gp33a-06 / 15.12.2004 fall agu meeting, san francisco, 13-17 december 2004 magnetic signals...

Fall AGU Meeting, San Francisco, 13-17 December 2004GP33A-06 / 15.12.2004

Magnetic signals generated by the ocean circulation and their variability .

Manoj, C.(1,2) ([email protected]), Kuvshinov, A.(3,4), Maus, S.(5,1) and Lühr, H.(1).

1) GeoForchungsZentrum - Potsdam, Germany2) National Geophysical Research Institute - Hyderabad, India3) Danish Space Research Institute - Copenhagen, Denmark4) Geoelectromagnetic Research Institute - Troitsk, Russia5) NOAA's National Geophysical Data Centre - Boulder, USA


•Electrically charged ions make ocean a conducting fluid.

•As the ocean-water flow through the ambient geomagnetic field, it generates secondary electric and magnetic fields (motional induction).

•Magnetic fields generated by ocean can be divided into “poloidal” and “toroidal” parts.

•The toroidal fields have higher strength (1-100 nT) than the poloidal fields (1-10 nT). However, only the poloidal fields are observable outside the ocean.

•The large spatial decay scales of the poloidal fields allow it to reach distant land and satellite based sensors.

Introduction


Recently Tyler et al (2003), for the first time identified ocean-magnetic signals in the satellite magnetic data.

This results confirm the non-trivial contribution of the ocean-magnetic signals to the geomagnetic fields.

Fig: Observed and predicted magnetic signal of the M2 ocean tide. (Tyler et al, Science, 2003). Figures A and C are observations and B and D are predictions.

Background


1. Numerical prediction of the magnetic fields. >> 3D EM code >> Sensitivity analysis

>> Ocean models

2. Magnetic fields generated by ocean circulation. >> Predicted fields at sea and satellite altitude

>> Effect of ocean eddies

3. Temporal variations.

>> Range of variability>> Monsoon & El Niño variations

4. Conclusions

Road map


We used a 3D global EM induction code to predict ocean-magnetic fields. (Kuvshinov et al., 2002; 2004)

Uses a volume integral equations approach to simulate EM fields over a 3D conductivity model of the Earth, excited by inducing currents.

)( mainwater BUJ where,

σ - Mean sea water conductivity 3.2 S/m

U - Depth integrated velocity m2/s

B - Geomagnetic field derived from IGRF 2000 in nT

For ocean-magnetic signals, this current J is calculated as,

Numerical predictions


Sensitivity of the ocean-magnetic signals

Log(∑|Br|) in nT

The sensitivity is influenced largely by the vertical component of the main geomagnetic field. This means ocean near geomagnetic equator produces least signals where as areas closer to geomagnetic poles produces maximum signals

In addition, the distribution of sensitivity is also influenced by the lateral conductivity contrast.


We used two state-of-the-art ocean models for the velocity data.

1) The horizontal velocity data from ECCO (www.ecco-group.org) adjoint model run (1992-2002) were used to estimate ocean-magnetic signals and their temporal variations on a 10 x 10 resolution. We use this model to predict the seasonal and interannual variations in ocean-magnetic signals.

2) To simulate the magnetic signatures of ocean eddies, the 0.250 x 0.250 resolution velocity data from OCCAM model (http://www.soc.soton.ac.uk/JRD/OCCAM).

Ocean circulation models used

log√(U2+V2)


Predicted ocean magnetic signals

ECCO OCCAM

0 km

430 km

nT nT

nT nT


Magnetic signals from ocean eddies

At sea level At satellite altitude

The simulation with OCCAM velocity data brought out the magnetic effects of ocean eddies.

However, effects of individual eddies do not show up in predicted the fields at satellite altitude.


Deviation from mean for predictions from 1992 - 2002

nT


0 50 100 150 200 250 300 350

-80

-60

-40

-20

0

20

40

60

80

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Range of variability

nT


Seasonal variations.

nT

Difference between the predictions for Summer and Winter, 2002.

BrThe difference map is dominated by a series of anomalies in Indian and western Pacific oceans. The variability in northern Indian ocean is probably related to Monsoon.


The difference map (El Niño (1997-1998) and La Niña (1999)) has lower magnitude than seasonal variation. Probable reasons:1) Weak vertical component of the geomagnetic field.2) El Niño affects only the surface currents.

El Niño variations

La Niña

El Niño Difference


Uses in Magnetic field models

We have applied corrections for tidal ocean signals that completely removed the tidal magnetic noise from observed data

In a similar way, the predicted magnetic signals from ocean circulation will be used to improve the crustal magnetic field models.

(Maus et al, 2004)


Conclusions

Magnetic signals generated from ocean circulation is within the range of the observational capability of satellites

Temporal variations in ocean – magnetic signals have lesser amplitudes.

Ocean eddies also contributes significantly to geomagnetic fields, but signatures from individual eddies are not shown up at satellite altitude.

High sensitivity of magnetic signals to ocean flow over ACC and western boundary currents improves their chances of detection.


Acknowledgements

ECCO and OCCAM modeling groups for making their data available

ESA for the financial support

gp33a-06 / 15.12.2004 fall agu meeting, san francisco, 13-17 december 2004 magnetic signals...

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