Interannual Variabilityof Shelf Water Entrainment

to the Slope Sea byGulf Stream Warm-Core Rings in

Response tothe North Atlantic Oscillation

Ph.D. Dissertation Proposalby

Ayan ChaudhuriAyan Chaudhuri

OutlineOutline Overview

Scientific Background

Preliminary Work

Hypotheses and Study Objectives

Data and Methods

Summary

OverviewOverview 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.

Entrainment occurs 69 ± 20% of times when WCRs approach shelf [Garfield and Evans, 1987]

Annual Volume transports of shelf water to SS by WCR entrainment estimated at 4700 km3 [Bisagni, 1982]

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

OverviewOverview Long term impacts of shelf

water entrainment is proposed

Study Domain between 75° and 50°W during a 22 year period from 1978 to 1999.

Preliminary results suggest a

significant response of WCR activity to climate variability related to the state of the North Atlantic Oscillation (NAO).

Source: John Hopkins Remote Sensing Lab05/16/97

Scientific BackgroundScientific Background The Gulf Stream (GS) forms large amplitude

meanders downstream of Cape Hatteras from baroclinic and barotropic instability processes. Individual meander crests, 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 WCRs [Saunders 1971, Csanady 1979].

WCRs transport active and passive substances that have a significant impact on the biogeochemistry within the Slope Sea region.

An important mechanism is the horizontal stirring associated with streamers off the continental shelf and GS waters that WCRs entrain into the SS. [Olson 2001, Ryan et al. 2001, Schollaert et al., 2004].

Source: http://kingfish.coastal.edu/mari

ne/gulfstream/p5.htm

Scientific BackgroundScientific Background

The Gulf Stream North Wall (GSNW) and the Shelf Slope Front (SSF) have been shown to exhibit considerable inter-annual variability (IAV) in their mean position on the order of 80-100 km. (Halliwell and Mooers [1979], Drinkwater [1994])

Given that WCRs originate from the meandering of the GS, does GSNW position IAV have implications on WCR activity over annual or decadal timescales? Longitude

Lat

itu

de

Scientific BackgroundScientific Background

The North Atlantic Oscillation (NAO) may be driving the IAV observed in the mean position of the GS [Taylor and Stephens, 1998, Rossby and Benway, 2000] at an observed lag of 1-2 years.

Given that the state of NAO is known to co-vary with the GSNW position, does the NAO influence WCR activity over interannual or decadal timescales?

Source: Dr. Martin Visbeck: http://www.ldeo.columbia.edu/NAO/

Preliminary WorkPreliminary Work 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])

Data were binned at each longitude between 75-50oW to obtain monthly mean GSNW and SSF positions.

The positions of all observed WCR edges located in the SS during 1978 -1999 and between 75° and 50°W were also obtained

WCR Occurrence

NAOWI1975 2000

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.

Preliminary WorkPreliminary WorkA

rea

An

om

aly

(km

2 )

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.

Preliminary WorkPreliminary Work

NAO vs. WCRs

R=-0. 5182 p-value = 0.0135

NAO vs. GSNW

R= -0.5293 p-value = 0.0113

GSNW vs. WCRs

R= -0.3844 p-value = 0.0773

SSF vs. NAO

R=-0. 3155 p-value =0 .1527

SSF vs. GSNW

R=-0. 6711 p-value = 0.0006

Co

rrel

atio

n C

oef

fici

ent

(R)

Does GSNW position IAV have implications on WCR activity over interannual or decadal timescales? 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

Does the NAO have any influence on WCR activity over interannual or decadal timescales? 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.

HypothesesHypotheses Preliminary results suggest that a positive (negative) NAOWI will

correspond to more (less) WCRs and consequently high NAO decades will see more (less) ring activity. Higher numbers of WCRs generated during positive NAO years would increase the probability of entrainment events and result in higher fluxes of shelf water being advected into the SS.

