IWMO-2010, May 24 Fanghua Xu and Leo Oey Princeton University The origin of along-shelf pressure gradient (ASPG) in the Middle Atlantic Bight (MAB) IWMO-2010, May 24 Fanghua Xu and Leo Oey Princeton University

ASPG  10-8~10-7 drives mean along-shelf flow towards the southwest (Stommel and Leetmaa,1972; Csanady, 1976; Lentz, 2008);

16-yr run: y x MeanASPG Momentum Balance: 10km & 25 sigma levels CCMP3 winds Daily rivers M2 tide SSHA (H>1000m) & SST assimilations x y MeanASPG Here is the result from our model that shows ASPG. Describe also the GS, coastal current, slope-sea gyre. Add arrow-heads on contours to indicate flow directions. Make sure you know about the momentum balance that drives the alongshelf flow by ASPG. Momentum Balance: Include here the balance that drives the alongshelf current

Distance from Cape Hatteras ASPG=8.4e-8, Std=8.6e-8 Distance from Cape Hatteras R=-0.52 @ 1 month

Tide-gauge data EOF mode 1: 67% Careful to say that this is not MEAN – only the amplitudes of fluctuations. So in winter, ASPG swings to become +ve, while in summer it swings to be weak or –ve. This means that the TOTAL ASPG is strongly +ve in winter and weak and could even reverses to be –ve in summer.

What drive the seasonal and inter-annual variations? how and why? Previous Works: Due to offshore pressure field (Beardsley and Winant,1979)? But, pressure field not likely to penetrate onto the shelf (Wang, 1982; Chapman et al. 1986); Due to be rivers? This was questioned by Csanady (1979); Upstream transport? (Wang, 1982; Chapman et al. 1986). The origin of (mean) ASPG remains unclear (Lentz, 2008)… what is its origin? What drive the seasonal and inter-annual variations? how and why? Possible candidates: Winds GS path Rings Rivers Labrador transport

First describe the means here

1. The variability of Gulf Steam path

2. Wind stress curl:

3. The Gulf Stream warm-core rings

4. Upstream Transport: 2-year low pass (the dash line)

5. Rivers

The Correlation coefficients (R) and coefficients (S) at 95% significant level R/S Along-shelf currents ASPG --- -0.44/0.20 GS latitude shifts 0.32/0.12 0.21/0.17 Wind stress curl 0.34/0.09 0.18/0.12 Transport 0.43/0.18 -0.09/0.2 River input -0.41/0.10 0.16/0.13 Model EKE 0.01/0.04 0.10/0.04 -0.54/0.18 0.52/0.19

River with reduced transport 34% River only River with reduced transport 34% Wind only Reduced transport 17% Std:3e-8 (top left)

An Idealized simulation with 3 warm-core rings initialized every 360 days 1cm contour interval 35*60 =2100 days FFT analysis indicate the spectrum peak at 700 days.

Summary The total freshwater discharge and upstream transport mainly contribute to the mean ASPG setup. The changes of rivers, GS path shifts, wind stress, and EKE create the ASPG seasonality. The southwestward propagating Rossby waves and warm-core rings can influence ASPG and the shelf currents in both seasonal and inter-annual time scale.

Thank you! Questions?

The mean ASPG is the primary driving force to the mean depth-averaged along-shelf currents. (Beardsley and Boicourt (1981), Lentz (2008)

Literature review An ASPG, 10-8~10-7 must exist to drive the mean along-shelf flow towards the southwest (Stommel and Leetmaa,1972; Csanady, 1976; Lentz, 2008). The ASPG could be imposed by the offshore large-scale circulation (LSC) based on model results (Beardsley and Winant,1979). However, LSC is hard to penetrate to the shelf (Wang, 1982; Chapman et al. 1986). The ASPG could be produced by a major freshwater source, like the St. Lawrence system (Beardsley and Winant,1979). This was questioned by Csanady (1979). An upstream transport can contribute to the along-shelf currents (Chapman et al. 1986). However, the source of the inferred ASPG remains unclear.

Questions Why is the mean ASPG southwestward? Why are there seasonal and inter-annual variations in ASPG? Gulf Stream path shifts; Wind stress; Warm-core rings; Upstream (Labrador Sea) transport; Freshwater discharge. A Northwestern Atlantic Ocean Model (NWAOM) (Oct, 92~Dec, 08)

POM Princeton Regional Ocean Forecast System (PROFS) PROFS domain Curvilinear grid Grid size: Horizontal: ~10km Vertical: 25σ levels Mid-Atlantic Model: Winds, rivers & tides Nesting from PROFS Finer grid size: ~5km DA for GS & Eddies MA domain

NWAOM : Along shelf currents and Along shelf pressure gradient (ASPG) x y NWAOM : Along shelf currents and Along shelf pressure gradient (ASPG) R=-0.52, S=0.33 The pressure contours are parallel to the shelf break and slope but bend as they ‘leak’ onto the shelf. ASPG=8.4e-8, Std=8.6e-8 U=-0.015 m s-1 m/s Std=0.03 m/s

Rivers: 1992-2008 4 8 13 17 16 3 7 11&12 2 6 10 15 1 5 9 14