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Knut Aagaard

Principal Oceanographer--Retiree

Professor Emeritus, Oceanography






Dr. Aagaard investigates high-latitude oceanic processes and their role in climate. He is currently studying oceanic convection and its relationship to ice distribution; the contribution of the Pacific Ocean to arctic circulation; the role of fresh water from the Arctic Ocean in the control of the thermohaline circulation in the North Atlantic; and the formation of dense water on the continental shelf and its control of ocean stratification. His work involves ocean observations and monitoring as well as analysis, all within a broad regional framework. His research has entailed fieldwork throughout the Arctic Ocean and its adjacent seas and in the subarctic. Dr. Aagaard holds a joint appointment with the School of Oceanography. He has published several books and his research papers appear in a variety of oceanographic and geophysical journals.

Department Affiliation

Polar Science Center


2000-present and while at APL-UW

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

Danielson, S.L., et al., including K. Aagaard and R. Woodgate, "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," Prog. Oceanogr., 125, 40-61, doi:10.1016/j.pocean.2014.04.006, 2014.

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1 Jun 2014

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.

Circulation on the central Bering Sea shelf, July 2008 – July 2010

Danielson, S.L., T.J. Weingartner, Kn. Aagaard, J. Zhang, and R.A. Woodgate, "Circulation on the central Bering Sea shelf, July 2008 – July 2010," J. Geophys. Res., 117, doi:10.1029/2012JC008303, 2012.

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1 Oct 2012

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 North Pole Environmental Observatory mooring

Aagaard, K., and J.M. Johnson, "The North Pole Environmental Observatory mooring," Oceanography, 24, 100-101, doi:10.5670/oceanog.2011.60, 2011.

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1 Sep 2011

Eulerian time series form an important element of the modern oceanographic toolbox. As part of the North Pole Environmental Observatory, we therefore maintained a bottom-anchored, instrumented mooring within ~55 km of the North Pole from 2001 to 2010. The mooring site was over the Pole Abyssal Plain in water ~4,300 m deep, a location that illuminated boundary current evolution along the Eurasian flank of the Lomonosov Ridge and events in the interior ocean away from the boundary. Standard measurements have included velocity, temperature, salinity, and pressure at various depths, as well as ice thickness. In 2005 and 2006, sensors for bio-optics and nutrients were added.

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Thermal and haline variability over the central Bering Sea shelf: Seasonal and interannual perspectives.

Danielson, S., L. Eisner, T. Weingartner, and K. Aagaard, "Thermal and haline variability over the central Bering Sea shelf: Seasonal and interannual perspectives." Cont. Shelf Res., 31, 539-554, doi: 10.1016/j.csr.2010.12.010, 2011.

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15 Apr 2011

We examine multi-year conductivity-temperature-depth (CTD) data to better understand temperature and salinity variability over the central Bering Sea shelf. Particular consideration is given to observations made annually from 2002 to 2007 between August and October, although other seasons and years are also considered. Vertical and horizontal correlation maps show that near-surface and near-bottom salinity anomalies tend to fluctuate in phase across the central shelf, but that temperature anomalies are vertically coherent only in the weakly or unstratified inner-shelf waters. We formulate heat content (HC) and freshwater content (FWC) budgets based on the CTD observations, direct estimates of external fluxes (surface heat fluxes, ice melt, precipitation (P), evaporation (E) and river discharge), and indirect estimates of advective contributions. Ice melt, P-E, river discharge, and along-isobath advection are sufficient to account for the mean spring-to-fall increase in FWC, while summer surface heat fluxes are primarily responsible for the mean seasonal increase in HC, although interannual variability in the HC at the end of summer appears related to variability in the along-isobath advection during the summer months. On the other hand, FWC anomalies at the end of summer are significantly correlated with the mean wind direction and cross-isobath Ekman transport averaged over the previous winter. Consistent with the latter finding, salinities exhibit a weak but significant inverse correlation between the coastal and mid-shelf waters. The cross-shelf transport likely has significant effect on nutrient fluxes and other processes important to the functioning of the shelf ecosystem. Both the summer and winter advection fields appear to result from the seasonal mean position and strength of the Aleutian Low. We find that interannual thermal and haline variability over the central Bering Sea shelf are largely uncoupled.

Upwelling in the Alaskan Beaufort Sea: Atmospheric forcing and local versus non-local response.

Pickart, R.S., M.A. Spall, G.W.K. Moore, T.J. Weingartner, R.A. Woodgate, K. Aagaard, and K. Shimada. "Upwelling in the Alaskan Beaufort Sea: Atmospheric forcing and local versus non-local response." Prog. Oceanogr., 88, 78-100, doi:10.1016/j.pocean.2010.11.005, 2011.

