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:10km & 25 sigma levelsCCMP3 windsDaily riversM2 tideSSHA (H>1000m) & SST assimilations

MeanASPG

Momentum Balance:Include here the balance that drives the alongshelf current

x

y

R=-0.52 @ 1 month

ASPG=8.4e-8, Std=8.6e-8

Distance from Cape Hatteras

Tide-gauge dataEOF mode 1: 67%

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

1. The variability of Gulf Steam path

2. Wind stress curl: /)( yx xy

3. The Gulf Stream warm-core rings

4. Upstream Transport:

HdyU

5. Rivers

The Correlation coefficients (R) and coefficients (S) at 95% significant level

R/S Along-shelf currents ASPG

Along-shelf currents --- -0.44/0.20

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

Model EKE -0.54/0.18 0.52/0.19

Wind only Reduced transport 17%

River only River with reduced transport 34%

An Idealized simulation with 3 warm-core rings initialized every 360 days

Summary1. The total freshwater discharge and upstream

transport mainly contribute to the mean ASPG setup.

2. The changes of rivers, GS path shifts, wind stress, and EKE create the ASPG seasonality.

3. 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?

1. Gulf Stream path shifts;

2. Wind stress;

3. Warm-core rings;

4. Upstream (Labrador Sea) transport;

5. Freshwater discharge.

A Northwestern Atlantic Ocean Model (NWAOM)(Oct, 92~Dec, 08)

POMPrinceton Regional Ocean Forecast System

(PROFS)

• 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

PROFS domain

MA domain

NWAOM : Along shelf currents and Along shelf

pressure gradient (ASPG)

ASPG=8.4e-8, Std=8.6e-8

U=-0.015 m s-1 m/sStd=0.03 m/s

xy

R=-0.52, S=0.33

Rivers: 1992-2008

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