic paths with the curvature variation in the arrival structure to validate our results and confirmed that they are consistent with each other.

The Southern Caribbean upwelling system off Colombia: Water masses and mixing processes

The upwelling system off the southern Caribbean coast is probably the main source of the nutrients that support biological productivity in the oligotrophic Caribbean Sea. Subtropical underwater (SUW) that forms the subsurface salinity maximum in the Caribbean Sea is the main source of upwelled waters in this system. Profiles of salinity and temperature with depth derived from four oceanographic cruises and Argo floats showed that upwelled waters have a salinity that is ~0.11 g kg−1 lower than the SUW in the central Caribbean Sea and a seasonal variation of approximately 0.09 g kg−1 that reflects the rainy/dry seasons. In addition, the SUW is ~50 m shallower on the continental shelf slope (~100 m) compared to the depth of the SUW in the central Caribbean Sea. The origin of these modified SUW was analyzed using the Mercator numerical model, which reproduces the main vertical characteristics of the subsurface salinity maximum. The modeled data showed that SUW upwelling off of the La Guajira Peninsula and Venezuela arrive into the system via an intense Caribbean Coastal Undercurrent (CaCU, mean speed ~0.28 m s−1). This current is formed in front of the Nicaragua platform from the divergence of subsurface water flow at the salinity maximum depth. The lower salinity observed in the upwelled waters may be the result of intense vertical mixing processes that could occur when the SUW are transported by the CaCU below the Panama-Colombia Gyre (PCG) region before reaching the upwelling zones. The mixing processes—involving double diffusion and mechanical turbulence driven by vertical shear of horizontal currents—were analyzed using the Turner Angle and the Thorpe scale, respectively. Below a depth of 200 m, double diffusion by salt fingers (diffusivities > 5 x 10−5 m2 s−1) was the main process of salt diffusion, generating a downward salt flux of >2 x 10−2 g kg−1 m d−1 between the SUW and the North Atlantic Central Waters (NACW). Above a depth of 100 m, mechanical turbulent diffusion generates a salt flux towards the surface ranging 0.5–4 x 10−2 g kg−1 m d−1, where double diffusion by salt finger is not possible. The diluted SUW is subsequently transported by the CaCU, connecting—at the subsurface level—the PCG region with the upwelling zones off of Colombia and Venezuela. As well as modifying the salt content of the coastal SUW, these mixing processes may also alter the nutrient content of upwelling waters, the ecosystem effects of which are still unknown.

The Deflection Angle of Surface Ocean Currents From the Wind Direction

AbstractWind stress drives the upper ocean circulation in nonequatorial regions by means of an interplay with the vertical turbulent friction and the Coriolis force, generating horizontal wind drift currents which spiral and decay with depth. Classical Ekman theory—applied almost universally in oceanography—predicts that the angle between the vectors of the surface current and surface wind is 45°, if the coefficient of vertical turbulent mixing is constant. However, observations show that the deflection angle is usually close to 30° in arctic regions and larger than 45° in some low‐latitude areas, because the vertical turbulent mixing varies with depth. In contrast to Ekman's classical solution, the solutions that are available for depth‐dependent eddy viscosity are quite involved and, as in data‐driven studies, it is difficult to sort out spurious correlations that obscure the underlying structure. We propose a perturbative approach, providing a formula for the deviation of the deflection angle from the 45° reference value and discussing its implications.

Study of the formation of the Black Sea haline stratification from the numerical simulations

The excess of river runoff and precipitation over evaporation (positive freshwater balance) and the supply of the Marmara Sea highly saline waters through the Bosphorus is the cause of the sharp halocline in the 100-300 m layer and low vertical mixing in the permanent pycnocline layer, where the contribution salinity to the seawater density prevails compared to temperature. Such features of the Black Sea waters make the description of the process of formation of the haline stratification an actual and inexhausted problem. Investigating the patterns of the formation of the haline stratification in the Black Sea, a laboratory model of a rotating cylindrical cavity and a numerical model are proposed. Laboratory studies have shown that in a simple configuration, the buoyancy flux across the basin boundaries leads to the formation of a sharp pycnocline and the formation of a three-layer vertical structure of horizontal currents. Numerical experiments demonstrate that halocline is formed at a depth of 10-40 m. The haline stratification has been established with a pronounced halocline for about hundred years which later slowly evolved.