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1 Jan 2011

The spin up and relaxation of an autumn upwelling event on the Beaufort slope is investigated using a combination of oceanic and atmospheric data and numerical models. The event occurred in November 2002 and was driven by an Aleutian low storm. The wind field was strongly influenced by the pack-ice distribution, resulting in enhanced winds over the open water of the Chukchi Sea. Flow distortion due to the Brooks mountain range was also evident. Mooring observations east of Barrow Canyon show that the Beaufort shelfbreak jet reversed to the west under strong easterly winds, followed by upwelling of Atlantic Water onto the shelf. After the winds subsided a deep eastward jet of Atlantic Water developed, centered at 250 m depth. An idealized numerical model reproduces these results and suggests that the oceanic response to the local winds is modulated by a propagating signal from the western edge of the storm. The disparity in wave speeds between the sea surface height signal - traveling at the fast barotropic shelf wave speed - versus the interior density signal - traveling at the slow baroclinic wave speed - leads to the deep eastward jet. The broad-scale response to the storm over the Chukchi Sea is investigated using a regional numerical model. The strong gradient in windspeed at the ice edge results in convergence of the offshore Ekman transport, leading to the establishment of an anti-cyclonic gyre in the northern Chukchi Sea. Accordingly, the Chukchi shelfbreak jet accelerates to the east into the wind during the storm, and no upwelling occurs west of Barrow Canyon. Hence the storm response is fundamentally different on the Beaufort slope (upwelling) versus the Chukchi slope (no upwelling). The regional numerical model results are supported by additional mooring data in the Chukchi Sea.

Evolution and dynamics of the flow through Herald Canyon in the western Chukchi Sea

Pickart, R.S., L.J. Pratt, D.J. Tores, T.E. Whitledge, A.Y. Proshutinsky, K. Aagaard, T.A. Agnew, G.W.K. Moore, and H.J. Dail, "Evolution and dynamics of the flow through Herald Canyon in the western Chukchi Sea," Deep-Sea Res. II, doi:10.1016/j.dsr2.2009.08.002, 2010.

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1 Jan 2010

The flow of summer and winter Pacific water masses through Herald Canyon is investigated using data from a high-resolution hydrographic/velocity survey conducted in summer 2004. The survey was part of the Russian-American Long Term Census of the Arctic (RUSALCA) program, and consisted of four cross-canyon transects occupied over a 2-day period.

At the time of the survey dense winter water was entering the western side of the canyon from the Chukchi Sea, flowing alongside a poleward jet of summer water on the canyon's eastern flank. As the dense water progressed northward it switched sides of the canyon and underwent a sudden increase in layer thickness. This coincided with vertical mixing near the interface of the winter and summer water, producing a new water mass mode exiting the canyon. All of these features are consistent with the notion of hydraulic activity occurring in the canyon.

A three-layer hydraulic theory is applied to the flow, which suggests that it is supercritical and that hydraulic control is likely. A lock-exchange formulation accurately predicts the northward transport of the winter water. The origin of the winter water and the manner in which it drains into the canyon is investigated using satellite ice-concentration data, atmospheric re-analysis fields, historical in-situ data, and a simple circulation model. Finally, the fate of the Pacific water exiting the canyon, and its connection to the Chukchi shelfbreak current, is discussed.

Glimpses of Arctic Ocean shelf-basin interaction from submarine-borne radium sampling

Kadko, D., and K. Aagaard, "Glimpses of Arctic Ocean shelf-basin interaction from submarine-borne radium sampling," Deep Sea Res. I, 56, 32-40, doi:10.1016/j.dsr.2008.08.002, 2009.

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1 Jan 2009

Evidence of shelf-water transfer from temperature, salinity, and 228Ra/226Ra sampling from the nuclear submarine USS L. Mendel Rivers SCICEX cruise in October, 2000 demonstrates the heterogeneity of the Arctic Ocean with respect to halocline ventilation. This likely reflects both time-dependent events on the shelves and the variety of dispersal mechanisms within the ocean, including boundary currents and eddies, at least one of which was sampled in this work. Halocline waters at the 132 m sampling depth in the interior Eurasian Basin are generally not well connected to the shelves, consonant with their ventilation within the deep basins, rather than on the shelves. In the western Arctic, steep gradients in 228Ra/226Ra ratio and age since shelf contact are consistent with very slow exchange between the Chukchi shelf and the interior Beaufort Gyre. These are the first radium measurements from a nuclear submarine.

A large eddy in the central Arctic Ocean

Aagaard, K., R. Andersen, J. Swift, and J. Johnson, "A large eddy in the central Arctic Ocean," Geophys. Res. Lett., 35, 10.1029/2008GL033461, 2008.

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1 May 2008

Long-term moored measurements of temperature, salinity, and velocity over the abyssal plain near the North Pole show a rich array of eddy-like structures over a wide range of depths. Here we demonstrate an anticyclone that extends from the surface to at least 1700 m, is about 60 km across, and has a likely origin along the Eurasian continental margin.

Atlantic water circulation over the Mendeleev Ridge and Chukchi Borderland from thermohaline intrusions and water mass properties

Woodgate, R.A., K. Aagaard, J.H. Swift, W.M. Smethie, and K.K. Falkner, "Atlantic water circulation over the Mendeleev Ridge and Chukchi Borderland from thermohaline intrusions and water mass properties," J. Geophys. Res., 112, doi:10.1029/2005JC003516, 2007.

