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 1.Observations of deep equatorial density layering 2.Interpretation in terms of Inertial instability 3.Consequences for the deep equatorial dynamics

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

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

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

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. 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 Another layering mechanism PV at 23°W 500m 2000m 1°S1°N equator + - 0 0 0 0 0

9. 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 Angular Momentum on an equatorial  -plane – defined as : – meridional homogenization of M detected by zero curvature of M

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

11. N umerical simulations of layering caused by Inertial Instability (Model of Hua et al. 1997) Results Layers coincide with regions of homogenized M Structures similar to the observed ones Forced by : temporally periodic barotropic shear to trigger instability + stationary vertically periodic jets to mimic Equatorial Deep Jets strong gradient of Mhomogenized 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. 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. d’Orgeville and Hua (2004) (submitted to JFM) Inertial parametric instability of zonally symmetric Yanai wave Impact of a large vertical-scale Yanai wave It leads to : westward flow at the equator extra-equatorial eastward flow barotropic U after instability

15. Large vertical westward flow below the thermocline Latitude Depth (m) 0 2500 5S6N 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