The energy spectrum of the current velocity in deep layers of the Black Sea

In 2016-2017, for the first time, nearly year-long measurements of the current velocities were carried out below the thermocline at a depth of 100 m and at a depth of 1700 m that was about 100 m above the seabed by using the mooring station in the deep central part of the Black Sea. Analysis of the new data set allowed us to uncover the followings. In the near-bottom layer, the significant flow velocities were recorded, reaching a peak magnitude of 0.13 m/s. Near the frequency of 110-6 Hz, in the transition region of the energy spectrum, the spectral energy of the deep current was close to that observed in the same range in the subthermocline layer of the sea. In the range of balanced geostrophic motions, the energy spectrum had a slope of -2,8 that should be compared to a spectrum slope of -1,8 obtained for current below the thermocline. The regular strong horizontal currents were observed near the local inertial frequency fI. There was a blue shift of the inertial maximum up to 5% with respect to fI in the energy spectrum.

Revisiting the coronal current sheet model: Parameter range analysis and comparison with the potential field model

Aims. We study the properties of the coronal magnetic field according to the current sheet source surface (CSSS) model in 1976–2017 for all physically reasonable values of the three model parameters (cusp surface radius Rcs, source surface radius Rss, and current parameter a), and compare the CSSS field with the potential field source surface (PFSS) model field. Methods. We used the synoptic maps of the photospheric magnetic field from the Wilcox Solar Observatory (WSO), National Solar Observatory/Kitt Peak (NSO/KP), and the NSO Synoptic Optical Long-term Investigations of the Sun Vector Spectromagnetograph (SOLIS/VSM) in order to calculate the coronal magnetic field according to the CSSS and PFSS models. We calculated the coronal field strength, its latitudinal variation and neutral line location, as well as its polarity match with the heliospheric magnetic field. Results. The CSSS model can correct the erroneous latitudinal variation of the PFSS model if the source surface is sufficiently far out with respect to the cusp surface (Rss ≥ 3 ⋅ Rcs). The topology of the neutral line only slightly depends on source surface radius or current parameter, but excludes very low values of the cusp surface (Rcs ≤ 1.5). A comparison of the polarities gives an optimum cusp surface radius that varies in time between 2 and 5; a stronger current yields a larger optimum Rcs. Interestingly, the optimum polarity match percentages and optimum radii vary very similarly in the two models over the four solar cycles we studied. Conclusions. The CSSS model can produce a stronger total coronal flux than the PFSS model and correct its latitudinal variation. However, the topology of the CSSS model is rather independent of horizontal currents and remains very similar to that of the PFSS model. Therefore, the CSSS model cannot improve the match of field polarities between corona and heliosphere.

Stimulation of Heterotrophic and Autotrophic Metabolism in the Mixing Zone of the Kuroshio Current and Northern South China Sea: Implications for Export Production

