Effects of the hydroelectric developments on the oceanographic surface parameters of Hudson Bay

Abstract

A one‐dimensional oceanic mixed‐layer model was used to simulate the annual surface layer properties of Hudson Bay. The model reproduces the sparse available data well and shows the equal importance of seasonal ice cover and run‐off on the pycnocline pattern. In spring, the large freshwater input from run‐off and local ice melt followed by summer heating slows the deepening of the pycnocline depth by wind mixing. As these stabilizing effects decrease and the wind strength increases, the pycnocline depth increases in the fall and continues to increase in the winter when the salt rejection effect during ice growth replaces the cooling effect. In the spring the salt rejection reduces and run‐off increases; the large pycnocline depth cannot be maintained and a shallow pycnocline is formed, starting a new seasonal cycle.

When the run‐off cycle includes the effects of hydroelectric developments, the results indicate that a new shallow surface pycnocline is formed earlier in the spring. This causes a decrease in surface layer temperature and salinity, thus stimulating more ice growth. On the other hand, in the summer the surface layer salinity is higher and the temperature lower. This decreases the stability, thus further deepening the pycnocline and increasing the deviations from normal conditions.     

Coupled ice‐ocean variability in the Greenland Sea

Abstract

A major surface feature of the Greenland Sea during winter is the frequent eastward extension of sea ice south of 75°N and an associated embayment to the north. These features are nominally connected with the East Greenland Current, and both the promontory and the embayment are readily apparent on climatic ice charts. However, there are significant changes in these features on time‐scales as short as a few days. Using a combination of satellite microwave images (SSM/I) of ice cover, meteorological data and in situ velocity, temperature and salinity records, we relate the ice distribution and its changes to the developing structure and circulation of the upper ocean during winter 1988–1989. Our measurements illustrate the preconditioning that leads to convective overturn, which in turn brings warmer water to the surface and results in the rapid disappearance of ice. In particular, the surface was cooled to the freezing point by early December and the salinity then increased through ice formation (about 0.016 m d^‐1) and brine rejection. Once the vertical density gradient was sufficiently eroded, a period of high heat flux (>300 W m^‐2) in late January provided enough buoyancy loss to convectively mix the upper water column to at least 200 m. We estimate vertical velocities at about 3 cm s^‐1 downward during the initial sinking. The deepening of the thermocline raised surface temperatures by over 1°C resulting in nearly 1.5 × 10⁵ km² of ice‐melt within two days. Average rates of ice retreat are about 11 km d^‐1 southwestward, generally consistent with a wind‐driven flow. Comparison of hydrographic surveys from before and after the overturning indicate the fresh water was advected out of the area, possibly to the south and east of our moorings.     

The effect of wind and temperature to phytoplankton biomass during blooming season in Barents Sea

The Barents Sea is the most productive sea in the Arctic. The main causes of phytoplankton spring blooms are studied for a decadal time period of 2003–2013 at the region of (70 °N-80 °N, 30 °E-40 °E) in Barents Sea. Due to the rapidly ice melt in the southern region (70 °N-75 °N), almost no ice left after year 2005, sea surface temperature (SST) and wind speed (WIND) are two main dominant factors influencing phytoplankton blooming in the southern region. Ice melt is another important factor of phytoplankton blooming in the northern region (75 °N–80 °N). SST and CHL had positive correlations during blooming season but negative correlations during summer time. The lower SST in spring could result in earlier blooming in the region. Higher SST and higher WIND could result in later blooming. Positive NAO after April 2013 caused higher SST in 2013. Increasing WIND would cause CHL reduced accordingly. Blooming period is from late April to late May in the southern region, and 1–2 weeks later in the northern region. During blooming season, SST was less than 4 °C and WIND was less than 10 m/s. The higher winds (over 15 m/s) in early spring would brought more nutrients from bottom to surface and cause higher blooming (near 10 mg/m³ in year 2010) after WIND is reduced to 5−8 m/s. Higher WIND (around 10 m/s) could generate longer blooming period (more than a week) during late May in the southern region. Decrease of WIND and increase of melting ice, with slightly increase of SST and decrease of mixed layer depth (MLD), are all the factors of phytoplankton blooming in late spring and early summer.     

