Exploring the mesoscale activity in the Solomon Sea: a complementary approach with a numerical model and altimetric data L. Gourdeau 1, J. Verron 2, A. Melet 1, W. Kessler 3, F. Marin 4, and B. Djath 2 1) IRD/LEGOS, Toulouse, France 2) LGGE, Grenoble France 3) NOAA/PMEL, Seattle, USA 4) IRD/LEGOS, Nouméa, New Caledonia

A study in the context of the SPICE program Decadal climate variability Connection subtropics to equator and Tasman Sea through WBCs Solomon Sea

Solomon Islands New Britain Papua New Guinea Solomon strait Vitiaz strait ? ? ? St Georges Strait Solomon Sea Unusual western boundary geometry:  5° latitude barrier in front of continental boundary  WBC transport may be limited by narrow straits An ongoing effort to understand the Solomon Sea circulation and its variability: - High resolution modelisation (1/12°, 1/36°) - In situ observation (Cruises, Glider experiments …) - Altimetry

A complex mean Circulation ( Melet et al.,2010a; Cravatte, 2011; Hristova, 2012) NGCC/NGCU mean transport from the 1/12° global DRAKKAR NICU Recent works A high variability area at: Seasonal variability ( Melet et al., 2010a, 2010b ) Interannual variability ( Melet et al., 2013; Davis et al., 2012 ) A place of water mass transformation ( Melet et al., 2011 ) A question: How turbulent is the Solomon Sea and for which impacts?

The Solomon Sea Suspected to be an area of high mesoscale activity but nothing is known Black arrows: Surface current from AVISORed arrows: Cross track surface current from glider Some evidence from altimetry and glider

Objective To give a first synoptic view of the mesoscale variability in the Solomon Sea ● EKE ● Eddy detection ● Seasonal and interannual eddy modulations ● Hypothesis for process Based on: ● model: 1/12° global DRAKKAR ● AVISO DT maps

Mean Eddy Kinetic Energy Thermocline layers ● Lower EKE level than at the surface ● In relation with the NGCU ● Interaction bathymetry/LLWBC NGCU

Mean Eddy Kinetic Energy Surface layers ● Model and altimetry: similar EKE level ● High EKE level (up to 400 cm 2 /s 2 ) ● EKE higher in the Northern basin ● In relation with the Solomon Strait Inflow (SSI) SSI

● CEs along the central Solomon Sea ● More CEs in the north Eddies detected at the surface Occurrence of CEstrajectory of CEs Nbre of eddies/month in 1°x1° box Two CEs types: ● propagative CEs generated in the south ● Stationary CEs generated in the north Cyclonic Eddies

Nbre of eddies/month in 1°x1° box Eddies detected at the surface Occurrence of AEstrajectory of AEs ● Less AEs than CEs ● Generated in the east and southeast ● Propagating westward Anticyclonic Eddies

Modulation in time of the mesoscale activity EKE Seasonal and interannual variability

Seasonal modulation EKE ● EKE in phase in the Solomon Sea ● Maximum in April-June ● Minimum in October-December SSI NVJ NGCC

● EKE seems driven by instability of the SSI (southward flow) ● When SSI is strong  Vertical/horizontal shears increase  EKE is maximum ● Barotropic/baroclinic instabilities?? Which mechanisms ?? Time/longitude of EKE and meridional velocity at 8°S NGCCSSI

Interannual variability: ENSO mode EKE ● EKE out of phase between the north and the south Solomon Sea ● EKE South in phase with the SOI (r=0.81)  EKE increases during La Niña ● EKE North lags El Niño by 6-8 months  EKE spikes during the El Niño/La Niña transition

Interannual variability: ENSO mode Eddy activity ● AEs in phase with the SOI (r=0.67)  in relation with EKE South  More AEs during La Niña than during El Niño ● CEs lags El Niño by 6-8 months  in relation with EKE North

● El Niño: The NGCC increases The SSI decreases  Low EKE ● La Niña: The NGCC/NVJ decreases The SSI increases  High EKE (AEs) in the South ● Transition phase (El Niño to La Niña): Strong NGCC Strong SSI  High EKE (CEs) in the North Interannual variability: ENSO mode Circulation anomalies in the surface layers Melet et al NGCC SSI NGCC NVJ Melet et al., 2010b Current anomalies and EKE during the transition phase between the El Nino and the La Nina

Conclusion ● A first description of the mesoscale activity in the Solomon Sea ● Coherent results between model and altimetric data ● High EKE level linked with eddy activity ● Cyclonic and Anticyclonic eddies behave differently ● Mesoscale activity modulated at seasonal and interannual timescale ● Linked with current instabilities: a main trigger appears to be the Solomon Strait Inflow more than the LLWBCs Gourdeau et al. (2014), JGR

● Need to work on the process ● What is the vertical structure of eddies? ● Is there an impact on water mass transformation? ● What is sub mesoscale? Looking at glider data A 1/36° Model Conclusion Djath et al. 2013, 2014

A pathway for Subtropical Cells Implication for ENSO and its low frequency modulation through changes in LLWBCs transport and properties Solomon Sea Transit zone for tropical/subtropical waters feeding The western Pacific warm pool and the EUC Lagrangian study from a numerical simulation: 87% of the EUC water comes from the western boundary currents 60% from the Solomon Sea 87% Grenier et al. 2011

The Solomon Sea Suspected to be an area of high mesoscale activity But nothing is known cm/s Cross track geostrophic velocity (positive to left of track, shaded) Potentiel density (minus 1000, contoured) Some evidence from altimetry and glider Black arrows: Surface current from AVISORed arrows: Cross track surface current from glider