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

We show that the observed, mean positive ASPG is caused by freshwater discharge and CLSW transport. The mean westerly wind produces a negative ASPG, though its effect is weak. The Gulf Stream produces a large sea-surface set-down to the north, ASPG < 0, but the Stream in this case separates from the coast too far north from Cape Hatteras, and clearly cannot by itself drive the observed positive ASPG. The CLSW and Gulf Stream are therefore closely tied, as a critical value of the former (= ­1.5 Sv (flowing southwestward) in our model) is necessary to maintain the separation near Cape Hatteras, and also to give a positive ASPG.

The seasonal and inter-annual variations in ASPG are produced by latitudinal shifts in the Gulf Stream, as well as by Gulf Stream’s warm-core rings that propagate southwestward in the Slope Sea, and that interact with the MAB shelf break. The Gulf Stream’s southward retreat produces a sea-level drop north of Cape Hatteras (ASPG > 0). The effects of rings on ASPG are demonstrated with an idealized experiment that isolates eddy processes. We show that shelf convergences and divergences are forced by rings that interact with the shelf break. Though the penetration of the ring’s sea-level signal across the shelf break is limited (because of the insulating effect of the slope; e.g. Wang, 1982; Csanady and Shaw, 1983; Chapman 1986), it is nevertheless sufficient to produce O(1~4×10^-8) fluctuations that are consistent with the observed fluctuations from tide-gauge. Vorticity analysis shows that the JEBAR term, caused by along-isobath density gradients by virtue of rings brushing against the MAB continental slope, is the dominant ageostrophic term accounting for the cross-isobath fluexs. We show that the production of warm-core rings peaks in spring~summer. Rings propagate southwestward and produce a northward set-down of the shelf’s sea-level (ASPG < 0) approximately 3 months later when the rings arrive over the slope north of Cape Hatteras. At inter-annual time scales, the response is opposite: the ASPG is positive in years of increased eddy kinetic energy N-EKE. The reason is that at long time scales, the net effect of eddies that spread throughout the entire northern portion of the Gulf Stream is to produce a sea-surface sloping downward towards Cape Hatteras.

A number of outstanding issues remain. While the mean ASPG as one of the important drivers of shelf’s currents is established, its seasonal and inter-annual effects are not (e.g. Lentz, 2008b). The present work suggests that the effects should be most apparent on shelves near Cape Hatteras, and this can be examined from observations and modeling. Our focus in the present work is on ASPG, and the connections between the Gulf Stream, eddies, CLSW and wind stress (curl) especially at the inter-annual time scales have only been briefly discussed. These should be pursued in a future work perhaps in conjunction also with a basin-scale model.

We thank the two reviewers and editor Dr. Barth for their helpful comments that improve the MS. This research is supported by the Minerals Management Service contract number M09PS20004. Dr. Jose Blanco processed the river and tidal data. Computations were done at NOAA/GFDL, Princeton.

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Figure 2. A locator map of the study region in the Mid-Atlantic Ocean with white contours showing the 50m and 200 m isobaths, and color background with black contours showing the 16-year mean sea surface height calculated from the experiment with data assimilation (EX.DA). Shown here is a sub-domain of the northwestern Atlantic Ocean model (NWAOM) which otherwise covers a larger region 98W-55W and 6N-50N (see text).

Figure 5. (a) 16-year (1993-2008) mean and variance ellipses of depth-averaged currents along the 50 m isobath; (b) 3-month running averaged time series of the ASPG (of fig.3c+the mean) estimated along the 50 m isobath; (c) 3-month running averaged time series of depth-averaged along-shelf current averaged along the 50 m isobath from Delaware to the east end of Long island (see Fig.3). The mean is 0.025 m s^-1 towards the southwest. Tick marks on (b) and (c) show January.

Figure 6. The linear best-fit of 16-year mean SSH vs. distance between DEL and ELS for the 8 numerical experiments in Table 1, computed as in Fig. 3b. The slopes represent the mean ASPGs. Their values are listed in Table 1. Symbols DEL: Delaware Bay, and ELS: eastern Long Island. For clarity, the y-scale has been shifted so that SSH = 0 at x = 450km (near New Jersey).

Figure 7. Three-month running average (black line) and one-year low pass (blue line) for (a) ASPG, (b) zonally-averaged Gulf Stream (GS) mean path shift, (c) wind stress curl over the open ocean (see text), (d) eddy kinetic energy density (N-EKE) for SSHA>0 (see text) north of the GS mean path estimated from AVISO satellite geostrophic currents, (e) upstream transport, and (f) total river discharge along the east coast of America north of Cape Hatteras.

Figure 8. (a) The eddy kinetic energy density for SSHA>0 (see text) north of the Gulf Stream (NEKE; unit is m²s^-2), averaged over the area west of 55W and from 1000m isobath south to the GS monthly mean path position. (b) The seasonal cycle of N-EKE.

Figure 9. Idealized simulation with three warm core rings injected every 360 days. (a) 8-year mean sea surface height (SSH); thick blue line denotes the zero contour; (b) mean ASPG along the 50 m isobath with linear-regression fit over the MAB (c.f. figure 2a); (c) 60-day low pass of ASPG variations (black line; mean removed).

Figure 10. Results from the idealized simulation of warm-core rings, at (a and b) day 1200 and (c and d) day 1610 (see text). Panels (a) and (c) show surface current trajectories superimposed on sea surface height (SSH) in color. Panels (b) and (d) show the SSH contours; yellow line indicates zero and negative regions are shaded in grey. Dark, white (a & c) and grey (b & d) contours indicate the 50m, 200m and 1000m isobaths.

Figure 11. From top to bottom: surface current anomalies (vectors) superimposed on shadings of SSH, CPVF and -JEBAR (denoted by “CJBAR” on plots) anomalies. Left columns (a, c & e) are weighted composites for the positive phase (PP) of ASPG anomaly (Fig.9c), and right (b, d & f) for the negative phase (NP). The shading scale for SSH (m) is shown to the right of “(b)”, and that for CPVF and –JEBAR (s^-2) to the right of “(d)”. Black and white contours indicate the 50m, 200m and 1000m isobaths. It is readily shown that the max/min of CPVF correspond to max/min of approximately 0.05 m s^-1, positive onshore across the 1000 m-isobath.