Detection of the Labrador current using ice‐floe movement in synthetic aperture radar imagery and ice beacon trajectories

Abstract

Two sets of Synthetic Aperture Radar (SAR) images were collected, as part of the Labrador Ice Margin Experiment (LIMEX), over the Newfoundland Shelf on consecutive days in April 1990. Ice movement is detected from the displacement of ice floes between the two images sets and compared with ice drift data from six satellite‐tracked beacons and in situ CTD data. The ice velocity data derived from the SAR images and the beacons are used to generate a map of ice velocity vectors. A streamfunction map of ocean currents is produced by removing the direct wind‐driven component in the ice movement data, and by using an objective analysis method. The resulting flow pattern contains the offshore branch of the Labrador Current with a speed of 30 to 50 cm s^?1. The current closely follows the shelf break topography from north to south through the study area (47–50.5°N) as a continuous flow. In comparison, if the wind effect was not removed from the ice velocity data, the calculated Labrador Current north of 50°N would stray from the shelf break. The position of the current axis and the current speed derived from the ice movement data are in good agreement with the geostrophic current computed from the CTD data.     

Wind Forcing of Ice Cover in the Labrador Shelf Marginal Ice Zone

Abstract

Anemometer‐measured winds for the period 5–13 March 1994 were used to study the coherence of observed and forecast coastal winds along the mid‐Labrador shelf. The reliability of these variables in predicting the response of the ocean and ice to wind forcing is an important issue for ice forecasting in this area. Two anemometer‐equipped 2‐m ice beacons were deployed on pack ice north of Wolf Island and a third beacon was deployed on Grady Island. The results indicate that due to the influence of local topography, 10‐m winds observed at the meteorological station in Cartwright, Labrador provide a poor estimate (r² = 0.2) of wind conditions over the offshore sea‐ice. In contrast, the σ = 1 level (～10 m) winds from the Canadian Meteorological Centre's Regional Finite Element (RFE) model provided a better correlation with anemometer beacon winds (0.90 for the 6‐hour forecast down to 0.45 at 36 hours). However, the RFE model overestimates the magnitude of the winds by 10–40%.

The response of the ocean and ice cover to wind forcing was measured by an ocean bottom‐mounted acoustic Doppler current proþler (ADCP). Relative to the 2‐m beacon winds, the ice moved at 2.5% of wind magnitude and turned 0.6° to the left of the wind. The ocean response decreased with depth until it reached a constant value of 0.9% of the wind speed. The turning angle increased from 0.3° to the right of the wind at 3.5 m to 50° at the lowest level measured by the ADCP (73 m depth). Approximately 57% of the variance in the ocean currents at 3 m below the surface can be attributed to the 2‐m winds; at 73 m the explained variance decreases to 27%.     

Ice drift in Southern Baffin Bay and Davis Strait: Research note

Abstract

Using satellite pictures of Baffin Bay and Davis Strait, ice‐floes were tracked in order to give weekly surface velocities for 1978–1979. The approximate location of the edge of the ice sheet was also determined.

In winter the direction of travel was mainly southward in Davis Strait then, as the summer approached, the edge of the ice sheet retreated northward and floe motion became less clearly defined — even going north on occasion in Baffin Bay.

Near shore speeds along Baffin Island exceeded 50 cm s^‐1 in Davis Strait during November and February. Typical values in the winter/spring period were 10–15 cm s^‐1 between Davis Strait and Hudson Strait. Wind records at nearby shore stations showed directions to be mainly from the northwest, roughly parallel to the Baffin Island coastline.

The study confirms the usefulness of satellite pictures as a data source for modelling surface ice movement and for selecting navigation routes in these northern waters.     

