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IARC Technical Report # 1

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7.1.2. Routine CTD measurements and water sampling

7.1.2.1. Objectives (I.Polyakov and I.Dmitrenko, IARC)

The major objectives of the 2003 field experiment are: _quantify the structure and spatial variability of the main water masses over the continental shelf of the Laptev Sea and adjoined EurasianBasin in 2003; and_enhance understanding of mechanisms by which the AW is transformed across and along the continental slope of the Eurasian Basin. The hydrographic survey also provides important background information for processing of the long-term mooring data.      

7.1.2.2. Methods ( I.Dmitrenko, IARC, and S.Kirillov, AARI)

Over the 10-day period 26 CTD casts were made. Location and sampling time for the CTD casts are listed in Table 7.1 and the locations are also depicted in Figure 7.3. Cross-section A (Figure 7.3) extended across the Russian Exclusive Economic Zone (REEZ) and the Laptev Sea continental slope to the Arctic Ocean. The measurements along the northern part of this transect outside the REEZ (stations KD0103-0403) were carried out  at the beginning of observations after first mooring recovery on September 1. The southern part of transect A is located within the REEZ and measurements at this part of the transect were carried out at the end of the observational program after deployment of a mooring at the position of station KD0103 on September 7. Cross-section B (stations KD0403-0803) connected the observational area in the central Laptev Sea with the region of research northward of the New Siberian Islands where cross-section C (stations KD0803-1303) crossed the continental slope. Cross-section D (KD1003, KD1403-1803) was carried out along the border of the REEZ. Due to winch technical problems most CTD casts were made from the sea surface to a depth of 800 -1000 m only. Continuous CTD profiles were measured on the downcast. The water sampling was carried out at all stations (Table 7.1) within 500 m of the surface water layer. Five-liter Niskin bottles were tripped on the upcast.  Sampling levels are shown in Appendix 1.

Figure 7.3. CTD  cross-sections on the NABOS-03 cruise.Blue circles represent CTD and CTD/Niskin bottle stations; yellow circles are mooring sites. Bathymetry is from IBCAO.

The winch site was situated on the helicopter deck of the icebreaker approximately 17 m forward of the three icebreaker propellers (Figure 2.3). The draft of the icebreaker at the position of the CTD winch varies between 8.5 and 9.5 m. During CTD sounding the propellers were left switched on to maintain the correct ship position relative to the ice floes. Data collection and processing software was the Sea-Bird Electronics, Inc. (SBE) SEASOFT software package for Windows. Derived variables include pressure (in db), water temperature (in °C), and conductivity (S/m). The original raw data (pressure, temperature and salinity) from the downcasts are presented in Appendix 2. Portions of poor quality data from the upper water layer have been removed. Poor quality was mainly determined by higher than normal noise levels, spikes or jumps in the data due to the strong impact of the rotating icebreaker propellers in the upper water layer. To avoid the spikes in the calculated salinity (which is dependent on temperature, conductivity, and pressure) caused by misalignment of temperature and conductivity with each other we used an alignment procedure. The best alignment of conductivity with respect to temperature was obtained when the salinity spikes were minimized. Some experimentation with different advances was required to find the best alignment, which we finally determined to be an advance of conductivity relative to temperature of -0.4 sec.Although in some cases the data were considered reliable, one should take into account that the noise from propellers and ship draft can affect the data within the upper 20 m layer. The ocean depth was reliably measured by an Odom Hydrographic System Dual Frequency 12 & 210 KHz Eco-Sounder only down to a depth of 2000 m. Otherwise the depth information was obtained from the navigation charts.    

7.1.2.3. Equipment (R.Chadwell, IARC, and M.Dempsey, OM)

Continuous CTD profiles were made using the SEACAT Profiler SBE19plus. This system continuously measures conductivity, temperature and pressure at 0.25 m vertical intervals. The technical description of the sensors is presented in Table 7.2. The water sampling was carried using a General Oceanic Rosette Model 1015-12 with twelve five-liter Niskin bottles in conjunction with a SEALOGGER CTD SBE 25 system. CTD is taken off line automatically during the 10-second Rosette sampling sequence. The Rosette was connected with the on-board computer via a deck command module and electromechanical cable. The engineering information regarding Rosette is available onhttp://www.generaloceanics.com/genocean/1015.htm. SEALOGGER CTD SBE 25 sensor data are shown in Table 7.3. Information in Tables 7.2 and 7.3 is presented according to the specifications of Sea-Bird Electronics, Inc. The full information can be downloaded from http://www.seabird.com/products/spec_sheets/19plusdata.htm and http://www.seabird.com/products/spec_sheets/25data.htm.

