recurrence statistics of great earthquakes 1 we investigate the sequence of great earthquakes over the past century to examine whether the earthquake record includes temporal clustering we identify aftershocks and remove those from the record we focus on the recurrence time defined as the time between two consecutive earthquakes we study the variance in the recurrence time and the maximal recurrence time using these quantities we compare the earthquake record with sequences of random events generated by numerical simulations while systematically varying the minimal earthquake magnitude mmin our analysis shows that the earthquake record is consistent with a random process for magnitude thresholds 7 0 mmin 8 3 where the number of events is larger interestingly the earthquake record deviates from a random process at magnitude threshold 8 4 mmin 8 5 where the number of events is smaller however this deviation is not strong enough to conclude that great earthquakes are clustered overall the findings are robust both qualitatively and quantitatively as statistics of extreme values and moment analysis yield remarkably similar results

circulation on the central bering sea shelf july 2008 to july 2010 1 we examine the july 2008 to july 2010 circulation over the central bering sea shelf using measurements at eight instrumented moorings hindcast winds and numerical model results at sub tidal time scales the vertically integrated equations of motion show that the cross shelf balance is primarily geostrophic the along shelf balance is also mainly geostrophic but local accelerations wind stress and bottom friction account for 10 40 of the momentum balance depending on season and water depth the shelf exhibits highly variable flow with small water column average vector mean speeds 5 cm s 1 mean peak speeds in summer 3 6 cm s 1 10 30 cm s 1 are smaller than in winter and fall 6 12 cm s 1 30 70 cm s 1 low frequency flows 1 4 cpd are horizontally coherent over distances exceeding 200 km vertical coherence varies seasonally degrading with the onset of summer stratification because effects of heating and freezing are enhanced in shallow waters warm summers increase the cross shelf density gradient and thus enhance northward transport cold winters with increased ice production and brine rejection increase the now reversed cross shelf density gradient and enhance southward transport although the baroclinic velocity is large enough to influence seasonal transports wind forced ekman dynamics are primarily responsible for flow variations the system changes from strong northward flow with coastal convergence to strong southward flow with coastal divergence for northerly and easterly winds respectively under northerly and northwesterly winds nutrient rich waters flow toward the central shelf from the north and northwest replacing dilute coastal waters that are carried south and west

the mechanisms of sea ice melt pond formation and evolution 1 a series of observations were made on melting first year landfast arctic sea ice near barrow alaska to explore the seasonal evolution of melt pond coverage observations of pond coverage albedo and ice properties are combined with terrestrial lidar measurements of surface topography and meltwater balance to quantitatively identify the timing and role of mechanisms driving pond coverage the formation of interposed fresh ice is found to eliminate meltwater percolation through early pond formation and allow widespread ponding well above sea level pond drainage to sea level occurs principally by horizontal meltwater transport over the ice surface to macroscopic flaws freeboard loss caused by buoyancy decline as the ice thins controls pond growth late in the melt season after percolation begins the majority of the macroscopic flaws that drain melt ponds to sea level are observed to develop from brine drainage channels within the ice a simple thermodynamic model of meltwater percolation illustrates that fresh meltwater inflow causes pores in the ice to either shrink and freeze shut or enlarge based on initial size and ice temperature this threshold behavior of pore diameter controls both the blockage of smaller pores with interposed ice and the enlargement of larger brine drainage channels to allow meltwater drainage the results identify links between the temporal evolution of pond coverage and ice temperature salinity and thickness providing new opportunities to realistically parameterize ponds and summer ice albedo within sea ice models

hydrographic conditions during the 2002 sbi process experiments abstract a review of the hydrographic data from the 2002 western arctic shelf basin interactions sbi process cruises permits the following conclusions 1 temperature salinity relationships were similar to canonical descriptions but at five stations in the outer shelf slope region warm high salinity atlantic layer water appeared to have risen displaced the lower halocline and mixed with shelf upper halocline water 2 primary production in the sbi study region was strongly influenced by the advection of dissolved inorganic nitrogen din entering via bering strait this import of din ammonium nitrate nitrite is modified by local processes but without the bering strait inflow biological productivity in the sbi region would be much lower 3 in comparison to the inflowing atlantic waters din urea phosphate and din urea silicate ratios in the pacific waters that dominated the upper 150 m of the water column were low they were also low relative to redfield uptake ratios for phytoplankton 4 microbial processes continue to destroy din in significant quantities as the pacific waters transit the sbi region 5 nitrate and ammonium were the principal contributors to din nitrite concentrations were always

