T jpo, 2011, in press he origin of along-shelf pressure gradient in the Middle Atlantic Bight



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T
JPO, 2011, in press
he origin of along-shelf pressure gradient in the Middle Atlantic Bight

F.-H. Xu and L.-Y. Oey*

Prin­­­­ceton University

*Corresponding Author: lyo@princeton.edu

September 20, 2010

January 25, March 18, 2011


Abstract

It is quite widely accepted that the along-shelf pressure gradient (ASPG) contributes in driving shelf currents in the Middle Atlantic Bight (MAB) off the U.S. northeastern coast; its origin, however, remains a subject for debate. Based on analyses of sixteen-year (1993-2008) satellite data, tide-gauge, rivers, wind and numerical experiments the authors suggest that rivers and Coastal Labrador Sea Water (CLSW) transport contribute to a positive mean ASPG (tilt up northward) approximately in the ratio 1:7 (i.e. CLSW dominates), whereas wind and Gulf Stream tend to produce a negative mean ASPG, approximately 1:6.

Data also indicate seasonal and inter-annual variations of ASPG that correlate with the Gulf Stream’s shift and eddy-kinetic-energy (N-EKE) north of the Gulf Stream due to warm-core rings. A southward shift in the Gulf Stream produces sea-level drop north of Cape Hatteras which is most rapid in winter. The N-EKE peaks in late spring to early summer, and is larger in some years than others. A process model is used to show that ring-propagation along the MAB slope and ring-impingement upon the shelf break north of Cape Hatteras generate along-isobath density gradients and cross-shelfbreak transports that produce sea-level change on the shelf; the dominant ageostrophic term in the depth-integrated vorticity balance is the JEBAR term. In particular, shelf’s sea-surface slopes down to the north when rings approach Cape Hatteras.


  1. Introduction

The Middle Atlantic Bight (MAB) is the continental shelf region off the northeastern coast of the United States stretching between Nantucket Shoals to the northeast and Cape Hatteras to the south (Beardsley and Boicourt, 1981). The MAB is a dynamically complex region where cooler and fresher shelf water is separated from warmer and saltier slope water by a shelf break front (Csanday and Hamilton, 1988). Understanding water properties and currents in MAB is of interest: for navigation, fisheries, and coastal ecosystems.

The MAB circulation has been extensively studied through observations and modeling (Csanady, 1976; Beardsley and Boicourt 1981; Chapman 1986; Csanday and Hamilton, 1988; Linder and Gawarkievicz 1998; Flagg et al. 2006; Lentz 2008a). Depth-averaged mean currents are predominantly along-isobath directed equatorward, with speeds 0.03-0.1 m s-1 that increase with distance offshore (Lentz, 2008a, 2010). Currents are westward on the New England shelf, southwestward in the middle of MAB, and veer offshore just north of Cape Hatteras.



An important driving force for currents in the MAB is the along-shelf pressure gradient (ASPG) (Beardsley and Boicourt 1981). Stommel and Leetmaa (1972) concluded that an ASPG of order of 10-7 is required to drive the southwestward flow. Csanady (1976) argued also that an ASPG must exist to account for the observed circulation in the MAB. Lentz (2008a) extended Csanady’s model, analyzed observations, and showed quite convincingly that the southwestward along-shelf current is consistent with an along-shelf sea surface slope; he estimated an ASPG value of approximately 3.710-8. Lentz (2008a) discussed the possibility of other types of forcing, but the hypothesis that ASPG exists seems reasonable.

Lentz (2008a) showed that ASPG is mainly due to the sea surface slope. The Gulf Stream and Slope Sea gyre (Csanady and Hamilton, 1988) may drive an ASPG at the shelf break, but the penetration of the pressure field onto the shelf is limited (Wang 1982; Csanady and Shaw, 1983; Chapman 1986). What drive(s) the ASPG?

Observations also show seasonal variations in the depth-averaged along-shelf currents which are different in different sub-regions of the MAB (Lentz 2008b). Over the southern flank of Georges Bank, the along-shelf flow is maximum southwestward in September (Butman and Beardsley 1987; Brink et al. 2003; Flagg and Dunn 2003; Shearman and Lentz 2003). Further west and south in the MAB, the seasonal variation is less clear (Mayer et al. 1979; Beardsley et al. 1985; Aikman et al. 1988). Along the Oleander line, Flagg et al. (2006) observed a shelfbreak jet (offshore of 100m-isobath) which was stronger southwestward in fall and winter and weaker in spring and summer. ADCP measurements at station 5 of the Coastal Ocean Bio-optical Buoy (COBY) transect (75.029W, 37.833N) show maximum southwestward currents in spring, and weak currents in summer and fall (Xu et al., manuscript in preparation). From analyses of 27 long-term measurements, many of which were taken in the New England Shelf, Lentz (2008b) found that the alongshore currents have amplitudes of a few cm s-1. The residual alongshore flow after the removal of the wind-driven component is maximum southwestward in spring onshore of the 60m isobath. He suggested that the seasonality of the along shelf currents is primarily driven by the cross-shelf density gradient induced by freshwater discharge. Does ASPG also have seasonal and inter-annual variations, and if it does, how are they produced?

In this study, we carry out a set of model experiments and analyze them in conjunction with satellite, tide-gauge, rivers and wind data. We attempt to provide answers to the origin of ASPG: its mean as well as seasonal and inter-annual variability. Although the focus is on the shelf, it seems reasonable (from the literature) that the ASPG can be due to larger-scale process(es) that requires careful considerations of forcing outside the MAB. We will examine mean, seasonal, and inter-annual variability, and attempt to relate them to driving mechanisms: wind stress curl, Gulf Stream’s latitudinal shifts, rings, Coastal Labrador Sea Water (CLSW; Csanady and Hamilton, 1988) transport, and river discharge.



Section 2 describes observational data and section 3 the numerical model. In section 4, we analyze the mean, seasonal, and inter-annual variations of currents and ASPG in the MAB. Driving mechanisms are discussed in section 5, and section 6 is conclusion.


  1. Observations

Sixteen-year (1993-2008; same for other data, below) sea level data at 12 stations (excluding Bermuda and Wilmington NC, Figure 1) off the eastern coast of the United States are obtained from the University of Hawaii Sea Level Center (UHSLC, http://ilikai.soest.hawaii.edu/uhslc/datai.html). The data have been corrected for atmospheric pressure, and are then monthly running-averaged for analyses.

Gridded sea surface heights (SSH) and corresponding geostrophic velocities are from AVISO (http://www.aviso.oceanobs.com/duacs/). This dataset has a temporal resolution of 7 days and spatial resolution of (Le Traon et al. 1998).

The Cross-Calibrated, Multi-Platform (CCMP) ocean surface wind data is used to force the numerical ocean model (below). This is a 6-hourly gridded (1/4o×1/4o) product that combines ERA-40 re-analysis with satellite surface winds from Seawinds on QuikSCAT, Seawinds on ADEOS-II, AMSR-E, TRMM TMI and SSM/I, as well as wind from ships and buoys.

Daily river data at 25 U.S. northeastern stations were downloaded from USGS (http://waterdata.usgs.gov/nwis). Missing data longer than 1 week were filled by regression using nearby stations, while shorter gaps were filled by linear interpolations.

The M2 tidal data is from Oregon State University’s (OSU) global assimilation model (http://www.coas.oregonstate.edu/research/po/research/tide/index.html) on 1/4o×1/4o grid. The data is not directly used for analysis; rather it is used to drive the numerical model at its open boundary (see below).






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