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