Dr. Robert Pickart
Woods Hole Oceanographic Institution, Woods Hole, MA 02543
Program Manager: Dr. John Calder
Related NOAA Strategic Plan Goal:
Goal 2. Understand climate variability and change to enhance society’s ability to plan and respond.
In summer 2004 the Russian icebreaker Khromov carried out the inaugural cruise of the Russia-US Long-term Census of the Arctic (RUSALCA) program. The expedition was a great success, consisting of multi-disciplinary sampling from Bering Strait northward into the Chukchi Sea. Part of the hydrographic component consisted of a detailed survey of the flow through Herald Canyon (Figure 1). This was the first time that the canyon had been sampled at high cross-stream resolution (station spacing less than the Rossby radius of deformation), enabling us to resolve fully the currents and water masses. Using the hydrographic and velocity data obtained from the survey, the dynamics of the flow of dense water through the canyon, and the potential impact that this has on the ventilation of the western Arctic, was investigated.
The focus of the study was on the cold, dense winter-transformed Pacific water (< -1.6oC) that drains out of Herald Canyon and ultimately ventilates the upper halocline of the western Arctic Ocean. This water was likely formed the previous winter due to strong surface buoyancy loss over the Chukchi Sea, both before the onset of the ice, and during polynya events throughout the season. (Influx of winter water from the Bering Sea likely contributed as well.) During the RUSALCA survey the dense water was flowing northward through the canyon, entering the head of the canyon on the western side (Figure 2). A lateral view of the winter-transformed layer shows that this cold water switched to the eastern side of the canyon as it progressed northward (Figure 3). The consequences of this are huge, since it means that the newly-ventilated winter water entering the Arctic will flow to the east along the Chukchi shelfbreak rather than to the west along the East Siberian Sea. This in turn means that the water has a “direct route” to Fram Strait and the North Atlantic Ocean, where it can contribute to the global overturning circulation.
Why did the dense water cross to the eastern side of the canyon? The topography of the canyon is such that in addition to the lateral constriction (greatest near section 2) there is a local sill near section 3. Both of these features suggest that hydraulic control could be active, and one of the consequences of this is that the northward-flowing water should switch sides of the canyon.
There is indication in the hydrographic data that hydraulic adjustment was indeed active during the time of the survey. In Figure 3, the core of the dense layer transiting through the canyon has been marked with a symbol at each section, and these stations were used to construct a vertical section along the length of the canyon (Figure 4). Within the dense layer, which roughly corresponds to the magenta shading in Figure 4a, the deepest isopycnals ground as the water flows northward through the canyon. However, this trend reverses near the canyon mouth where the isopycnals abruptly lift off the bottom. This has the character of a hydraulic jump. Furthermore, some of the properties of the layer (e.g. turbidity, fluorescence, buoyancy frequency) show a tongue emanating from offshore in this region (near 60m at station 74 in Figure 4b) which is another characteristic of a hydraulic jump.
Roughly a month after the RUSALCA survey, a hydrographic section was occupied across the Chukchi shelfbreak at 166oW (approximately 350 km to the east of Herald Canyon, Figure 1) as part of the Western Arctic Shelf-Basin Interactions Program (SBI). If one assumes an advective speed of 10-15 cm/s for the water exiting Herald Canyon (a plausible speed based on previous SBI measurements), it means that the 166oW section likely sampled the same water measured during the RUSALCA survey. This gives us the opportunity to investigate the far-field product. An analysis in T-S space (not shown) reveals that the water became warmer, fresher, and less dense, along the same mixing line that was observed in the canyon. Overall, however, relatively little mixing occurred over this large distance, indicating that the evolution of the dense water within Herald Canyon sets, to first order, the properties of the water that eventually ventilates the upper halocline. This demonstrates the importance of canyon dynamics in influencing both the water mass product, and where the water ultimately ends up in the Arctic basin.
Pickart, R.S., Pratt, L., and Whitledge, T. “Dynamics of the flow of Pacific water through Herald Canyon: Implications for ventilation of the western Arctic halocline”. Manuscript in preparation.
SUMMARY OF INTERACTION WITH NOAA
Subsequent to the RUSALCA field program the PI participated in a synthesis workshop and presented preliminary results from the study. A report describing the processing of the hydrographic data was distributed to the PIs in the project, and vertical sections were constructed and made available to the program manager and PIs via a website.