HYP(I): Volume fluxes of shelf water into the SS due to WCR entrainment vary over interannual and interdecadal timescales with higher (lower) fluxes in years when the NAO is positive (negative) supported by increased (decreased) WCR activity.

Test of HYP(I) will examine the importance of water fluxes entrained by WCRs to the transport balance between the SS and the continental slope.

HypothesesHypotheses The entrainment of nutrient-rich waters from the outer continental

shelf by WCRs to the SS may significantly influence the nutrient budgets of both the shelf and slope regions.

The variability and availability of macro nutrients in WNA have significant impacts on all trophic levels.

HYP(II): Fluxes of nitrate (NO)3 and silicate (Si(OH)4) transported by shelf waters entrained into the SS by WCRs vary interannually with higher advective (lower) fluxes in years when the NAO is positive (negative) augmented by increased (decreased) volume transport.

Test of HYP(II) will provide bounds for the potential nutrient fluxes from the shelf to the SS assuming steady-state dynamics and neglecting losses due to diffusion, dissipation, mixing and uptake.

HypothesesHypotheses Since the lateral movement of the Gulf Stream forced by the NAO does not show

significant impact on WCR formation, other forcing mechanisms need to be addressed.

Recent studies (Penduff et al. 2004, Brachet 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 like GS.

HYP(III): NAO induced variability of wind stress and net heat flux to the WNA significantly affects the EKE distribution of the GS. GS EKE co-varies interannually with the NAOWI such that high (low) EKE distributions are found when the NAO is positive (negative) resulting in the production of more (fewer) WCRs.

Data and MethodsData and MethodsWCR Database The positions of all observed WCR edges

located in the SS during 1978 -1999 and between 75° and 50°W digitized at Bedford Institute of Oceanography (BIO)

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 will be 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.

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

Data and MethodsData and MethodsRing Entrainment Model (REM) Stern [1987] suggests that large scale amplitude disturbances like ambient water currents,

topography, -effects or horizontal shear induced by other eddies transfer energy to the cross stream velocity of WCRs

Large scale disturbances cause an increase in relative vorticity due to large scale and stretching of the potential vorticity (PV) isopleths of WCRs, subsequent lateral wave-breaking of these disturbances creates a PV imbalance

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).

Entrainment would occur when the gradient of relative vorticity is dominant, while detrainment would occur when the gradient of isopycnal thickness is dominant.

Data and MethodsData and MethodsRing Entrainment Model (REM) 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.

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

Data and MethodsData and MethodsPV = PV = f - r’/Rf - r’/Rmm

PV=0 is considered steady state

If PV > 0 during the lifetime of the WCR, it implies that the /f term (Rossby number) dominates the r’/ Rm term, thus causing instability

To revert back to steady state, the WCR will need to increase r’ i.e. increase its radius or entrain ambient water.

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

The increase in the parameter r’ will provide an area-scale of entrainment from which a streamer length-scale can be derived, assuming an appropriate width for streamers (12 km for Bisagni [1982])

Since streamer length scale is the most dominant for calculating volume, assumption of typical streamer depth (60m from Schlitz [2003]) will be used to derive shelf water volume fluxes.

Individual streamer fluxes will be temporally integrated to provide annual estimates of shelf water volume transport.

Data and MethodsData and MethodsMacro-nutrient Fluxes A comprehensive biogeochemical oceanographic database called

BIOCHEM prepared by Bedford Institute of Oceanography (BIO) is composed of in-situ observations of macro-nutrients nitrate (NO)3 and silicate (Si(OH)4) for the WNA.

BIOCHEM and World Ocean Data (WOD) datasets have been combined to create separate seasonal macro-nutrient climatologies for high and low NAO years.

Nutrient concentrations available in shelf water for advection to the SS due to WCR entrainment will be deduced by matching the time series of entrainment events each year given by REM to the quality controlled seasonal nutrient climatologist.

Nutrient Flux = Nutrient concentrations X streamer transport

Temporal integration of individual streamer nutrient fluxes will provide annual estimates of potential nutrient transport to the SS and will be used to test HYP (II).

