Average Drift Speed Research Articles

Why is summertime Arctic sea ice drift speed projected to decrease?

Abstract. Alongside declining Arctic sea ice cover during the satellite era, there have also been positive trends in sea ice Arctic average drift speed (AADS) during both winter and summer. This increasing sea ice motion is an important consideration for marine transportation as well as a potential feedback on the rate of sea ice area decline. Earlier studies have shown that nearly all modern global climate models (GCMs) produce positive March (winter) AADS trends for both the historical period and future warming scenarios. However, most GCMs do not produce positive September (summer) AADS trends during the historical period, and nearly all GCMs project decreases in September AADS with future warming. This study seeks to understand the mechanisms driving these projected summertime AADS decreases using output from 17 models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) along with 10 runs of the Community Earth System Model version 2 Large Ensemble (CESM2-LE). The CESM2-LE analysis reveals that the projected summertime AADS decreases are due to changes in sea surface height (SSH) and wind stress which act to reduce sea ice motion in the Beaufort Gyre and Transpolar Drift. During March, changes in internal stress and wind stress counteract tilt force changes and produce positive drift speed trends. The simulated wintertime mechanisms are supported by earlier observational studies, which gives confidence that the mechanisms driving summertime projections are likely also at work in the real world. However, the precise strength of these mechanisms is likely not realistic during summer, and additional research is needed to assess whether the simulated summertime internal stress changes are too weak compared to changes in other forces. The projected summertime wind stress changes are associated with reduced sea level pressure north of Greenland, which is expected with the northward shift of the jet streams. The projected summertime SSH changes are primarily due to freshening of the Arctic Ocean (i.e., halosteric expansion), with thermal expansion acting as a secondary contribution. The associated ocean circulation changes lead to additional piling up of water in the Russian shelf regions, which further reinforces the SSH increase. Analysis of CMIP6 output provides preliminary evidence that some combination of wind stress and SSH changes is also responsible for projected AADS decreases in other models, but more work is needed to assess mechanisms in more detail. Altogether, our results motivate additional studies to understand the roles of SSH and wind stress in driving changes in Arctic sea ice motion.

Iceberg drift and melting rates in the northwestern Weddell Sea, Antarctica: Novel automated regional estimates through machine learning.

Global warming and its consequences on polar regions have been thoroughly discussed in recent times. One of those consequences is the freshwater flux and the associated cooling and freshening that result from iceberg melting. Despite the potential impact, large uncertainties exist resulting mostly from the complexity to follow icebergs from space, which make the few existing estimates essentially model-based. This study takes advantage of state-of-art machine learning methods to present novel prevalent trajectories and potential freshwater input from 450 icebergs ranging from 1 to 2765 km2 across the northwestern Weddell Sea, Antarctica. The main results highlight the predominance of a northward flux and the entrance of icebergs up to 10 km2 into Bransfield Strait associated with the main current systems along the Antarctic Peninsula. The present analysis on such a large number of icebergs unveils an average drift speed of 3.4 ± 2.7 km day-1 and an average disintegration rate of ~62% per year, representing an integrated potential regional freshwater input of 133.62 Gt yr-1. Altogether, this study adds new knowledge to the complex problem of autonomous applications for iceberg detection and tracking, further exploring such methods on a very dynamic region of singular importance for the ocean and climate studies.

Arctic sea ice motion change and response to atmospheric forcing between 1979 and 2019

AbstractQuantification of the spatial variability and long‐term changes of Arctic sea ice motion is important for understanding the mechanisms of rapid Arctic sea ice decline because sea ice motion determines ice mass advection, outflow, thickness redistribution, as well as the formation of leads and ridges associated with ice deformation. The spatiotemporal changes in Arctic sea ice motion between 1979 and 2019 and their responses to atmospheric forcing were analysed using satellite‐derived sea ice motion products and atmospheric reanalysis data. The pan‐Arctic average sea ice drift speed increased significantly for all seasons between 1979 and 2019 (p < .001). Rates of increase were higher in autumn and winter than in spring and summer. Spatially, rates of increase in the peripheral seas in the Pacific sector—the Beaufort, Chukchi and East Siberian Seas—were higher than in the central Arctic Ocean and the peripheral seas in the Atlantic sector—the Kara and Laptev Seas. On the contrary, Arctic wind speed increased significantly only in autumn (p < .01). However, the correlation between wind speed and ice speed was the lowest in this season, suggesting that wind forcing is unable to completely account for drift speed increase. In general, the trends in above‐average drift speeds—retrieved from grid cells with the relatively high drift speeds—were statistically significant and were larger than that in average drift speeds probably because of enhanced response of ice motion to extreme wind forcing. The influence of the Arctic Oscillation, Beaufort High, and North Atlantic Oscillation on the zonal ice speed was symmetrical between the Pacific and Atlantic sectors of the Arctic Ocean, while the influence of the Dipole Anomaly and the east–west surface air pressure gradient in central Arctic on the meridional ice speed was distributed in an annular pattern and was the strongest along the Transpolar Drift Stream.

