Interannual Variability of Gulf Stream Warm Core Rings In Response to the North
Atlantic Oscillation
by
Ayan H. Chaudhuri1, Avijit Gangopadhyay1 and James J. Bisagni1
Draft Version 2.0
1. Inter-campus Graduate School, University of Massachusetts and School for Marine
Science and Technology, University of Massachusetts, New Bedford, Massachusetts,
02744.
Keywords: Gulf Stream, Warm Core Rings, Western North Atlantic, North Atlantic
Oscillation
ABSTRACT
A database of quality-controlled Gulf Stream Warm Core Rings (WCRs) between 75° and
50°W during 1978 -1999 is analyzed to demonstrate that a significant response between
WCR formation and variations in large scale atmospheric forcing is related to the state of
the North Atlantic Oscillation (NAO). The influences of the NAO on Gulf Stream (GS)
position, transport and eddy kinetic energy (EKE) are presented as possible pathways
linking the NAO with rate of WCR formation. We provide evidence that the lateral
movement of the GS may not be forcing the rate of WCR formation as both parameters are
seen to respond at different timescales to the state of the NAO. The adjustment to NAOinduced wind forcing is observed to considerably impact the total transport and baroclinic
instability structure of the GS. Thus, the inter-annual variability in GS transport and EKE
are the mostly likely mechanisms affecting WCR formation.
1. INTRODUCTION
The Gulf Stream (GS) is a turbulent jet residing quasi-permanently along the western
boundary of the North Atlantic transporting heat and mass northward until its migration to
Cape Hatteras. Downstream of Cape Hatteras the GS separates from the coast and traverses
northeastward into deeper waters and thus forming large amplitude meanders due to
baroclinic and barotropic instability processes. Individual meanders, if large enough (surface
radii of 2-4 times the internal Rossby radius) can separate from the main GS current, loop
back onto themselves and form independent rings [Saunders, 1971; Csanady, 1979]. Warm
Core Rings (WCRs) form from GS meander crests engulfing parcels of warm, salty
Sargasso Sea water in their core [Parker, 1971].
The GS is seen to have considerable spatial variability in its transport mainly due to
the presence of two recirculation gyres on either flank of the GS, namely the cyclonic
Northern Recirculation Gyre, north of the GS and the anticyclonic Southern Recirculation
Gyre, south of the GS. The recirculation cells contribute in increasing the mean baroclinic
transport of the GS from 30 Sv in the Florida Straits [Niiler and Richardson, 1973] to about
70 Sv near Cape Hatteras [Halkin and Rossby, 1985] and further reaching a maximum of
150 Sv at 55°W [Hogg, 1992].
On time scales on the order of decades, a prominent factor affecting the circulation
of the North Atlantic basin is the North Atlantic Oscillation (NAO). NAO is the north south
alternation of atmospheric masses centered near the quasi-permanent Azores (subtropical)
high and Icelandic (subpolar) low [Hurrell et al., 2003]. The NAO becomes most dominant
in winter, modifying climate parameters like wind, irradiation, precipitation, and also air and
water temperatures. The NAO has profound basin scale biological and physical linkages like
IAV of buoyancy and wind induced momentum fluxes affecting the water temperature and
nutrient concentrations in the euphotic zone that significantly impact marine ecosystems in
the North Atlantic region [Drinkwater et al. 2003].
The common occurrence of WCRs in the Slope Sea (SS) and their role in affecting
the physical, chemical and biological oceanography of the SS region have been well
documented through satellite imagery [Halliwell and Mooers, 1979; Brown et al., 1986;
Auer, 1987], theoretical models [Flierl, 1977; Csanady, 1979; Olson et al., 1985] and field
observations [Saunders, 1971; Lai and Richardson, 1977; Joyce, 1985]. However, most
reported results concerning WCRs have been deduced from single surveys or time-series
surveys from individual WCRs. Long term impacts of these energetic features and their
inter-annual variability (IAV) have not been studied. Halliwell and Mooers [1979] had
suggested large scale atmospheric circulation as a possible forcing mechanism affecting the
shedding of WCRs by the GS on inter-annual scales. Thus, the NAO may be an important
dynamic in WCR activity and related shelf water entrainment.
We present a comprehensive analysis of 22-years of WCR data (1978-1999)
between 75o and 50oW. Atmospheric forcing related to the variation in the state of the NAO
is seen to significantly co-vary with occurrence of WCRs in the SS. NAO-induced IAV in
(1) the GS position, (2) GS transport and (3) GS eddy kinetic energy (EKE) are analyzed to
obtain a better understanding of the dynamical pathway involved in the observed covariance
between the NAO and WCR occurrences.