Spatial and Temporal Variability of Ocean Temperature over the Labrador Shelf

Abstract

The mid‐to‐bottom waters of the Labrador Shelf are shown to exhibit an anomalous along‐shelf temperature gradient, with warmer waters found in the north. This feature is present in summer and autumn but appears to reverse in December. Inadequate data are available during winter and spring to draw firm conclusions regarding this feature. A time averaged heat loss of the shelf waters to the atmosphere would result on average, in colder waters in the south (because of north‐south advection); however, it is shown that there is a net annual‐mean input of heat to the shelf waters. An examination of the seasonal temperature cycle at standard depths reveals that its phase is almost uniform below 30‐m depth on the northern banks of the Labrador Shelf. The limited phase variation suggests the influence of a plume of well mixed water originating near Hudson Strait. It thus appears that mixing at the entrance to Hudson Strait imparts a phase anomaly to the seasonal cycle in the north that contributes to the observed inversion of the expected latitudinal temperature gradient.     

The formation and maintenance of the North Water Polynya

Abstract

This paper investigates the formation and maintenance of the North Water Polynya, Baffin Bay in winter using a multi‐category sea‐ice model coupled with the Princeton ocean model. Monthly climatological atmospheric data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis provides the forcing. An objectively‐analysed climatology provides the initial ocean temperature and salinity. Wind stress drives the ice in a cyclonic gyre around northern Baffin Bay. Localized regions of thin ice form where wind drives ice away from coastlines or fast ice. The regions of thin ice are characterized by enhanced ice growth, exceeding 1.2 m mo^?1. In the regions of thin ice, surface ocean heat flux is also enhanced and is between 30–60 W m^?2. Surface heat flux is, in part, attributable to convective mixing and entrainment driven by ice growth. The surface heat flux reflects advection of the warm West Greenland Current. Heat and salt balances show that horizontal advective exchange counterbalances surface fluxes of heat and salt.     

Laboratory simulation of the ocean currents in the Barents sea

A rotating laboratory model of the Barents Sea was forced by computed inflows of Atlantic Water and Arctic Surface Water for the period 1979–1984. Ad hoc tidal excursions over the shoals north of Bear Island and deep water production as a result of winter cooling and salt rejection in the eastern part of the basin were calibrated in the model. The high spatial resolution in the basin, which was 5 m in diameter, provided the basis for simulating several physical scales simultaneously. The simulated current features of interest include (1) the spreading of the Norwegian Coastal Current over Tromsøflaket, (2) a warm-core jet along the southeastern slope of the Svalbardbanken, which pushes the ice front far to the NE of Hopen Island, (3) the anticyclonic circulation around Sentralbanken, which drives Arctic Surface Water and ice far south in the eastern basin, (4) Norwegian Coastal Water flowing north across the Bear Island Channel, (5) deep water outflows north through the Franz-Victoria Trough and west through the Bear Island Channel, (6) the dependence of dense water accumulation and flushing on the variable Atlantic inflow, and (7) a robust, tidally driven circulation on the Svalbardbanken and around Bear Island. The Polar Front along the Svalbardbanken is fairly stationary, although its location is highly variable in the Sentralbanken area as a result of underflows (and winds—which were not simulated). The residence time for the Arctic Surface Water on Sentralbanken is about 8 months. Comparisons with available field measurements show a validation that is better than existing numerical model simulations.Entrainment of Arctic Surface Water on Svalbardbanken to the Atlantic inflow holds the Polar Front sharp and modifies the Atlantic Water as it flows to the Arctic Ocean. The simulated warm-core jet along this slope had a core speed up to 85 cm s⁻¹, whereas the best available current measurements near the core show surges up to about 30 cm s⁻¹. The simulated vorticity of the current is −0.33f, where f is the planetary vorticity. This can be provided from the conservation of potential vorticity. Both field data and laboratory simulations show that particles trapped in the Bear Island Current take 5–8 days to circle the island, which is 20 km in diameter. Except for surface confetti, agreement between model and field data was good for the southern flow east of Sentralbanken, but poor for the Murman Current. A model ‘wind’ caused a significant departure in this region and may be responsible for an exaggerated warm-core jet past Svalbardbanken.     

