Interannual Variability and Shelf Water Entrainment by

Gulf Stream Warm-Core Rings

by

Ayan ChaudhuriAyan Chaudhuri

James J. BisagniJames J. Bisagni

Avijit GangopadhyayAvijit Gangopadhyay

School for Marine Science and Technology School for Marine Science and Technology (SMAST)(SMAST)

andand

School of Marine Sciences, UMASSSchool of Marine Sciences, UMASS

IntroductionIntroduction Common occurrence of

Gulf Stream warm-core rings (WCRs) within the western North Atlantic’s (WNA) Slope Sea (SS) and their role in causing seaward entrainment of outer continental shelf water is well documented.

Most reports concerning WCRs and their associated shelf water entrainments have been based upon single surveys or time-series from individual WCRs. Long term impacts are unknown.

Source: John Hopkins Remote Sensing Lab05/16/97

Shelf Water Entrainment

DataData Data consist of hand-digitized weekly

frontal charts produced from satellite-derived sea surface temperature (SST) and charts produced by NOAA and the U.S. Navy ([Drinkwater et al. 1994]). Dataset contains WCR location, GS position and Shelf Slope Front position from 1978-1999.

A total of 459 quality controlled WCRs show significant inter-annual variability from 1978-1999 with an average of 21 WCRs per year.

A significant positive correlation (p-value=0.0135, n=43, two-tailed test) is observed between annual WCR occurrences and NAO Index at 0-year lag. Higher (lower) occurrences of WCRs are likely during high (low) NAO years.

How does the NAO impact WCR occurrences ?

WCR Occurrence

AnalysisAnalysis

NAO -> GS Position -> WCRs: [Frontal Analysis] The lateral movement of the GS is most likely not forcing the rate of WCR formation as they respond to the NAO at different phases

NAO -> GS EKE -> WCRs: [Modeling Study] The NAO is observed to be significantly correlated to GS EKE at 0-year lag and thus provide a strong indication that GS EKE variability is the most probable mechanism affecting the annual occurrences of WCRs.

GS EKE

NAO

GS Position

WCRShelf Water

Entrainment

(1) Brachet et al., 2004

NAO->Wind Stress-> Circulation

(2) Penduff et al., 2004

NAO-> Combination of Non-

linear modes

(1) Taylor and Stephens, 1998

NAO->Wind Stress->Rossby

Propagation

(2) Rossby and Benway, 2000

NAO->Buoyancy Flux->Labrador

Spilling

(1) Spall and Robinson, 1990

Baroclinic Instability modes->WCR formation

(2) Stammer, 1998

EKE->Baroclinic instability

(1) Richardson, 1980

GS->New England Seamounts

(2) Teague and Hallock, 1990

Topography-> GS Meandering

+1 -1

0 0

0

Do more (less) WCRs cause more (less) shelf water entrainment?

WCR Center, Radius and Velocity WCR Center, Radius and Velocity EstimationEstimation

Satellite-derived data only has positions of rings

Surface or subsurface momentum or tracer observations for the WCRs are not available

Key characteristics like WCR center position, radius and orientations are determined by analyzing the WCR observations using an ellipse-fitting feature model proposed by Glenn et al. [1990] and implemented by Gangopadhyay et al. [1997]

Swirl velocities will be computed by finite differencing WCR orientations () obtained from the feature model time series.

Vi = [(i-i-1)/(ti-ti-1)] * Ri , , where Ri =[a*b]1/2

(After: Gangopadhyay et al. 1997, Figure 10 (a))

WCR Spatial StatisticsWCR Spatial Statistics

WCR observations are averaged over 1o latitude X 1o longitude bins based on WCR center positions obtained from Ellipse-Fitting Model

Most WCR activity occurs offshore of Georges Bank

Most WCRs are formed between 65-55oW WCRs swirl most just after GS

separation, eventually slowing as they traverse southwestward and shoreward

First WCR OccurrenceMean WCR Occurrences

WCR Mean Swirl Velocities

cm sec -

1

Ring Entrainment ModelRing Entrainment Model Stern [1987] suggests that after a WCR is formed, it achieves

steady state, such that, its potential vorticity (PV) is conserved. However, over time as WCRs become slower and smaller, the PV balance is disrupted.

The PV imbalance is compensated by lateral entrainment or detrainment of ambient water in order to re-establish steady state.

A 3-D Quasi-Geostrophic Potential Vorticity (QGPV) Model was proposed as follows:

PV =/f - h’/Hm

where, is the WCR relative vorticity, f is the planetary vorticity, Hm is the WCR mean vertical thickness and h’ is the deviation from the time-averaged mean thickness (Hm).

Balanced Case: PV = 0, if /f (Velocity) = h’/Hm (Volume) Unbalanced Case: PV > 0, if /f (Velocity) > h’/Hm (Volume) To regain balance, volume change would have to increase, the

WCR thus entrains water from outside to increase its volume

Since subsurface data for most of the WCRs are not available, the three-dimensional model cannot be used in this proposed study.

