Circulation through the Solomon Sea ... origins and consequences William S. Kessler (U. Washington, Seattle USA) Sophie Cravatte (IRD, Nouméa NC) Frédéric Marin (IRD, Nouméa NC) Russ Davis (Scripps, La Jolla USA) Davis, R.E., W.S. Kessler and J.T. Sherman, 2012: Gliders measure western boundary current transport from the South Pacific to the equator. J.Phys.Oceangr., 42(11), 2001-2013. Cravatte, S., W.S. Kessler and F. Marin, 2012: Intermediate jets in the tropical Pacific Ocean observed by Argo floats. J.Phys.Oceanogr., 42(9), 1475-1485. Kessler, W.S. and S. Cravatte, 2013: ENSO and short-term variability of the South Equatorial Current entering the Coral Sea. J.Phys.Oceanogr., in press. Kessler, W.S. and S. Cravatte, 2013: Mean circulation of the Coral Sea. J.Geophys.Res., submitted. Collaborators: With thanks to Stuart Godfrey, Ernest Frohlich, ‣ Solomon Islands Meteorological Service and CSIRO Hobart .... who started me on this path in 2001 ‣ University of Papua New Guinea ‣ Bureau of Meteorology (Australia) The Spray glider is essentially an Argo float Antennas in wings with wings and movable batteries Moveable battery packs control orientation and The Spray glider is developed and built by the flight CTD Instrument Development Group at Scripps. GPS Iridium (Aft of wings shown rotated 90°) Very dense sampling (~ resolve tides) Argo-comparable T-S profiles: geostrophic relative currents 000 m 1 Infer vertical-average absolute currents We operate 500- by the glider’s drift: to 700m Inferred current Actual motion 3 km (3-4 hr) Range 4-5 months at 20 cm/s = 2000+km Motion relative to the water Davis, Kessler and Sherman (2012) The only propulsion is the external bladder. A pump inflates and deflates it like a balloon (but with oil not air). South Pacific average circulation Western Boundary ~25Sv Interior Geostrophic ~15Sv Ekman Divergence ~30Sv New Guinea Coastal Current Island Rule (ERS winds) South Equatorial Current East Australian Current Zoom in Interocean net ~10Sv Next zoom PNG Solomon Solomon Is. Sea Glider experiments cross the Solomon Sea NVJ NFJ Coral South and measure the flow Sea Vanuatu Equatorial NCJ towards the equator. Current Fiji SFJ Australia NC Glider currents in the Solomon Sea 21 missions since mid-2007 (just deployed #22-23) (2 failures ... but recovered the gliders) Davis, Kessler and Sherman (2012) Glider currents in the Solomon Sea Vectors = 0-700m average Shallow currents Deep currents Zoom From NQC From SEC Davis, Kessler and Sherman (2012) Vertical-average currents at the tip of PNG The most consistent observation is a strong current towards the equator at the tip of Louisiade Chain the Louisiades. Rossel The current is very Sudest close to the reef line. Davis, Kessler and Sherman (2012) Vertical-average currents at the tip of PNG Now, we’ll look at the vertical structure of the NGCU .... four 100km sections (light purple lines) Louisiade Chain Rossel Tagula Davis, Kessler and Sherman (2012) The equatorward flow is an Louisiades undercurrent Currents at the tip of the Louisiades Each section shows 100km from the coast on the left. Glider obs Dec 08-Jan 09 Opposite flow in upper layer 22 23 24 Note isopycnal spreading at 25 the coast (white) 26 Maximum 80 km near 450m 27 Positive (red) = Equatorward Davis, Kessler and Sherman (2012) How to be quantitative with the Contours of Φ (by 0.1) irregularly-distributed glider tracks? 2 Define a function Φ, such that: ∇ Φ=0, and: 0.4 0.2 Φ=0 at PNG coast, Φ=1 at Solomons coast. Consider velocity parallel to Φ contours “equatorward” 0.6 0.8 Mean From FromSEC eq-ward SEC uϕ Depth (m) NGCU core 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Define northern EOF 1 and southern tracks Variability is surface-intensified, dominantly ENSO 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Davis, Kessler and Sherman (2012) By contrast, the Mindanao Current is surface-trapped, and shallow Average !" (ADCP) Reversed shear only above 100m Southward shear at 250-550m –90 –10 –80 –40 –70 –30 –60 –50 5° Wijffels, Firing and Toole (1995) 13° Solomon Sea transport has a strong ENSO cycle Lags SOI by “a few months” Individual transport samples and SOI SOI (inverted) Transport (Sv) Transport Transport anomalies and SOI SOI (inverted) Transport anomalies (Sv) anomalies Transport (Also high correlation between glider transport anomalies and AVISO velocity) La Niña transport anomalies ENSO modifies western boundary transports: La Niña tends to weaken the WBC in the west Rossby Anticyclonic curl Downwelling curl east of the Solomon Sea during La Niña: northward interior Sverdrup flow ... Rossby waves ... → expect southward WBC anomalies a few months later. ENSO variation of SEC at ~160°E is surface-intensified Examples of ENSO signal (ug160°E) 1.5-layer