Automated iceberg tracking with a machine learning approach applied to SAR imagery: A Weddell sea case study

Drifting icebergs represent a significant hazard for polar navigation and are able to impact the ocean environment around them. Freshwater flux and the associated cooling from melting icebergs can locally decrease salinity and temperature and thus affect ocean circulation, biological activity, sea ice, and –on larger spatial scales– the whole climate system. However, despite their potential impact, the large-scale operational monitoring of drifting icebergs in sea ice-covered regions is as of today typically restricted to giant icebergs, larger than 18.5 km in length. This is due to difficulties in accurately identifying and following the motion of much smaller features in the polar ocean from space. So far, tracking of smaller icebergs from satellite imagery thus has been limited to open-ocean regions not covered by sea ice. In this study, a novel automated iceberg tracking method, based on a machine learning-approach for automatic iceberg detection, is presented. To demonstrate the applicability of the method, a case study was performed for the Weddell Sea region, Antarctica, using 1213 Advanced Synthetic Aperture Radar (ASAR) satellite images acquired between 2002 and 2011. Overall, a subset of 414 icebergs (3134 re-detections in total) with surface areas between 3.4 km2 and 3612 km2 were investigated with respect to their prevalent drift patterns, size variability, and average disintegration. The majority of the tracked icebergs drifted between 1.3 km and 2679.2 km westward around the Antarctic continent, following the Antarctic Coastal Current (ACoC) and the Weddell Gyre, at an average drift speed of 3.6 ± 7.4 km day−1. The method also allowed us to estimate an average daily disintegration (i.e. iceberg area decrease) rate of ~0.13% (~37% year−1) for all icebergs. Using the sum of all detected individual surface area reductions, we estimate a total iceberg mass decrease of ~683 Gt year−1, which can be freshwater input and/or new ‘child’ icebergs calved from larger icebergs. The extension to an automated long-term tracking method for icebergs is challenging as the iceberg shape can vary significantly due to abrupt disintegration or calving of bergy bits. However, our machine learning approach extended by automatic shape-based tracking capabilities proved to be a reliable alternative for automatic detection and tracking of icebergs, even under the ambiguous SAR background signatures often found in the Southern Ocean. In particular, the method works in the challenging near-coastal environment where the presence of sea ice and coastal ocean dynamics such as surface waves usually pose major obstacles for other approaches.

Statistical analysis of long-lived mesoscale eddies in the Lofoten basin from satellite altimetry

The paper presents a statistical analysis of long-lived detected mesoscale eddies in the Lofoten Basin (LB). An automated eddy identification and tracking method is applied to the altimeter data during the time-period 1993–2017 to detect and track anticyclonic (ACEs) and cyclonic eddies (CEs) in the LB. Our analysis found that only one percent of the eddies detected are long-lived (eddy-life > 35 days). Even though only 1%, the detected 330 long-lived mesoscale eddies (CEs, 120; ACEs, 210) account for >11,550 daily individual eddy-observations. The lifetime, occurrence, generation sites, size, intensity, and drift of these long-lived eddies are quantified. The average drift speed of long-lived eddies is found to show a pronounced seasonal variation with a maximum from October to March. Long-lived eddies in the LB are further divided into four groups based on their region of generation and dissipation. Long-lived eddies generated and dissipated outside the region of the Lofoten Vortex (group 2 eddies) are found to be the predominant type (CEs, 73.3%; ACEs, 69.5%). The eddies generated and dissipated in the region of the permanent Lofoten Vortex form the second dominant type (CEs, 14.3%; ACEs, 26.2%). Based on their lifetime, properties of the two predominant groups of eddies are examined in detail. The difference found in the temporal variability of the eddy characteristics of the two groups reflects their different genesis. The analysis revealed that the mesoscale eddies of group 2 have a longer life than eddies of group 1, and ACEs are more long-lived in comparison to CEs. The analysis also found three main areas of eddy generation in the frontal zone of the NwASC from where mesoscale eddies propagate to the north-west, forming three main corridors of trajectories. The study further provides evidence of long-lived cyclonic CEs surrounding the large quasi-permanent Lofoten Vortex (LV) and forming a shield around it. Small CEs located in two areas with centers at 69.5° N, 4° E, and 70° N, 2.5° E survive nearby the LV resisting the vortex-vortex interaction with the large and strong LV.