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3 Feb 2007

Hydrographic and tracer data from 2002 illustrate Atlantic water pathways and variability in the Mendeleev Ridge and Chukchi Borderland (CBLMR) region of the Arctic Ocean. Thermohaline double diffusive intrusions (zigzags) dominate both the Fram Strait (FSBW) and Barents Sea Branch Waters (BSBW) in the region. We show that details of the zigzags' temperature-salinity structure partially describe the water masses forming the intrusions. Furthermore, as confirmed by chemical tracers, the zigzags' peaks contain the least altered water, allowing assessment of the temporal history of the Atlantic waters. Whilst the FSBW shows the 1990s warming and then a slight cooling, the BSBW has continuously cooled and freshened over a similar time period. The newest boundary current waters are found west of the Mendeleev Ridge in 2002. Additionally, we show the zigzag structures can fingerprint various water masses, including the boundary current. Using this, tracer data and the advection of the 1990s warming, we conclude the strongly topographically steered boundary current, order 50 km wide and found between the 1500 m and 2500 m isobaths, crosses the Mendeleev Ridge north of 80°N, loops south around the Chukchi Abyssal Plain and north around the Chukchi Rise, with the 1990s warming having reached the northern (but not the southern) Northwind Ridge by 2002. Pacific waters influence the Atlantic layers near the shelf and over the Chukchi Rise. The Northwind Abyssal Plain is comparatively stagnant, being ventilated only slowly from the north. There is no evidence of significant boundary current flow through the Chukchi Gap.

Boundary current circulation in the Mendeleev Ridge and Shukchi Borderland region of the Arctic Ocean: Atlantic water zigzags and the influence of shelf processes at the arctic crossroads

Woodgate, R.A., K. Aagaard, J.H. Swift, W.M. Smethie, and K.K. Falkner, "Boundary current circulation in the Mendeleev Ridge and Shukchi Borderland region of the Arctic Ocean: Atlantic water zigzags and the influence of shelf processes at the arctic crossroads," Eos Trans. AGU, 87(Abstr.), S33N-02, 2006.

1 Dec 2006

Control of the Bering Strait throughflow and its salinity

Aagaard, K., R.A. Woodgate, T.J. Weingartner, "Control of the Bering Strait throughflow and its salinity," Eos Trans. AGU, 87, Abstr. 0332P-06, 2006.

1 Dec 2006

Joint effects of wind and ice motion in forcing upwelling in Mackenzie Trough, Beaufort Sea

Williams, W.J., E.C. Carmack, K. Shimada, H. Melling, K. Aagaard, R.W. Macdonald, and R.G. Ingram, "Joint effects of wind and ice motion in forcing upwelling in Mackenzie Trough, Beaufort Sea," Cont. Shelf Res., 26, 2352-2366, doi:10.1016/j.csr.2006.06.012, 2006.

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1 Dec 2006

Mackenzie Trough, a cross shelf canyon in the Beaufort Sea shelf, is shown to be a site of enhanced shelf-break exchange via upwelling caused by wind- and ice-driven ocean surface-stresses. To characterize flow within the Trough, we analyze current meter mooring data and concurrent wind and ice velocity data from 1993 to 1996, and show CTD/ADCP sections from 2002. Mackenzie Trough is approximately 400 m deep and 60 km wide, but dynamically it is only 2–3 times the baroclinic Rossby radius at its mouth, and patterns of upwelling and downwelling flow within the canyon are similar to dynamically 'narrow' canyons. Large upwelling events within the canyon are associated with wind in the short ice-free summer season and with ice motion in winter. Ice motion does not necessarily reflect the wind-stress because of internal ice stresses that differentially block downwelling-causing ice motion. The asymmetry between upwelling and downwelling flow within the canyon combined with the predominance of upwelling-causing ice motion, suggests that Mackenzie Trough is a conduit for deeper, nutrient-rich water to the shelf.

The large-scale freshwater cycle of the Arctic

Serreze, M.C., A.P. Barrett, A.G. Slater, R.A. Woodgate, K. Aagaard, R.B. Lammers, M. Steele, R. Moritz, M. Meredith, and C.M. Lee, "The large-scale freshwater cycle of the Arctic," J. Geophys. Res., 111, 10.1029/2005JC003424, 2006.

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21 Nov 2006

This paper synthesizes our understanding of the Arctic's large-scale freshwater cycle. It combines terrestrial and oceanic observations with insights gained from the ERA-40 reanalysis and land surface and ice-ocean models. Annual mean freshwater input to the Arctic Ocean is dominated by river discharge (38%), inflow through Bering Strait (30%), and net precipitation (24%). Total freshwater export from the Arctic Ocean to the North Atlantic is dominated by transports through the Canadian Arctic Archipelago (35%) and via Fram Strait as liquid (26%) and sea ice (25%). All terms are computed relative to a reference salinity of 34.8. Compared to earlier estimates, our budget features larger import of freshwater through Bering Strait and larger liquid phase export through Fram Strait. While there is no reason to expect a steady state, error analysis indicates that the difference between annual mean oceanic inflows and outflows (~8% of the total inflow) is indistinguishable from zero. Freshwater in the Arctic Ocean has a mean residence time of about a decade. This is understood in that annual freshwater input, while large ~8500 km3), is an order of magnitude smaller than oceanic freshwater storage of ~84,000 km3. Freshwater in the atmosphere, as water vapor, has a residence time of about a week. Seasonality in Arctic Ocean freshwater storage is nevertheless highly uncertain, reflecting both sparse hydrographic data and insufficient information on sea ice volume. Uncertainties mask seasonal storage changes forced by freshwater fluxes. Of flux terms with sufficient data for analysis, Fram Strait ice outflow shows the largest interannual variability.