On the relationship between Arabian Sea warm pool and formation of onset vortex over east-central Arabian Sea

The interannual variability in the formation of mini warm pool (MWP, SST ≥ 30.5°C) and its impact on the formation of onset vortex (OV) over the east-central Arabian Sea (ECAS) are addressed by analyzing the NCEP OIV 2-weekly SST data and NCEP–NCAR reanalysis 850 hPa wind fields from May to June (prior to the onset of monsoon) over the north Indian Ocean for a period of 12 years from 1992 to 2003. Strong interannual variability in the formation and intensification of MWP was observed. Further, the 850 hPa wind fields showed that OV developed into an intense system only during 1994, 1998 and 2001. It formed in the region north of the MWP and on the northern flank of the low-level jet axis, which approached the southern tip of India just prior to the onset of monsoon, similar to the vortex of MONEX-79. The area-averaged zonal kinetic energy (ZKE) over the ECAS (8–15°N, 65–75°E) as well as over the western Arabian Sea (WAS, 5°S–20°N, 50–70°E) showed a minimum value of 5–15 m² s^?2 prior to monsoon onset over Kerala (MOK), whereas a maximum value of 280 m² s^?2 (40–70 m² s^?2) was observed over the ECAS (WAS) during and after MOK. The study further examined the plausible reasons for the occurrence of MWP and OV.     

Summer circulation of the waters in Queen Charlotte sound

Abstract

An extensive set of measurements of currents, winds, subsurface pressures and water properties was undertaken in the summer of 1982 in Queen Charlotte Sound on the west coast of Canada. At most observation sites the summer‐averaged currents are found to be about 10 cm s^?1, smaller than the tidal currents but comparable to the standard deviation of the non‐tidal currents. The strongest average flow was the outflow of surface water past Cape St James at the northwestern corner of the Sound. During strong winds from the north or northwest a strong outflow of near‐surface fresher water was also observed over Cook Bank in the south. Eddies dominate the motion in the interior of the Sound, as shown by the behaviour of a near‐surface drifter that remained in mid‐Sound for 40 days before a storm pushed it into Hecate Strait. The disorganized, weak currents in the central Sound will likely allow surface waters or floating material to remain there for periods of several weeks in summer.

Empirical orthogonal function analyses of fluctuating currents, subsurface pressures and winds reveal that a single mode explains most of the wind and pressure variance but not the current variance. The first two pressure modes represent two distinct physical processes. The first mode is a nearly uniform, up‐and‐down pumping of the surface, while the second mode tilts across the basin from east to west, likely due to geostrophic adjustment of wind‐driven currents. This mode also tilts from south to north, owing to along‐strait wind stress. Most contributions to the first mode currents come from meters near shore or the edge of a trough. Coherence is high between these second mode pressures and first mode currents and winds, and lower but still significant between first mode pressures and first mode currents and winds. It is therefore difficult to predict the behaviour of currents in Queen Charlotte Sound in summer from pressure measurements at a single site, but the difference in sea‐level across Hecate Strait is a more reliable indicator.     

Glacial/interglacial changes in the East Australian current

The East Australian Current (EAC) is the western boundary current of the south Pacific gyre transporting warm tropical waters to higher southern latitudes. Recent modelling shows that the partial separation of the EAC (~32°S) and the coupled formation of the Tasman Front (~34°S) are caused by a steep gradient in the zonally integrated wind stress curl. Analysis of oxygen isotope ratios (δ¹⁸O) in the planktonic foraminifer, Globigerinoides ruber, from sediment cores from the Coral Sea and Tasman Sea indicates that the EAC separation shifted northward to between 23 and 26°S during the last glacial. We suggest these results indicate a significant change in the Pacific wind stress curl during the glacial. Given recent evidence for El Niño-like conditions in the Pacific during the last glacial, with a reduction in the east–west sea surface temperature (SST) gradient, we suggest that weaker trade winds combined with more northerly, stronger westerlies were associated with a change to the wind stress curl, which repositioned the EAC separation and Tasman Front. In contrast, by ~11 ka BP, the EAC separation was forced south of 26°S. This southward shift was synchronous with a rapid warming of tropical SSTs, and the onset of a La Niña-like SST configuration across the tropical Pacific. It appears that the south Pacific trade winds strengthened accordingly, causing the EAC to readjust its flow. This readjustment of the EAC marks the onset of modern surface-ocean circulation in the southwest Pacific, but the present EAC transport was only achieved in the late Holocene, after 5 ka BP.     