Relaxation Processes in Submicron Heterotransistors with a System of Quantum Wells

The analysis of relaxation processes in submicrometer heterostructure transistors with the system of quantum walls is investigated. The methods of nano heterostructure design are developed including quantum effects and specific for ternary alloys scattering mechanism. Modeling based on the relaxation equations system for submicrometer heterostructures with quantum walls is fulfilled.The optical, acoustic, intervalley and doping scattering mechanisms is considered. New approach allows to study the influence of alloy scattering as well as. The relaxation times are expected for scattering mechanisms and the influence on field-speed and output characteristics of submicrometer heterostructure transistor are estimated.The modeling results take into account influence of casual fluctuations of the molar concentration of aluminium in the structure of transistor. The analysis of the scattering mechanisms is considered in the 2D-model of structure, that allows to estimate influence of the separate scattering mechanisms on the electronic gas in heterostructures with the systems of quantum walls. Polar optical phonons scattering is basic in such structures, while in an area gate-drain does not begin to prevail the intervalley scattering mechanism.It is shown that the average drift speed in the heterotransistor with two channel structure are more high, than in heterotransistor with one potential wall and two heterojunctions transistors have higher values of output current.The growth of the average drift speed in a structure with two quantum walls in comparison from heterotransistor with one quantum walls makes to 30-40%. It is related to the redistribution of electrons between potential walls with the loss of the energy at overcoming of barriers.The modeling results shown that warming-up of electronic gas and growth of the electrons temperature is a greater degree related to the electrons which drift in potential walls at heterojunctions.It is also necessary to take into account the dimensional quantization effect. This effect in heterostructure is require the calculation of discrete energy levels in quantum walls.Diminishing of the influence of strong electric-field effects in a two channel structure is related to the diminishing of scattering probability both on polar optical phonons and as a result of the injecting of electrons to lower heterojunction with high initial speed and small values of energy which diminishes also probability of the intervalley scattering.The model of calculation of the quantum walls influence on the longitudinal transport in the heterojunction transistors with submicrometer gate length are developed. The mathematical model can be used for the calculation of not only heterostructures transistor but also other constructions of multi-layered devices with the systems of quantum walls in the conditions of the strong electric fields.Ref. 13, fig. 19.

Reassessing Sea Ice Drift and Its Relationship to Long‐Term Arctic Sea Ice Loss in Coupled Climate Models

AbstractResults of an earlier study suggest that sea ice drift in climate models is unrealistic, and this has undermined confidence in model projections of “long‐term” (i.e., secular) Arctic sea ice loss. We revisit this by analyzing 22 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is shown that, when consistent temporal sampling is applied, sea ice drift speed in models and observations come into closer agreement than previously suggested. There is still considerable intermodel scatter in climatological drift speed, and we show that much of this likely relates to prescribed parameters in the sea ice models. Since 1979, observations show a long‐term positive trend of annual mean Arctic average sea ice drift speed resulting primarily from sea ice thinning, and most of the CMIP5 models qualitatively reproduce this. The simulated annual mean drift speed trends reflect strong cancellation between winter trends (which are positive in most models and in good agreement with observations) and summer trends (which are negative in most models and in poor agreement with observations). Positive Arctic average drift speed trends do not consistently coincide with positive trends of Fram Strait outflow. The simulated regional relationship between sea ice strength and drift speed changes dramatically as the Arctic transitions from full to partial ice cover, and this “sea ice extent effect” likely influences simulated summer drift speed trends. Altogether, these results highlight aspects in which models show encouraging agreement with observations, while pinpointing aspects in which models require improvement.