AbstractTo evaluate the influences of the Kuroshio Current intrusion on the biogeochemistry of the northern South China Sea (NSCS), we conducted field observations of the responses of microbial metabolism to the intrusions of the Kuroshio Current into the NSCS. We used an isopycnal mixing model to quantitatively assess the extent of the Kuroshio intrusion into the NSCS and found that bacterial abundance, production, and growth efficiency were unimodal functions of the fraction of Kuroshio water. Values were maximal at ~60% Kuroshio water and decreased monotonically as the percent of Kuroshio water deviated from 60%. The patterns of gross primary production and nitrate concentration were similar, but the peaks occurred at ~50% Kuroshio water. The Kuroshio intrusion, however, had little impact on bacterial and community respiration. The observed elevation of nitrate concentrations at ~50% Kuroshio water was quantitatively consistent with estimates of the amount of inorganic nitrogen released by remineralization of dissolved organic nitrogen by bacteria. Based on these observations, we hypothesize that dissolved organic matter in the Kuroshio water stimulates bacterial activity, and catabolism of the dissolved organic matter releases inorganic nutrients that stimulate primary production in the NSCS basin. The concomitant elevation of net community production subsequently enhances export production and thereby strengthens the biological pump in the nitrogen‐limited NSCS basin. Recognition of this mechanism underscores the role of horizontal currents in regional carbon and nitrogen cycles and broadens understanding of the processes that sustain export production in the oligotrophic ocean.

The impact of a new high-resolution ocean model on the Met Office North-West European Shelf forecasting system

Abstract. The North-West European Shelf ocean forecasting system has been providing oceanographic products for the European continental shelf seas for more than 15 years. In that time, several different configurations have been implemented, updating the model and the data assimilation components. The latest configuration to be put in operation, an eddy-resolving model at 1.5 km (AMM15), replaces the 7 km model (AMM7) that has been used for 8 years to deliver forecast products to the Copernicus Marine Environment Monitoring Service and its precursor projects. This has improved the ability to resolve the mesoscale variability in this area. An overview of this new system and its initial validation is provided in this paper, highlighting the differences with the previous version. Validation of the model with data assimilation is based on the results of 2 years (2016–2017) of trial experiments run with the low- and high-resolution systems in their operational configuration. The 1.5 km system has been validated against observations and the low-resolution system, trying to understand the impact of the high resolution on the quality of the products delivered to the users. Although the number of observations is a limiting factor, especially for the assessment of model variables like currents and salinity, the new system has been proven to be an improvement in resolving fine-scale structures and variability and provides more accurate information on the major physical variables, like temperature, salinity, and horizontal currents. AMM15 improvements are evident from the validation against high-resolution observations, available in some selected areas of the model domain. However, validation at the basin scale and using daily means penalized the high-resolution system and does not reflect its superior performance. This increment in resolution also improves the capabilities to provide marine information closer to the coast even if the coastal processes are not fully resolved by the model.

Deep-sea Sediment Resuspension by Internal Solitary Waves in the Northern South China Sea

Internal solitary waves (ISWs) can cause strong vertical and horizontal currents and turbulent mixing in the ocean. These processes affect sediment and pollutant transport, acoustic transmissions and man-made structures in the shallow and deep oceans. Previous studies of the role of ISWs in suspending seafloor sediments and forming marine nepheloid layers were mainly conducted in shallow-water environments. In summer 2017, we observed at least four thick (70–140 m) benthic nepheloid layers (BNLs) at water depths between 956 and 1545 m over continental slopes in the northern South China Sea. We found there was a good correlation between the timing of the ISW packet and variations of the deepwater suspended sediment concentration (SSC). At a depth of 956 m, when the ISW arrived, the near-bottom SSC rapidly increased by two orders of magnitude to 0.62 mg/l at 8 m above the bottom. At two much deeper stations, the ISW-induced horizontal velocity reached 59.6–79.3 cm/s, which was one order of magnitude more than the seafloor contour currents velocity. The SSC, 10 m above the sea floor, rapidly increased to 0.10 mg/l (depth of 1545 m) and 1.25 mg/l (depth of 1252 m). In this study, we found that ISWs could suspend much more sediments on deepwater areas than previously thought. Specifically, we estimated that ISWs could induce and suspend 787 Mt/yr of sediment from shelf to deep-sea areas of the northern South China Sea. The total amount of sediment resuspended by shoaling ISWs was 2.7 times that of river-derived sediment reaching the northern South China Sea. This accounted for 6.1% of the global river-discharged sediment (16.4% of that from Asian rivers) transported to the sea.