2. DATA AND METHODS
The positions of all WCR edges located in the Slope Sea (Fig. 1) between 75° and
50°W during 1978 -1999 are obtained by hand digitizing satellite images from both NOAA
and U. S. Navy frontal charts produced at Bedford Institute of Oceanography. The WCRs
are binned into individual years based on the year of formation. The WCR features present
in the dataset are quality controlled by visual inspection such that features like shingles and
filaments are discarded. A total of 459 quality controlled WCRs show significant IAV from
1978-1999 with maximum occurrence of 31 WCRs in 1990 and minimum occurrence of 7
WCRs in 1978 (Fig. 2a). An average of 21 WCRs are seen to occur each year. The WCR
dataset also has positions of the Shelf Slope Front (SSF) and the Gulf Stream North Wall
(GSNW) for the same space-time scales. The Slope Sea region remains dynamic in area due
to fluctuations in position of the SSF and the GSNW.
The NAO wintertime index (NAOWI) (Fig. 2b) is used as a proxy to depict longterm atmospheric and possible ocean property and circulation patterns. NAOWI is
calculated by subtracting the normalized mean winter sea-level pressure from December to
March at Akureyri, Iceland from that of Ponta Delgada, Azores. A positive NAOWI for a
specific year implies that the mean winter pressure is above average resulting in a colder and
drier conditions over the northwestern Atlantic and Mediterranean regions, whereas
conditions are warmer and wetter than average in northern Europe, the eastern United
States, and parts of Scandinavia. Conversely, negative NAOWI index implies lower than
average winter pressure and reversal of patterns in the aforementioned regions [Hurrell,
1995].
Investigation into possible impacts of the GSNW position and the state of the NAO
on WCR activity is conducted by using the SSF and GSNW frontal position database
mentioned earlier. The data is binned at each longitude between 75-50oW to obtain monthly
mean GSNW and SSF positions [Drinkwater et al. 1994] for 22 years ranging from 1978 –
1999. Long term SSF and GSNW mean positions are calculated by averaging data from all
months at each of the 26 longitudes (Fig. 1) and a line mid-way between the mean positions
of both fronts is computed. This mid-line is assumed to be a position where neither the SSF
(in its extreme seaward position) nor the GSNW (in its extreme shoreward position) crosses
at any point in time for the monthly averaged data. Area anomalies bounded by the SS mid
line and the annual mean position for each individual year are calculated for both the
GSNW and SSF and differenced subsequently from respective long term mean areas
bounded by each front’s long term mean position and the mid-line. Area anomalies signify
the movement of the GSNW and the SSF for each year and show their individual
contributions in affecting IAV of the net Slope Sea area (Fig. 3). The influence of GS
transport is studied by using annual averages of 16 years of daily observations from 19821998 measured off the Florida Straits using submerged submarine cables [Baringer and
Larsen, 2001].
3. DISCUSSION AND RESULTS
A visual comparison between the NAOWI and the WCR time-series (Fig. 2)
demonstrates discernible phase coherence for the period from 1978-1999. During years
when the NAO was largely in its positive phase like 1989-1990 period display higher
occurrences of WCRs and conversely, the single largest drop in the phase of the NAO
during 1996 coincides with lesser number of WCRs having spun-off from the GS. Lagged
correlations between the NAOWI and WCR occurrences (Fig. 4a) confirm the similarity in
trends of both time-series. The NAOWI is observed to be significantly positively correlated
(r = 0.51, 95% significance, two-tailed test) to WCR activity at zero lag. The correlation
analysis suggests that positive (negative) phases of the NAO correspond to more (less)
WCRs and consequently high-NAO (low-NAO) decades see more (less) ring activity.
The near immediate response of the WCRs to variations in the state of the NAO is
somewhat surprising as it is widely understood that the GS takes on the order of 1-3 years to
adjust to fluctuations in the state of the NAO, as reflected in the lateral variations of the
mean position of the GS. Gangopadhyay [1992] and Taylor and Stephens [1998] suggest
that NAO-induced fluctuations in zonal westerly wind stress generate westward traveling
baroclinic Rossby waves that deflect off the western Atlantic coast and impact the GS. This
probable mechanism for observed IAV in GSNW mean position is shown to occur at lags of
2-3 years in phase with the NAO. Rossby [1999] and Rossby and Benway [2000] speculate
that the observed displacement in GS mean position is due to variations in NAO-induced
buoyancy fluxes. They suggest that the southward extent of “spilling” and transport of
Labrador Seawater outflow into the SS occurs at lags of 1.5 years. Frankignoul et al. [2001]
studied GS mean positions obtained from altimeter data and concluded that the GS position
lags the NAO by 11-18 months. They attributed variations in both buoyancy and wind
forcing to explain the observed lag. Our analysis of lagged correlations between the NAO
and GS area anomalies display significant positive correlations at 0,1 and 2 years, with
maximum correlation observed (Fig. 4b) when the GS area anomalies lags the NAO by 1
year (r = 0.56, 95% significance, two-tailed test). The statistical correlation signifies that an
increase (decrease) in the GS area anomaly (GSNW moving southward (northward) of its
mean position) responds to NAO variations at a lag of more than 1 year. A cross correlation
between WCR occurrences and GS area anomalies (Fig. 4c) shows a maximum significant
correlation (r = 0.63, 95% significance, two-tailed test) with the rate of WCR formation
leading the GS area anomalies by 1 year. Results from previous contemporary work and
this present study suggests that the lateral movement of the GS is presumably not affecting
the generation of WCRs as they respond on different time-scales to variations in the state of
the NAO.
