Richard A. Krishfield

Influences of the ocean surface mixed layer and thermohaline stratification on Arctic Sea ice in the central Canada Basin

[1]xa0Variations in the Arctic central Canada Basin mixed layer properties are documented based on a subset of nearly 6500 temperature and salinity profiles acquired by Ice-Tethered Profilers during the period summer 2004 to summer 2009 and analyzed in conjunction with sea ice observations from ice mass balance buoys and atmosphere-ocean heat flux estimates. The July–August mean mixed layer depth based on the Ice-Tethered Profiler data averaged 16 m (an overestimate due to the Ice-Tethered Profiler sampling characteristics and present analysis procedures), while the average winter mixed layer depth was only 24 m, with individual observations rarely exceeding 40 m. Guidance interpreting the observations is provided by a 1-D ocean mixed layer model. The analysis focuses attention on the very strong density stratification at the base of the mixed layer in the Canada Basin that greatly impedes surface layer deepening and thus limits the flux of deep ocean heat to the surface that could influence sea ice growth/decay. The observations additionally suggest that efficient lateral mixed layer restratification processes are active in the Arctic, also impeding mixed layer deepening.

Automated Ice-Tethered Profilers for Seawater Observations under Pack Ice in All Seasons

An automated, easily deployed Ice-Tethered Profiler (ITP) instrument system, developed for deployment on perennial sea ice in the polar oceans to measure changes in upper ocean water properties in all seasons, is described, and representative data from prototype instruments are presented. The ITP instrument consists of three components: a surface subsystem that sits atop an ice floe; a weighted, plastic-jacketed wire-rope tether of arbitrary length (up to 800 m) suspended from the surface element; and an instrumented underwater unit that employs a traction drive to profile up and down the wire tether. ITPs profile the water column at a programmed sampling interval; after each profile, the underwater unit transfers two files holding oceanographic and engineering data to the surface unit using an inductive modem and from the surface instrument to a shore-based data server using an Iridium transmitter. The surface instrument also accumulates battery voltage readings, buoy temperature data, and locations from a GPS receiver at a specified interval (usually every hour) and transmits those data daily. Oceanographic and engineering data are processed, displayed, and made available in near–real time (available online at http://www.whoi.edu/ itp). Six ITPs were deployed in the Arctic Ocean between 2004 and 2006 in the Beaufort gyre with various programmed sampling schedules of two to six one-way traverses per day between 10- and 750–760-m depth, providing more than 5300 profiles in all seasons (as of July 2007). The acquired CTD profile data document interesting spatial variations in the major water masses of the Canada Basin, show the double-diffusive thermohaline staircase that lies above the warm, salty Atlantic layer, measure seasonal surface mixed layer deepening, and document several mesoscale eddies. Augmenting the systems already deployed and to replace expiring systems, an international array of more than one dozen ITPs will be deployed as part of the Arctic Observing Network during the International Polar Year (IPY) period (2007–08) holding promise for more valuable real-time upper ocean observations for operational needs, to support studies of ocean processes, and to facilitate numerical model initialization and validation.

Ice‐Tethered Profiler observations of the double‐diffusive staircase in the Canada Basin thermocline

vertical heat fluxes through the staircase are in the range 0.05–0.3 W m � 2 , only about one tenth of the estimated mean surface mixed layer heat flux to the sea ice. It is thus concluded that the vertical transport of heat from the Atlantic Water in the central basin is unlikely to have a significant impact to the Canada Basin ocean surface heat budget. Icebreaker conductivity-temperature-depth data from the Beaufort Gyre Freshwater Experiment show that the staircase is absent at the basin periphery. Turbulent mixing that presumably disrupts the staircase might drive greater flux from the Atlantic Water at the basin boundaries and possibly dominate the regionally averaged heat flux.

Eddies in the Canada Basin, Arctic Ocean, Observed from Ice-Tethered Profilers

Five ice-tethered profilers (ITPs), deployed between 2004 and 2006, have provided detailed potential temperature and salinity S profiles from 21 anticyclonic eddy encounters in the central Canada Basin of the Arctic Ocean. The 12–35-m-thick eddies have center depths between 42 and 69 m in the Arctic halocline, and are shallower and less dense than the majority of eddies observed previously in the central Canada Basin. They are characterized by anomalously cold and low stratification, and have horizontal scales on the order of, or less than, the Rossby radius of deformation (about 10 km). Maximum azimuthal speeds estimated from dynamic heights (assuming cyclogeostrophic balance) are between 9 and 26 cm s 1 , an order of magnitude larger than typical ambient flow speeds in the central basin. Eddy –S and potential vorticity properties, as well as horizontal and vertical scales, are consistent with their formation by instability of a surface front at about 80°N that appears in historical CTD and expendable CTD (XCTD) measurements. This would suggest eddy lifetimes longer than 6 months. While the baroclinic instability of boundary currents cannot be ruled out as a generation mechanism, it is less likely since deeper eddies that would originate from the deeper-reaching boundary flows are not observed in the survey region.