Some controls on flow and salinity in Bering Strait

Aagaard, K., T.J. Weingartner, S.L. Danielson, R.A. Woodgate, G.C. Johnson, and T.E. Whitledge, "Some controls on flow and salinity in Bering Strait," Geophys. Res. Lett., 33, 10.1029/2006GL026612, 2006.

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3 Oct 2006

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.

The St. Lawrence polynya and the Bering shelf circulation: New observations that test the models

Danielson, S., K. Aagaard, T. Weingartner, S. Martin, P. Winsor, G. Gawarkiewicz, and D. Quadfasel, "The St. Lawrence polynya and the Bering shelf circulation: New observations that test the models," J. Geophys. Res., 111, 10.1029/2005JC003268, 2006.

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19 Sep 2006

Using 14 yearlong instrumented moorings deployed south of St. Lawrence Island, along with oceanographic drifters, we investigate the circulation over the central Bering shelf and the role of polynyas in forming and disseminating saline waters over the shelf. We focus also on evaluating the Gawarkiewicz and Chapman (1995) model of eddy production within coastal polynyas. Principal results include the following. (1) The northern central shelf near-surface waters exhibit westward flow, carrying low-salinity waters from the Alaskan coast in fall and early winter, with consequences for water mass formation and biological production. (2) Within the St. Lawrence polynya the freshening effect of winter advection is about half as large as the salting effect of surface brine flux resulting from freezing. (3) Brine production over the Bering shelf occurs primarily offshore, rather than within coastal polynyas, even though ice production per unit area is much larger within the polynyas. (4) We find little evidence for the geostrophic flow adjustment predicted by recent polynya models. (5) In contrast to the theoretical prediction that dense water from the polynya is carried offshore by eddies, we find negligible cross-shelf eddy density fluxes within and surrounding the polynya and very low levels of eddy energy that decreased from fall to winter, even though dense water accumulated within the polynya and large cross-shore density gradients developed. (6) It is possible that dense polynya water was advected downstream of our array before appreciable eddy fluxes materialized.

Trajectory shifts in the arctic and subarctic freshwater cycle

Peterson, B.J., J. McClelland, R. Curry, R.M. Holmes, J.E. Walsh, and K. Aagaard, "Trajectory shifts in the arctic and subarctic freshwater cycle," Science, 313, 1061-1066, 2006.

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25 Aug 2006

Manifold changes in the freshwater cycle of high-latitude lands and oceans have been reported in the past few years. A synthesis of these changes in freshwater sources and in ocean freshwater storage illustrates the complementary and synoptic temporal pattern and magnitude of these changes over the past 50 years. Increasing river discharge anomalies and excess net precipitation on the ocean contributed ~20,000 cubic kilometers of fresh water to the Arctic and high-latitude North Atlantic oceans from lows in the 1960s to highs in the 1990s. Sea ice attrition provided another ~15,000 cubic kilometers, and glacial melt added ~2000 cubic kilometers. The sum of anomalous inputs from these freshwater sources matched the amount and rate at which fresh water accumulated in the North Atlantic during much of the period from 1965 through 1995. The changes in freshwater inputs and ocean storage occurred in conjunction with the amplifying North Atlantic Oscillation and rising air temperatures. Fresh water may now be accumulating in the Arctic Ocean and will likely be exported southward if and when the North Atlantic Oscillation enters into a new high phase.

Interannual changes in the Bering Strait fluxes of volume, heat and freshwater between 1991 and 2004

Woodgate, R.A., K. Aagaard, and T.J. Weingartner, "Interannual changes in the Bering Strait fluxes of volume, heat and freshwater between 1991 and 2004," Geophys. Res. Lett., 33, 10.1029/2006GL026931, 2006.

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15 Aug 2006

Year-round moorings (1990 to 2004) illustrate interannual variability of Bering Strait volume, freshwater and heat fluxes, which affect Arctic systems including sea ice. Fluxes are lowest in 2001 and increase to 2004. Whilst 2004 freshwater and volume fluxes match previous maxima (1998), the 2004 heat flux is the highest recorded, partly due to ~0.5°C warmer temperatures since 2002. The Alaskan Coastal Current, contributing about 1/3rd of the heat and 1/4th of the freshwater fluxes, also shows strong warming and freshening between 2002 and 2004. The increased Bering Strait heat input between 2001 and 2004 (>2 x 1020 J) could melt 640,000 km2 of 1-m thick ice; the 3-year freshwater increase (~800 km3) is about 1/4th of annual Arctic river run-off. Weaker southward winds likely explain the increased volume flux (~0.7 to ~1 Sv), causing ~80% of the freshwater and ~50% of the heat flux increases.

