HOW IS THE SOLOMON SEA IMPACTED BY ENSO ? A. MELET1,2,, L. GOURDEAU 3,, N. DJATH1, W. KESSLER4, J. VERRON1 1. CNRS/LEGI, France 3. IRD/LEGOS, France 2. Currently at Princeton University / GFDL, USA 4. NOAA/PMEL, USA

1. Movaon 2. Methodology 3. Thermocline circulaon and ENSO The interannual variaons of the Solomon Sea WBC The Solomon Sea is a key region of the southwest Pacific where Low Latude An original modeling strategy based on model nesngs (Fig 2) has been thermocline transports are related to ENSO. A Western Boundary Currents (LLWBCs) connect the subtropical and equatorial implemented to realiscally resolve the complex bathymetry of the comprehensive diagnosis of ENSO influence on the circulaons through narrow straits (Fig 1). Since the LLWBCs are the main sources Solomon Sea, notably the network of narrow straits connecng it to the thermocline circulaon is performed through the of the Equatorial Undercurrent (EUC), they could play a major role in the low Equatorial Pacific. construcon of composite anomalies for El Niño (Apr 87, frequency modulaon of El Niño Southern Oscillaon (ENSO). Therefore, the A 1/12° model of the Solomon Sea is interacvely nested (AGRIF 92, 98, Jan 2003) and La Niña (Apr 89 & 97, Jan 2000) Solomon Sea is of parcular interest in a climac context and is a focal point in soware, 2-way) in a regional ¼° model of the southwest Pacific, itself states. The ENSO related thermocline circulaon the South PacifIc Circulaon and Climate Experiment (SPICE). In recent work, the embedded in a ¼° global simulaon (Drakkar project). variability in the Solomon Sea mainly consists in a mean circulaon and the seasonal cycle have become beer known from modulaon of the WBC strength. Due to the ENSO wind modelling studies (Melet et al. 2010a, 2011, Fig 3) whose results are confirmed curl anomalies, the NGCU (Fig 3) strengthens during El m2/s by increasing observaonal evidence from in situ (Cravae et al. 2011) and Niño and weakens during La Niña (Fig 4). Viaz Strait (Fig NEMO-OPA code 1/4° Fig 3. Solomon Sea circulaon vercally integrated satellite data (Melet et al. 2010b). But, as shown by almetry, the highest 4, 5) thermocline transport exhibits a weak interannual 46 z-levels with global over the thermocline (24.0≤σ ≤ 26.5) from a 1/12° variability is at interannual me scales and represents a response of the LLWBCs variability: the NGCU variaons are mainly transmied to 0 paral step model (see secon 2) for the 1986-2004 period. in phase with ENSO that counterbalances equatorial-driven changes in western the NBCU (Fig 3) and Solomon Strait (Fig 4). Pacific warm water volume. At this stage, lile is known about the sensivity of + Bathymetry : the Solomon Sea circulaon to ENSO. In this work we invesgate how the 1/4° GEBCO pathways of the LLWBCS and their water masses are affected by ENSO. Southwest Solomon St Etopo2v1 Pacific Viaz St Fig 1. Origins of water parcles (handmodified emerging in June 1998 in the for the 1/12° grid) El Niño La Niña Nino-3 region in a numerical + simulaon (Izumo et al, 2002). Forcing : EUC The Solomon Sea is located in CORE ERA40 + NGCU 1/12° South inflow SEC the black rectangle. Simulaons Solomon EUC : Equatorial UnderCurrent 2 over Sea m /s SEC: South Equatorial Current 1984-2004 Fig 4. Composite of the anomalies of the circulaon integrated over the thermocline (24.0≤σ ≤ 26.5) in m2/s for El NGCU : New Guinea Coastal 0 Fig 2. Hierarchy of models used to reach high- Nino condions (le) and La Nina condions (right). Anomalies are relave to the 1986-2004 mean (Fig 3). UnderCurrent resoluon modeling of the Solomon Sea. The asymmetry of the composites of El Niño/La Niña anomalies (Fig 4) suggests a bathymetric 4. Temperature modificaons control at Viaz Strait. To test this assumpon, a sensivity simulaon was run in which Viaz St was SOI Transit me The variability of the temperature of The temperature modificaons from the south inflow to widened (from 42km at 200m depth to 67km). A the water