The origin of along-shelf pressure gradient in the Middle Atlantic Bight
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Summary In this work, the question of what physical mechanisms contribute to the mean, seasonal and inter-annual variability of the along-shelf pressure gradient (ASPG) in the MAB is addressed by analyzing observational data and results of numerical experiments both realistic (including data assimilations) and idealized. The realistic experiment simulates the circulation in the northwest Atlantic Ocean (including the Gulf Stream and the MAB) from October 1992 to December 2008. Realistic atmospheric forcing, freshwater discharge and tidal forcing are included. Our results show that the model ASPG agrees with that deduced in other studies (Stommel and Leetmaa, 1972; Scott and Csanady, 1976; Lentz 2008a), and its variation is consistent with that deduced from tide-gauge data. The mean ASPG is positive, about 5~8×10-8, but it also has seasonal and inter-annual variations of 10-7. We show that the mean ASPG is caused by freshwater discharge and CLSW transport. On the other hand, the seasonal and inter-annual variations in ASPG are mainly produced by Gulf Stream’s warm-core rings that propagate southwestward in the Slope Sea, and that interact with the MAB shelf break. The effects of rings on ASPG were demonstrated with an idealized experiment that isolates the 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 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. The influence of large-scale wind pattern on ASPG is also examined. Though wind does not directly affect ASPG, it is the major mechanism accounting for the GS path variations (Dong and Kelly, 2003), as well as the seasonality of EKE (Zhai et al. 2008). We show that the production of warm-core rings peaks in spring~summer. These rings propagate southwestward and produce a northward set-down of the shelf’s sea-level approximately 3 months later when the rings arrive over the slope north of Cape Hatteras. This variation appears to be in good agreement with our analysis of (limited) current-meter data over the shelf north of Cape Hatteras (Xu et al., manuscript in preparation). References Auer, S.J., 1987: Five-year climatological survey of the Gulf Stream system and its associated rings, J. Geophys. Res., 92, 11709-11726. Aikman, F., III, H.W. OU, and R.W. Houghton, 1988: Current variability across the New England continental shelf-break and slope. Continental Shelf Research, 8, 625-651. Bane, J. M., O.B. Brown, R.H. Evans, and P. Hamilton, 1988: Gulf Stream remote forcing of shelfbreak currents in the Mid-Atlantic Bight, Geophys. Res. Lett., 15, 405-407. Beardsley, R.C., and W.C. 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Kohlmann, 2008: On the seasonal variability of eddy kinetic energy in the Gulf Stream region. Geophysical Research Letter, 35, L24609, doi:10.1029/2008GL036412,2008. Table 1. Model experiments. 1UA corresponds to a CLSW transport of 1.5 Sv specified at the northeastern boundary, Assim: data assimilation, GS = Gulf Stream, and ASPG = Along-shelf pressure gradient. Y = Yes, N = No.
Table 2. Correlation coefficients (R) and time lags (shown as R/Lag in months) for 3-month running averages minus 1-year averages for GS path-shifts, wind stress curl, eddy kinetic energy north of the Gulf Stream (N-EKE), CLSW transport computed off the southern coast of Nova Scotia (see text), and total river discharge. See also Figure 7. The last row indicates the correlation coefficients R for 1-year low pass of N-EKE with ASPG, wind stress curl, and GS path shifts. A positive (negative) lag indicates that the variable listed in the first column leads (lags) that listed in the first row.
Figure 1. Map of the western Mid-Atlantic Ocean and locations of the 14 tide-gauge stations.  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).(color); and the. 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).
(a) (b) 
(c) Figure 3. (a) 16-year mean SSH (contours; interval = 0.005 m on shelves, but = 0.05m elsewhere). (b) The mean SSH at stations (red dots in (a)) along the 50 m isobath with distance measured from the southernmost location near Cape Hatteras. Symbols are, CHS: Chesapeake Bay, DEL: Delaware Bay, ELS: east end of Long Island, and CC: Cape Cod. The solid line indicates a linear regression from DEL to ELS, and the dash line indicates a linear regression from CHS to CC. (c) Time series of along-shelf pressure gradient fluctuations (mean is removed). The time tick marks indicate January 1st. 
(a) (b) 
(c) Figure 4. (a) The EOF mode 1 of 12 monthly running-averaged tide-gauge sea level anomalies; (b) the principle component 1 (PC1); and (c) the linear regression of the EOF mode 1, where distance is from Wilmington (NC). The estimated sea surface slope is about 4.8  10-8. Tick marks on the PC1 plot show January.
(a) (b) (c) 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 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. distances between DEL and ELS for 8 numerical experiments in Table 1 as in Fig. 3b. The slopes represent the mean ASPGs. Their values are listed in Table 1. Symbols DEL: Delaware Bay, and ELS: east end of Long Island. 
(a) (b) (c) (d) (e) (f) (f)
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 (N-EKE) 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. 
(a) 
(b) (c) Figure 8. (a) The eddy kinetic energy (EKE) offshore from 1000m isobaths averaged over winter, spring, summer, and fall from AVISO satellite geostrophic currents. Thick black line indicates the corresponding seasonal mean GS position. (b) Monthly mean of averaged N-EKE over the area in the rectangular box north of the GS monthly mean path. (c) The seasonal cycle of N-EKE; unit is m2s-2. 
(a) (b) (c) 
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).    
(a) (b) (c)
(d) 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 and grey contours indicate the 50m, 200m and 1000m isobaths. A backup N-EKE plot for Figure 8a 
1 This same formula was used in Oey et al. (2006), but the coefficient for |ua| 2 was erroneously rounded off to 0.0002. 2 Redundancy experiments were also done for checking consistencies and ideas. Those summarized in Table 1 suffice, however, for explaining the contributions of various forcing to ASPG. 3 A more complicated function may give a better “fit” but this is not attempted. The goal here is to summarize in a succinct way the general response due to each of the different forcing. Directory: ftp -> fxu -> paper -> ASPG ftp -> Tr-41. 4-03-05-024 Telecommunications ftp -> Message Race Detection for Web Services by an smt-based Analysis ftp -> A proposal submitted to the ftp -> National reports on data buoy activities ftp -> Louisiana barrier island comprehensive monitoring program (bicm) Volume 1: Barrier Shoreline Post-Storm Assessment ftp -> Jcomm expert team on ASPG -> The origin of along-shelf pressure gradient in the Middle Atlantic Bight ASPG -> The origin of along-shelf pressure gradient in the Middle Atlantic Bight ASPG -> The origin of along-shelf pressure gradient in the Middle Atlantic Bight Download 134.95 Kb. Share with your friends:
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