Scope of fieldwork
There will be three August-September cruises every two years (2013, 2015 and 2017), with repeated oceanographic sections and deployment of moorings and Lagrangian drifters (Fig. 1). Annual observations provide a higher sampling frequency, which is very attractive. However, due to the high cost of ship operations, we will have a cruise every two years.
Extensive measurements are planned reaching from Svalbard to the Lomonosov Ridge and eastward into the Makarov Basin (MB, Fig. 1), both to answer fundamental questions about circulation and transformation of Atlantic Water (AW), and to provide context for the oceanographic, biological, and chemical sampling programs. Ship-based sampling will include conductivity-temperature-depth (CTD)/rosette and geochemistry sampling and attendant sea-ice and atmospheric observations. Repeated cross-sections, some of which were initiated as early as 2002, have proved to be a powerful tool for detecting and documenting the climate change signal [e.g. Polyakov et al. 2005, 2011a,b]. There are, however, some important additions to the routine NABOS survey. Two cross-sections at 90°E and 160°E will be expanded further north to merge with the two NPEO sections (Fig. 1). This addition links our observations with another Arctic Observing Network (AON) element, NPEO, and provides the large-scale spatial coverage that is much needed for data interpretation. Higher (~several km) spatial density of sampling is planned for the shelf edge vicinity and coarser sampling (~10-15km) will be used over the slope area, thus resolving fast lateral changes of water properties at the shelf margins. In the basin interior the sampling will be even coarser (~50km). CTD observations will be complemented with lowered Acoustic Doppler Current Profiler (LADCP) measurements. These will be compared to XCP velocity profiles at selected stations on the 160°E and 90°E line
Figure 1. Locations of moorings, cross-sections, and Lagrangian drifters. We will have three August-September cruises every two years, with repeated oceanographic sections and deployment of Lagrangian drifters and moorings (including mooring section at the Laptev Sea slope). We refer to the Amundsen and Nansen Basins collectively as the Eurasian Basin, distinct from the Makarov Basin.
Chemical tracers can be used in conjunction with temperature-salinity (T-S) measurements to address variability in halocline water formation and circulation [McLaughlin et al. 2004; Woodgate et al. 2005; Itoh et al. 2007; Alkire et al. 2010]. By combining high-resolution measurements of the NO parameter and focused bottle chemistry sampling, variability in the halocline can be studied. Fractional contributions of meteoric water, net sea-ice melt, and AW will be calculated via a simple water-type analysis [Ostlund and Hut 1984; Schlosser et al. 1994; Yamamoto-Kawaii et al. 2008] incorporating conservative tracers (salinity, δ18O, and total alkalinity). The fractions of these water types will be used to assess the contribution of shelf waters to halocline layers. Combined with sensor-based, high-resolution measurements of NO, temperature, and salinity, these data will be used to gauge the role of shelf waters in halocline layer formation.
While the deployment of nitrate and dissolved oxygen sensors on moorings M1, M5, and M9 should capture the temporal variability in NO at fixed locations, particularly the seasonal cycles of biological production versus respiration and sea-ice melt and formation, the multi-disciplinary cruises will provide unparalleled detail about the distribution of both NO and freshwater sources over the Siberian shelf/slope and Eurasian Basin (EB). Together, these data should help to elucidate variability in the role of shelf waters in halocline ventilation in both time and space. An aggressive chemical sampling program is essential to the success of the analysis. For example, samples for nutrient determination (nitrate, nitrite, ammonia, phosphate, and silicate), chlorophyll and dissolved inorganic carbon/total alkalinity will be collected. Highly accurate DIC and TA data will be also collected allowing changes in oceanic CO2 to be documented. Bottle samples of nitrate and dissolved oxygen are necessary for sensor calibration. Salinity and stable oxygen isotopes (δ18O) will be utilized to separate and quantify fractional contributions of net sea-ice melt, meteoric water, and AW as described above. Total alkalinity (TA) samples, collected as part of the carbon biogeochemistry program, will also serve to supplement the limited number of stable oxygen isotope samples available to estimate contributions of net sea-ice melt and meteoric water [Anderson et al. 1994, 2004; Yamamoto-Kawaii et al. 2005].
Moorings are an important element of the program (Figs. 1, 2a). Some of these moorings have been maintained since 2002. Moorings from these locations have already provided key information about strong EB warming and have helped estimations of transit times required for the signal to travel from Fram Strait to the basin interior. Moorings will be deployed for two years, with which we have accumulated much experience.