A year in the physical oceanography of the Chukchi Sea. Moored measurements from autumn 1990-1991

Woodgate, R.A., K. Aagaard, and T.J. Weingartner, "A year in the physical oceanography of the Chukchi Sea. Moored measurements from autumn 1990-1991," Deep-Sea Res. II, 52, 3116-3149, doi:10.1016/j.dsr2.2005.10.016, 2005

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1 Dec 2005

Year-long time-series of temperature, salinity and velocity from 12 locations throughout the Chukchi Sea from September 1990 to October 1991 document physical transformations and significant seasonal changes in the throughflow from the Pacific to the Arctic Ocean for one year. In most of the Chukchi, the flow field responds rapidly to the local wind, with high spatial coherence over the basin scale — effectively the ocean takes on the lengthscales of the wind forcing. Although weekly transport variability is very large (ca. –2 to 3 Sv), the mean flow is northwards, opposed by the mean wind (which is southward), but presumably forced by a sea-level slope between the Pacific and the Arctic, which these data suggest may have significant variability on long (order a year) timescales. The high flow variability yields a significant range of residence times for waters in the Chukchi (i.e. one to six months for half the transit) with the larger values applicable in winter.

Temperature and salinity (TS) records show a strong annual cycle of freezing, salinization, freshening and warming, with sizable interannual variability. The largest seasonal variability is seen in the east, where warm, fresh waters escape from the buoyant, coastally trapped Alaskan Coastal Current into the interior Chukchi. In the west, the seasonally present Siberian Coastal Current provides a source of cold, fresh waters and a flow field less linked to the local wind. Cold, dense polynya waters are observed near Cape Lisburne and occasional upwelling events bring lower Arctic Ocean halocline waters to the head of Barrow Canyon. For about half the year, at least at depth, the entire Chukchi is condensed into a small region of TS-space at the freezing temperature, suggesting ventilation occurs to near-bottom, driven by cooling and brine rejection in autumn/winter and by storm-mixing all year.

In 1990–1991, the ca. 0.8 Sv annual mean inflow through Bering Strait exits the Chukchi in four outflows — via Long Strait, Herald Valley, the Central Channel, and Barrow Canyon — each outflow being comparable (order 0.1–0.3 Sv) and showing significant changes in volume and water properties (and hence equilibrium depth in the Arctic Ocean) throughout the year. The clearest seasonal cycle in properties and flow is in Herald Valley, where the outflow is only weakly related to the local wind. In this one year, the outflows ventilate above and below (but not in) the Arctic halocline mode of 33.1 psu. A volumetric comparison with Bering Strait indicates significant cooling during transit through the Chukchi, but remarkably little change in salinity, at least in the denser waters. This suggests that, with the exception of (in this year small) polynya events, the salinity cycle in the Chukchi can be considered as being set by the input through Bering Strait and thus, since density is dominated by salinity at these temperatures, Bering Strait salinities are a reasonable predictor of ventilation of the Arctic Ocean.

Circulation on the north central Chukchi Sea shelf

Weingartner, T., K. Aagaard, R. Woodgate, S. Danielson, Y. Sasaki, and D. Cavalieri, "Circulation on the north central Chukchi Sea shelf," Deep-Sea Res. II, 52, 3150-3174, 2005

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1 Dec 2005

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.

Pacific ventilation of the Arctic Ocean's lower halocline by upwelling and diapycnal mixing over the continental margin

Woodgate, R.A., K. Aagaard, J.H. Swift, K.K. Falkner, and W.M. Smethie, "Pacific ventilation of the Arctic Ocean's lower halocline by upwelling and diapycnal mixing over the continental margin," Geophys. Res. Lett., 32, 10.1029/2005GL023999, 2005

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29 Sep 2005

Pacific winter waters, a major source of nutrients and buoyancy to the Arctic Ocean, are thought to ventilate the Arctic's lower halocline either by injection (isopycnal or penetrative) of cold saline shelf waters, or by cooling and freshening Atlantic waters upwelled onto the shelf. Although ventilation at salinity (S) > 34 psu has previously been attributed to hypersaline polynya waters, temperature, salinity, nutrient and tracer data suggest instead that much of the western Arctic's lower halocline is in fact influenced by a diapycnal mixing of Pacific winter waters (with S ~ 33.1 psu) and denser eastern Arctic halocline (Atlantic) waters, the mixing taking place possibly over the northern Chukchi shelf/slope. Estimates from observational data confirm that sufficient quantities of Atlantic water may be upwelled to mix with the inflowing Pacific waters, with volumes implying the halocline over the Chukchi Borderland region may be renewed on timescales of order a year.