Field‐Aligned and Horizontal Currents in the Northern and Southern Hemispheres From the Swarm Satellite

AbstractWe present statistical investigation of the high‐latitude ionospheric current systems in the Northern Hemisphere (NH) and Southern Hemisphere (SH) during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly 4 years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system method. The ionospheric horizontal and field‐aligned currents (FACs) for each auroral oval crossing are calculated. The distributions of the mean values of FACs as well as the horizontal curl‐free and divergence‐free currents in magnetic latitude and magnetic local time for each hemisphere and activity level are presented. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling. This is done in such a manner that there are equal number of samples from each season in each Kp bin. We find that for the low activity level, the currents in the NH are stronger than in the SH by 12±4% for FAC, 9±2% for the horizontal curl‐free current, and 8±2% for the horizontal divergence‐free current. During the high activity level, the hemispheric differences are not statistically significant. This suggests that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation, may be important and play a larger role during quiet than disturbed periods. This issue must be studied further.

Chemical composition and water quality of the Baikal ecosystem in 2018

Analysis of the ion composition in the water of the Baikal ecosystem (Lake Baikal, the Angara River estuary and 35 tributaries) in the spring and autumn of 2018 has shown that the water from Baikal and the Angara estuary maintains low mineralization (94-98 mg/l) and corresponds to the calcium bicarbonate composition. The average chemical composition of the water from the Angara estuary is within the range of minimum-maximum values of the Baikal water. In the water from some tributaries, mineralization is two-three times higher compared to Baikal and the estuary. The average ion composition of the Baikal water intermediates in the range of minimum-maximum values of the studied tributaries. Microbiological studies have shown that the average number of hygiene indicator microorganisms in the water from the tributaries is much higher than in Baikal and the Angara estuary. Unlike the tributaries, the water from Baikal and the Angara estuary contain less number of organotrophic microorganisms. The most extensive Baikal tributaries, which drain the significant and status diverse (anthropogenic and natural) areas around Lake Baikal, definitely have a negative impact on the lake. However, many smaller rivers and streams flowing into Baikal, as well as horizontal and cyclonic currents existing in Baikal, preserve its status as the site with pure drinking water, but only in the pelagic zone of the lake.

Energy Flux Observations in an Internal Tide Beam in the Eastern North Atlantic

AbstractLow‐mode internal waves propagate over large distances and provide energy for turbulent mixing when they break far from their generation sites. A realistic representation of the oceanic energy cycle in ocean and climate models requires a consistent implementation of their generation, propagation, and dissipation. Here we combine the long‐term mean energy flux from satellite altimetry with results from a 1/10° global ocean general circulation model that resolves the low modes of internal waves and in situ observations of stratification and horizontal currents to study energy flux and dissipation along a 1000 km internal tide beam in the eastern North Atlantic. Internal wave fluxes were estimated from twelve 36‐ to 48‐hr stations in along‐ and across‐beam direction to resolve both the inertial period and tidal cycle. The observed internal tide energy fluxes range from 5.9 kW m−1 near the generation sites to 0.5 kW m−1 at distant stations. Estimates of energy dissipation come from both finestructure and upper ocean microstructure profiles and range, vertically integrated, from 0.5 to 3.3 mW m−2 along the beam. Overall, the in situ observations confirm the internal tide pattern derived from satellite altimetry, but the in situ energy fluxes are more variable and decrease less monotonically along the beam. Internal tides in the model propagate over shorter distances compared to results from altimetry and in situ measurements, but more spatial details close the main generation sites are resolved.

Currents Associated With Saturn's Intra‐D Ring Azimuthal Field Perturbations

AbstractDuring the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn's D ring and the denser layers of the planet's atmosphere, revealing the presence of an unanticipated low‐latitude current system via the associated azimuthal perturbation field peaking typically at ~10–30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field‐aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field‐aligned current densities above the ionosphere are typically ~5–10 nA/m2, more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field‐aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north‐south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller‐scale structures, possibly due to rapid temporal or local time variations.

