Gulf of Mexico Circulation: Eddy Formation, Vertical Motion, and Source of Yucatan Outflow

1. Weekly resultEda20100129 Circulation in GOM 5years runs with and without wind from the same IC.

2. Outline for today’s issue • The characteristic of accumulated eddy (rossby wave) forming WBC in nowind case. (p3-6) • The vertical profile of time-area averaged vertical velocity (p7-8) and the vertical motion along the eddy passage (p15-17) • Why do the isotherms in wind case deeper in both downfront and upfront sides? (p9-14) • The source of Yucatan outflow. (p18-21)

3. Eddy vertical size ∫∫dydz Eddy pass through 90W, integrating EL>0.1m, from surface to Z7(~700m). Wind:2.18.e8m2 Nowind:1.81e8m2 Since this is 90W, so the propagating speed need to modify to u-component. See p22. Teddy/Ttotal~1/3 Qeddy_wind=2.18e8m2*7.3cm/s*cos23˚ (transform to u)=14.65Sv Qeddy_nowind=1.81e8m2*8.7cm/s*cos16˚ =15.13Sv Note: the eddy is not a prefect circle, it is longer in y-axis, so that’s why we get larger number than the previous one (assume eddy is symmetric) (see next page)

4. Western boundary current (WBC) • In nowind case, the eddy accumulate at the western Gulf, forming the WBC. • WBC flows anti-cyclonic along the Gulf, it merges into LC , and going out from Florida Strait.

5. WBC Vertical Profile LHS are the temperature with v-velocity, indicating the size and the strength of WBC. ~100km, Vmax=0.47m/s LHS-bottom is the same but showing topography. section position with mean surface v-velocity WBC transport ~5.4Sv

6. Summary for WBC and Qeddy • WBC is formed in nowind case, the width is 100km, and the Vmax=0.47m/s, transport is 5.4Sv. • Qeddy is about 15Sv in nowind case WBC_transport=Qeddy*Teddy/Total~5Sv is about the same as actual calculation.

7. Area-time-averaged wExclude the shelf (only for h>200m) B C A A:western Gulf B:eastern Gulf C:west of 94w A+B: total Gulf B C A A+B

8. Summary for w The downward motion in the western Gulf is mainly from the eddy effect (from p7, figC, and from the map). The stronger downward motion can also been seen from EG near the surface(figb, black), resulting in deeper isotherm. From 500 to 2500m, WG has larger downwelling (more negative), but EG has larger upwelling (more positive) compare to the nowind case.

9. 90w mean, with eddy pass by Why does the isotherm also deep in the upfront side? T U PV downfront upfront Wind Nowind Wind-nowind

10. 90w mean, with eddy pass by Due to vorticity shear induced vertical motion? V vor/f W ???? Does not coicide Wind Nowind Wind-nowind

11. Other reason? From idealized case 1day • dx=1km (0~400km), dz=1m • X=600km, H=350m • Nowall, spinning up for 60days After 20 days, the warm water propagate eastward, and the isotherm does not deepen. 200km But, as you notice, this is a “nowall” case, the GOM is closed at the northern coast. Does it change result?

12. Idealize case 2- with wall • dx=2km, kb=301 • X=500km, H simulates similar as 90W in GOM. • Both side close, spinning up for 60days • Only a thermal front, so the velocity structure is different from a real warm eddy, and also f is constant. day120 day70

13. Let’s see the other upfront case • BC case, but constant downwelling wind. • dx=0.25km,kb=101 This is in close the coast region. After the warm water (bulge) touch the wall, the downwelling together with the keep moving warm water will deepen the isotherm.

14. Summary for deep isotherm in 90W • The deep isotherm is induced by the wind. • Down-front wind offer large mixing deepens the isotherm, the up-front wind moves the warm water to the coast, the downwelling and keep moving warm water also deepens the isotherm. • (p11, white contour in upper 2 figs also explain why the upfront part may even has deeper isotherm than downfront side) • The wind-induced Ekman transport strengthens the v-velocity, and the minimum of the v goes into subsurface (p10, v, wind)

15. 5 yrs mean Vertical velocity • The belt shape vertical velocity is along the eddy passage. • Why are the w positive in the southern eddy passage but negative in the northern eddy passage? wind nowind

16. Again, let’s see from 90W vor/f W The strong upwelling and downwelling is agree well with the position of vor/f. Due to PV conserve, so that the negative vor/f has downwelling, and positive vor/f has upwelling. In nowind case, the negative vor/f is stronger, so the downwelling is stronger. Wind Nowind Wind-nowind

17. Summary of vertical motion in the eddy passage • The vorticity of the eddy is positive in the southern eddy and negative in the northern side. • For PV conservation, the positive vorticity induces upwelling and negative vorticity for downwelling. • In nowind case, the negative vorticity is stronger than wind case, so the Ertel’s PV(p8, @26N) can go as deep as wind case even without wind.

18. Source of Yucatan’s outflow (Qy) • The outflow is from deep ocean, so the source will be from? Under inflow of LC? • Set up the section northward of Yucatan e a

19. Vertical section plotcross section velocity (v) a b c

20. Schematic for deep ocean outflow, green lines denote outflow d e Black curve arrow simulates from ZB’s plot

21. Summary for Yucatan outflow • The deep Yucatan outflow comes from the cyclonic circulation in the Gulf, and flows out from the deep basin to Yucatan Channel.

22. θ1 θ2