Dissolved oxygen extrema in the Arctic Ocean halocline from the North Pole to the Lincoln Sea

Falkner, K.K., M. Steele, R.A. Woodgate, J.H. Swift, K. Aagaard, and J. Morison, "Dissolved oxygen extrema in the Arctic Ocean halocline from the North Pole to the Lincoln Sea," Deep Sea Res. I, 52, 1138-1154, doi:10.1016/j.dsr.2005.01.007, 2005

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30 Jul 2005

Dissolved oxygen (O2) profiling by new generation sensors was conducted in the Arctic Ocean via aircraft during May 2003 as part of the North Pole Environmental Observatory (NPEO) and Freshwater Switchyard (SWYD) projects. At stations extending from the North Pole to the shelf off Ellesmere Island, such profiles display what appear to be various O2 maxima (with concentrations 70% of saturation or less) over depths of 70–110 m in the halocline, corresponding to salinity and temperature ranges of 33.3–33.9 and ~1.7 to ~1.5°C. The features appear to be widely distributed: Similar features based on bottle data were recently reported for a subset of the 1997–1998 SHEBA stations in the southern Canada Basin and in recent Beaufort Sea sensor profiles. Oxygen sensor data from August 2002 Chukchi Borderlands (CBL) and 1994 Arctic Ocean Section (AOS) projects suggest that such features arise from interleaving of shelf-derived, O2-depleted waters. This generates apparent oxygen maxima in Arctic Basin profiles that would otherwise trend more smoothly from near-saturation at the surface to lower concentrations at depth. For example, in the Eurasian Basin, relatively low O2 concentrations are observed at salinities of about 34.2 and 34.7. The less saline variant is identified as part of the lower halocline, a layer originally identified by a Eurasian Basin minimum in "NO," which, in the Canadian Basin, is reinforced by additional inputs. The more saline and thus denser variant appears to arise from transformations of Atlantic source waters over the Barents and/or Kara shelves. Additional low-oxygen waters are generated in the vicinity of the Chukchi Borderlands, from Pacific shelf water outflows that interleave with Eurasian waters that flow over the Lomonosov Ridge into the Makarov Basin and then into the Canada Basin. One such input is associated with the well-known silicate maximum that historically has been associated with a salinity of %u224833.1. Above that (322-depleted.

We propose that these low O2 waters influence the NPEO and SWYD profiles to varying extents in a manner reflective of the large-scale circulation. The patterns of halocline circulation we infer from the intrusive features defy a simple boundary-following cyclonic flow. These results demonstrate the value of the improved resolution made feasible with continuous O2 profiling. In the drive to better understand variability and change in the Arctic Ocean, deployment of appropriately calibrated CTD-O2 packages offers the promise of important new insights into circulation and ecosystem function.

Long-term variability of Arctic Ocean waters: Evidence from a reanalysis of the EWG data set

Swift, J.H., K. Aagaard, L. Timokhov, and E.G. Nikiforov, "Long-term variability of Arctic Ocean waters: Evidence from a reanalysis of the EWG data set," J. Geophys. Res., 110, 10.1029/2004JC02312, 2005.

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12 Mar 2005

We have examined interannual to decadal variability of water properties in the Arctic Ocean using an enhanced version of the 1948–1993 data released earlier under the Gore–Chernomyrdin environmental bilateral agreement. That earlier data set utilized gridded fields with decadal time resolution, whereas we have developed a data set with annual resolution. We find that beginning about 1976, most of the upper Arctic Ocean became significantly saltier, possibly related to thinning of the arctic ice cover. There are also indications that a more local upper ocean salinity increase in the Eurasian Basin about 1989 may not have originated on the shelf, as had been suggested earlier. In addition to the now well-established warming of the Atlantic layer during the early 1990s, there was a similar cyclonically propagating warm event during the 1950s. More remarkable, however, was a pervasive Atlantic layer warming throughout most of the Arctic Ocean from 1964–1969, possibly related to reduced vertical heat loss associated with increased upper ocean stratification. A cold period prevailed during most of the 1970s and 1980s, with several very cold events appearing to originate near the Kara and Laptev shelves. Finally, we find that the silicate maximum in the central Arctic Ocean halocline eroded abruptly in the mid-1980s, demonstrating that the redistribution of Pacific waters and the warming of the Atlantic layer reported from other observations during the 1990s were distinct events separated in time by perhaps 5 years. We have made the entire data set publicly available.

Monthly temperature, salinity, and transport variability of the Bering Strait through flow

Woodgate, R.A., K. Aagaard, and T.J. Weingartner, "Monthly temperature, salinity, and transport variability of the Bering Strait through flow," Geophys. Res. Lett., 32, 10.1029/2004GL021880, 2005.

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16 Feb 2005

The Bering Strait through flow is important for the Chukchi Sea and the Arctic and Atlantic oceans. A realistic assessment of through flow properties is also necessary for validation and boundary conditions of high-resolution ocean models. From 14 years of moored measurements, we construct a monthly climatology of temperature, salinity and transport. The strong seasonality in all properties (–31.9 to 33 psu, ~ –1.8 to 2.3°C and ~0.4 to 1.2 Sv) dominates the Chukchi Sea hydrography and implies significant seasonal variability in the equilibrium depth and ventilation properties of Pacific waters in the Arctic Ocean. Interannual variability is large in temperature and salinity. Although missing some significant events, an empirical linear fit to a local (model) wind yields a reasonable reconstruction of the water velocity, and we use the coefficients of this fit to estimate the magnitude of the Pacific-Arctic pressure-head forcing of the Bering Strait through flow.

