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Andreas Münchow

Plot of the Day

Central Channel - 9/29/04 - PDF
High winds (~40 kts) and waves from the north were insufficient to reverse the flow to the north in a central channel of the Chukchi Sea between Herals and Hanna Shoals. While the weather forced us to discontinue all over-the-side data collection, the ship steamed back and forth to wait for a break in the weather. During this "idle" steaming we crossed a major circulation feature, said channel. The near surface flow is generally in the direction of the wind (blue colors) with little veering in shallow waters, however, a dramatic, vertically strongly sheared flow against the winds (red and yellow colors) emerges over slightly deeper water in the south that represent a portion of a deeper channel connecting the southern Chukchi Sea and the Bering Strait throughflow with the northern Chukchi shelf. Surface salinity and temperature are color coded as lines at 7-m of the section of northward (top panel) and eastward (middle panel) flow. They indicated that the sheared northward flow is much warmer and slightly fresher than the surrounding waters. These features identify the water as Bering Strait water that probably contains Yukon River water as well. The remarkable feature of this northward flow is that it opposes very strong winds in the opposite direction that dominate the circulation elsewhere over the shelf where there is little velocity shear in the water.

Eddy Section - 9/27/04 - PDF
We just finished a detailed and complete hydrographic section through the center of the eddy that we had surveyed previously. The anti-cyclonic (clockwise) circulation still persists. It becomes a paticularly clear if the mean westward flow of about 13 cm/s is removed as I have done in the graph here. This is why I call it "anomaly," the spatial and temporal mean flow is removed from this section. Both horizontal and vertical shears are preserved. The eddy has a strong eastward (red) velocity component in the north and an eastward velocity (blue) tendency in the south. Neither the shelf nor the slope appear to participate much in the circulation (not shown). It will become quite exciting how water and biogeochemical properties vary both within and across the eddy.

Eddy 160m - 9/25/04 - PDF
Eddy Section 4 - 9/25/04 - PDF
Snow, sleet, freezing temperatures, winds gusting at 40kts, and rocking and rolling motions Snow, sleet, freezing temperatures, winds gusting at 40kts, and rocking and rolling motions of a ship in heavy 5+ foot seas did not prevent us from making a major discovery today. The eddy-like feature reported earlier did materialize just to the west of our initial intense survey. An even finer scale survey (still ongoing) verified that this feature is indeed an isolated anti-cyclonic subsurface eddy, that is, a roughly 60-m thick lense of cold shelf-derived water rotates clockwise about 160-m below the surface. It is embedded in a dominantly westward flow of similar (20 cm/s) magnitude. It is strongly sheared in both vertical and horizontal directions. It carries anomolous characteristics of winter shelf water in a deep water environment.

Our direct velocity observations and concurrent expendible CTD drops (magenta circles) will provide the first ever synpotic and comprehensive descriptions of such an eddy in the Arctic Ocean. We already have much data on hand to actually prove prior claims of a more speculative nature. Tommorrow we will probably start a comprehensive biogeochemical sampling of the fully surveyed eddy. The eddy feature in the section is the subsurface lense of yellow shading of the along-shore velocity component which represents an eastward flow component embedding in a generally westward (presumably partly wind-driven) circulation.

Slope Survey 100m - 9/24/04 - PDF
Slope Survey 55m - 9/24/04 - PDF
Slope 1 - 9/23/04 - PDF
Slope 2 - 9/23/04 - PDF
We just completed the first ever intense slope survey of the Chukchi Sea within about 30 hours. The resulting velocity (and temperature) fields reveal a most dynamic current regime that drastically contrasts canonical views of the Arctic circulation. (Please note the WHOI update for 9/23.) Winds were light, less than 10m/s most of the time. Consistent with more sparsely distributed observations both during this and the prior SBI process cruise, we find a general westward flow. This flow is especially strong, persistent, and spatially coherent inshore of the 200-m isobath where it closely follows isobaths with little vertical shear. Seaward of the steeply sloping shelfbreak, we find a generally westward flow also, however, a meandering flow appears that exhibits substantial vertical shear across the halocline into the Atlantic layer. It also appears more variable and is little constrained by the bottom topography. A perhaps diurnal signal appears at the offshore edges that could be the result of remotely forced Kelvin or topographic Rossby waves. Note that the large across-shore velocities extend into the individual sections as well as the offshore dog legs.

These subinertial motions associated with such planetary waves are geostrophic to first order for the alongshore flow while the across-shore flow is strongly time-dependent. The temperature field (not shown) moves vertically by perhaps 20-40m both up and downward from section to section. If these are indeed planetary waves, then they have periods longer than the inertial period that at our present latitude is near 12 hours. These waves transport particles both on and offshore, however, their net transport or flux is zero. In contrast, eddies are separate parcels of water with anomalous properties relative to their environments. We set out to find such eddies over this slope, however, we did not find any. Neither did we find a pronounced "boundary current" that prominently features in all Arctic circulation schemes even in regions where few measurements are available. So, the Arctic holds its mystic of not yet revealing to us how exactly eddies form and exactly what scales (time and space) determine particle exchanges. SBI will be hard-pressed to answer these questions. Rapid eddy-resolving surveys and geophysical fluid dynamics are much needed to progress towards more predictive Arctic sciences.