Исследование динамики электрических токов и полярных шапок в ионосферах двух полушарий во время геомагнитной бури 17 августа 2001 г.

The magnetogram inversion technique (MIT), developed at ISTP SB RAS more than forty years ago, has been used until recently only in the Northern Hemisphere. In recent years, MIT has been improved and extended to make instantaneous calculations of 2D distributions of electric fields, horizontal and field-aligned currents in two polar ionospheres. The calculations were carried out based on one-minute ground-based geomagnetic measurements from the worldwide network of stations in both hemispheres (SuperMAG). In this paper, this extended technique is used in the approximation of uniform ionospheric conductance and is applied for the first time to calculations of equivalent and field-aligned currents in two hemispheres through the example of the August 17, 2001 geomagnetic storm. We have obtained the main and essential result: the advanced MIT-ISTP can calculate large-scale distributions of ionospheric convection and FACs in the Northern (N) and Southern (S) polar ionospheres with a high degree of expected hemispheric similarity between these distributions. Using the said event as an example, we have established that the equivalent and field-aligned currents obtained with the advanced technique exhibit the expected dynamics of auroral electrojets and polar caps in two hemispheres. Hall current intensities in polar caps and auroral electrojets, calculated from the equivalent current function, change fairly synchronously in the N and S hemispheres throughout the magnetic storm. Both (westward and eastward) electrojets of the N hemisphere are markedly more intense than respective electrojets of the S hemisphere, and the Hall current in the north polar cap is almost twice as intense as that in the south one. This interhemispheric asymmetry is likely to be due to seasonal conductance variations, which is implicitly contained in the current function. From FAC distributions we determine auroral oval boundaries and calculate magnetic fluxes through the polar caps in the N and S hemispheres. These magnetic fluxes coincide with an accuracy of about 5 % and change almost synchronously during the magnetic storm. In the N hemisphere, the magnetic flux in the dawn polar cap is more intense that that in the dusk one, and vice versa in the S hemisphere. These asymmetries (dawn–dusk and interhemispheric) in the polar caps are consistent with the theory of reconnection for IMF By>0 and with satellite images of auroral ovals; both of these asymmetries decrease during the substorm expansion phase.

Studying the dynamics of electric currents and polar caps in ionospheres of two hemispheres during the August 17, 2001 geomagnetic storm

The magnetogram inversion technique (MIT), developed at ISTP SB RAS more than forty years ago, has been used until recently only in the Northern Hemisphere. In recent years, MIT has been improved and extended to make instantaneous calculations of 2D distributions of electric fields, horizontal and field-aligned currents in two polar ionospheres. The calculations were carried out based on one-minute ground-based geomagnetic measurements from the worldwide network of stations in both hemispheres (SuperMAG). In this paper, this extended technique is used in the approximation of uniform ionospheric conductance and is applied for the first time to calculations of equivalent and field-aligned currents in two hemispheres through the example of the August 17, 2001 geomagnetic storm. We have obtained the main and essential result: the advanced MIT-ISTP can calculate large-scale distributions of ionospheric convection and FACs in the Northern (N) and Southern (S) polar ionospheres with a high degree of expected hemispheric similarity between these distributions. Using the said event as an example, we have established that the equivalent and field-aligned currents obtained with the advanced technique exhibit the expected dynamics of auroral electrojets and polar caps in two hemispheres. Hall current intensities in polar caps and auroral electrojets, calculated from the equivalent current function, change fairly synchronously in the N and S hemispheres throughout the magnetic storm. Both (westward and eastward) electrojets of the N hemisphere are markedly more intense than respective electrojets of the S hemisphere, and the Hall current in the north polar cap is almost twice as intense as that in the south one. This interhemispheric asymmetry is likely to be due to seasonal conductance variations, which is implicitly contained in the current function. From FAC distributions we determine auroral oval boundaries and calculate magnetic fluxes through the polar caps in the N and S hemispheres. These magnetic fluxes coincide with an accuracy of about 5 % and change almost synchronously during the magnetic storm. In the N hemisphere, the magnetic flux in the dawn polar cap is more intense that that in the dusk one, and vice versa in the S hemisphere. These asymmetries (dawn–dusk and interhemispheric) in the polar caps are consistent with the theory of reconnection for IMF By>0 and with satellite images of auroral ovals; both of these asymmetries decrease during the substorm expansion phase.