Observations support the notion that WCR radius can be assumed to be a good proxy to WCR depth or thickness.

A 3-D QGPV Model is transformed to a 2-D QGPV as follows:

PV = /f - r’/Rm

= V/R + dV/dR

is relative vorticity for WCRs [Csanady, 1979], where V is the swirl velocity of the WCR, f = 2 sin is the planetary vorticity, Rm is mean radius of the WCR and r’ is the deviation from the mean radius (Rm) in time.

The 2-D model identifies WCRs that entrain ambient water which may not necessarily be from the shelf. The model also provides a deformation radius scale which is used to estimate volume transport

Distance of a WCR to the position of the SSF (considered the outer boundary of continental waters) determines whether the ambient water entrained is derived from the outer continental shelf.

After: Olson 1985, Figure 3(g))Ring 82-B

Ring Entrainment ModelRing Entrainment Model

Shelf Water EntrainmentShelf Water Entrainment

Volume fluxes derived from deformation radius of entrainment and assuming a constant streamer depth of 50m [Bisagni, 1982].

Near-zero volume flux from 1978-1980 due to large sample size of 7 days during this period.

Mean Annual Volume flux due to shelf water entrainment is 0.75 Sv (23700 km3/year) Given total alongshore transport near the shelf break is 3 Sv (Flagg et al. [2006]), WCRs

entrain 25% of this transport.

WCRs vs. Volume Flux

R= 0.51 p-value = 0.0217

CONCLUSIONSCONCLUSIONS

The lateral movement of the GS is most likely not forcing the rate of baroclinic instability of the GS and hence rate of WCR formation.

High (low) phases in the state of the NAO exhibit higher (lower) EKE in the GSR, providing a greater (lesser) source of baroclinic instability to the GS front, resulting in higher (lower) occurrences of WCRs.

Higher (lower) occurrences of WCRs during high (low) NAO years results in higher (lower) incidences of shelf water volume transport.

Given total alongshore transport near the shelf break is 3 Sv (Flagg et al. [2006]), WCRs entrain 25% of this transport.

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS Dr. K. Drinkwater, Institute for Marine Research, Bergen, Norway

R. Pettipas, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada

Dr. Stephen Frasier, UMASS Amherst

This work is being supported by the NASA’s Interdisciplinary Science (IDS) Program, under grant number NNG04GH50G and GLOBEC NWA Program under grant number NSF OCE 0535379.

THANK YOUTHANK YOU

hhhh

Preliminary WorkPreliminary Work

Long-term SSF and GSNW mean positions were calculated by averaging data from all months at each of the 26 longitudes (75-50oW) from 1978-1999

A line mid-way between the mean positions of both fronts 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.

hhhh

Preliminary WorkPreliminary Work

Area bounded by the SS mid-way line and the annual mean position for each individual year are calculated for both the GSNW and SSF

Long term mean area for both fronts is calculated from long term mid-line, SSF and GSNW mean positions by averaging data from all months at each of the 26 longitudes (75-50oW)

Area anomalies for GSNW and SSF are obtained by differencing long term mean areas of each front from the area bounded for each individual year.

NAO, GS Position and WCRsNAO, GS Position and WCRs

The lateral movement of the GS is most likely not forcing the rate of baroclinic instability of the GS and hence rate of WCR formation

The NAOWI is observed to be significantly correlated to WCR activity at a lag of under a year and maybe forcing WCR formation by means different from GS movement.

NAO vs. WCRs

R=0. 51 p-value = 0.0135

NAO vs. GS Position

R= 0.56 p-value = 0.0113

GS Position vs. WCRs

R= 0.54 p-value = 0.0120

Co

rrel

atio

n C

oef

fici

ent

(R)

NAO, GS EKE and WCRsNAO, GS EKE and WCRs

The response of GSR EKE to variability in NAO-induced ocean conditions is studied by numerical simulation of the North Atlantic basin (NAB).

The domain is implemented using a 1/6o ROMS model. 50 vertical levels

Southampton Ocean Center (SOC) ocean-atmosphere atlas [Josey, 2001] derived forcing.

Model spun-up using climatological forcing and then annual year simulations run from 1980-1999. Model output stored at 3 day intervals. Yearly time-series of 120 points

Model validation done by comparing results with Penduff et al. [2004] and TOPEX/POSIEDON data.

where

2/][22 y

itit

yitit

yit vvuuEKE

monthsiti

monthsiti i

iyit

yit

v

u

v

u 6

6120

1

NAO, GS EKE and WCRsNAO, GS EKE and WCRs

GS EKE Anomalies are calculated by removing the global mean EKE from annual GS EKE averages from 1980-1999

The NAO is observed to be significantly correlated to GS EKE at 0-year lag and thus provide a strong indication that GS EKE variability is the most probable mechanism affecting the annual occurrences of WCRs.

No significant correlation could be established between GS position and GS EKE

NAO vs. GS EKE

R= 0.52 p-value = 0.0127

GS Position vs. GS EKE

R= 0.46 p-value = 0.0542