AOTIM: 5km Arctic Inverse Tide Model ESROSU

1
Upper Ocean Currents in the SBI Study Area in the
Arctic Ocean During July-August 03 Robin Muench,
Eric Johnson, Laurie Padman, and Susan
Howard Earth Space Research, Seattle,
Washington, USA
Introduction
Impact of Tidal Currents?
Currents were measured throughout the Shelf-Basin
Interactions (SBI) study area during July-August
2003 from the RVIB Nathaniel B. Palmer.
Measurements were made using the vessel-mounted
150 kHz acoustic Doppler current profiler
(VM-ADCP) and an Lowered ADCP (LADCP) that was
attached to the CTD rosette frame. The VM-ADCP
returned data reliably down to about 300 m while
underway and on station. The LADCP returned data
throughout the water column at CTD stations.
Visual comparison between vector current plots
constructed for the upper ocean using the VM-ADCP
and LADCP data show good agreement between the
two independent measurement sets, lending
credibility to the data. Two dominant features
were observed. One was a down-canyon flow event
in Barrow Canyon that was present early during
the cruise but was not observed later in the
cruise. Current speeds during this event were of
order 100 cm s-1. The second feature was
presence along the upper continental slope of a
current jet some 200 km wide and having speeds of
order 25 cm s-1. This jet was easterly or
westerly at different times, and at different
cross-slope transects, during the cruise. The
reversals cannot be explained in terms of tidal
currents modeled tidal currents for the region
exceed 10 cm s-1 only in the vicinity of Herald
Island and above shoal areas associated with the
Chukchi Borderlands, and are far lower along the
slope and shelf break. Thus, we hypothesize that
flow reversals are subtidal, perhaps driven by
weather systems.
Western Transects
AOTIM 5-km Arctic Inverse Tide Model
(ESR/OSU) AOTIM is a depth-integrated model of
Arctic tides on a 5-km grid. 8 constituents are
modeled 4 semidiurnal and 4 diurnal. AOTIM
assimilates tide height data from 300 tide gauge
sites, plus TOPEX/Poseidon and ERS satellite
radar altimetry. The model is available with a
Matlab graphical user interface see
http//www.esr.org/arctic_tides_index.html or
contact Laurie Padman (padman_at_esr.org) for
further details. The time-averaged (mean) tidal
current, ?U? (Figure below), frequently exceeds
10 cm s-1 in the shelf seas. In the SBI region
(right side, below), mean currents are usually lt5
cm s-1. Spring tidal currents are 2?U?. These
values should be removed from ship and lowered
ADCP records for interpretation of mean
circulation from these data sets.
First Half of Cruise
Upper-Ocean (0-100 m Depth-Averaged) Currents
Vessel-Mounted ADCP
Lowered ADCP
Mean tidal speed
First Half of Cruise
Eastern Transects
Second Half of Cruise
SBI Region
(cm s-1)
Eastward flow along the upper slope off Alaska
was stronger earlier in the cruise than later.
Second Half of Cruise
Conclusions
A westward current jet over the upper slope in
the northwest portion of the study region (upper
panel) was replaced later during the cruise by an
easterly flow (lower panel) occupied at a
slightly different location but in the same
region.
Circulation in the SBI study region during early
summer 2003 was measured with ship-mounted and
lowered ADCP.  Typical currents over the shelf
were of order 10 cm s-1.  Stronger currents,
frequently exceeding 25 cm s-1, were found over
the shelf break and upper continental slope.  The
most intense current was a downslope flow of 100
cm s-1 in Barrow Canyon.  There were significant
differences in circulation patterns for the first
and second parts of the cruise.  For example, the
strong alongslope flow over the continental slope
reversed direction, and there was no evidence for
the strong Barrow Canyon flow in the second half
of the cruise.  Modeled depth-averaged tidal
currents along the cruise track rarely exceed 3
cm s-1, indicating that observed variability in
measured currents was probably related to wind
stress changes.  Because of the temporal
variability, no clear pattern of mean
circulation can be discerned from these data.
Results presented here were obtained through
research carried out under NSF Grant OPP-0125252
to Earth Space Research.
Strong down-canyon flow in Barrow Canyon early in
the cruise.
Vessel-mounted ADCP current vectors were computed
as one-hour means vertically averaged from 0-100
m, and include both underway and on-station data.
Lowered ADCP current vectors were computed as
0-100 m vertical means from data acquired during
each CTD cast while the vessel was stopped on
station.