coupled wind forced controls of the bering chukchi shelf circulation and the bering strait throughflow ekman transport continental shelf waves and variations of the pacific arctic sea surface height gradient abstract we develop a conceptual model of the closely co dependent bering shelf bering strait and chukchi shelf circulation fields by evaluating the effects of wind stress over the north pacific and western arctic using atmospheric reanalyses current meter observations satellite based sea surface height ssh measurements hydrographic profiles and numerical model integrations this conceptual model suggests bering strait transport anomalies are primarily set by the longitudinal location of the aleutian low which drives oppositely signed anomalies at synoptic and annual time scales synoptic time scale variations in shelf currents result from local wind forcing and remotely generated continental shelf waves whereas annual variations are driven by basin scale adjustments to wind stress that alter the magnitude of the along strait meridional pressure gradient in particular we show that storms centered over the bering sea excite continental shelf waves on the eastern bering shelf that carry northward velocity anomalies northward through bering strait and along the chukchi coast the integrated effect of these storms tends to decrease the northward bering strait transport at annual to decadal time scales by imposing cyclonic wind stress curl over the aleutian basin and the western subarctic gyre ekman suction then increases the water column density through isopycnal uplift thereby decreasing the dynamic height sea surface height and along strait pressure gradient storms displaced eastward over the gulf of alaska generate an opposite set of bering shelf and aleutian basin responses while ekman pumping controls canada basin dynamic heights proshutinsky et al 2002 we do not find evidence for a strong relation between beaufort gyre sea surface height variations and the annually averaged bering strait throughflow over the western chukchi and east siberian seas easterly winds promote coastal divergence which also increases the along strait pressure head as well as generates shelf waves that impinge upon bering strait from the northwest

thermal regimes in the chukchi sea from 1941 to 2008 the summer june october temperature observations in the surface 0 m and subsurface 20 mbottom chukchi sea layers collected from 1941 to 2008 have been analyzed using the self consistent data recovery procedure based on correlation analysis and iterative empirical orthogonal function eof decomposition the analysis of the surface and subsurface eofs identified cold normal and warm thermal states with variability of 2 3 years and also 4 7 years we found that the chukchi sea water temperature has gradually increased since 1941 warming in the surface layer since 1941 has been minimal in the bering strait 0 012 1 cy r 1 total 0 8 1c and maximal in long strait 0 030 0 036 1 cy r 1 total 2 0 2 4 1c in the subsurface layer temperatures have increased about half as much minimal 0 0030 0 0075 1 cy r 1 total 0 2 0 5 1c in long strait and rather uniform 0 010 0 015 1 cy r 1 total 0 7 1 0 1c for the remaining chukchi sea analysis of the satellite sea surface height anomaly data shows that during the warm periods there is a stronger flow through the bering strait and intensification of the northwestward currents in the central chukchi sea extensive correlation analysis shows that the thermal state of the chukchi sea agrees well with the flow of pacific water through the bering strait and by an increase of the global atmospheric temperature in addition typical circulation during warm and cold periods was reconstructed using four dimensional variational 4dvar data assimilation into the ocean model and estimates of volume and heat fluxes in the chukchi sea during cold and warm periods were derived which are consistent with eof and correlation analyses

circulation and thermohaline variability of the hanna shoal region on the northeastern chukchi sea shelf we analyzed velocity and hydrographic data from 23 moorings in the northeast chukchi sea from 2011 to 2014 in most years the eastern side of hanna shoal was strongly stratified year round while weakly stratified regions prevailed on the shelf south and west of the shoal stratification differences cause differential vertical mixing rates which in conjunction with advection of different bottom water properties resulted in seasonally varying along isobath density gradients in agreement with numerical models we find that bottom waters flow anticyclonically around the shoal whereas most of the shelf responded barotropically to wind forcing there was a strong baroclinic component to the flow field northeast of hanna shoal resulting in no net vertically integrated transport on average in contrast there is a net eastward transport from west of the shoal which implies convergence north of the shoal convergence and along isobath density gradients may foster cross shelf exchange north of hanna shoal modal analyses indicate that the shelf south of the shoal and barrow canyon responded coherently to local and remote winds whereas the wind current response around hanna shoal was less coherent barotropic topographic waves of 3 day period were generated episodically northeast of the shoal and propagate clockwise around hanna shoal but are blocked from entering barrow canyon and are possibly scattered by the horizontally sheared flow and converging isobaths on the western side of the shoal analysis of water properties on the western side of hanna shoal suggests that these include contributions from the western and southern portions of the chukchi sea

the north pacific a global scale estuary abstract the atmospheric net flow of water from the atlantic to the pacific ocean is supposed to maintain the salinity difference between the two oceans assuming the existence of a subsurface level of no horizontal pressure gradient in the ocean the mean sea level in the northern pacific must be higher than in the arctic ocean this mean sea level difference is supposed to drive the observed mean flow through the bering strait the estimated flow of freshwater through the bering strait is approximately equal to the estimated atmospheric net flow of water from the north atlantic to the north pacific this justifies the formulation of a simple estuary model for the north pacific in which the brackish water exits through the bering strait the salinity difference between the two oceans is shown to be controlled by the topography of the bering strait the estuary model gives residence times for water in the upper layer 1000 m thick of approximately 1000 years and in the lower layer of approximately 400