Table 7.2: SEACAT Profiler SBE19 plus technical information.

Sensors

Measurement
Range

Initial
Accuracy

Typical
Stability
(per month)

Resolution

Conductivity (S/m)

0 - 9

0.0005

0.0003

0.00005 (most oceanic waters)

0.00007 (high salinity waters)

0.00001 (fresh waters)

Temperature (°C)

-5 to +35

0.005

0.0002

0.0001

Pressure

3500 m

0.1% of full scale range

0.004% of full scale range

0.002% of full scale range

 

Table 7.3: SEALOGGER CTD SBE 25 technical information.

Sensors

Measurement
Range

< /td>

Initial
Accuracy

Resolution

Conductivity (S/m)

0 - 7

0.0003

0.00004

Temperature (°C)

-5 to +35

0.002

0.0003

Pressure

1000 m

0.1% of full scale range

0.015% of full scale range

SBE 43 Dissolved Oxygen (DO) Sensor

120% of surface saturation in all natural waters

2% of saturation

increased by on-board temperature compensation

Sea Tech Chlorophyll "a" fluorometer

N/A

N/A

N/A

7.1.2.4. Preliminary Results

The early fall water column structure in the area of our oceanographic survey is characterized by a cold (-1.8°C), fresh (28-32 psu) surface layer, overlying a halocline in which low surface salinities increase to 34.8 psu at about 250 m depth (Figures 7.4, 7.6, 7.8).  The surface mixed layer is about 18-20 m thick (Figure 7.4, right panel). The shallow halocline centered at about 90 m depth is found above the thermocline (centered at 130 m); this layer is believed to play a crucial role in insulating near-surface waters and overlying ice from upward heat fluxes.

Figure 7.4: Salinity (blue) and temperature (red) vertical distribution at station KD0103. Data from CTD SBE19plus. Right panel presents the zoomed upper 250 db of the left panel.

Surface temperature is close to the freezing point down to the depth of the surface mixed layer. An increase of temperature by 0.2 to 0.5 °C at a depth of 20-30 mwas observed in most cases directly on the upper surface of the pycnocline layer (Figure 7.4, right panel; Figure 7.5, left panel). Apparently this temperature increase is associated with summer solar radiation heating of the surface water layer [Kassens et al., 1997]. The temperature at this level (about 20 m) corresponds to the temperature of the surface mixed layer during summer. Farther downward a temperature decrease is observed down to a depth of about 50 m. This depth could be considered the depth of seasonal temperature variations. This layer lies above a strong thermocline reaching 250 m depth, where a prominent temperature maximum of 1.0 to 1.2 °C is found which corresponds well to the previous studies (EWG, 1997, see also Figure 7.4). This mid-depth temperature maximum marks the core of the AW, typically found between 150 and 800 m depth in this region (Figures 7.4, 7.5, 7.7, see also Schauer et al., 1997 for more details). AW core salinity varies between 34.82 and 34.90 psu (Figures 7.9-7.12). Beneath the AW core, temperature decreases to about -0.8 °C close to the bottom while salinity slightly increases to 34.92 psu.

The spatial variations of water temperature and salinity along the transects across the continental slope (marked as A and C in Figure 7.3) are presented in Figures 12 and 13. Ice melting southward of cross-section A (Figures 5.1 and 7.3) seemed to result in freshening of the surface water layer up to 28-29 psu at stations KD2603, KD2503 and KD2303 (Figure 7.6, left panel). Although this freshening could also be caused by the input of river runoff water, the pack ice edge coincides very well with the zone of freshening (Figure 5.1). Open water conditions also allowed radiative heating of the surface water layer up to -0.7°C (Figure 7.5, left). Northward from station KD2603 the cooling of the most superficial layer is also well pronounced (Figure 7.5, left). Under the ice cover (for example, all stations along the cross-sections B and D, Figure 7.7) these patterns were not observed. 