On the Factors Driving Upper-Ocean Salinity Variability at the Western Edge of the Eastern Pacific Fresh Pool.

The tropical Eastern Pacific Fresh Pool (EPFP) has some of the highest precipitation rates and lowest sea surface salinities found in the open ocean. In addition, the sea surface salinity in the EPFP exhibits one of the strongest annual cycles in the world ocean. The region is strongly affected by the meridionally migrating Intertropical Convergence Zone and is also influenced by large-scale ocean currents and wind-driven Ekman currents. Recognizing the complexity of competing regional influences and the importance of sea surface salinity as an integrator of freshwater forcing, the Salinity Processes Upper-ocean Regional Study (SPURS) was undertaken to better understand how ocean processes and surface freshwater fluxes set surface salinity. Instrumentation on a surface mooring, deployed for 14 months near the western edge of the EPFP, allowed estimation of the surface fluxes of momentum, heat, and freshwater. Subsurface instrumentation on the mooring provided upper-ocean vertical structure and horizontal currents. These observations, along with horizontal gradients of surface salinity from the Soil Moisture Active Passive (SMAP) satellite instrument, were used to estimate the surface-layer salinity budget at the western edge of the EPFP. While the low salinity associated with the presence of the EPFP at the mooring site was sustained by heavy rainfall, it was found that seasonal variability in large-scale currents was important to controlling the transition between the “salty” and “fresh” seasons. Ekman advection was important to prolonging local high salinity as rainfall decreased. Although illuminating some key processes, the temporal variability of the surface-layer salinity budget also shows significant complexity, with processes such as surface freshwater fluxes and vertical mixing making notable contributions. The surface flux term and the terms involving mixing across the base of the surface layer oppose and nearly cancel each other throughout the deployment, such that the horizontal advection term effectively accounts for most of the variability in surface salinity at the site on monthly to seasonal timescales. Further investigation, taking advantage of additional observations during SPURS-2, will be needed to more thoroughly examine the relevant physical processes.

Oceanic heat transport into the Arctic under high and low hbox {CO}_2 forcing

Enhanced ocean heat transport into the Arctic is linked to stronger future Arctic warming and polar amplification. To quantify the impact of ocean heat transport on Arctic climate, it is imperative to understand how its magnitude and the associated mechanisms change in other climate states. This paper therefore assesses the ocean heat transport into the Arctic at 70^{circ }hbox {N} for climates forced with a broad range of carbon dioxide concentration levels, ranging from one-fourth to four times modern values. We focused on ocean heat transports through the Arctic entrances (Bering Strait, Canadian Archipelago, and Nordic Seas) and identified relative contributions of volume and temperature to these changes. The results show that ocean heat transport differences across the five climate states are dominated by heat transport changes in the Nordic Seas, although in the warmest climate state heat transport through the Bering Strait plays an almost equally important role. This is primarily caused by changes in horizontal currents owing to anomalous wind responses and to differential advection of thermal anomalies. Changes in sea ice cover play a prominent role by modulating the surface heat fluxes and the impact of wind stresses on ocean currents. The Atlantic meridional overturning circulation and its associated heat transport play a more modest role in the ocean heat transport into the Arctic. The net effect of these changes is that the poleward ocean heat transport at 70^{circ }hbox {N} strongly increases from the coldest climate to the warmest climate state.