westward intensification of the mean circulation on the bering sea shelf abstract most of the water that eventually flows northward through bering strait originates about 500 km south seaward of the shelfbreak in the bering sea cumulative observational evidence supports the idea that most of this northward flow across the gently shoaling eastern bering sea continental shelf occurs as a western boundary current along the siberian coast a homogeneous rotating laboratory model and a barotropic numerical model each demonstrate this westward intensification of the mean flow the intensification results from the well known topographic effect the combination of rotation and the depth decrease in the direction of flow acts in a similar fashion to the meridional gradient of the coriolis parameter for reasonable values of bering strait transport and shelf bottom friction current speeds of 10 20 cm s 1 and a current width of 50 km are predicted

circulation and water mass transformation in a model of the chukchi sea 1 the circulation and water mass transformation in a regional ocean ice model of the chukchi sea are discussed the model has horizontal resolution of o 4 km is forced by fluxes derived from daily ncep reanalysis fields and has seasonally varying transport temperature and salinity imposed at bering strait many of the observed characteristics of the mean circulation and seasonal cycle in the chukchi sea are reproduced the discussion focuses on the branching of the inflow transport into pathways following herald canyon central channel and the alaskan coast the pattern of ice melt and the water mass transformation and formation of winter water and hypersaline water the ice melt pattern and timing is strongly influenced by advection through bering strait high frequency forcing results in a larger region of ice melt particularly over the shoals and in the northern chukchi sea compared to monthly mean forcing in the model the seasonal cycle of salinity in the southern and central chukchi sea is dominated by advection through bering strait while local atmospheric forcing and brine rejection are more important north of herald and hanna shoals and in barrow canyon however since the residence time in the chukchi sea is generally less than 1 year interannual variability in the bering strait salinity will be reflected in the salinity across the chukchi sea and at barrow canyon

lateral mixing across ice meltwater fronts of the chukchi sea shelf summer and fall hydrographic sections in the northeastern chukchi sea frequently capture 5 20 m thick intrapycnocline lenses or horizontal plumes of warm moderately salty summer bering sea water flowing northward from bering strait these features occur within the shallow 20 m depth pycnocline separating cold dilute surface meltwater from near freezing salty winter formed waters beneath the pycnocline an idealized numerical model suggests that the features arise from eddies and meanders generated by instability of the surface front separating meltwater from bering sea water warm bering sea water is transported across the front and into the pycnocline by the cross frontal velocities associated with the instabilities the accompanying lateral eddy heat fluxes may be important both in summer for promoting ice melt and in fall by delaying the onset of ice formation over portions of this shelf lateral heat flux magnitudes depend upon the stratification of the bering sea water

a wind driven hybrid latent and sensible heat coastal polynya off barrow alaska the nature of the barrow coastal polynya bcp which forms episodically off the alaska coast in winter is examined using mooring data atmospheric reanalysis data and satellite derived sea ice concentration and production data we focus on oceanographic conditions such as water mass distribution and ocean current structure beneath the bcp two moorings were deployed off barrow alaska in the northeastern chukchi sea from august 2009 to july 2010 for sea ice season from december to may a characteristic sequence of five events associated with the bcp has been identified 1 dominant northeasterly wind parallel to the barrow canyon with an offshore component off barrow 2 high sea ice production 3 upwelling of warm and saline atlantic water beneath the bcp 4 strong up canyon shear flow associated with displaced density surfaces due to the upwelling and 5 sudden suppression of ice growth a baroclinic current structure established after the upwelling caused enhanced vertical shear and corresponding vertical mixing the mixing event and open water formation occurred simultaneously once sea ice production had stopped thus mixing events accompanied by ocean heat flux from the upwelled warm water into the surface layer played an important role in formation maintenance of the open water area i e sensible heat polynya the transition from a latent to a sensible heat polynya is well reproduced by a high resolution pan arctic ice ocean model we propose that the bcp previously considered to be a latent heat polynya is a wind driven hybrid latent and sensible heat polynya with both features caused by the same northeasterly wind

decline in arctic sea ice thickness from submarine and icesat records 1958 2008 1 the decline of sea ice thickness in the arctic ocean from icesat 2003 2008 is placed in the context of estimates from 42 years of submarine records 1958 2000 described by rothrock et al 1999 2008 while the earlier 1999 work provides a longer historical record of the regional changes the latter offers a more refined analysis over a sizable portion of the arctic ocean supported by a much stronger and richer data set within the data release area dra of declassified submarine sonar measurements covering 38 of the arctic ocean the overall mean winter thickness of 3 64 m in 1980 can be compared to a 1 89 m mean during the last winter of the icesat record an astonishing decrease of 1 75 m in thickness between 1975 and 2000 the steepest rate of decrease is 0 08 m yr in 1990 compared to a slightly higher winter summer rate of 0 10 0 20 m yr in the five year icesat record 2003 2008 prior to 1997 ice extent in the dra was 90 during the summer minimum this can be contrasted to the gradual decrease in the early 2000s followed by an abrupt drop to 55 during the record setting minimum in 2007 this combined analysis shows a long term trend of sea ice thinning over submarine and icesat records that span five decades