The thickness of the AW layer, traced by the zero temperature isotherm, varies from 600 m (KD2303) to 320 m (KD0103) (Figure 7.5, left).The increase of AW layer thickness is generally accompanied by a salinity and temperature increase of 0.02 psu and 0.2°C respectively. The location of the AW core in the central Laptev Sea (cross-section A) coincides roughly with the continental slope. At the same time on the east (cross-section C) the thickness of the AW layer was only 470 m (Figure 7.5, right panel). Note that in 2002 (NABOS expedition) and 1993 (ARK IX/4 Polarstern expedition; Schauer et al., 1997) the maximum temperature in the central Laptev Sea AW core was about 250-300 km northward from the edge of the continental shelf. The northward deepening of the AW upper boundary by 45 m was recorded along cross-section A (Figure 7.5). The same features were observed in the westward direction along cross-sections B (80 m) and D (31 m, Figure 7.7).

Figure 7.5: Temperature (°C) distribution across the continental slope of the Laptev Sea along the cross-sections A and C. The0 °C isotherm  (white curve) traces the boundaries of the AW layer.

Figure 7.6: Salinity (psu) distribution across the continental slope of the Laptev Sea along the cross-sections A and C. The0 °C isotherm (black curve) traces the boundaries of the AW layer.

Figure 7.7: Temperature (°C) distribution along the continental slope of the Laptev Sea on cross-sectionsB and D. The0 °C isotherm (white curve) traces the boundaries of the AW layer.

Figure 7.8: Salinity (psu) distribution along the continental slope of the Laptev Sea on the cross-sections B and D. The 0 °C isotherm (black curve) traces the boundaries of the AW layer.

The AW core temperature of 1.2 °C in 2003 was similar to the climatic mean temperature presented in [EWG, 1997]. It also corresponds well to the temperature recorded on the continental slope to the north of the New Siberian Islands from January - August 1996[Woodgate et al., 2001]. Note that in the earlier part of their record the temperature of the AW core was warmer, as warm as1.7°C.Our measurements show the AW temperature cooler by 1 °C and fresher by 0.05 psu compared with observations from the1995 and 1997 Polarstern cruises in this area [Schauer et al., 1997; Rudels et al., 2000] (Figure 7.12). A period of cooler AW inflow through Fram Strait began after 1995 [e.g. Karcher et al., 2003]. Apparently, this cooler AW inflow later resulted in cooler and fresher water at the continental slope in the northern Laptev Sea. Nevertheless, the recent 2000-2001 temperature data from the North Pole Environmental Observatory measured on the downstream flank of the Lomonosov Ridge still show increased AW temperature inherited from the 1990s [Morison et al., 2002]. Our data from the 2002 and 2003 NABOS cruises confirm that for the Laptev Sea continental slope the large warming event of late 1980s - early 1990s is over. This also corresponds well with other observations and modeling studies [e.g. Karcher et al., 2003].   

Figure 7.9: Temperature versus salinity from NABOS-02 (black) and NABOS-03 (red) expeditions on cross-section A. The measurements were carried out at approximately the same positions.

Comparing data from the 2002 and 2003 NABOS cruises, substantial differences in temperature and salinity in the AW core were observed only along cross-section A. The temperature and salinity in the AW core in 2003 was higher than in 2002 by about 0.5 °C and 0.2 psu respectively (Figure 7.9, right). The upper surface of the AW layer was also elevated by about 60 m in 2003 compared to 2002 (not shown). Northward of cross-section A the temperature at the upper surface of the AW layer was slightly lower than in 2002 (Figure 7.9, left and central panels).Along cross-section B thethermohaline conditions within the AW layer in 2002 and 2003 were very similar (Figure 7.10). Along cross-section D a considerable decrease of the AW temperature was observed in 2003 at the lower layer beneath the AW core compared to 2002 (Figure 7.12). At the same time the temperatures measured in the AW core in 2002 and 2003 were comparable.

Figure 7.10: Temperature versus salinity from NABOS-02 (black) and NABOS-03 (red) expeditions along cross-section B.The measurements were carried out at approximately the same positions.

Figure 7.11: Temperature versus salinity from NABOS-02 (black) and NABOS-03 (red) expeditions along cross-section C.The measurements were carried out at approximately the same positions.