Iceberg drift and ocean circulation in the northwestern Weddell Sea, Antarctica

Icebergs are distinctive features in the Southern Ocean and can influence the physicochemical properties of the ocean environment around them. Their trajectories are influenced by ocean currents, sea ice, bathymetry and wind action. Here, twenty-five icebergs were identified and tracked by applying image classification methods to a series of 142 Advanced Synthetic Aperture Radar (ASAR) images obtained during 2008 and 2009. Tracking these icebergs made it possible to associate the patterns of iceberg drift with the main ocean currents and front systems in this region, including the Antarctic Coastal Current, the Antarctic Slope Front and the Weddell Front. A case study investigated here revealed the recirculation of a Weddell Sea iceberg within the Bransfield Strait in the Northern Antarctic Peninsula as well as its corresponding mass loss starting in the Weddell Sea and extending throughout the monitoring period. The icebergs studied experienced an average disintegration percentage of 21.15% and an average drift speed of 0.22 ± 0.11 km h–1. The icebergs that drifted at least 500 km had an average meltwater injection rate of 62.76 ± 38.27 m3 s–1 in the ocean around the Northern Antarctic Peninsula.

Nonquasineutral electron vortices in nonuniform plasmas

Electron vortices are observed in the numerical simulation of current carrying plasmas on fast time scales where the ion motion can be ignored. In plasmas with nonuniform density n, vortices drift in the B × ∇n direction with a speed that is on the order of the Hall speed. This provides a mechanism for magnetic field penetration into a plasma. Here, we consider strong vortices with rotation speeds Vϕ close to the speed of light c where the vortex size δ is on the order of the magnetic Debye length λB=|B|/4πen and the vortex is thus nonquasineutral. Drifting vortices are typically studied using the electron magnetohydrodynamic model (EMHD), which ignores the displacement current and assumes quasineutrality. However, these assumptions are not strictly valid for drifting vortices when δ ≈ λB. In this paper, 2D electron vortices in nonuniform plasmas are studied for the first time using a fully electromagnetic, collisionless fluid code. Relatively large amplitude oscillations with periods that correspond to high frequency extraordinary modes are observed in the average drift speed. The drift speed W is calculated by averaging the electron velocity field over the vorticity. Interestingly, the time-averaged W from these simulations matches very well with W from the much simpler EMHD simulations even for strong vortices with order unity charge density separation.

Global‐scale observations of ionospheric convection variation in response to sudden increases in the solar wind dynamic pressure

We have used a superposed epoch analysis to study 205 sudden commencement (SC) events detected with ground‐based magnetometers between the years 2000 and 2007. The strength of the SC events was clearly correlated to the magnitude of the jump in the solar wind dynamic pressure, regardless of whether or not the SC events were followed by a magnetic storm. Data from the Super Dual Auroral Radar Network (SuperDARN) demonstrated that both the ionospheric plasma drift speed and the number of echoes increased in the noon sector in response to the increase in solar wind dynamic pressure. In contrast, the number of SuperDARN echoes in the midnight sector decreased as the solar wind dynamic pressure increased, even though the average drift speed in the midnight sector also increased. We also uncovered that the ionosphere and ring current evolve differently in response to the pressure pulses. The SYM‐H index, which represents changes in both the magnetopause and ring currents, responded immediately and either rapidly returned to pre‐SC values or progressed into the main phase of a geomagnetic storm. In contrast, the ionospheric convection data were affected for a much longer time. The implication is that the ring current reacts to a sudden compression of the magnetosphere on a time scale of 10 min, while the convection pattern itself is affected for as long as the increase in solar wind dynamic pressure is sustained, or until a geomagnetic storm was triggered, as is the case in the sudden storm commencement (SSC) subset of events.