observed increases in bering strait oceanic fluxes from the pacific to the arctic from 2001 to 2011 and their impacts on the arctic ocean water column 1 mooring data indicate the bering strait throughflow increases 50 from 2001 0 7 sv to 2011 1 1 sv driving heat and freshwater flux increases increase in the pacific arctic pressure head explains two thirds of the change the rest being attributable to weaker local winds the 2011 heat flux 5 1020j approaches the previous record high 2007 due to transport increases and warmer lower layer ll temperatures despite surface temperature sst cooling in the last decade warmer ll waters arrive earlier 1 6 1 1 days yr though winds and sst are typical for recent decades maximum summer salinities likely set in the bering sea remain remarkably constant 33 1 psu over the decade elucidating the stable salinity of the western arctic cold halocline despite this freshwater flux variability strongly driven by transport exceeds variability in other arctic freshwater sources remote data winds sst prove insufficient for quantifying variability indicating interannual change can still only be assessed by in situ year round measurements

summertime synoptic variability of frontal systems in the northern bering sea hydrographic observations in the northern bering sea during august and september 1987 indicate the presence of a front dividing relatively warm fresh bering shelf water from relatively cool saline anadyr water along the western and northern coasts of st lawrence island near anadyr strait a buoyant layer 20 m thick with surface salinities as low as 29 5 practical salinity units and a maximum temperature of 10 c was present adjacent to the island the surface outcrop of the front migrated 80 km north during the nine day time period of the hydrographic observations surface thermal patterns suggest that this front may extend the length of the northern coastline of st lawrence island during the summer the front veers north and passes through the bering strait where temperature differences as large as 6 c exist across the strait an examination of the water mass properties of the bering sea suggests that the buoyant water north of st lawrence island is bering shelf water which has been carried northward through anadyr strait the baroclinic transport assuming no flow at the bottom associated with the front is 0 07 sv which is roughly a third of the seasonal increase in transport through the bering strait in the summer

on the wind driven variability of the flow through bering strait analysis of time series from 1981 to 1982 near bering strait shows the meridional wind component the current and the oceanic pressure gradient to be well correlated we argue that because of the constraining coastal geometry there are wind driven flow divergences which perturb the oceanic pressure field and account for most of the current variability for example northerly winds cause a pronounced set down of sea level southeast of bering strait and if the winds are strong enough they will reverse the mean slope downward to the north through the strait winds during 1976 1977 were also well correlated with the bering strait transport estimates of coachman and aagaard 1981 and we therefore use the calculated winds from 1946 to 1982 to examine the low frequency transport variability there is a marked seasonal cycle with summer transport about 50 greater than during winter the long term mean transport is calculated as less than 0 6 sv which is much lower than earlier estimates abnormally large transports apparently occurred during a number of years from 1948 to 1967 but during the past 15 years transports have tended to be considerably less corresponding to the stronger northerly winds during these years the interannual wind variability near bering strait together with the corresponding variability in the transport through the strait is in fact part of the large scale variability of the atmospheric circulation over the north pacific and its oceanic effects

circulation on the north central chukchi sea shelf abstract mooring and shipboard data collected between 1992 and 1995 delineate the circulation over the north central chukchi shelf previous studies indicated that pacific waters crossed the chukchi shelf through herald valley in the west and barrow canyon in the east we find a third branch through the central channel onto the outer shelf the central channel transport varies seasonally in phase with bering strait transport and is 0 2 sv on average although some of this might include water entrained from the outflow through herald valley a portion of the central channel outflow moves eastward and converges with the alaskan coastal current at the head of barrow canyon the remainder appears to continue northeastward over the central outer shelf toward the shelfbreak joined by outflow from herald valley the mean flow opposes the prevailing winds and is primarily forced by the sea level slope between the pacific and arctic oceans current variations are mainly wind forced but baroclinic forcing associated with upstream dense water formation in coastal polynyas might occasionally be important winter water mass modification depends crucially on the fall and winter winds which control seasonal ice development an extensive fall ice cover delays cooling limits new ice formation and results in little salinization in such years bering shelf waters cross the chukchi shelf with little modification in contrast extensive open water in fall leads to early and rapid cooling and if accompanied by vigorous ice production within coastal polynyas results in the production of high salinity 33 shelf waters such interannual variability likely affects slope processes and the transport of pacific waters into the arctic ocean interior

