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In situ evidence of deep equatorial layering due to inertial instability

In situ evidence of deep equatorial layering due to inertial instability. M. d’Orgeville, B. L. Hua & R. Schopp Laboratoire de Physique des Océans, IFREMER, Brest L. Bunge Laboratoire d’Océanographie et de Dynamique du Climat, Paris. Outline. Observations of deep equatorial density layering

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In situ evidence of deep equatorial layering due to inertial instability

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  1. In situ evidence of deep equatorial layering due to inertial instability M. d’Orgeville, B. L. Hua & R. Schopp Laboratoire de Physique des Océans, IFREMER, Brest L. Bunge Laboratoire d’Océanographie et de Dynamique du Climat, Paris

  2. Outline • Observations of deep equatorial density layering • Interpretation in terms of Inertial instability • Consequences for the deep equatorial dynamics

  3. N2 profiles 100m N2 (10 –5 .s-2) Equatorial density layering Stairs-like structure in density • best detected by maxima in N2 (heigth scale of 50-100 m) 5 adjacent equatorial stations Density profiles Depth (m) Potential density (kg.m-3) EQUALANT 1999 DATA 10°W

  4. 1 range of density for all stations Equatorial density layering Layering better detected in isopycnal coordinates Density profiles different depths for each station Depth (m)

  5. Layers occur in the equatorial band over the whole water column under the thermocline Homogeneous density density step (layers extension can reach 2°) normalised N2 anomalies Potential Density (kg.m-3) equivalent depth (m) EQUALANT 1999 DATA 23°W

  6. Characteristics of equatorial deep layering are true for all the available Atlantic equatorial CTD sections (WOCE dataset and EQUALANT cruises) How can we explain such layering observations ?

  7. Layering mechanisms • Double diffusion in our data the T/S profiles are stable or marginally unstable between 600m and 2000m depth (Turner angle lies inside critical values range) • Not plausible between 600m and 2000m depth • Strain of internal wave field • deep equatorial layering observed in individual CTD-profiles • (Indian Ocean, Dengler & Quadfasel 2002) • Here we focus on large-scale layering (2°, 100m) =>dynamics influenced by Earth rotation

  8. PV at 23°W 500m 0 0 0 0 2000m equator 1°S 1°N - + 0 Another layering mechanism • Inertial Instability • already invoked for equatorial upper layers interleaving (Richards & Banks 2002) • in our data, the flow is clearly unstable over the whole water column in the equatorial band • (Negative values of f *PV easily triggered at the equator) • Plausible but requires further quantifications

  9. Angular Momentum on an equatorial -plane • defined as : • meridional homogenization of M detected by zero curvature of M Mechanism of Inertial Instability • Inertial Instability • Triggered whenever the maximum of angular momentum (M) appears north or south of the equator • Its non-linear evolution leads to • meridional homogenization of angular momentum on isopycnals • vertical homogenization of density causing layers

  10. equivalent depth (m) Potential Density (kg.m-3) strong gradient of M homogenized M • Large zones of homogenization of Angular Momentum (Myy~0) • Westward EDJs Observational signatures of Inertial Instability (10°W) • Layers are located only in region of homogenized M EQUALANT 1999 DATA 10°W

  11. Numerical simulations of layering caused by Inertial Instability (Model of Hua et al. 1997) • Forced by : • temporally periodic barotropic shear to trigger instability • + • stationary vertically periodic jets to mimic Equatorial Deep Jets • Results • Layers coincide with regions of homogenized M • Structures similar to the observed ones strong gradient of M homogenized M

  12. In all ADCP sections and numerical simulations : • spatial correlation between • large-scale layering and • angular momentum homogenization • => evidence of inertial instability • Consequences for deep equatorial dynamics ?

  13. Consequences for deep equatorial dynamics ? • Numerical simulations • Inertial instability = transfer of energy • from the barotropic time-variable shear forcing • to the westward EDJ • Observations • Oscillating shear flow  large vertical-scale equatorial waves • Can large vertical-scale shear variability impact the deep equatorial dynamics ?

  14. Impact of a large vertical-scale Yanai wave • It leads to : • westward flow at the equator • extra-equatorial eastward flow d’Orgeville and Hua (2004)(submitted to JFM) Inertial parametric instability of zonally symmetric Yanai wave barotropic U after instability

  15. 0 Depth (m) 2500 5S 6N Latitude Large vertical westward flow below the thermocline Zonal Velocity at 23°W (Gouriou et al. 2001)

  16. Large vertical westward flow below the thermocline Zonal Velocity at 23°W vertical mean between 400 and 2500m depth • westward flow at the equator • extra-equatorial eastward flow ? due to the large vertical-scale equatorial waves ?

  17. Conclusions • Evidence of large scale equatorial layering • (2° of latitudinal extension and up to 100m vertical scale) • Layering due to inertial instability • Possible importance of large vertical shear variability for deep equatorial dynamics

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