Large parallel electric fields, currents, and density cavities in dispersive Alfvén waves above the aurora

Large parallel electric fields (E∥ > 10 mV/m) are sometimes observed with intense filamentary field‐aligned currents (J∥ > 10 μA/m2 mapped to 100‐km altitude) on transverse scales where kxc/ωpe ≈ 1 above the auroral oval. The ratio of the transverse electric (Ex) and magnetic field (By) variations is approximately the local Alfvén speed and increases with decreasing transverse scale in a manner consistent with inertial Alfvén wave dispersion. These fields are typically observed in association with density cavities. Statistically, it is shown that the depth of the cavities observed over the altitude range from 350 to 4175 km by the Fast Auroral Snapshot (FAST) satellite typically lies between 30 and 50% of the background plasma with 90% depletions being observed for 1% of the recorded events. The wave currents embedded within these cavities exceed 100 μA/m2 for 2% of the ensemble and are typically found over transverse widths of ∼2πλe. Using a fluid description of the plasma, we show statistically that electron pressure gradients provide a probable means for balancing the large E∥ found in these structures with a likely but unknown contribution from anomalous resistivity. Furthermore, we find that the ratio of the average drift speed of the bulk electron plasma (vd) to the electron thermal speed (ve) inside these structures is vd/ve ≥ 0.1 and that in at least 2% of cases veh05d/ve ≥ 1.0. On the basis of recent simulation results and the Bohm criterion for double layer formation, these statistics indicate that the large E∥ sometimes observed in density cavities may occur in double layers driven by the Alfvén wave current.

Tropical Cyclone Motion in Response to Land Surface Friction

Abstract Numerical experiments are performed with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) to study the effects of surface-moisture flux and friction over land on the movement of tropical cyclones (TCs). On an f plane, the TCs are initially placed 150 km due east of a north–south-oriented coastline in an atmosphere at rest. It is found that a TC could drift toward land when the roughness length is 0.5 m over land, with an average drift speed of ∼1 m s−1. Friction, but not surface-moisture flux over land, is apparently essential for the movement toward land. The friction-induced asymmetry in the large-scale flow is the primary mechanism responsible for causing the TC drift. The mechanism responsible for the development of the large-scale asymmetric flow over the lower to midtroposphere (∼900–600 hPa) appears to be the creation of asymmetric vorticity by the divergence term in the vorticity equation. Horizontal advection then rotates the asymmetric vorticity to give a northeasterly flow in the TC periphery (∼500–1000 km from the TC center). The flow near the TC center has a more northerly component because of the stronger rotation by the tangential wind of the TC at inner radii. However, the TC does not move with the large-scale asymmetric flow. Potential vorticity budget calculations indicate that while the horizontal advection term is basically due to the effect of advection by the large-scale asymmetric flow, the diabatic heating and vertical advection terms have to be considered in determining the vortex landward drift, because of the strong asymmetry in vertical motion. Two mechanisms could induce the asymmetry in vertical motion and cause a deviation of the TC track from the horizontal asymmetric flow. First, the large-scale asymmetric flow in the upper troposphere differs from that in the lower troposphere, both in magnitude and direction, which results in a vertical shear that could force the asymmetry. A vertical tilt of the vortex axis is also found that is consistent with the direction of shear and also the asymmetry in rainfall and vertical motion. Second, asymmetric boundary layer convergence that results from the internal boundary layer could also force an asymmetry in vertical motion.

Locations of auroral kilometric radiation bursts inferred from multispacecraft wideband Cluster VLBI observations. 1: Description of technique and initial results

The Cluster Wideband Data instrument has been used to determine the locations of auroral kilometric radiation (AKR) using very long baseline interferometry. The technique involves cross‐correlating individual AKR bursts from all six Cluster baselines using time and frequency filtered waveforms. The resulting differential delay peaks are used to determine source locations with an uncertainty as small as 500 km in a plane perpendicular to the source‐spacecraft line of sight or about 200 km when the burst position is projected onto the auroral zone along the magnetic field lines passing through the source. The uncertainty along the line of sight is much larger, but this is mitigated by assuming that the emission arises from a height corresponding to the electron gyrofrequency. We report the locations of over 1700 individual AKR bursts during six observing epochs between 10 July 2002 and 22 January 2003 when the Cluster constellation was high above the Southern or Northern Hemisphere. In general we find that the AKR burst locations lie along magnetic field lines which map onto the nighttime auroral zone as expected from previous AKR studies. For the three observing epochs viewing the Northern Hemisphere, there is a strong tendency for AKR burst locations to be centered within the auroral oval and in the evening sector. The Southern Hemisphere burst locations favor magnetic local midnight to early morning and have somewhat higher invariant magnetic latitudes. The distribution of AKR auroral footprint locations at each epoch had a overall spatial scale between 1000 and 2000 km, much larger than the positional uncertainty of an individual AKR burst location magnetic footprint, but a small fraction of the auroral oval. This indicates that on a timescale of 1–3 hours conditions for suitable generation of AKR emission are found on only a fraction of all magnetic field lines crossing through the auroral oval. For two of the six epochs, there was a significant drift in the mean location of AKR activity over a period of 1–2 hours. The drift was predominantly in latitude at one epoch and in longitude at the other, with average drift speed V ∼ 80–90 m s−1 at the AKR emission location.