the encoding of wind forcing into the pacific arctic pressure head chukchi sea ice retreat and late summer barrow canyon water masses abstract barrow canyon which incises the northeast corner of the chukchi sea shelf is a major conduit through which pacific origin waters carrying nutrients biota freshwater and heat enter the arctic ocean as such barrow canyon was adopted as a long term monitoring site for the distributed biological observatory dbo in 2010 however annual hydrographic surveys across barrow canyon conducted during late august 2005 2015 along a location near what is the barrow canyon dbo survey line and in support of other research programs extend and complement the dbo hydrographic record these complementary hydrographic surveys show that volumes of pacific origin and melt water masses in barrow canyon are significantly correlated with daily sea ice areas in the eastern chukchi sea for most of the may august ice retreat season year to year differences in the timing and pattern of sea ice retreat across the chukchi sea are also shown to be well correlated with changes in seasonally averaged regional winds particularly as defined by the strength and longitudinal location of the beaufort sea high pressure cell these interdependent wind ice retreat water mass relationships are largely predicated on wind stress curl driven changes to sea level in the east siberian sea northwestern chukchi sea statistically significant correlations among wind forced sea surface heights ice areas and water mass volumes suggest that during the ice retreat season the east siberian sea western chukchi sea region serves as the arctic terminus for the pacific arctic pressure head

the role of sea ice and other fresh water in the arctic circulation salinity stratification is critical to the vertical circulation of the high latitude ocean we here examine the control of the vertical circulation in the northern seas and the potential for altering it by considering the budgets and storage of fresh water in the arctic ocean and in the convective regions to the south we find that the present day greenland and iceland seas and probably also the labrador sea are rather delicately poised with respect to their ability to sustain convection small variations in the fresh water supplied to the convective gyres from the arctic ocean via the east greenland current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past the north atlantic salinity anomaly of the 1960s and 1970s is a recent example it must have had its origin in an increased fresh water discharge from the arctic ocean similarly the freshening and cooling of the deep north atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the arctic ocean into the convective gyres finally we note that because of the temperature dependence of compressibility a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean

seasonal to mesoscale variability of water masses and atmospheric conditions in barrow canyon chukchi sea abstract twenty four repeat hydrographic transects occupied across barrow canyon from 2010 to 2013 are used to study the seasonal evolution of water masses in the canyon from july october as well as the occurrence of upwelling the mean sections revealed that the alaskan coastal water is mainly confined to the eastern flank of the canyon corresponding to a region of sloped isopycnals indicative of the surface intensified alaskan coastal current which advects the water the pacific origin winter water is found at depth banked against the western flank of the canyon its isopycnal structure is consistent with a bottom intensified flow of this dense water mass out of the canyon for the months that were sampled the alaskan coastal water is most prevalent in august and september while the coldest winter water is observed in the month of august it is argued that this newly ventilated winter water is delivered to the canyon via pathways on the central chukchi shelf as opposed to the coastal pathway roughly a third of the hydrographic sections were preceded by significant up canyon winds and hence were deemed to be under the influence of upwelling during these periods anomalously salty water is found throughout the eastern flank of the canyon and on occasion atlantic water fills the deepest part of the section using atmospheric reanalysis data it is shown that upwelling occurs when the beaufort high is strengthened and the aleutian low is deepened two modes of storm tracks were identified northward progressing storms mode 1 and eastward progressing storms mode 2 both of which can drive upwelling mode 1 is prevalent in july august while mode 2 is more common in september october these seasonal patterns appear to be dictated by regional variations in blocking highs

phytoplankton blooms beneath the sea ice in the chukchi sea in the arctic ocean phytoplankton blooms on continental shelves are often limited by light availability and are therefore thought to be restricted to waters free of sea ice during july 2011 in the chukchi sea a large phytoplankton bloom was observed beneath fully consolidated pack ice and extended from the ice edge to 4100 km into the pack the bloom was composed primarily of diatoms with biomass reaching 1291 mg chlorophyll a m 2 and rates of carbon fixation as high as 3 7 g c m 2 d 1 although the sea ice where the bloom was observed was near 100 concentration and 0 8 1 2 m thick 30 40 of its surface was covered by melt ponds that transmitted 4 fold more light than adjacent areas of bare ice providing sufficient light for phytoplankton to bloom phytoplankton growth rates associated with the under ice bloom averaged 0 9 d 1

a comparison between late summer 2012 and 2013 water masses macronutrients and phytoplankton standing crops in the northern bering and chukchi seas abstract survey data from the northern bering and chukchi sea continental shelves in august september 2012 and 2013 reveal inter annual differences in the spatial structure of water masses along with statistically significant differences in thermohaline properties chemical properties and phytoplankton communities we provide a set of water mass definitions applicable to the northern bering and chukchi continental shelves and we find that the near bottom bering chukchi summer water bcsw was more saline in 2012 and alaskan coastal water acw was warmer in 2013 both of these water masses carried higher nutrient concentrations in 2012 supporting a larger chlorophyll a biomass that was comprised primarily of small