Spatio‐temporal variability in a mid‐latitude ocean basin subject to periodic wind forcing

Abstract

The mid‐latitude ocean's response to time‐dependent zonal wind‐stress forcing is studied using a reduced‐gravity, 1.5‐layer, shallow‐water model in two rectangular ocean basins of different sizes. The small basin is 1000 km × 2000 km and the larger one is 3000 km × 2010 km; the aspect ratio of the larger basin is quite similar to that of the North Atlantic between 20°N and 60°N. The parameter dependence of the model solutions and their spatio‐temporal variability subject to time‐independent wind stress forcing serve as the reference against which the results for time‐dependent forcing are compared.

For the time‐dependent forcing case, three zonal‐wind profiles that mimic the seasonal cycle are considered in this study: (1) a fixed‐profile wind‐stress forcing with periodically varying intensity; (2) a wind‐stress profile with fixed intensity, but north–south migration of the mid‐latitude westerly wind maximum; and (3) a north–south migrating profile with periodically varying intensity. Results of the small‐basin simulations show the intrinsic variability found for time‐independent forcing to persist when the intensity of the wind forcing varies periodically. It thus appears that the physics behind the upper ocean's variability is mainly controlled by internal dynamics, although the solutions’ spatial patterns are now more complex, due to the interaction between the external and internal modes of variability. The north–south migration of wind forcing, however, does inhibit the inertial recirculation; its suppression increases with the amplitude of north–south migration in the wind‐stress forcing.

Model solutions in the larger rectangular basin and at smaller viscosity exhibit more realistic recirculation gyres, with a small meridional‐to‐zonal aspect ratio, and an elongated eastward jet; the low‐frequency variability of these solutions is dominated by periodicities of 14 and 6–7 years. Simulations performed in this setting with a wind‐stress profile that involves seasonal variations of realistic amplitude in both the intensity and the position of the atmospheric jet show the seven‐year periodicity in the oceanic circulation to be robust. The intrinsic variability is reinforced by the periodic variations in the jet's intensity and weakened by periodic variations in the meridional position; the two effects cancel, roughly speaking, thus preserving the overall characteristics of the seven‐year mode.     

Inertial oscillations near the coast of Nova Scotia during CASP

Abstract

Analysis of current, temperature and salinity records in the nearshore region of the Scotian Shelf during the Canadian Atlantic Storms Program (CASP), reveals that the inertial wave field is highly intermittent, with comparable amplitudes in the surface and deep layers. Clockwise current energy in the surface layer is concentrated at a frequency slightly below inertial, consistent with Doppler shifting by the strong mean current and/or straining by the mean flow shear, whereas the spectral peak in deep water is at the local inertial frequency. Clockwise coherence is high (γ² ≥ 0.8) horizontally over the scale of the array (60 km × 120 km) and in the vertical, with upward phase propagation rates of 0.15–0.50 × 10^?12 ms^?1, inversely proportional to the local value of the Brunt Väisälä frequency. Clockwise current energy decreases in the onshore direction and appears to be completely inhibited on the 60‐m isobath.

A case study of the response to the CASP IOP 14 storm indicates that the inertial waves may be generated by a strong wind shift propagating onshore at a speed of 10 ms^?1. On the eastern side of the array (Liscomb line), clockwise current oscillations propagate onshore in the surface layer at a rate (8.1 ± 0.9 m s^?1) comparable with the speed of the atmospheric front, while waves in the pycnocline move offshore at a lower (internal wave) speed (1.8 m s^?1). Furthermore the temperature and salinity fluctuations are in (out) of phase with longshore current in the deep (surface) layer. However, on the western side of the array (Halifax line), the inertial waves are more complex. A sharp steepening of phase lines at the coast indicates that the phase speed of clockwise current oscillations is considerably reduced and the evidence for offshore propagation of internal waves is less clear. The discrepancies between observations on the two lines suggest that the internal wave field is three‐dimensional.

Results of simple mixed‐layer models indicate that the inertial response near the surface is sensitive to the accurate definition of the local wind field, but not to certain model physics, such as the form of the decay term. The observations also show some qualitative similarities with models for two‐dimensional response to a moving front (e.g. Kundu, 1986), but the actual forcing terms are more complicated, based on IOP 14 wind measurements.     

