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Wind-Driven Cross-Shelf Transport on a Shelf with Curvature

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Presentation on theme: "Wind-Driven Cross-Shelf Transport on a Shelf with Curvature"— Presentation transcript:

1 Wind-Driven Cross-Shelf Transport on a Shelf with Curvature
Steven Morey Center for Ocean – Atmospheric Prediction Studies The Florida State University

2 Geostrophic currents on a shelf are dominantly along-isobath (PV conservation)
Large cross-shelf gradients in physical and biochemical variables elevate the importance of the secondary cross-shelf flows. Example: Cross-shelf transport off of California yields cold filaments… … and offshore export of chlorophyll-rich waters.

3 Gag grouper spawning locations
Gag grouper (Mycteroperca Microlepis) spawn along the outer shelf edge in winter through early spring. Courtesy C. Koenig, FSU Larvae are transported onshore to nursery habitat (sea grass) in the coastal region by some undetermined mechanism. Gag grouper spawning locations

4 Idealized Coastal Upwelling
Wind Offshore Ekman Transport Upwelled Onshore Transport

5 Coastal Upwelling on a Realistic Shelf
y x τx τy τ Cross-isobath surface Ekman transport is proportional to, and to the right of, τy

6 Coastal Upwelling on a Realistic Shelf
y x τy τ τx Little cross-shelf surface Ekman transport, and hence upwelling/downwelling, when winds are mainly blowing across-shelf

7 This does not look like an idealized straight shelf
Decrease in Shelf Width (following the Shelf Wave propagation direction) Curvature

8 Cross-shelf Transport due to Changing Shelf Width
Alongshore depth-averaged velocity (from along-shore shallow-water momentum equation with a flat shelf) Wind Alongshore transport W

9 U y Cross-shelf Transport due to Changing Shelf Width U y Wind
Change in along-shelf transport must be balanced by cross-shelf transport where the shelf width changes. U y

10 over an idealized shelf - upwelling wind case
Solution to a linear, steady homogeneous potential vorticity equation over an idealized shelf - upwelling wind case Cross-shelf velocity component  From Pringle, JPO, 2002

11 y x τx τy τ Thus, under an upcoast wind regime, one might expect enhanced onshore transport at the shelf constriction.

12 Upwelling on the Northern West Florida Shelf
3m 15m N7 Tower observations

13 Upwelling Index for the northern West Florida Shelf
A spatially-varying upwelling index is computed as the pseudostress projected onto the along-isobath direction Positive in the topographic wave propagation direction  downwelling QuikSCAT winds can be used due to the large width of the shelf The topographic gradient is computed over a ~30km length scale (comparable to Rd) QuikSCAT Level 2B winds are converted to pseudstress |u|u Pseudostresses are bin-averaged and projected onto the local along-isobath direction (Computed between 10m and 300m isobaths) ~200 km

14 SeaWinds/QuikSCAT Along-Shore Pseudostress
Downwelling Upwelling 01 Jan :23 UTC

15 SeaWinds/QuikSCAT Along-Shore Pseudostress
Downwelling Upwelling 03 Jan :46 UTC

16 SeaWinds/QuikSCAT Along-Shore Pseudostress
Downwelling Upwelling 05 Jan :36 UTC

17 SeaWinds/QuikSCAT Along-Shore Pseudostress
Downwelling Upwelling 05 Jan :58 UTC

18 SeaWinds/QuikSCAT Along-Shore Pseudostress
Downwelling Upwelling 09 Jan :36 UTC

19 1999 – 2009 Mean Downwelling Upwelling

20 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

21 Dec-Jan-Feb Climatology
(spawning season) North-northeasterly winds produce a dominant upwelling pattern over the shelf with onshore surface Ekman transport in the western Apalachee Bay region

22 July Climatology Only month with upwelling-favorable winds over the northern And western Apalachee Bay region

23 Water Temperature (°C)
Near-bottom (17 m) Water Temperature (°C) 9 m

24 Summary QuikSCAT winds are useful for studies of the West Florida shelf due to its width. The curvature of the northern West Florida Shelf produces significant along-shore variability in inferred cross-shelf transport The narrowing of the shelf suggests regions of enhanced cross-shelf transport due to simplified dynamics A localized region of upwelling is suggested by the analysis, and is evident in the observations This work would be further supported by extending the NGI observational array westward toward the narrower portion of the shelf to provide a better characterization of the cross-shelf transport pathways. Un-instrumented towers

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