The images served by UMO/GoMOOS (below left) and others can't
provide
the frontal details we need.
But, the Gulf of Maine SST data
from UMO can be processed for the smaller GSC region to focus on
frontal features in that area. One example from the summer of
2004 is plotted (below right).
Access to actual pixel data provides detailed image of the study
area from 5 April through 20 June 2005 (below). Unfortunately, very
little imagery is available for this period due to the poor weather
conditions. Right whale sightings near those days are posted in purple
dots. The
sightings usually fall directly on the
very subtle (<1 degC) temperature fronts. The black lines
are the 60m isobath. The white lines often depict drifter tracks
(when some are present at that time).
The operations in 2005 went as planned. We deployed two sets of 3
units (~week apart) as denoted by the white
dots in the figure below, to span the coastal current. To see where the
inshoremost unit is located on a navigational chart
click here.
Drifters
On deciding where to deploy drifters
We
considered
historical
drifter data and some arbitrary box. Of the seven historical tracks
shown below (most from the "Mass Bay" , only the three
colored "flow through".
They did so in 5.3 days (mean). One unit goes "inshore" and
three go "offshore". So, we could say, based on this anyway, that there
is nearly 50% chance
that drifters deployed off the backside of Truro will arrive in the GSC
area in about
a week.
Please note this type of analysis can be easily done for other boxes to
determine other probabilities. Drifter
Results in General
A
total of 9 drifters entered the study area during the cruise period and
4 more later in June.
All drifters were surface
followers set to report GPS fixes every 30-60
minutes. The first set of three
(545291,545292,
and 545293), deployed by the R/V ALBATROSS on 29 April off
Truro, did not perform as hoped but two of them did survive a fairly
intense northeaster on 4 May and ended up east of Nantucket.
After being drawn offshore away from the normal coastal current route,
the storm relocated them in the area they were intended to go. These
first three units had strobes attached which evidently weighted them
down such that they did not transmit in heavy seas. After removing the
strobes and shortening the flotation tethers on the second set of three
(55291,55292, and 55293 deployed 11
May by lobstermen Billy Souza), the frequency of transmission improved
significantly. This second set took the more-traditional route quickly
down the backside of Cape Cod and then stalled on reaching the Great
South Channel area. One of these drifters (55292), fitted with telemetered temperature, documented
the cold water mass it occupied until malfunctioning on 14 May
(possibly due to the temperature sensor port allowing seawater seeping
into the electronics). It starts out in relatively warm (~8-9
degC) but, as it got entrained into the core of the coastal current, it
registered near 6 degC. While this result is interesting, it should be
noted that this experimental unit is only accurate to about 1.5 deg
C. The next set of three (55201,55202, and 55203)
were deployed off Cape Ann on the morning of 10 May by the R/V OCEANUS.
They proceeded rapidly down to Great South Channel in less than a weeks
time. Finally, four more units (55381, 55383,55385,55386)
entered the study area in June after coming from a mid-coast Maine
deployment on 13 May 2005.
Results
of individual drifters
The individual tracks and estimates of velocity are presented in the
following table. The "deployer" entry is a link to a figure showing the
drifters track while the "ID#" entry is a link to the time series
plot. Also included here are the results of other drifters deployed by the
R/V OCEANUS north of the study area that eventually flow down into
the GSC. These drifters were deployed to track the advection of
harmful algal blooms.
That study is reported elsewhere.
Individual detail
drifter tracks, velocity, and integral time scales