Generation and atmospheric heat exchange of coastal polynyas in the Weddell Sea

The forcing mechanisms for Antarctic coastal polynyas and the thermodynamic effects of existing polynyas are studied by means of an air-sea-ice interaction experiment in the Weddell Sea in October and November 1986.Coastal polynyas develop in close relationship to the ice motion and form most rapidly with offshore ice motion. Narrow polynyas occur frequently on the lee side of headlands and with strong curvature of the coastline. From the momentum balance of drifting sea ice, a forcing diagram is constructed, which relates ice motion to the surface-layer wind vector v _z and to the geostrophic ocean current vector c _g . In agreement with the data, wind forcing dominates when the wind speed at a height of 3 m exceeds the geostrophic current velocity by a factor of at least 33. This condition within the ocean regime of the Antarctic coastal current usually is fulfilled for wind speeds above 5 m/s at a height of 3 m.Based on a nonlinear parameter estimation technique, optimum parameters for free ice drift are calculated. Including a drift dependent geostrophic current in the ice/water drag yields a maximum of explained variance (91%) of ice velocity.The turbulent heat exchange between sea ice and polynya surfaces is derived from surface-layer wind and temperature data, from temperature changes of the air mass along its trajectory and from an application of the resistance laws for the atmospheric PBL. The turbulent heat flux averaged over all randomly distributed observations in coastal polynyas is 143 W/m². This value is significantly different over pack ice and shelf ice surfaces, where downward fluxes prevail. The large variances of turbulent fluxes can be explained by variable wind speeds and air temperatures. The heat fluxes are also affected by cloud feedback processes and vary in time due to the formation of new ice at the polynya surface.Maximum turbulent fluxes of more than 400 W/m² result from strong winds and low air temperatures. The heat exchange is similarly intense in a narrow zone close to the ice front, when under weak wind conditions, a local circulation develops and cold air associated with strong surface inversions over the shelf ice is heated above the open water.     

Decadal variability of characteristics of the Black Sea pycnocline and geostrophic circulation in the wintertime

The Black Sea density fields and geostrophic circulation in the 0–300-dbar layer are reconstructed from the February archive hydrological data for several decades from 1951 to 1995 and then their interdecadal variability is studied. A gradual rise of the pycnocline is noted in the dome region of large-scale cyclonic gyrals from 5 m at the top of the pycnocline to 10–15 m at 100–300-m levels. Differently directed tendencies of long-term variability of the winter circulation are revealed: circulation intensification in the upper 0–50-m layer (except for the southwestern sea) and weakening in the lower pycnocline 200–300-m layer. The connection with the wind vorticity variability, river runoff, precipitation, and air temperature is analyzed. Strengthening of the cyclonic wind vorticity in the 1960s and the early 1970s is in good agreement with the circulation intensification of that time in the eastern sea.     

Airborne surveys of the atmospheric boundary layer above the marginal ice zone on the Newfoundland shelf

Abstract

Airborne measurements in the atmospheric boundary layer (ABL) above the marginal ice zone (MIZ) on the Newfoundland Shelf reveal strong lateral variations in mean wind, temperature and the vertical fluxes of heat and momentum under conditions of cold, off‐ice wind. Flux measurements in (and near) the surface layer indicate that the neutral 10‐m drag coefficient depends on ice concentration, ranging from 2 × 10^‐3 at 10% coverage to 5 × 10^‐3 at 90%. Furthermore, cross‐ice‐edge transects consistently show increasing wind speed, temperature and heat flux in the off‐ice direction, but the momentum flux may either increase or decrease, depending on the relative importance of surface buoyancy flux and roughness. For the conditions encountered in this experiment, it appears surface wave maturity does not have a significant influence on the drag coefficient in fetch‐limited regimes near the ice edge.     

Are polynyas self‐sustaining?

