Ichiro Fukumori Dimitris Menemenlis Tong Lee (JPL/Caltech)

1. “Dynamics of the Near-Uniform Basin-Wide Wind-Driven Sea Level Fluctuation of the Mediterranean Sea” Ichiro Fukumori Dimitris Menemenlis Tong Lee (JPL/Caltech) Venice Symposium, 13 March 2006

2. Gist • A basin-wide fluctuation is found, • Near-uniform sea level & bottom pressure, • Net inflow/outflow through Gibraltar Strait, • Driven by winds near Gibraltar Strait, • Coherent with atmospheric pressure. I.Fukumori/JPL

3. Basin-Wide Oscillation TOPEX/POSEIDON reveals a basin-wide sea level fluctuation nearly uniform across the Mediterranean Sea. Jan. 7, 1996 Jan. 19, 1996 Sea level anomalies (cm) relative to mean annual cycle

4. Empirical Orthogonal Functions (EOFs) 1st EOF of T/P (explains 54% of non-annual variance) correspond to a near-uniform basin-wide fluctuation. A similar variability is found in an ocean circulation model (82%). T/P ocean model

5. MIT General Circulation Model (MITgcm) (Marshall et al., 1997) • Nonlinear primitive equation model • Advanced physics (e.g., KPP & GM mixing) • NCEP reanalysis forcing • Global, moderate resolution • 73°S~ 73°N • 0.3°10m grid in tropics Model grid (360 224 46)

6. Time-Series of Basin-Mean Sea Level (non-annual) Model is coherent (correlation of 0.75) with T/P with similar amplitudes (3.1 & 3.2 cm non-seasonal rms for model & T/P). model T/P

7. Diabatic Forcing vs Wind Forcing Wind-forced barotropic changes account for most of the model’s non-seasonal sea level fluctuation. wind-driven bottom pressure all forcing diabatic

8. Transport through Gibraltar Strait Simulated wind-driven sea level fluctuation consistent with net transport changes through model’s Strait of Gibraltar. net transport sea level Non-seasonal sea level variability Zonal flow @ Gibraltar

9. Sensitivity to Wind Forcing mean Med sea level x(0) wind at different locations and lags x(-20 day) Units in cmm2/N

10. Oscillation due to winds in & near Gibraltar Strait Zonal Wind 16% 74% 12% Meridional Wind -4% 36% 16% % of explained variance

11. Dynamic Mechanism The sensitivity analysis suggests a balance between zonal wind and zonal pressure gradient about the Strait; Integrating horizontally and vertically within the region connecting the Mediterranean and Atlantic basins (width W, length L, depth D), Wind-driven sea level difference is independent of width W of the connecting region, and rest of the Mediterranean Sea geometry.

12. Oscillation insensitive to model geometry half basin baroclinic wide strait deep strait barotropic

13. Comparison to Pressure Loading 1st EOF of IB (explains 73% of non-annual variance) is not as uniform as that of T/P (or model). T/P IB

14. Comparison to Pressure Loading T/P IB model model vs T/P IB vs T/P Frequency (cpd)

15. Conclusion • A near-uniform, basin-wide fluctuation of sea level across the Mediterranean Sea has been identified, • The fluctuation is barotropic and is driven by winds in the VICINITY of the Strait of Gibraltar, • To first approximation, the oscillation is independent of the Strait’s width and the basin geometry of the Mediterranean Sea.

16. Implications • The wind-driven basin-wide barotropic sea level fluctuation contributes to the apparent deviation of the Mediterranean Sea from an inverse barometer response (e.g., Le Traon and Gauzelin, 1997), • The fluctuation may help account for apparent observed discrepancies between sea level and the hydraulic state of the strait (e.g., Garrett et al., 1990), • The rapid sea level change can be aliased if not properly accounted for (e.g., GRACE).