masses entering the Viaz St were analyzed in a Lagrangian framework. lagrangian analysis was used to analyze the paron Solomon Sea is maximum in the Numerical parcles were seeded in the Solomon Sea south of the thermocline south inflow between Viaz and upper thermocline, at σ0 24.4 (Fig 6). inflow (10.5°S, Fig 4), between σ0 24.2 and 24.6, and Solomon straits (Fig 4), and its response to the integrated forward in me. Only the parcles joining Viaz bathymetric constraint. Fig 5 shows that when Viaz St were selected. Fig 7a shows that the interannual St is widened, the interannual variability of the variability of the transit me between the south inflow transport through this strait is increased while the and Viaz St is strongly correlated with the SOI (proxy for ΔT Transit me one through Solomon St is weakened. Thus, the Fig 5. Interannual anomalies of thermocline transport ENSO), in agreement with the strenghening/weakening of transport limitaon at Viaz St impacts the paron (Sv) for water inially at the south inflow between σ0 24.0 the NGCU during El Niño/La Niña (Fig 4). The Lagrangian of the south inflow between the different and 26.5 for the: analysis allows to follow the temperature variaons along equatorward straits. It plays a stronger role during El - regular Viaz St bathymetry : Viaz St, Solomon St. the path from the south inflow to Viaz St. fig 7b shows Niño, when the NGCU is strenghened. - widened Viaz St : Viaz St, Solomon St. that the thermocline temperature modificaons are Fig 6. Secon of the RMS of the weaker during El Niño (SOI < -0.5), with weak warming, temperature (in °C) at the south whereas a cooling is observed during La Niña (SOI > 0.5). Fig 7. Interannual variability of the transit me between the 5. Conclusions inflow (10.5°S, Fig 4) from the This could be related to the me transit anomalies : the south inflow and Viaz St (in blue for the two plots, unit in The highest variability in the Solomon Sea circulaon is at the interannual me scale, in relaon to ENSO. In this 1986-2004 monthly fields. The y-axis longer the water stays in the Solomon Sea, the more it days, le axis). In red : (top) SOI and (boom) temperature study, we invesgate the variability of the Solomon Sea circulaon at thermocline level (which is where the WBCs is the density. could be modified. difference (ΔT, °C) between Viaz St and the south inflow. connect to the EUC, Fig 1) and the modificaons of water masses in response to ENSO using both Eulerian and Lagrangian analysis of a high-resoluon simulaon (Fig 2). We show that although the subtropical water entering References the Solomon Sea from its south inflow preferenally join the Equatorial Pacific through Viaz St in the mean state, Cravae S.et al. 2011 « Observed circulaon in the Solomon Sea from ADCP data », Progress in Oceanog, 88, 116-130. the interannual variability is mainly transmied to Solomon St, especially during El Niño events (Fig 4). This is Izumo T. Picaut J. Blanke B. 2002 « Tropical pathways, Equatorial Undercurrent variability and the 1998 La Niña »,Geophys Res Le, 29. partly due to transport limitaon through Viaz St (Fig 5). During El Niño, the NGCU is increased, leading to shorter Melet A. Gourdeau L. Kessler W. Verron J. Molines J.M. 2010a «Thermocline circulaon in the Solomon Sea: A modeling study », J Phys Oceanog, 40,1302-1319. transit mes of water masses from the south inflow to Viaz St (Fig 7). As a result, the cooling of the upper Melet A. Gourdeau L. Verron J. 2010b « Variability in Solomon Sea circulaon derived from almeter sea level data », Ocean Dyn, 60, pp 883-900. thermocline water, originang fromthe subtropics and concentrang the highest temperature interannual Melet A. Verron J. Gourdeau L. Koch-Larrouy A. 2011 « Solomon Sea water masses pathways to the Equator and their modificaons », J Phys Oceanog, 41, 810-826. variability, is reduced (Fig 7). The reverse is observed during La Niña.  Correspondance : [email protected], [email protected]