Abstract

In this paper, 441 Conductivity Temperature Depth (CTD) casts from the North Water (NOW) Polynya study were used to calculate geostrophic currents between the 10 and 200 dbar surface during April, May and June 1998. Results for April and May indicated a surface intensified southward flow of 10 to 15 cm s^–1 with a small return flow along the Greenland coast in agreement with inferred currents described by Melling et al. (2001) and surface ice drifts found by Wilson et al. (2001). Southward transports at this time were 0.4–0.55 Sv in April and May. In June, however, surface currents diminished markedly: southward transports declined to 0.1–0.35 Sv, coincident with a decrease in directly measured winds over the polynya and in the surface barometric pressure difference between Grise Fjord and the Carey Islands that was used as a surrogate for the local north wind speed. There was no evident decrease in air pressure difference between Resolute and Grise Fjord, indicative of the strength of the north wind over the eastern Arctic in general. The results are consistent with present thinking that the NOW Polynya is primarily a latent heat polynya, forced by dominant north winds. The idea, broached here, is that the polynya creates its own microclimate which sustains the polynya's ice‐free condition after its initial formation. The mechanism is identified by an anomalous low pressure region associated with surface buoyancy flux in the polynya and is pursued through the application of a simple geostrophic adjustment model that suggests two self‐sustaining mechanisms. Firstly, the frontal intrusion of the cold ambient terrestrial air mass drives a significant surface wind that transports frazil ice to the edge of the polynya before it can congeal. Secondly, rotation at these high latitudes restricts the penetration of the front into the polynya, essentially insulating the centre from freezing temperatures.     

Vertical Distribution Characteristics of PM<Subscript>2.5</Subscript> Observed by a Mobile Vehicle Lidar in Tianjin,China in 2016

We present mobile vehicle lidar observations in Tianjin, China during the spring, summer, and winter of 2016. Mobile observations were carried out along the city border road of Tianjin to obtain the vertical distribution characteristics of PM_2.5. Hygroscopic growth was not considered since relative humidity was less than 60% during the observation experiments. PM_2.5 profile was obtained with the linear regression equation between the particle extinction coefficient and PM_2.5 mass concentration. In spring, the vertical distribution of PM_2.5 exhibited a hierarchical structure. In addition to a layer of particles that gathered near the ground, a portion of particles floated at 0.6–2.5-km height. In summer and winter, the fine particles basically gathered below 1 km near the ground. In spring and summer, the concentration of fine particles in the south was higher than that in the north because of the influence of south wind. In winter, the distribution of fine particles was opposite to that measured during spring and summer. High concentrations of PM_2.5 were observed in the rural areas of North Tianjin with a maximum of 350 μg m^–3 on 13 December 2016. It is shown that industrial and ship emissions in spring and summer and coal combustion in winter were the major sources of fine particles that polluted Tianjin. The results provide insights into the mechanisms of haze formation and the effects of meteorological conditions during haze–fog pollution episodes in the Tianjin area.     

On the interannual variability of arctic sea‐level pressure and sea ice

Abstract

Monthly mean sea‐level pressure (SLP) data from the Northern Hemisphere for the period January 1952‐December 1987 are analysed. Fluctuations in this field over the Arctic on interannual time‐scales and their statistical association with fluctuations farther south are determined. The standard deviation of the interannual variability is largest compared with that of the annual cycle along the seaboards of the major land masses. The SLP anomalies are generally in phase over the entire Arctic Basin and extend south over the northern Russia and Canada, but tend to be out of phase with fluctuations at mid‐latitudes. The anomalies are most closely associated with fluctuations over the North Atlantic and Europe except near the Chukchi Sea to the north of Bering Strait. The associations with the North Pacific fluctuations become increasingly more prominent at most Arctic sites (e.g. the Canadian Arctic Archipelago) as the time‐scale increases.

Associations between the SLP fluctuations and atmospheric indices that represent processes affecting sea‐ice drift (wind stress and wind stress curl) are determined. In every case local associations dominate, but some remote ones are also evident. For example, changes in the magnitude of the wind stress curl over the Beaufort Sea are increased if the atmospheric circulation over the North Pacific is intensified; wind stress over the region where sea ice is exchanged between the Beaufort Gyre and the Transpolar Drift Stream is modulated by both the Southern and North Atlantic Oscillations.

Severe sea‐ice conditions in the Greenland Sea (as measured by the Koch Ice Index) coincide with a weakened atmospheric circulation over the North Atlantic.