Statistical study of auroral spirals

We present results of a statistical study of 189 auroral spiral forms. The data set was collected by a systematic examination of the images recorded at five all‐sky camera (ASC) stations in northern Fennoscandia and Svalbard during the winters 1996–1997 and 1997–1998. For every spiral we defined the time of occurrence, diameter, direction of motion, arid distance between the adjacent spirals (wavelength) in cases of spiral streets. The magnetic activity level was determined by using the magnetic north component (Bx) recorded at the ASC station and the local auroral electrojet index (IL index) computed by using the data from the IMAGE magnetometer network. The typical values of the diameter, wavelength, and ratio between diameter and wavelength were 25–75 km, 125‐175 km, and 2–6, respectively. According to our ASC data set, spirals are more common in the magnetic postmidnight sector than previous satellite observations suggest. Spirals drift predominantly in the same direction as the large‐scale ionospheric convection. Our observations of the average drift speed of spirals of 4 km/s are comparable to the corresponding values of other auroral small‐scale structures. Statistical analysis revealed that spirals occur most frequently under magnetically quiet (IL > −200 nT) conditions, and thus they cannot always be considered as precursors of substorm activity.

Investigation with surface drift-cards to identify the wind effects on the oceanic conditions of the eastern Seto Inland Sea.

In this paper, we discuss wind-induced surface drift inferred from drift-card returns and its implications on the variation of the oceanic conditions in the eastern Seto Inland Sea: the Sea of Harima and Osaka Bay. The wind stress factor estimated for the average drift speed of the cards was about 0.02, which suggests the wind effect is the most dominant influ nce on the movement of the surface water in this sea. In fact, the distribution of low salinity water on the surface of Osaka Bay showed quite different patterns according to the prevailing winds. This may be due to the effect of the wind-induced current coupled with a density current which is found throughout the year. It is also noticeable that prevailing W to NW winds which blow towards the southern coast of Osaka Bay will reduce the advective speed and dispersion of low salinity water, which may result in comparatively small water-exchange in winter. On the other hand, the northward drift of the surface water which is induced by a strong southerly wind in summer possibly cause an upwelling of the bottom water on the southern coast of the Sea of Harima.

Latitude Variation of the Effect of Magnetic Activity on Ionospheric Drifts

THE effect of magnetic activity on horizontal drifts in the ionosphere has been reported by several investigators1. This communication deals with the effect of magnetic activity on ionospheric drifts at various latitudes with the objective of making apparent the variation in drift speeds with latitude under active magnetic conditions. We have used data on horizontal drifts available from World Data Centers for the IGY period and the drift stations are selected so as to represent a particular range of latitude. The drift speeds and the corresponding K index values are tabulated for all the drift recording stations and corresponding nearest magnetic observatories (Table 1). The tabulated values are then averaged and the average drift speed values for each corresponding K index are plotted as shown in Fig. 1.

Irregularities and Their Horizontal Drifts in the Night Time F Region of the Ionosphere over Ahmedabad

Results of horizontal drift of the irregularities in the night time F region of the ionosphere over Ahmedabad are presented in this paper. Average drift speed of the irregularities is of the order of 65 M/s. The most probable direction of drift is towards south east.Irregularities seem to show considerable elongation and are found to align themselves in the direction NNE.Size of the irregularities is found to be larger than the wavelength of the radio waves used for the investigations.