Figure 2: (A) Mooring-based element of the observational network. Conventional moorings will be maintained spanning from Severnaya Zemlya to East Siberian Sea (Fig. 1). A mooring-based cross-slope section will be deployed at the central Laptev Sea (~125°E). Note that the vertical scale is distorted in order to emphasize observations proposed for the AW and overlying layers. (B) Time series of AW temperature anomalies (°C) relative to the time-series means from 125°E section (blue) and mooring (yellow) and heat content of the layer overlying the AW (~50–125-m, mJ/m3, green, from Polyakov et al. 2010).
The mooring locations in the Laptev Sea (M1 and M3 moorings, Fig. 1) and East Siberian Sea (M9 mooring) are reliable climatological sites; data from these locations were used in the past for monitoring climatic changes [e.g. Polyakov et al. 2005; Dmitrenko et al. 2008a]. The mooring site north of Severnaya Zemlya (M5 mooring) is close to the frontal zone between the AW Fram Strait and Barents Sea branches and to a polynya region; therefore, it is more susceptible to local processes like down-slope cascading of dense shelf waters. Data from this mooring bridges snapshot CTD observations taken at the 90°E cross-slope section carried out every other year. These moorings will use “conventional,” reliable equipment: fixed-depth CTDs, SBE-37s (“microcats”) to measure temperature and salinity, and ADCPs to measure currents. Previous observations have demonstrated rather uniform vertical structure of the boundary current in the AW and halocline; thus, the proposed design should be sufficient to capture AW transports. These moorings complement snapshot ship-based observations at distributed cross-sections providing climatologic context to the snapshot measurements. Two of the moorings (M1 and M9) will be equipped with upward-looking bottom-tracking ADCPs to measure current profile and to monitor ice drift; they will also have Upward Looking Sonar (ULS) for ice draft measurements. CTD chains affixed to two moorings (M5 and M3) will be located above major flotation to measure surface boundary layer T and S.
Ocean Bottom Pressure (OBP) variations will be tracked at the M1 and M9 moorings using Sea-Bird BPR-53 meters. We will compare these observations with Dynamic Ocean Topography (DOT) derived from the CryoSat-2 altimeter to test satellite-derived estimates of Arctic Ocean circulation [Kwok and Morison, 2011] and freshwater distribution [Morison et al., 2011].
Cross-slope mooring section
We will deploy several moorings shaped in a cross-isobath section at 125°E (Laptev Sea slope, Fig. 2a). A total number of six moorings will be deployed in 2013 at this cross-section, covering the depth range from 300m to 4000m. Climatology shows that the AW core defined by the temperature maximum is located over the 2500-3000m isobaths along the Laptev Sea slope. Seven NABOS CTD cross-sections (i.e. 2002, 2004-09) captured the core within this range. The only exception was in 2003 when there were two AW cores; the deeper one was shifted to the north, and was not resolved by the section. However, the northern station of this section was located over the ~3500-m isobath. Thus, the mooring section will capture the major AW transports. The McLane Moored Profiler (MMP) climbs up and down along a mooring line providing high-resolution T, S, and velocity profiles. We plan to use MMPs within the ~50-750m (i.e. AW+halocline) depth range with one profile per three days, thus obtaining a good climatological record and preserving MMP battery power (which was a problem in the past). The shallowest and central moorings will use conventional equipment (Fig. 2a). Data from these moorings will be used to define the spatial structure of the boundary current, for estimating along-slope transports and cross-slope AW core displacements.
Satlantic V2 ISUS nitrate sensors will be added to moorings and deployed alongside Seabird CTDs (Fig. 2a); additional dissolved oxygen measuring capabilities in lower halocline waters will provide year-round monitoring of the quasi-conservative tracer NO. Alkire et al.  found variability in vertical profiles of NO to be highly complex and illuminating in the study of halocline ventilation and its variability. Such detail cannot be captured without sensor-based measurements.
Lagrangian buoys (partners’ programs)
Lagrangian buoys will be deployed in the eastern EMB, providing the longest drift trajectories through the eastern Arctic Ocean (Fig. 1). These buoy observations, an essential part of AON and this project.
Ice-Tethered Profilers (ITPs)
ITPs will be provided by WHOI and will be deployed during each cruise (Fig. 1). We plan to deploy the instruments on ice. However, during the NABOS cruise in 2009, for the first time, two ITPs were deployed in open water; they incorporated a new-style surface package designed to be more robust in Marginal Ice Zone conditions. Thus, ice conditions will not preclude ITP deployments. The ITP profilers operate on typical sampling schedules of two one-way CTD profiles between 7 and 760 m depth each day. Data from the ITPs are broadcast within hours of acquisition and made available on the ITP website at: http://www.whoi.edu/itp/data/.
Figure 3: Ice-Tethered Profiler (ITP).