Temporal variations in GS transport most likely respond to different forcing
mechanisms than the lateral movement of the GS. Consequently, variations in GS transport
may influence the rate of baroclinic instability of the GS and hence the rate of WCR
formation. Rossby et al. [2005] imply that GS transport is largely wind-driven whereas the
movement of the GS position is more thermohaline driven. Gangopadhyay et al. [1992]
tested the two-layer steady state wind driven Parsons-Veronis model and found reasonable
covariance between the GS separation latitude near Cape Hatteras and zonally integrated
wind stress. GS separation was attributed to the surface outcropping of the GS thermocline
in response to variations in geostrophic and Ekman transport. The GS separation latitude
may thus be used as a metric to study variations in GS transport. Gangopadhyay et al.
[1992] observe a 30 km longitudinal envelope in GS separation that is oriented along the
2000m isobath and hence slightly seaward of 74.5oW. Unfortunately, due to the one degree
resolution of the GS frontal charts we chose 74oW to study GS separation latitudinal
variations. Using annually averaged transport estimates off the Florida Straits as indicative
of the GS transport signature upstream of Cape Hatteras, we notice significant negative
correlation (r = -0.48, 95% significance, two-tailed test) between GS transport and GS
separation latitude at zero lag (Fig. 4d). The correlation suggests that enhanced (diminished)
transport upstream of Cape Hatteras leads to a more southward (northward) separation
latitude for the GS which is in agreement with Veronis [1973].
Transport estimates upstream of Cape Hatteras demonstrate noticeable IAV off the
Florida Straits observed from submerged submarine cables [Baringer and Larsen, 2001].
They found significant negative correlations between the GS transport and the state of the
NAO at lags ranging from 1-25 months. However, it must be noted that they correlated
monthly mean transports with monthly values of the NAO in contrast to the annual mean
transport and NAOWI used in our study. The GS transport off the Florida Straits thus shows
significant IAV with enhanced (diminished) volume transport seen during low (high) NAO
years. Contribution of the westward Slope Sea flow, north of the GS, and total GS transport
downstream of Cape Hatteras provides further evidence to the observed IAV in GS
transport. Rossby et al. [2005] present an eleven-year record of Slope Sea, GS and Sargasso
Sea transport observed from Acoustic Doppler Current Profiler (ADCP) aboard the ship
Oleander. The period from 1993-1999 of the transport record (their Fig. 5) is coincident
with our analysis and is seen to display remarkable IAV. The total GS transport is seen to
decrease from 1993-95, followed by an increase during 1996-97 and then decrease in 1998
followed by an increase in 1999. Surprisingly, this trend appears to be in precise phase with
the state of the NAO (Fig. 2b). A similar pattern is seen in the westward transport in the
Slope Sea although the magnitude of variability is noticeably lower than the total GS
transport. We conclude that there is suggestive evidence that the GS transport is enhanced
(diminished) during low (high) NAO years. Consequently, weaker (stronger) transport may
provide more (less) suitable conditions for growth of baroclinic instabilities that leads to
formation of more (less) number of WCRs.
The growth of baroclinic instability in the GS system predominantly depends on the
meridional gradient of potential vorticity (PV). The dominating terms in the PV on the
cyclonic side of the GS are shear and stretching vorticity [Rajamony et al. 2001].
Furthermore, Rossby and Gottlieb [1998] suggest that the region of maximum velocity of
the core GS is temporally invariant, however significant IAV is seen off the cyclonic and
anticyclonic regimes. It can thus be assumed that the variations in total transport and related
horizontal and vertical shear are mainly due to contributions from the cyclonic and
anticyclonic sides of the GS rather than from the core of the GS current. We thus
hypothesize that higher (lower) transport in low (high) NAO years result in lower (higher)
horizontal and vertical shear between the cyclonic side and the core of the GS such that the
PV gradient becomes diminshed (enhanced) (Fig. 5). The enhanced PV gradient during high
NAO years effect baroclinic instability conditions conducive for larger occurrences of
WCRs.