pathways of pacific water across the chukchi sea a numerical model study 1 pathways of pacific water flowing from the north pacific ocean through bering strait and across the chukchi sea are investigated using a two dimensional barotropic model in the no wind case the flow is driven only by a prescribed steady northward flow of 0 8 sv through bering strait the resulting steady state circulation consists of a broad northeasterly flow basically following the topography with a few areas of intensified currents about half of the inflow travels northwest through hope valley while the other half turns somewhat toward the northeast along the alaskan coast the flow through hope valley is intensified as it passes through herald canyon but much of this flow escapes the canyon to move eastward joining the flow in the broad valley between herald and hanna shoals another area of slightly intensified currents there is a confluence of nearly all of the flow along the alaskan coast west of pt barrow to create a very strong and narrow coastal jet that follows the shelf topography eastward onto the beaufort shelf thus in this no wind case nearly all of the pacific water entering the chukchi sea eventually ends up flowing eastward along the narrow beaufort shelf with no discernable flow across the shelf edge toward the interior canada basin travel times for water parcels to move from bering strait to pt barrow vary tremendously according to the path taken e g less than 6 months along the alaskan coast but about 30 months along the westernmost path through herald canyon this flow field is relatively insensitive to idealized wind forcing when the winds are from the south west or north in which cases the shelf transports tend to be intensified however strong northeasterly to easterly winds are able to completely reverse the flows along the beaufort shelf and the alaskan coast and force most of the throughflow in a more northerly direction across the chukchi sea shelf edge potentially supplying the surface waters of the interior canada basin with pacific water the entire shelf circulation reacts promptly to changing wind conditions with a response time of 2 3 days the intense coastal jet between icy cape and pt barrow implies that dense water formed here from winter coastal polynyas may be quickly swept away along the coast in contrast there is a relatively quiet nearshore region to the west between cape lisburne and icy cape where dense water may accumulate much longer and continue to become denser before it is carried across the shelf

spatial patterns of primary production on the shelf slope and basin of the western arctic in 2002 in the spring and summer of 2002 primary production in the chukchi sea was measured using 14 c uptake experiments our cruise track encompassed the shelf and continental slope area of the chukchi and beaufort seas progressing into deep water over the canada basin the study area experienced upwards of 90 ice cover during the spring with ice retreating into the basin during the summer production in the spring was light limited due to ice cover with average euphotic zone production rates of o0 3 g c m 2 d 1 values of 8 g c m 2 d 1 were observed in association with surface bloom conditions during the initial ice breakup considerable nutrient reduction in the surface waters took place between the spring and summer cruise and although not observed this was attributed to a spring bloom decreased ice cover and increased clarity of surface waters in the summer allowed greater light penetration the highest rates of production during the second cruise were found at 25 30 m coincident with the top of the nutricline daily euphotic zone productivity in the summer averaged 0 78 g c m 2 d 1 on the shelf and 0 32 g c m 2 d 1 on the edge of the canada basin these data provide an estimated annual production of 90 g c m 2 yr 1 in the study area

tidal currents in the st lawrence island region 1 vertical and spatial variability of tidal currents in the northern part of the bering sea near st lawrence island were examined using observations made at 17 new and historical current meter mooring locations the semidiurnal currents dominate over the diurnal currents semidiurnal currents exhibit a seasonal modulation with currents being somewhat stronger during the summer and fall when the density structure displays two layer stratification diurnal currents exhibit a weak semiannual modulation examination of the vertical structure of tidal currents shows that maximum semidiurnal currents are observed in the upper or middle layers down to a depth of about 20 m and below this depth the energy of the semidiurnal currents decreases rapidly however the energy of diurnal band currents remains almost constant with depth decreasing only in close proximity to the bottom rotary spectral analysis applied to the records shows further differences the m2 clockwise cw component of rotation dominates in the upper and middle water column but the counterclockwise component ccw dominates near the bottom interpretation of these observed results points to the different bottom boundary layers bbl for the cw and ccw motions in addition to the role of density stratification in organizing the bbl we compare results of a vertically averaged two dimensional model to the observationally derived tidal ellipse parameters and show that the barotropic formulation provides a first order description of the tidal regime within the domain

ocean circulation and exchanges through the northern bering sea 1979 2001 model results abstract we have developed and run a model with sufficiently high resolution 9 km and 45 levels and a large enough spatial domain to allow for realistic representation of flow through the narrow and shallow straits in the northern bering sea this is potentially important for quantification of long term mean and time dependent ocean circulation and water mass and property exchanges between the pacific and arctic oceans over a 23 year interval 1979 2001 mean transport through bering strait is estimated to be 0 65 sv comparison of our model results with published observations indicates that ocean circulation is not only variable at seasonal to interdecadal scales but it is also responsive to short term atmospheric forcing one of such events occurred during the winter of 2000 2001 with reversed oceanic flow in some areas and much reduced sea ice cover analyses of eddy kinetic energy fields identify some high biological productivity regions of the chirikov basin coincident with persistent high energy up to 2700 cm 2 s 2 in the surface layer and up to 2600 cm 2 s 2 at mid depth throughout the annual cycle model output in the bering strait region is validated against several time series of moored observations of water mass properties comparison with shipboard observations of near bottom salinity from late winter through autumn indicates that the model reasonably represents the major water mass properties in the region the modeled vertical water column structure in the northern bering sea allows increased understanding of the mechanisms of water transformation and transport northward through bering strait into the chukchi and beaufort seas we conclude that the long term model results for the northern bering sea provide important insights into the ocean circulation and fluxes and they are a useful frame of reference for limited observations that are short term and or cover only a small geographic region

