Characteristics of mesoscale eddies in the Southwest Pacific Research by: Lydia Keppler (MSc student at PaCE-SD, USP-based in Germany) Supervisors: Dr. Awnesh Singh (USP, Fiji), Dr. Sophie Cravatte and Dr. Alexis Chaigneau (IRD, France)

Outline Introduction Data & methods Horizontal properties of eddies: Where are the eddies Characteristics Summary Outline of this talk (more in MSc thesis)

The study region Colour: bathymetry Vectors: absolute geostrophic velocity between 0 m and 1000 m from Kessler and Cravatte (2013) We can explain that the bathymetry is very dynamic, especially in the northern part are many islands and shallow water, that might influence eddies. The Kessler & Cravatte product ends at 180°W. general westwards flow, stronger in the north and strong EAC going south

What is a mesoscale eddy Eddy: turbulent, spinning flow Mesoscale (=not very small, not very big): between ~40 to ~200km in diameter Cyclonic eddies (CE): lower sea level in the ocean Anticyclonic eddies (AE): higher sea level in the ocean The spinning of the earth creates a current around the low / high which is an eddy They propagate westwards and can trap water and transport heat, salt & nutrients from their area of formation They have an effect on ecosystems & the climate Can explain that they are a bit like a cyclone, but less violent.

Jason-2 spacecraft (image credit: CNES) Data Satellite altimetry: Aviso Duacs2014 (1993 to 2015) Multi-satellite product: HY-2A, Saral/AltiKa, Cryosat-2, OSTM/Jason-2, Jason- 1, Topex/Poseidon, Envisat, GFO and ERS-1&2 Measures sea surface height Provides Sea Level Anomaly (SLA) in reference to a 20-year mean (1993 to 2012) Daily, 1/4° x 1/4° global product Jason-2 spacecraft (image credit: CNES) For the Jason2 picture: it’s a NASA satellite that measures SSH as part of the Ocean Surface Topography Mission. Sea surface height on 28th August 1996 (Source: Chelton et al., 2011)

Methodology Eddy detection algorithm by Pegliasco et al. 2015 Finds local minimum and maximum (eddy centre) Finds outermost closed contour line around the eddy centre (eddy edge) Finds the same eddies on consecutive days and tracks them Filters: Min. amplitude: 2cm Min. lifetime: 14 days Min. radius: 40km The picture on the top is the Jason 2 satellite, which measures sea level height. Filters were applied to remove small detected structures that are not technically eddies. We can see on the picture that the eddies are everywhere. Animation to add detected contours & centres Sea level anomaly (in m) from Aviso on Jan 01 1993 in colour shading; Detected eddies: Contour lines show the detected eddy edge; * mark AE centres (high) o mark CE centres (low)

Eddy occurrence Number of days (between1993 to 2015) that have an eddy at a given location (AE & CE) (white regions are shallower than 200m) Number of eddies per latitude (AE: red, CE: blue) Start of results More eddies are counted in the South as they propagate slower there. They are also not interrupted by islands.

Eddy genesis Left: Location of Eddy genesis (AE and CE): West of islands 5° were added to the box towards the east so that it doesn’t appear like the eddies formed at the edge if they propagate into the region. Most eddies form inside the study region (green line), but many also propagate into the study region Right: Number of eddies that form at each latitude: red: form East of the study region, green: form inside Why? Explained on next slide…

Top: Location of eddy decay (AE and CE): in the lee of islands Eddies often decay when encountering an island. This explains, why eddies are generated west of an island (previous slide): a pervious eddy decayed, a new one can form Again, 5° was added towards the north and south, so they eddies don’t appear to die at the edge if they leave the region labels of bottom plot Bottom: Number of eddies that decay at each longitude: red: decay towards the South, blue: decay towards the North, green: decay inside the study region

Amplitude Left: Mean amplitude of AE and CE at a given location (absolute value) Right: Mean amplitude at each latitude: red: AE, blue: CE Higher towards the south, especially in the EAC. More eddies are higher in the South, because they live longer there (no interruption by islands). They are especially high in the EAC, as this Western Boundary currents creates strong and energetic eddies

Summary Number of eddies More eddies in the South Genesis In the lee (West) of islands or East of the study region More in the North Decay East of the islands and plateaus or move North or South of the study region Amplitude Mean: approx. 8 cm Higher in the South, especially in the EAC Add plots

Thank you, Vinaka vakalevu, Dhanyavaad & Dankeschön

Extra: Movie of the eddies in the study region https://youtu.be/PYSA8QMG5Gg Maybe you could run this while people ask questions, if it’s not too distracting?