Surface freshening in the Arctic Ocean's Eurasian Basin : an apparent consequence of recent change in the wind-driven circulation

[1]xa0Data collected by an autonomous ice-based observatory that drifted into the Eurasian Basin between April and November 2010 indicate that the upper ocean was appreciably fresher than in 2007 and 2008. Sea ice and snowmelt over the course of the 2010 drift amounted to an input of less than 0.5 m of liquid freshwater to the ocean (comparable to the freshening by melting estimated for those previous years), while the observed change in upper-ocean salinity over the melt period implies a freshwater gain of about 0.7 m. Results of a wind-driven ocean model corroborate the observations of freshening and suggest that unusually fresh surface waters observed in parts of the Eurasian Basin in 2010 may have been due to the spreading of anomalously fresh water previously residing in the Beaufort Gyre. This flux is likely associated with a 2009 shift in the large-scale atmospheric circulation to a significant reduction in strength of the anticyclonic Beaufort Gyre and the Transpolar Drift Stream.

Ice-tethered profilers sample the upper Arctic Ocean

Studies conducted over the past decade indicate that the Arctic may be both a sensitive indicator of climate change and an active agent in climate variability. Although progress has been made in understanding the Arctics coupled atmosphere-ice-ocean system, documentation of its evolution is hindered by a sparse data archive. This observational gap represents a critical shortcoming of the ‘global’ ocean observing systems ability to quantify the complex interrelated atmospheric, oceanic, and terrestrial changes now under way throughout the Arctic and that have demonstrated repercussions for society [Symon et al., 2005]. n nMotivated by the Argo float program, an international effort to maintain an ensemble of approximately 3000 autonomous profiling instruments throughout the temperate oceans (see http://w3.jcommops.org), a new instrument, the ‘Ice-Tethered Profiler’ (ITP) was conceived to repeatedly sample the properties of the ice-covered Arctic Ocean at high vertical resolution over time periods of up to three years.

Motion tracking in an acoustic point-measurement current meter

Measurements of velocity structure in the water column under Arctic ice from an Ice-Tethered Profiler (ITP) employed an acoustic point-measurement current meter, MAVS (Modular Acoustic Velocity Sensor) [1]. With the velocity sensor it becomes the Ice-Tethered Profiler with Velocity (ITPV). The profiler, containing a Seabird CTD, MAVS, batteries, an inductive modem, and a wire crawling engine, integrated by McLane Labs, was constrained to be deployed through a 24″ diameter hole drilled in the ice. The anchor to the ice via a buoy with a satellite transmitter fixed the top of the mooring to a drifting but GPS tracked location while the profiler descending to a depth of 800m measured velocity relative to the moving mooring and the climbing profiler. A large current-orienting alignment fin was not possible on the ITP due to the limit of the ice hole diameter yet it was known that vortex shedding by the profiler body in the current would cause the instrument to swing and the current sensor to measure horizontal velocities due to the rotation of the profiler around the center of gyration of the package. To remove this platform motion from the current measurement, an inertial sensor, Analog Devices ADIS 16355 [2], was added to the MAVS current meter and three axes of angular velocity and three axes of linear acceleration were added to each data record of time, velocity, temperature, three-axis magnetic vector components and two axes of tilt. From the rate gyro value of angular velocity around the vertical axis, the platform rotation is determined and using the distance that the velocity sensor is displaced from the axis of rotation the horizontal current sensor velocity can be subtracted from the horizontal velocity measured by the sensor. This ITP was deployed in October, 2009 and data from the first profile indicates expected and unexpected performance.

Ice-Tethered Profiler Measurements of Dissolved Oxygen under Permanent Ice Cover in the Arctic Ocean

Abstract Four ice-tethered profilers (ITPs), deployed between 2006 and 2009, have provided year-round dissolved oxygen (DO) measurements from the surface mixed layer to 760-m depth under the permanent sea ice cover in the Arctic Ocean. These ITPs drifted with the permanent ice pack and returned 2 one-way profiles per day of temperature, salinity, and DO. Long-term calibration drift of the oxygen sensor can be characterized and removed by referencing to recently calibrated ship DO observations on deep isotherms. Observed changes in the water column time series are due to both drift of the ITP into different water masses and seasonal variability, driven by both physical and biological processes within the water column. Several scientific examples are highlighted that demonstrate the considerable potential for sustained ITP-based DO measurements to better understand the Arctic Ocean circulation patterns and biogeochemical processes beneath the sea ice.

Design and operation of automated ice-tethered profilers for real-time seawater observations in the polar oceans

Funding was provided by the National Science Foundation under Contract Nos. OCE-0324233 and ARC-0519899.

Deployment operation procedures for the WHOI Ice-Tethered Profiler

Funding was provided by the National Science Foundation under Grant No. OCE-0324233 and by the Office of Polar Programs under award numbers ARC-0519899 and ARC-0631951.