Revising the Bering Strait freshwater flux into the Arctic Ocean

Woodgate, R.A., and K. Aagaard, "Revising the Bering Strait freshwater flux into the Arctic Ocean," Geophys. Res. Lett., 32, 10.1029/2004GL021747, doi:10.1029/2004GL021747, 2005

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20 Jan 2005

The freshwater flux through the Bering Strait into the Arctic Ocean is important regionally and globally, e.g. for Chukchi Sea hydrography, Arctic Ocean stratification, the global freshwater cycle, and the stability of the Atlantic overturning circulation. Aagaard and Carmack [1989] estimated the Bering Strait freshwater flux as 1670 km3/yr (relative to 34.8 psu), assuming an annual mean transport (0.8 Sv) and salinity (32.5 psu). This is ~1/3rd of the total freshwater input to the Arctic. Using long-term moored measurements and ship-based observations, we show that this is a substantial underestimate of the freshwater flux. Specifically, the warm, fresh Alaskan Coastal Current in the eastern Bering Strait may add ~400 km3/yr. Seasonal stratification and ice transport may add another ~400 km3/yr. Combined, these corrections are larger than the interannual variability observed by near-bottom measurements and near-surface measurements will be necessary to quantify this flux and its interannual variability.

Dissolved oxygen extrema in the Arctic Ocean halocline from the North Pole to the Lincoln Sea

Falkner, K.K., M. Steele, R.A. Woodgate, J.H. Swift, K. Aagaard, and J. Morison, "Dissolved oxygen extrema in the Arctic Ocean halocline from the North Pole to the Lincoln Sea," Eos Trans. AGU, 85(47), Abstract OS41A-0465, 2004.

15 Dec 2004

The freshwater flux to the Arctic via the Bering Strait

Woodgate, R.A., and K. Aagaard, "The freshwater flux to the Arctic via the Bering Strait," Eos Trans. AGU, 85(47), Abstract C54A-04, 2004.

15 Dec 2004

North Pole Environmental Observatory delivers early results

Morison, J.H., K. Aagaard, K.K. Falkner, K. Hatakeyama, R. Mortiz, J.E. Overland, D. Perovich, K. Shimada, M. Steele, T. Takizawa, and R. Woodgate, "North Pole Environmental Observatory delivers early results," Eos Trans. AGU, 83, 357-361, 2002.

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1 Aug 2002

Scientists have argued for a number of years that the Arctic may be a sensitive indicator of global change, but prior to the 1990s, conditions there were believed to be largely static. This has changed in the last 10 years. Decadal-scale changes have occurred in the atmosphere, in the ocean, and on land [Serreze et al., 2000]. Surface atmospheric pressure has shown a declining trend over the Arctic, resulting in a clockwise spin-up of the atmospheric polar vortex. In the 1990s, the Arctic Ocean circulation took on a more cyclonic character, and the temperature of Atlantic water in the Arctic Ocean was found to be the highest in 50 years of observation [Morison et al., 2000]. Sea-ice thickness over much of the Arctic decreased 43% in 1958–1976 and 1993–1997 [Rothrock et al., 1999].

The Arctic Ocean Boundary Current along the Eurasian slope and adjacent to the Lomonosov Ridge: Water mass properties, transports and transformations from moored instruments

Woodgate, R.A., K. Aagaard, R.D. Muench, J. Gunn, G. Bjork, B. Rudels, A.T. Roach, U. Schauer, "The Arctic Ocean Boundary Current along the Eurasian slope and adjacent to the Lomonosov Ridge: Water mass properties, transports and transformations from moored instruments," Deep Sea Res. I, 48, 1757-1792, 2001.

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1 Aug 2001

Year-long (summer 1995 to 1996) time series of temperature, salinity and current velocity from three slope sites spanning the junction of the Lomonosov Ridge with the Eurasian continent are used to quantify the water properties, transformations and transport of the boundary current of the Arctic Ocean. The mean flow is cyclonic, weak (1 to 5 cm s-1), predominantly aligned along isobaths and has an equivalent barotropic structure in the vertical. We estimate the transport of the boundary current in the Eurasian Basin to be 5 ± 1 Sv . About half of this flow is diverted north along the Eurasian Basin side of the Lomonosov Ridge. The warm waters (>1.4°C) of the Atlantic layer are also found on the Canadian Basin side of the ridge south of 86.5°N, but not north of this latitude. This suggests that the Atlantic layer crosses the ridge at various latitudes south of 86.5°N and flows southward along the Canadian Basin side of the ridge.

Temperature and salinity records indicate a small (0.02 Sv), episodic flow of Canadian Basin deep water into the Eurasian Basin at ~1700 m, providing a possible source for an anomalous eddy observed in the Amundsen Basin in 1996. There is also a similar flow of Eurasian Basin deep water into the Canadian Basin. Both flows probably pass through a gap in the Lomonosov Ridge at 80.4°N.

A cooling and freshening of the Atlantic layer, observed at all three moorings, is attributed to changes (in temperature and salinity and/or volume) in the outflow from the Barents Sea the previous winter, possibly caused by an observed increased flow of ice from the Arctic Ocean into the Barents Sea. The change in water properties, which advects at ~5 cm s-1 along the southern edge of the Eurasian Basin, also strengthens the cold halocline layer and increases the stability of the upper ocean. This suggests a feedback in which ice exported from the Arctic Ocean into the Barents Sea promotes ice growth elsewhere in the Arctic Ocean.