A step-like structure of vertical temperature distribution was observed within the AW layer in 2002 and 2003.Typical thickness of temperature steps is about 20-25 m (Figure 7.4, right). Similar steps were observed by Rudels et al. [1999]. It was hypothesized that these steps are formed as a result of evolution of temperature isopycnical intrusions. These intrusions represented by small-scale temperature inversions are widely observed in the AW layer (Figure 7.4, see also Rudels et al., 1999). One may suggest that instability of the boundary separating water masses of the same density but different temperatures may result in these intrusions. Most interesting is that these intrusions were located at the same isopycnical surfaces in 2003 and 2002 (Figures 7.10 and 7.12).     

Figure 7.12: Temperature versus salinity from NABOS-02 (black), NABOS-03 (red) and (blue) Polarstern ARK XIII/3 (left and right panels, 1997) and XI/1 (central panel, 1995) cruises along cross-section D.  The measurements were carried out at approximately the same positions.

7.1.3. Moorings observations

7.1.3.1. Objectives (I.Polyakov and I.Dmitrenko, IARC)

The overall purpose of mooring observations is to provide observationally based information on temporal variability of water circulation and water mass transformation on    the continental slope of the Laptev Sea. The major objectives are: _quantify the structure and temporal variability of main water masses over the continental shelf of the Laptev Sea; and_    obtain detailed information about AW layer dynamics and seasonal variations.

7.1.3.2. Mooring design and equipment (R.Chadwell, IARC, and M.Dempsey, OM)

Mooring design and oceanographic equipment is presented in Figure 7.13. The most notable feature of these moorings is the specially modified avalanche beacons (Figure 7.14) used for recovery if the mooring is released in ice covered waters. The beacons are modified with mercury tilt switches and three additional batteries to ensure sufficient battery life during the expected one year deployment. The beacons are placed in a heavy gauge Schedule 80 PVC housing with rounded slip caps. Squared slip caps have had a tendency to succumb to the pressure in past experience The PVC housing is then ballasted to remain upright while buoyant which in turn leaves the mercury tilt switch deactivated to conserve power. When the mooring is released and the beacon reaches the keel of the ice covering, the beacon will then self-right and wait in the receive mode. Searchers on the ice can then use a non-modified avalanche beacon to locate the mooring trapped below.

Figure 7.13: NABOS-02 (M1A) and NABOS-03 (M1B and M2A) mooring design and equipment. 

Figure 7.14: Beacon (left) and beacon placed in a heavy gauge Schedule 80 PVC housing with rounded slip caps (right).

Figure 7.15: Sketch of Mclane Moored Profiler (MMP), _ of McLane Research Laboratories, Inc.

The Mclane Moored Profiler (Figure 7.15) designed and manufactured by McLane Research Laboratories, Inc. is the main component of all NABOS moorings. The full technical information and description are available on http://www.mclanelabs.com. The sensor data of the profiler along with other mooring equipment information is presented in Tables 7.4 and 7.5 and Figures 7.16 and 7.17.

Table 7.4: Sensors for Mooring M1B Deployed 8 September, 2003.

Equipment

Serial #

Parameters

Last Calibration

Sampling Rate

Target depth (db)

Comments

Top Microcat CTD SBE-37SM

1672

Conductivity Temperature Pressure

July 16, 2003

15

Minutes

71

 

Mclane Moored Profiler (MMP)

11474

Current

Conductivity

Temperature

Pressure

N/A

One

profile

per day

65

to

1500

 

MMP FSI ACM Sensor

1661

Current

April

2003

-

N/A

MMP sub-sensor

MMP FSI EMCTD

Sensors

1360

Conductivity

Temperature

Pressure

June

2003

-

N/A

MMP sub-sensor

Table 7.5: Mooring M2A Deployed 10 September, 2003.

Equipment

Serial #

Parameters

Last Calibration

Sampling Rate

Target depth (db)

Comments

Mclane Moored Profiler

11622

Current

Conductivity

Temperature

Pressure

N/A

One

profile

per day

90

to

1390

 

MMP FSI ACM Sensor

1660

Current

April

2003

-

N/A

MMP sub-sensor

MMP FSI EMCTD

Sensors

1359

Conductivity

Temperature

Pressure

June

2003

-

N/A

MMP sub-sensor

 

7.1.3.3. Mooring deployments (R.Chadwell, IARC, and M.Dempley, OM)

Two sub-surface moorings were deployed from the I/B "Kapitan Dranitsyn" during the NABOS-03 expedition. The first mooring (M1B, Figure 7.16) was deployed near station KD0103 (Figure 7.3) and replaced mooring M1A (see part 7.1.2.2), deployed during the NABOS-02 expedition in 2002. The second mooring (M2A, Figure 7.17) deployed during 2003 was located near station KD2303 (Figure 7.3).

