The role of cyclones and topography in Loop Current ring shedding Yves morel – Eric Chassignet.

Plan 1. Introduction 2. Analysis of Loop Current Ring formation in MICOM simulation 3. Idealized model of the Loop Current Ring   effect and interaction with topography 5. Summary and conclusions

Observation of cyclones Cyclones are observed during the separation of the Loop Current Rings (Cochrane, 1972, Vukovich et al., 1979)

Cyclones in numerical models Numerical experiments showed that cyclones are also involved in the separation process (Hurlburt, 1985, 1986; Wallcraft, 1985, 1986; Oey, 1996; Chérubin et al., 2005) ECMWF MICOM SIMULATION  Where are they coming from?

Rationale Evidence in both observations and numerical experiments that cyclones play an important role in the separation process Mechanism of formation of cyclones during the separation process of the Loop Current Rings

MICOM simulation  ECMWF daily-wind forced very high resolution simulation (1/12 degree)  Good agreement with observations in the GOM: - Yucatan channel transport (27 Sv) - Vertical and horizontal current distribution and variability - Loop Current ring size - shedding period - cyclones involved in the separation process

Analysis of the instability of a Loop Current ring during a shedding event  Chérubin et al. (2005) found a mode 4 instability growing around the rim of the Loop Current  Cyclones were formed as the product of this instability from the cyclonic rim of the vortex.

Instability properties  Mixed barotropic- baroclinic instability.  Barotropic instability is intensified in the upper layers.  Baroclinic instability is intensified in the deep layers. Baroclinic conversion

Idealized model of the LC ring: potential vorticity anomaly distribution Necessary conditions for instability: Charney-Stern and Rayleigh criteria

 4 layers, piecewise constant potential vorticity anomaly strips. Vortex of type “R” (Morel and Mc Williams, 1997): anticyclonic core surrounded by a cyclonic belt.  5th layer at rest.  R = 250 km, R’ = 200 km.  Type “R” vortex: the most unstable mode depends on the width of  Type “R” vortex: the most unstable mode depends on the width of the cyclonic belt (Flierl, 1988). Idealized QG model H1 H2 H3 H4 H5 c1 c2 c3c4 c5 c6 c7 PVA1<0 PVA3<0 PVA4<0 PVA7>0 PVA2 >0 PVA4 > 0 PVA5 >0 R R’ PVA8 = 0 z PVA2 >0 PVA4 >0 PVA5 >0

Idealized model  Dependence of instability on the vorticity profile parameter , equivalent to the ratio R/R’. Carton and Legras,  Potential vorticity profiles: define an isolated vortex Vorticity jumps. And also the layer thicknesses for the numerical model: MICOM in adiabatic mode Maximum PVA in each layer

Idealized model on a f-plane  Linear and nonlinear most unstable mode: m=4.  Baroclinic instability bottom intensified.  Barotropic instability surface intensified in the linear model.

 -effect  The end product of the instability different than on a f-plane   effect: tendency for a mode 1 perturbation (Dewar and Killworth, 1995; Morel and Mc Williams, 1997)

 effect and interaction with topography

Southward slope 2500 exp hbhb L

Westward propagation.. no  Exp. Vortex east of the cape + Camp. Bank shelf + Vortex north of the cape + Camp. Bank shelf ^ MICOM

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Summary and Conclusions  Observations and numerical modeling show the presence of cyclones before the separation of the Loop Current ring, and during its westward propagation.  In MICOM simulation we showed that the generation of cyclones and the separation of the Loop Current ring are the end product of barotropic and baroclinic instabilities.  The Loop Current Ring is a shielded vortex with a core (rim) of negative (positive) potential vorticity anomaly.  This provides us with a characteristic unstable behavior for such vortex: multipolar nonlinear steady state (pentapole) and sensitivity of the instability to the width (or shear) of the rim (Flierl, 1988).

Summary and Conclusions  Mode 4 is the most unstable mode on a f-plane.  In the nonlinear regime, under the  -effect, the vortex rim separates from the anticyclonic core, and forms a cyclone north of the anticyclone. The dipole drifts westward. This result also explains the origin of the modon often observed in numerical experiments.  Campeche Bank’s cape enhances filamentation and vortex splitting. Both effects increase with the shelf slope.

Summary and Conclusions  In presence of a southward slope north of the ring, scattering in the deep layers is increased with the steepness of the slope.  The exponential slope favors differential scattering between the anticyclone and the cyclone to the north.  Fast dispersion of the cyclone reduces its splitting effect on the anticyclone, which maintains its coherence.  When the northern Campeche Bank shelf is taking into account, the westward propagation of the anticyclone is increased and realistic propagation speeds are obtained.

Summary and Conclusions  The Campeche Bank shelf contributes to the crossing of the Loop Current ring around the Campeche Bank cape.  The Florida shelf tends to keep the Loop Current ring in its vicinity and compete with Campeche Bank shelf.  As in our idealized model, the Loop Current ring in MICOM simulation follows the northern Campeche Bank shelf before interacting with the western Gulf circulation.  SSH maps confirms the above results.

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