mesozooplankton abundance and distribution in association with hydrography on hanna shoal ne chukchi sea during august 2012 and 2013 abstract hanna shoal in the northeastern chukchi sea is potentially vulnerable to ecosystem disruption under ongoing climate change however aspects of its ecology particularly of its zooplankton have been poorly understood mesozooplankton distribution taxonomic composition and abundance were described from across hanna shoal in august 2012 and 2013 as part of the multidisciplinary comida hanna shoal program zooplankton were collected using vertical tows of paired bongo nets equipped with 150 m and 500 m mesh nets samples from the 150 m mesh nets were enumerated to identify and count taxa copepod species and copepod life stages haplotypes of the mitochondrial cytochrome c oxidase i mtcoi gene were used to differentiate bering and arctic haplotype groups of the copepod calanus glacialis and to differentiate c glacialis and its close congener c marshallae the meroplankton particularly bivalve larvae and in 2012 echinoderm larvae were an important component of the zooplankton and were of greater abundance on the eastern portion of the shoal than elsewhere regions identified on the basis of different taxonomic compositions were associated with different water masses and current pathways the northeast corner of the shoal in particular was distinct from the remainder of the shoal and from barrow canyon with both different life stage compositions and unique arctic haplotypes of the mtcoi gene for c glacialis marshallae suggesting populations at those locations originated in the arctic ocean rather than the bering sea bering sea summer water and intrinsic plankton was observed in the southwest portion of the shoal comparisons with historic and recent studies done near hanna shoal demonstrated that similar plankton compositions were present across a broad region of the chukchi sea and that abundances of the copepod c glacialis appear to be increasing on the time scale of decades potentially through increased input from the northern bering sea because the chukchi sea is highly advective it is likely that zooplankton populations over hanna shoal are lost to the canada basin to the north and must be re established annually through input from the northern bering sea under this scenario enhancement of a resident hanna shoal zooplankton community that would retain a significant proportion of the primary production in the water column is unlikely

northward flow in the bering and chukchi seas the ocean circulation on the bering and chukchi sea shelves is investigated using a barotropic numerical model in the presence of the winter seasonal wind stress from the northeast observed northward transport through bering strait of 0 6 sv for the 8 month winter season 1981 1982 can be driven by a sea level difference of 0 4 m between the pacific and arctic oceans this sea level difference between basins is in agreement with hydrographic estimates in the absence of wind stress a 0 4 m sea level difference drives a northward transport through bering strait of 1 1 sv maximum transport through bering strait is geostrophically limited not frictionally or inertially limited major qualitative circulation features of the region are evident in the simulations westward intensification of the northward transport across the bering sea shelf is confirmed with the major transport occurring in the relatively deep gulf of anadyr in the absence of wind anadyr strait contributes 72 of the northward transport through bering strait with northeasterly winds there is a net clockwise circulation around saint lawrence island within the 20 m isobath norton and kotzebue sounds are little influenced by the general circulation northward transport in the chukchi sea bifurcates in the vicinity of point hope the predominant branch is directed toward the northwest in a broad canyon feature south of herald shoal and the balance becomes the alaskan coastal current between cape lisburne and point barrow

buoyancy reversal decoupling and the transition from stratocumulus to shallow cumulus topped marine boundary layers the transition in a marine boundary layer mbl from stratocumulus topped to shallow cumulus topped is investigated by using a large eddy simulation les model the experiments performed aim to examine the influence on the transition of 1 the probability of buoyancy reversal at the mbl top i e situations in which the mixture of two air parcels becomes denser than either of the original parcels due to phase change or other nonlinear processes involved in the mixing and 2 the degree of decoupling in the mbl i e the strength of a shallow stably stratified layer near cloud base our results suggest that a stratocumulus topped mbl is most likely to transit to a cumulus topped one when 1 there exists high probability of buoyancy reversal at the mbl top and 2 the mbl is decoupled due to large surface evaporation we argue that a parameterization that includes representation of those two effects combined has the potential to provide a simple way of predicting the mbl transition in climate models