Recent studies [Penduff et al., 2004; Volkov 2005] have shown the eddy kinetic
energy (EKE) varying interannually over the North Atlantic in phase with the NAO index.
Instability processes from the mean flow generate excess energy and is the source of EKE
and lead to the formation of eddies and rings [Gill et al. 1974, Stammer 1998]. Stammer
[1998] has verified that baroclinic instability is a major eddy source term throughout the
ocean, especially for the western boundary currents. Thus EKE estimates can be useful to
study baroclinic instability processes for the GS. Penduff et al. [2004] used model
simulations and altimeter data to study EKE IAV and presented strong positive covariance
between NAO and EKE during 1994-2001 at a lag of 4-12 months. They hypothesized that
the observed redistribution of EKE at lags of 4-12 months enveloped time-scales for
adjustment of the ocean to wind-induced forcing (several months) as well as growth rate of
mesoscale eddies like WCRs (few weeks). Their results supported by our work provide a
concomitant indication that during high (low) NAO years the GS responds to intensification
(weakening) of wind stress in the North Atlantic region such that the adjustment enhances
(diminishes) the growth of baroclinic instabilities.
In summary, we present a significant annual response of WCR occurrences to
variability in the state of the NAO. We provide evidence that the lateral movement of the
GS may not contribute to the IAV seen in the rate of WCR formation. The growth of
baroclinic instabilities necessary for WCR generation show sensitivity to NAO related
wind-induced GS EKE and GS transport variability. The NAO, during its high phase is
associated with stronger winds and diminished transport on the cyclonic side of the GS.
These high NAO conditions increase both the EKE and the meridional gradient of PV across
the GS front, hence supporting the growth of baroclinic instabilities and resulting in more
occurrences of WCRs. Conversely, weaker winds and enhanced transport along the
cyclonic side of the GS during the low NAO phase lead to decrease in GS EKE and
meridional gradient of PV. The low NAO conditions, thus oppose the growth of baroclinic
instability within the GS system and hence lesser occurrences of WCRs are observed.
ACKNOWLEDGEMENTS
The authors would like to thank Dr. K. Drinkwater, Institute for Marine Research, Bergen,
Norway and R. Pettipas, Bedford Institute of Oceanography, Dartmouth, Nova Scotia,
Canada, for providing digitized Gulf Stream north wall, shelf-slope front data and warmcore ring data. This work is being supported by the NASA’s Interdisciplinary Science
(IDS) Program, under grant number NNG04GH50G.
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FIGURES
Figure 1. Study domain including positions of SS mid line (dashed black) and mean GSNW
(bold red) and SSF (bold blue) from 22 years (1978-1999) of frontal position data. Also
shown are the annual mean positions of the GSNW (dotted red) and SSF (dotted blue) for
1995. Some important hydrographical regions in the WNA include the Tail of Grand Banks
(TGB), Georges Bank (GB) and the Mid Atlantic Bight (MAB) and the Gulf of Maine
(GoM). Some important geographical regions include Newfoundland (NFLD), Nova Scotia
(NS) and Cape Hatteras (CH).
Figure 2. (a): The total number of warm core rings per year from 1978-1999. (b): The
NAOWI for the same time period.
Figure 3. GSNW and SSF area anomalies in km2, showing the movement of both fronts
with respect to each other. Positive anomalies signify northward or shoreward movement
whereas negative anomalies signify southward or seaward migration. It must be noted that
the magnitude of the GS area anomalies have been reversed to understand the frontal
movement.
Figure 4. A series of correlations are presented along with bounds of 95% significance.(a):
lagged correlation between NAO and the number of WCRs showing significant positive
correlation at zero lag. (b): lagged correlation between NAO and GSNW area anomaly
showing significant negative correlation with GSNW area anomalies lagging NAO by 0-2
years. (c): lagged correlation between number of WCRs and GSNW area anomalies
showing maximum significant correlation at 1-year lag. (d): lagged correlation between
annual mean transport of the GS off the Florida Straits and GS separation latitude taken at
74oW, showing moderate to significant negative correlation at zero lag.
Figure 5. An idealized schematic
representing Gulf Stream shear
(arrows) and meridional potential
vorticity gradient (dotted line) for (a) high NAO and (b) low NAO periods. The shear
profiles oriented along the axis of maximum velocity (um) at the core of the GS (bold
arrows) are assumed to be similar for both periods. However, the high shear low velocity
cyclonic side of the GS is seen to be different such that, although u1(y) > u2(y),
u1 y u2 y . Therefore, enhanced transport and velocity on the cyclonic side of GS
during low NAO results in diminished velocity shear and meridional potential vorticity
gradient , whereas diminished transport and velocity on the cyclonic side of GS during high
NAO results in enhanced velocity shear and meridional potential vorticity gradient.