Building on the ongoing success of ice drifters that support multiple discrete subsurface sensors on tethers and the WHOI-developed Moored Profiler instrument capable of moving along a tether to sample at better than 1-m vertical resolution, WHOI researchers designed and field tested an automated, easily-deployed ITP for Arctic study. The system consists of a small surface capsule housing a controller interfaced to an Iridium data telemetry unit and inductive modem, a plastic-jacketed wire rope tether extending down 500 to 800 m into the ocean terminated by a ballast weight, and a new variation of the WHOI Moored Profiler (in shape and size much like an Argo float) that mounts on the tether and cycles vertically along it. Communication between the Profiler and surface controller is supported by an inductive modem (utilizing the wire tether and seawater return), and between the surface unit and shore via a satellite link.
Figure and caption are from IABP web page:
Ice Mass Buoys
Figure 4. The IMB is an autonomous, ice-based system, designed to measure and attribute thermodynamic changes in the mass balance of the sea ice cover. The instrumentation of the autonomous mass balance buoys typically consists of a Campbell scientific datalogger, an Argos transmitter, a thermistor string, and above ice and below ice acoustic sounders measuring the positions of the surface and bottom within 5 mm. In addition to the mass balance instrumentation the buoys also have a GPS, a barometer, and an air temperature sensor. Thermistor strings were PVC rod with YSI thermistors spaced every 10 cm. These rods could easily be connected to assemble strings that extended from the air through the snow and ice into the upper ocean. The thermistor accuracy is better than 0.1°C.
Meteorological buoys SVP (Surface Velocity Profiler)
Several meteorological buoys (IABPs) provided by APL (I. Rigor), and Meteo France (Pierre Blouch) will be available for deployment during each cruise (17 for 2013 cruise, Fig. 5). The SVP are buoys that have been used for the World Ocean Circulation Experiment (WOCE) buoys for many years, and given the increasing amounts of open water in the Arctic, the IABP has been using these buoys on sea ice and in the open water with very good success. The basic SVP measure ocean currents (or ice motion) and sea surface temperature, but we have been deploying SVP buoys that have been upgraded with a barometer (SVP-B). Some of these buoys have also been upgraded to include a GPS which is more accurate (better than 5 m) than the Argos positioning (typically +/- 120 m). The basic SVP typically reports for 18 months in the wet ocean, but depending on the deployment conditions, these buoys typically report for about 9-12 months. As always, one of the trade offs for additional instruments (barometer, GPS) is a shorter battery life. Most buoys report via Argos, but many have started using Iridium (e.g. the ITP, and possibly the IMB this year).
Figure 5: Deployment of an SVP buoy during NABOS-09 expedition from the IB Kapitan Dranitsyn.
The focus of this work is on understanding the behavior of O3 and CO2, as two of the most important greenhouse gases that are as yet poorly understood. The buoy also measures bromine oxide (BrO), a key species responsible for the extraordinary polar springtime O3 and Hg atmospheric depletion , both of which have strong consequences for human and ecosystem health in the Arctic region.
Figure 6: O-Buoys
There are a variety of potential connections between these three species. O3 is a critical and central molecule in the troposphere. It indirectly but profoundly affects the abundance of most trace gases, including many greenhouse gases. Lower atmosphere depletions in O3 seen in the Arctic (Bottenheim et al., 2002) during springtime are highly correlated to deposition of Hg (Lu et al., 2001; Lindberg et al., 2002), a toxic and bio-accumulative pollutant; thus it also indicates and possibly determines Hg behavior. This coupled Hg and O3 chemistry is driven by bromine radicals (Br and BrO). Therefore, observations of BrO directly quantify a key catalyst in Arctic atmospheric chemistry and provide a link to the cycling of O3 and Hg in this region. A long standing hypothesis is that both micro and macro-algae emit organo-halogen species that may be initiators of the halogen chemistry that destroys O3, producing the intermediate BrO (Bottenheim et al., 1990). The fluxes of organo-halogen compounds are tied to primary productivity and thus to some extent to CO2 fluxes. Fluxes of halogen precursors and CO2 may or may not be tied to the presence of open leads. CO2 is exchanged by biological processes and thus is an indicator of biological activity, coupled to the physical characteristics of the open ocean and sea ice surfaces. While CO2 is readily exchanged between the atmosphere and the sea water, the role of sea ice as a barrier to, or integral player in, this process is only poorly understood. In spite of their importance, very little data exists on O3, BrO and CO2 concentrations in the Arctic Ocean region, and very little indeed over the Arctic Ocean surface. However, the scarce data for these key atmospheric species in the Arctic Ocean region itself is primarily due to the lack of capability. More information may be found at www.o-buoy.org.