6.9.2 Ocean eddies as seen by satellite altimetry: the Kuroshio current

Altimetry data processing for mesoscale studies.

Mesoscale variability typically refers to oceanic signals with space and time scales of 50 km to 500 km and 10 days to 100 days, respectively. An example of this are ocean eddies, fronts, and meanders. The Kuroshio is a strong western boundary current in the North Pacific Ocean, similar to the Gulf Stream in the North Atlantic. In this western boundary current, there is considerable mesoscale variability which tends to be dominated by meanders and eddies. Satellite altimetry offers a high-performance technique for studying such phenomena.

Data used

Merged Maps of Sea Level Anomalies (MSLAs) have been used in this study:
– they are already gridded and easy to use, moreover no specific data reprocessing is required here,
– they have been specifically processed for mesoscale studies, i.e they have been filtered from small-scale signals and long wavelength errors, and have been sub-sampled,
– merged datasets provide a better description of mesoscale activities thanks to improved accuracy.

Methodology

Geographic extraction

Extracting data from MSLAs will strongly greatly simplifiesy our study as the MSLAs are available by from FTP. and Our area of interest is defined by the following coordinates: 25°N-40°N, 135°E-180°E (fig 1 & fig 2).

Computation of geostrophic currents

The components of geostrophic currents are deduced from the geostrophic balance hypothesis:

u=-(g/f)xd(SLA)/dy
v=(g/f)xd(SLA)/dx

where g is gravity and f the Coriolis parameter. To generate a map of geostrophic currents from SLA fields, algorithms can be based on a centred finite difference method (fig 3).

Computation of MADTs

Maps of Absolute Dynamic Topography (MADT) are obtained using:

MADT=MSLA+MDT

Where MDT is the Mean Dynamic Topography. There are several MDT models, some of which are available on Aviso web site.
‘Absolute Dynamic Topography’ represents the general ocean dynamics, whereas ‘Sea Level Anomalies’ focus on its variable component (fig 4).

fig 1: Area for data extraction
fig 2: Maps of Sea Level Anomaly, Kuroshio area, cm

fig 3: Maps of geostrophic velocity anomalies, Kuroshio area, cm/s

fig 4: Maps of Absolute Dynamic Topography, Kuroshio area, cm

Western boundary currents such as the Kuroshio convey a lot of energy and generate strong turbulence systems. While SLAs and geostrophic currents illustrate eddies, EKE (Eddy Kinetic Energy) fields allow us to focus on mesoscale variability; statistics are especially necessary to quantify these phenomena.