Eddy Train Section - 9/23/04 - PDF
Eddy Train 100m - 9/23/04 - PDF
Slope 1 - 9/23/04 - PDF
Slope 2 - 9/23/04 - PDF
Just after I woke up this morning and plotted up the night's data on a long southward transit back to the Chukchi slope, a most beautiful section appeared. Three eddy-like features appear near km-120, km-180, and km-260. All three suggest clockwise (anti-cyclonic) rotation. The fact that currents below these features are close to zero gives great confidence that the navigational systems (and my processing and calibration) are working superbly as even a small misalignment or scaling offset would introduce large errors in the measurements; the ship is moving close to 14 kts at the time. These eddy-like features are very prominent in the Arctic Ocean and several competing theories exist to explain their formation. They probably constitute a major mechanism to transport material across the shelves into the deep ocean. Yet, no eddy formation has been observed in situ, I believe. Most are accidental crossings like the present ones. Nevertheless, we are presently on a closely spaced rapid survey of the three-dimensional current and temperature fields over the Chukchi slope without stopping the ship. This element of rapidly mapping quasi-synoptic fields are critical in observing eddy formations. We found tentalizing evidence so far, however, I will report on those tommorrow when 5 closely spaced XBT/ADCP lines will be complete. The red dots and triangles are XBT locations, that is, we are measuring vertical temperature profiles at thos locations without stopping the ship that just keeps steaming at 10-kts.

Borderland - 9/22/04 - PDF
Ice Tide - 9/22/04 - PDF
Both plots depict currents at 100-m depth that we collected over the last 4-5 days (Borderland) and the last 24 hours (Ice Tide). The Chukchi Borderland is a topographically complex domain of many valleys, basins, banks, and slopes. Our excursion was officially billed as "sightseeing" ice from the end of a CTD-line (red dots) northward. The ice excursion is described in more detail at the WHOI update for 9/21.

Bin - 9/21/04 - PDF
We are breaking ice and it's freezing out here, below +20F. And the section data today shows, I believe, a tentalizing surface feature of enhanced northward surface currents along the ice-edge that we entered about 4 hours ago near km-150. The ice is to the north-east of us as the surface flow is to the north-west of us about 20 cm/s. Theoretically, the jet could be the result of a freshwater release during the melting season (now ending, sea water is freezing outside) and/or that of a melting and retreating ice-edge. The horizontal density gradients imply horizontal pressure gradients that are often balanced by the Coriolis force. As a result, a geostrophically balanced jet may coincide with the region of largest horizontal density gradients.

Tra - 9/20/04 - PDF
We are heading north into deeper water towards 76 23' N. The plot shows our crossing today of an extension of a shelf-like bank with very few scatterers in the water column. I am working on trying to retrieve a velocity signal from this low-scattering environment and I think my new processing can do so. I am particularly intrigued by a strong northward bottom flow near the center of the bank that is strongly sheared in the vertical.

Eddy - 9/19/04 - PDF
The graph depicts what I believe to be an eddy. It coincides with temperatures very near the freezing point, that is colder than -1.7 deg. C. There is not much of a density signal associated with it, that is, the feature does not appear to be in thermal wind balance. Also remarkable is the generally westward flow which is contrary to expectations and the many circulation sketches that have been made over the last decades. There are no winds and there have not been any storms the last couple of days either.

Inertial Time Series - 9/18/04 - PDF
Inertial Section - 9/18/04 - PDF
The plot is something I worked on today from data in exactly the same location at exactly the same day last year. It's shows a nice clean section (Inertial Section) across the Chukchi shelf and slope from 50-m to 1000-m, however, the beautiful structure may very well be dominated by inertial oscillations that at 74 degrees latitude have a period of 12.45 hours (Inertial Time Series), very close to the lunar semi-diurnal tidal constituent. The problem with inertial oscillation is that they are both intermittend and difficult to predict. At the "critical" latitude (we are pretty much on it) it is possible to encounter resonant interactions between tidal and inertial motions. Less dramatic cases are probably part of all ADCP records and careful examination and interpretation with physical processes in mind is always crucial to avoid misinterpretaion of temporal features as jets and eddies in a spatial context.

Barrow_north - 9/17/04 - PDF

Barrow Plot - 9/16/04 - PDF

Barrow Plot_38m - 9/16/04 - PDF

Wainwright - 9/16/04 - PDF

Backscatter_Beaufort Slope - 9/16/04 - PDF



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