Data and MethodsData and MethodsNumerical Model Simulations and Analysis HYP(III) will be tested by forcing a 1/6° high resolution eddy resolving North Atlantic

basin scale model with high NAO and low NAO heat flux and wind stress field components.

Regional Ocean Model System (ROMS) model for the domain of 15°S – 75°N and 100°W – 20°E will be used.

The net heat flux (Qnet) along with the meridional and zonal components of wind stress are needed to force the model during spin-up.

Qnet and wind stress are obtained from a dataset made available by Southampton Oceanographic Center (SOC) for the largely high NAO period from 1980–1993.

However, SOC data is not available for late 1950’s through 1960’s when the NAO index was lower than its mean value.

National Center for Environmental Predictions (NCEP) re-analysis data are used to compute monthly mean climatologies from 1958-1971 of Qnet and wind stress components for the low NAO period.

SOC and NCEP monthly mean climatologies for same period show differences on the order of 100 W/m2

Data and MethodsData and Methods

NCEP climatology grossly overestimated the sensible (QH) and latent heat (QE) flux components and underestimated the shortwave (Qsw) component.

SOC Qnet Climatology(1980-1993) for JAN NCEP Qnet Climatology(1980-1993) for JAN

SOC Climatology – NCEP Climatology NCEP Qnet Climatology(1958-1971) for

JAN

Data and MethodsData and MethodsNumerical Model Simulations and Analysis The adjusted NCEP low NAO and adjusted NCEP high NAO climatology using Type

II regression (Ricker, 1973) will be used as initial conditions to force the model during spin-up for the Low and High NAO periods, respectively.

Each simulation (low NAO and high NAO) will run for a 10 year period in order to stabilize the basic ocean circulation and location of fronts.

Individual year model simulations from 1978-1999 will be forced by actual monthly Qnet and wind stress components.

The simulations will result in producing diagnostic fields like temperature (T), salinity (S), zonal velocity (u), and meridional velocity (v) for the period of 1978-1999.

The EKE time series will be computed at 55m depth (Penduff et al. [2004]) as a running window of variances of the velocity fields (u,v) at overlapping 1-year time intervals as given in equation below:

EKEit = [(uit – uit’)2 + (vit – vit’)2]/2

uit and vit are the instantaneous zonal and meridional velocities, respectively, at model time step “it”. uit’and vit’ are the mean zonal and mean meridional velocities, computed by averaging all zonal and meridional velocity (u,v) observations, respectively, from “it”- 6 months to “it”+ 6 months.

SummarySummary HYP(I): Volume fluxes of shelf water into the SS due to WCR

entrainment vary over inter-annual and inter-decadal timescales with higher (lower) fluxes in years when the NAO is positive (negative) supported by increased (decreased) WCR activity.

HYP(II): Fluxes of nitrate (NO)3 and silicate (Si(OH)4) transported by shelf waters entrained into the SS by WCRs vary interannually with higher advective (lower) fluxes in years when the NAO is positive (negative) augmented by increased (decreased) volume transport.

HYP(III): NAO induced variability of wind stress and net heat flux to the WNA significantly affects the EKE distribution of the GS. GS EKE co-varies interannually with the NAOWI such that high (low) EKE distributions are found when the NAO is positive (negative) resulting in the production of more (fewer) WCRs.

SummarySummary• HYP 3 HYP 3 HYP 1 HYP 1 HYP 2 HYP 2

NAO

Wind Stress

GS EKE

WCR Occurrence

Shelf Water Entrainment

Volume Transport

Nutrient Advection

THANK YOUTHANK YOU

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

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

Dr. Avijit Gangopadhyay, UMASS Dartmouth

Dr. J. J. Bisagni, UMASS Dartmouth

Dr. Stephen Frasier, UMASS Amherst

This work is being supported by the NASA’s Interdisciplinary Science (IDS) Program, under grant number NNG04GH50G.

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