mechanisms of pacific summer water variability in the arctic s central canada basin pacific water flows northward through bering strait and penetrates the arctic ocean halocline throughout the canadian basin sector of the arctic in summer pacific summer water psw is modified by surface buoyancy fluxes and mixing as it crosses the shallow chukchi sea before entering the deep ocean measurements from ice tethered profilers moorings and hydrographic surveys between 2003 and 2013 reveal spatial and temporal variability in the psw component of the halocline in the central canada basin with increasing trends in integrated heat and freshwater content a consequence of psw layer thickening as well as layer freshening and warming it is shown here how properties in the chukchi sea in summer control the temperature salinity properties of psw in the interior by subduction at isopycnals that outcrop in the chukchi sea results of an ocean model forced by idealized winds provide support to the mechanism of surface ocean ekman transport convergence maintaining psw ventilation of the halocline

the siberian coastal current a wind and buoyancy forced arctic coastal current we describe circulation and mixing in the siberian coastal current scc using fall shipboard measurements collected between 1992 and 1995 in the western chukchi sea the scc forced by winds siberian river outflows and ice melt flows eastward from the east siberian sea it is bounded offshore by a broad 60 km front separating cold dilute siberian coastal water from warmer saltier bering sea water the alongshore flow is incoherent because the current contains energetic eddies and squirts probably generated by frontal baroclinic instabilities these enhance horizontal mixing and weaken the cross shore density gradient along the scc path eventually the scc converges with the northward flow from bering strait whereupon it deflects offshore and mixes with that inflow deflection occurs where the alongshore pressure gradient vanishes that location varies on synoptic and seasonal timescales because this gradient depends on the winds buoyancy fluxes and the sea level difference between the pacific and arctic oceans deflection usually occurs on the chukchi shelf but the scc occasionally flows southward through bering strait such events are short lived 1 10 days and occur mainly in fall and winter under northerly winds scc transport is likely small 0 1 sv but its dilute waters could substantially freshen the bering strait inflow and affect the disposition of pacific waters in the arctic ocean arctic river outflows should preferentially form surface advected fronts rather than bottom advected fronts because vertical mixing energy is low on arctic shelves surface advected fronts are more susceptible to upwelling winds and for the scc the pressure gradient between the pacific and arctic oceans than bottom advected fronts the scc never developed in fall 1995 because of anomalously steady upwelling winds the western chukchi shelf could have formed upper halocline source water in the winter of 1995 1996

direct evidence for northward flow on the northwestern bering sea shelf during the summer of 1994 a satellite tracked drifter transited from the southeastern bering sea slope through bering strait by a route westward along the slope and then northward through anadyr canyon and strait the trajectory emphasizes the importance of a western location of northward flow on the bering sea shelf the transit time was 2 months from cape navarin to bering strait with northward drift velocities of 5 40 cm s

variability of physical and chemical characteristics along the 70 m isobath of the southeastern bering sea abstract from observations of ice cover temperature salinity currents and nitrate it is evident that along shelf variability was significant over the middle shelf of the eastern bering sea but less distinct than that observed in the cross shelf domains along the 70 m isobath three zones were evident in the summer the southeastern cold pool centered at 57 n an intermediate zone consisting of warmer water with weaker stratification and the northern cold pool extending northward from 58 n small scale 20 km horizontal features that persisted for months were common nutrient concentrations were related to salinity and were replenished more uniformly over the southern shelf than north of the pribilof islands although mean currents were weak 1 cm s 1 short energetic advective events impacted the temperature and salinity structure

the effect of possible taylor columns on the summer ice retreat in the chukchi sea the chukchi sea topography consists of a broad flat 50 m deep plain with two prominent shoals herald and hanna which have horizontal dimensions of about 100 km and rise to depths of about 30 m herald shoal in particular is a prominent isolated seamount which because of the warm water flux in summer through bering strait has a warm 0 1 m s 1 flow incident upon it examination of active and passive microwave imagery of the region for 1992 1994 shows that as the general ice cover recedes the ice remains preferentially over herald shoal for 3 4 weeks after the surrounding ice has melted a scale analysis suggests that the cause of the ice persistence is the formation of a taylor column over the shoal which traps cold water and ice above it a similar trapping of ice occurs over hanna shoal and on its eastern slope but the response is complicated by a very different topography and the northward flow down barrow canyon

some controls on flow and salinity in bering strait 1 during 1993 1994 steric forcing of flow through bering strait represented a northward sea level drop of 0 7 m from the bering sea basin to the adjacent deep arctic ocean of which 2 3 was due to the salinity difference between the basins seasonal variability of steric forcing appears small 0 05 m in contrast to large seasonal wind effects interannual changes in steric forcing may exceed 20 however and warm inflow from the north atlantic accumulation of freshwater in the southwest canada basin and temperature and salinity changes in the upper bering sea have all contributed to recent changes the mean salinity balance in bering strait is primarily maintained by large runoff to the bering shelf dilute coastal inflow from the gulf of alaska and on shelf movement of saline and nutrient rich oceanic waters from the bering sea basin in bering strait therefore both the throughflow and its salinity are affected by remote events