Deploying moorings in the ice-covered waters is problematic, and we used anchor-first deployment. This method prevents towing the mooring through floating ice floes thereby placing the towed array in danger of fouling on ice obstacles while it is being maneuvered toward the target deployment site. Similarly, our moorings were designed with buoyancy located only at the top to prevent the array from surfacing during recovery with floats stranded under the ice on opposite sides of ice keels, complicating retrieval.

Unfortunately, anchor-first operations also have their disadvantages. Anchor-first deployments require that the crane and rigging bear the weight of both the anchor and 0.25-inch galvanized Nilspin wire. The tension on the wire hanging vertically over the side makes it more difficult to manipulate the wire while attaching instruments. Additionally, the tension poses greater risk for damaging the plastic wire jacket which isessential for the unencumbered vertical movement of our primary instrument, the Mclane Moored Profiler.

The initial deployment attempt was thwarted by what the Handbook of Oceanographic Winch, Wire and Cable Technology [2001] describes as "pull down" or "cutting in." The previous year's deployment had presented challenges because the ship's capstans are not in the best position relative to the ship's cranes. Therefore, it had been decided to use a LEBUS winch, as opposed to a capstan, for the 2003 field season. This year's mooring team anticipated that "the potential for cutting in of a cable can be reduced if the cable is wound on the drum so that the inner layers have a high tension" [Handbook of Oceanographic Winch, Wire and Cable Technology, 2001]. Therefore, a Reel-O-Matic pay out tensioning machine was used to transfer the wire from a wooden spool to the winch drum. Nevertheless, the high loads caused by the anchor and weight of the wire caused "cutting in" where the outer wrap of cable damaged the inner wraps in response to a high tension load; under extremely high tension, the cable pulled down with a force great enough to penetrate existing cable layers on the winch drum. 

Figure 7.16: NABOS-03 M1B mooring design and equipment. 

Figure 7.17: NABOS-03 M2A mooring design and equipment.

The effort to deploy from the winch was therefore abandoned in favor of using the ship's docking capstan located on the aft deck. Several minor problems with side loading using the main crane from the capstan located on the aft deck were quickly overcome and both moorings were successfully deployed. One attempt to use the bare drum of the winch as an impromptu capstan was discontinued since the drum didn't provide enough friction to bear the weight of the anchor. The purchase of a Brail Winch, or horizontal capstan, is a high priority for the next season and should alleviate all obstacles associated with anchor-first deployments.

7.1.3.4. Mooring recovery (R.Chadwell, IARC, and M.Dempley, OM)

A mooring recovered during the NABOS-03 cruise expedition was originally deployed on September 1, 2002. After one year it was successfully recovered on September 1, 2003. The mooring location coincides with stations KD0103/KD1803 shown in Figure 7.3. Data obtained from this mooring are summarized in Table 7.6.

 

TABLE 7.6: Data from NABOS-02 (mooring M1A).

Equipment

Serial #

Parameters

Data recovered

Time of observations

mm/dd/yy

Actual depth (db)

Comments

Surface Floating CTD SBE-37

1759

Conductivity Temperature Pressure

Yes

09/02/02

-

09/01/03

136

Wrong pressure sensor range. Actual depth is guessed

Surface Anderaa RCM-7

9640

Conductivity Temperature Pressure Currents

Data lost

-

-

Battery problem

Surface CTD SBE-16

168350-1408

Conductivity Temperature

Yes

09/02/02

-

09/01/03

161

The depth was accounted respectively MMP most upper position

McLANE Profiler

11622

Profiles of:

Conductivity Temperature

Currents

Yes

See Figure 17

from 164 to 2607

Record of currents may be wrong. Actual profiling track shown on Figure 7.18

Bottom CTD SBE-37

2368

Conductivity Temperature Pressure

Yes

09/02/02

-

09/01/03

2742

Recorded depth (2742 db) seems to be wrong

Bottom Anderaa RCM-7

1800

Conductivity Temperature Pressure Currents

Yes

09/02/02

-

09/01/03

2670

Current records may be underestimated.

Figure 7.18: Actual profiling track of the NABOS-02 moored McLANE MMP Profiler (station M2A) according to pressure sensor data.

 

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