Sensitivity Analysis and Building Laterally-Variable Ocean Conductivity Grid 1 N. R. Schnepf (UoC/CIRES) C. Manoj (UoC/CIRES) A. V. Kuvshinov (ETHZ)

Outline 3-D modeling of tidal signals Sensitivity analysis Laterally-variable ocean conductivity 2

Modeling tidal signals Simulate tidal magnetic fields under different conductivity regimes – Satellite height (430 km) – Sea-level – Seafloor 3

Modeling tidal signals 3-D global EM induction code to predict ocean magnetic fields on 1°x1° grid (Kuvshinov, 2008) 4 TidePeriod (hours) TypeVertical amplitude (mm) M Lunar semi-diurnal A. V. Kuvshinov (2008). 3-D Global Induction in the Oceans and Solid Earth: Recent Progress in Modeling Magnetic and Electric Fields from Sources of Magnetospheric, Ionospheric and Oceanic Origin. Surveys in Geophysics, 29(2):139–186.

Modeling tidal signals 3-D global EM induction code to predict ocean magnetic fields on 1°x1° grid (Kuvshinov, 2008) 5 Tidal depth-integrated velocity model for Average ocean conductivity for (3.2 S/m) Local conductance map and underlying 1-D conductivity for World Magnetic Model (year 2014) for Kuvshinov & Olsen (2006) for 1-D conductivity model A. Kuvshinov and N. Olsen (2006). A global model of mantle conductivity derived from 5 years of CHAMP, Ørsted, and SAC-C magnetic data. Geophysical Research Letters, 33(18):L18301.

Tidal depth-integrated velocity model 6 HAMTIDE: depth-integrated assimilated tidal model (0.25 o x0.25 o interpolated to 1 o x1 o ) (Taguchi et al., 2014) E. Taguchi, D. Stammer, and W. Zahel (2014). Inferring deep ocean tidal energy dissipation from the global high- resolution data-assimilative HAMTIDE model. Journal of Geophysical Research: Oceans, 119: 4573–4592.

Local conductance map 7 C. Manoj, A. Kuvshinov, and S. Maus. Ocean circulation generated magnetic signals. Earth Planets Space, 58:429–437, 2006.

World Magnetic Model (year 2014, B r ) 8 Chulliat, A., Macmillan, S., Alken, P., Beggan, C., Nair, M., Hamilton, B., … Thomson, A. (2015). The US/UK World Magnetic Model for Boulder, CO. doi: /V5TH8JNW

CM5 vs Modeled tidal signals 9 T. J. Sabaka, N. Olsen, R. H. Tyler, and A. Kuvshinov (2015). CM5, a pre-Swarm comprehensive geomagnetic field model derived from over 12 yr of CHAMP, Orsted, SAC-C and observatory data. GJI, 200:1596–1626.

Sensitivity analysis 10

Sensitivity analysis 11 is a 1°x1° grid Layer 1 k= C5 M2 radial field at satellite height Continents masked out for analysis

Sensitivity analysis: M2 results 12 Promising for probing upper mantle. Promising for probing upper mantle & lithosphere.

Incorporating noise into the sensitivity analysis Randomly add noise to each grid point – 0 to 0.2 nT – 0 to 0.5 nT 13

Sensitivity analysis with noise added: M2 results 14

Sensitivity analysis conclusions 15 Satellite Br is sensitive to the lithosphere & upper mantle  Satellite and seafloor tidal data may be used to constrain lithospheric and upper mantle conductivity

Laterally-variable ocean conductivity (LVOC) 16 For models used in sensitivity analysis: was the average ocean conductivity (3.2 S/m) For LVOC: Method of Fofonoff (1985) to create 1°x1° map of depth-averaged conductivity from salinity, temperature and pressure data NOAA’s 2009 World Ocean Atlas salinity (s) & temperature (t)  3-D 1°x1° with 33 layers (depths of m) Pressure (p) determined from depth and latitude (Saunders 1981). N. P. Fofonoff (1985). Physical Properties of Seawater: A New Salinity Scale and Equation of State for Seawater. Journal of Geophysical Research, 90(C2):3332–3342. P. M. Saunders (1981). Practical conversion of pressure to depth. Journal of Physical Oceanography, 11:573–574.

Laterally-variable ocean conductivity (LVOC) 17 Conductivity may be determined for a given salinity (s), temperature (t) & pressure (p) using the reference conductivity of 4.29 S/m.

Laterally-variable ocean conductivity (LVOC) 18 Annual average

Laterally-variable ocean conductivity (LVOC) 19 Different between LVOC annual average and 3.2 S/m

Laterally-variable ocean conductivity (LVOC) 20 Difference between summer & annual average

Laterally-variable ocean conductivity (LVOC) 21 Difference between winter & annual average

Laterally-variable ocean conductivity (LVOC) conclusions 22 Minimal seasonal difference (under 0.1 S/m) Standard dev of 0.37 S/m, average S/m Greater conductivity  In warmer, saltier regions  By river mouths Lower conductivity in cooler, fresher regions

Laterally-variable ocean conductivity (LVOC) conclusions 23  Improve accuracy of inversions  Local variations in conductivity especially important when incorporating seafloor data