The strongest currents recorded at the moorings (up to 40 cm s-1) are related to eddy features which are predominantly anticyclonic and, with a few exceptions, are of two main types: cold core eddies, confined to the upper 100–300 m, probably formed on the shelf, and warm core eddies of greater vertical extent, probably related to instabilities of an upstream front.

Some thoughts on the freezing and melting of sea ice and their effects on the ocean

Aagaard, K. and R.A. Woodgate, "Some thoughts on the freezing and melting of sea ice and their effects on the ocean," Ocean Modelling, 3, 127-135, 2001.

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31 May 2001

The high-latitude freezing and melting cycle can variously result in haline convection, freshwater capping or freshwater injection into the interior ocean. An example of the latter process is a secondary salinity minimum near 800 m-depth within the Arctic Ocean that results from the transformation on the Barents Sea shelf of Atlantic water from the Norwegian Sea and its subsequent intrusion into the Arctic Ocean. About one-third of the freshening on the shelf of that initially saline water appears to result from ice melt, although the actual sea ice flux is small, only about 0.005 Sv. A curious feature of this process is that water distilled at the surface of the Arctic Ocean by freezing ends up at mid-depth in the same ocean. This is a consequence of the ice being exported southward onto the shelf, melted, and then entrained into the northward Barents Sea throughflow that subsequently sinks into the Arctic Ocean. Prolonged reduction in sea ice in the region and in the concomitant freshwater injection would likely result in a warmer and more saline interior Arctic Ocean below 800 m.

Long-tern near-bed observations of velocity and hydrographic properties in the northwest Barents Sea with implications for sediment transport

Sternberg, R.W., K. Aagaard, D. Cacchione, R.A. Wheatcroft, R.A. Beach, A.T. Roach, and M.A.H. Marsden, "Long-tern near-bed observations of velocity and hydrographic properties in the northwest Barents Sea with implications for sediment transport," Cont. Shelf Res., 21, 509-529, 2001.

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1 Mar 2001

Combined results from time-series observations of currents and water properties from two moorings and a benthic tripod measuring near-bed velocity profiles are used to evaluate the potential for sediment transport in the Storfjord, east of Hopen Island, and in Olgastretet in the Barents Sea near the southeast Svalbard Archipelago. Current observations include a 15-month time series from each mooring with the lowermost current meter on each mooring positioned at 6 mab and a 5-month time series from the tripod with four current meters located within 1.2 mab. Threshold of grain motion was estimated from seabed sediment characteristics sampled at each site, bed roughness length was calculated from the benthic tripod velocity profiles. Results from the Storfjord and east of Hopen Island suggest that near-bed currents and bottom stresses cannot resuspend sediment in the summer months. Currents exceed the threshold of grain motion during the fall and winter months in response to strong flows forced by surface cooling and winds. Threshold of grain motion occurs for approximately 10 days per year in both the Storfjord and east of Hopen Island. In Olgastretet, measured bottom currents had distinct reversals from north to south over periods of 3–8 days throughout the record. The highest currents (and largest bottom stresses) were directed southward and were high enough to resuspend bottom sediment about 19 days during the deployment period. Near-bottom flows are dominantly southward at all stations during times that sediment threshold velocities are exceeded, thus strong flows exiting the fjords in southern Svalbard during winter may also transport significant quantities of sediment into the deep northern Norwegian Sea.

Recent environmental changes in the Arctic: A review

Morison, J., K. Aagaard, and M. Steele, "Recent environmental changes in the Arctic: A review," Arctic, 53, 359-371, 2000.

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1 Dec 2000

Numerous recent observations indicate that the Arctic is undergoing a significant change. In the last decade, the hydrography of the Arctic Ocean has shifted, and the atmospheric circulation has undergone a change from the lower stratosphere to the surface. Typically the eastern Arctic Ocean, on the European side of the Lomonosov Ridge, is dominated by water of Atlantic origin. A cold halocline of varying thickness overlies the warmer Atlantic water and isolates it from the sea ice and surface mixed layer. The western Arctic Ocean, on the North American side of the Lomonosov Ridge, is characterized by an added layer of water from the Pacific immediately below the surface mixed layer. Data collected during several cruises from 1991 to 1995 indicate that in the 1990s the boundary between these eastern and western halocline types shifted from a position roughly parallel to the Lomonosov Ridge to near alignment with the Alpha and Mendeleyev Ridges. The Atlantic Water temperature has also increased, and the cold halocline has become thinner. The change has resulted in increased surface salinity in the Makarov Basin. Recent results suggest that the change also includes decreased surface salinity and greater summer ice melt in the Beaufort Sea. Atmospheric pressure fields and ice drift data show that the whole patterns of atmospheric pressure and ice drift for the early 1990s were shifted counterclockwise 40°-60° from earlier patterns. The shift in atmospheric circulation seems related to the Arctic Oscillation in the Northern Hemisphere atmospheric pressure pattern. The changes in the ocean circulation, ice drift, air temperatures, and permafrost can be explained as responses to the Arctic Oscillation, as can changes in air temperatures over the Russian Arctic.

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