Inter-annual variability of currents and water properties offshore of the mid-Atlantic bight Charlie Flagg (SBU)-Kathy Donohue (GSO)-Jon Hare (NMFS) URI Climate Change Science Symposium - May 5, 2011

1. Inter-annual variability of currents and water
properties offshore of the mid-Atlantic bight
Charlie Flagg (SBU)-Kathy Donohue (GSO)-Jon Hare (NMFS)
URI Climate Change Science Symposium - May 5, 2011
Objective: To gain a
quantitative handle on long
term variability of currents in
the Gulf Stream and adjacent
waters. Weekly roundtrips
between Bermuda and New
Jersey yield good ADCP data
about 50% of the time.
1992-present

2. Acoustic remote sensing - currents
The 75 kHz ADCP can reach to 600-700 m depth in good
weather and in the absence of bubbles blocking the beams. This
figure highlights the vertical coherence of the Gulf Stream, and
the westward flow in the Slope and Sargasso Seas.
cont. slo
pe

3. Acoustic remote sensing - biology
The Gulf Stream
Slope Sea Sargasso Sea
Top: currents (m/s).
Bottom: raw
backscatter count (not
range corrected) - a
measure of particle
density.

4. Acoustic remote sensing - biology
The Gulf Stream
Slope Sea Sargasso Sea
Top: currents (m/s).
Bottom: raw
backscatter count (not
range corrected) - a
measure of particle
density. Note multiple
- DSL - layers of diurnal
migration - deep
scattering layer and
complexity in the Gulf
Stream.

5. A typical transit A very unusual transit
CCR
AVHRR SST from http://fermi.jhuapl.edu/avhrr/gs (excellent website)

6. Hovmöller diagram of velocity (m/s) in 67°T from shelfbreak to
Bermuda. Note lateral shifting of the GS.
ea
op eS m
Sl trea
lf S
Gu
1 m/s
1 J/kg
ea
oS
ass
Sarg
Warm colors to the
NE, cool colors the
west east
the SW. Heavy line
= 0 m/s.
The ‘beaded’ structure of the GS due to meandering.

7. Directly measured transports
Seasonal variation in
GS transport. The
15% scatter is the
reason frequent
4.3% sampling is crucial to
(JMR - July 2010) construct means and
their LF variability.
The 3 subzones exhibit
very different LF
variability. GS shows no
long-term trend: it is stable.
Slope Sea exhibits
significant interannual
variability - most likely
NAO related.

8. Zooming in on the shelf-Slope Sea subsection
Significant spatial
variation in mean SW
flow, shelfbreak front
shows up clearly at
100 m.
Note that variance
ellipses do not scale
with mean flow.
1993-2002
Flagg et al., JGR 2006

9. Hovmöller diagram of along- and offshore currents between 130
and 270 km from 1993 through 2001.
Upper layer (14 to 54 m), long-
term current fluctuations after
the removal of the mean and
seasonal fluctuations. The data
were averaged into 60-day
intervals and low-passed filter
with a one-year half-power
point.

10. Hovmöller diagram of along- and offshore currents between 130
and 270 km from 1993 through 2001.
Upper layer (14 to 54 m), long-
NAO-
term current fluctuations after
the removal of the mean and
seasonal fluctuations. The data
were averaged into 60-day
intervals and low-passed filter
with a one-year half-power
point. Note strong SW flow
following low NAO in early
1996.

11. 1979-2003
Upper ocean temperature and
salinity anomalies from
monthly XBT and surface
salinity observations from the
CMV Oleander carried out by
the NMFS. The data have
been binned into 10km
intervals along the Oleander
track between New York and
the mean position of the Gulf
Stream. Note cold, fresh
waters following low NAO.

12. How the NAO may be impacting our waters:
NAO+
Storm tracks run north, mild here,
very cold in Labrador. Deep
NAO+ convection in L.S, waters spread
east. Less water flows west, GS
NAO- shifts north.
NAO-
Storm tracks run south, cold here,
mild in Labrador. Less ice
production, more fresh water on shelf
and slope. We subsequently
Mean dynamic topography
experience colder fresher waters on
or sea level in cm.
the New England shelf.
But T/S variability may also have local cause. Many questions..!

13. Thank you for listening!