Rossby model gives much of the transport a b SOI XBT line (green) and schematic circ. c. La Niña d. El Niño Fig. 5. a) Time series of transport anomalies (Sv) from the Rossby model (black), XBT data (green) and Argo (red). The XBT and Argo time series are the same as in Fig. 3. b) Southern Oscillation Index, where red and blue shading indicates El Niño (SOI less than −1) and La Niña (SOI greater than +1) respectively. c) Mean wind stress vectors and curl (colors, scale at right) averaged over the La Niña periods shown in panel (b). Units given at bottom. d) As panel c but for El Niño periods. Kessler and Cravatte (2013) What are the sources of the two Solomon Sea inputs? Upper layer Highly schematic circulation Subthermocline Bismarck Sea Papua New New Britain ? Pacific Ocean Guinea Solomon Solomon Islands Sea Louisiade Archipelago Coral Sea Bathymetry: 1000m, 500m, 100m Two distinct high-salinity tongues At the level of the subtropical high-S max At the base of the thermocline (NVJ) (NCJ) 35.2 36 35.15 35.9 35.1 35.8 35.05 35.7 35 35.6 34.95 35.5 34.9 σ 24.75 35.4 σ 26.5 Mean z = 160m Mean z = 400m 34.85 35.3 Subducted in the eastern subtropics Subducted in the Tasman Front (~ 20°S,120°W) (~ 35°S,160°W) Enters the Coral Sea north of Vanuatu Enters the Coral Sea in the N. Caledonian Jet Splits, flowing: Flows as a western boundary current - directly into the Solomon Sea, and - along the Queensland and PNG coasts - south into the EAC. into the Solomon Sea. Kessler and Cravatte (2013) Salinity anomalies on isopycnals: 2 glider sections from the Solomons Louisiades Glider 2007-2011 Solomons Glider Jul-Oct 2005 N. Caledonia Salinity anomalies from section means Sigmatheta Sigmatheta Glider sections from the Solomons to: - New Caledonia (Jul-Oct 2005) - Louisiades (15 sections, 2007-ongoing) σ23-25 (green lines at right) σ25.5-27 High-S tongues carried across and around the Coral Sea at shallow and mid-depth isopycnals Kessler and Cravatte (2013) Absolute geostrophic currents CARS ug referenced with Argo trajectory motion (more later) d e f σ24.5 c g a: EAC σ26.5 b: EAC b c: GPC(GBRUC) h d: GPC (NQC) e: GPC (Hiri) f: NGCU g: NVJ h: NCJ i: SCJ σ27.2 a Current speed (u or v) (cm/s) i Kessler and Cravatte (2013) If NGCU is subthermocline, and (relatively) invariant at ENSO timescales, where does that water go? Upper layer Highly schematic circulation Subthermocline Bismarck Sea Southern Tsuchiya Jet? Papua New New Britain ? Pacific Ocean Guinea Solomon Solomon Islands Sea Louisiade Archipelago Coral Sea Bathymetry: 1000m, 500m, 100m 188 W.S. Kessler / Progress in Oceanography 69 (2006) 181–217 a large upwelling transport was impossible, commenting ‘‘the amount of water upwelled along the equator certainly does not exceed a few 1012 cm3/s [Sv], otherwise the thermal structure would break down’’. There- fore, he postulated that the EUC water must move into the lower levels of the SEC without rising to the surface. Wyrtki corrected himself 15 years later with a since-confirmed estimate of 50 Sv of upwelling over 170–100°W, based on the meridional divergence of Ekman and geostrophic transports (Wyrtki, 1981). A remaining item of confusion that was not cleared up until much later concerned the spreading of isotherms about the EUC (Fig. 6 and Section 4). It was thought at the time that this spreading must be a signature 188 W.S. Kessler / Progress in Oceanography 69 (2006) 181–217 of vertical mixing, which seemed reasonablea large upwelling in transport such was a impossible, strong commenting current. ‘‘the However, amount of water turbulence upwelled along measurements the equator certainly does not exceed a few 1012 cm3/s [Sv], otherwise the thermal structure would break down’’. There- fore, he postulated that the EUC water must move into the lower levels of the SEC without rising to the beginning with Gregg (1976) showedsurface. that Wyrtki mixing corrected is inhimself fact 15 years very later weak with a since-confirmedat the EUC estimate core; of 50 instead Sv of upwelling being over strong 170–100°W, based on the meridional divergence of Ekman and geostrophic transports (Wyrtki, 1981). A remaining item of confusion that was not cleared up until much later concerned the spreading of isotherms about the EUC (Fig. 6 and Section 4). It was thought at the time that this spreading must be a signature uADCP (colors)of vertical and mixing, T which (contours) seemed reasonable in such a strong at current. 125 However,°W turbulence measurements Tsuchiya Jets overview beginning with Gregg (1976) showed that mixing is in fact very weak at the EUC core; instead being strong SEC SEC NECC NEC Tsuchiya Jets flow across the entire Pacific EUC at about 13°C (2-300m), feeding upwelling TJ TJ in the Costa Rica Dome and Peru coast.