Glider measurements of the Solomon Sea: A piece of the El Niño puzzle William S. Kessler (NOAA/PMEL, Seattle USA) A collaboration: Lionel Gourdeau (Institut de Recherche pour le Developpement, Noumea, New Caledonia) Russ Davis and Jeff Sherman (Scripps Institution of Oceanography, La Jolla USA)
(With inspiration from Patricia Pepena and Masio Nidung and help from the PNG MSR Committee) Outline of the talk • What a glider is and how it works • The big picture: circulation of the South Pacific Ocean • El Niño: why the Solomon Sea is important • What we have learned so far • Future plans. What would be useful to PNG? The ocean glider “Spray”: Schematic diagram
The Spray glider is developed and built by the Instrument Development Group at the Scripps Institution of Oceanography in La Jolla, Ca, USA.
2 meters long, weighs 50kg ⇒ The work can be done from small boats near shore, much cheaper than a ship. Cost to build: about USD50K How the glider propels itself
Temperature The glider is precisely Battery packs + Salinity balanced to be just barely measurements buoyant. Its only propulsion is a pump and external oil bladder. The pump inflates and Pump deflates the bladder like a balloon.
When the bladder is inflated, The battery packs move forward and back the glider becomes a little to control the orientation in the water. lighter, and floats up. When To rise, it points up as it floats up, and it is deflated, it sinks. glides forward, and vice versa. It moves very slowly (20 km/day).
The only propulsion is the external bladder. A pump inflates and deflates it like a balloon (but with oil not air). A dive of the Spray glider
A glider dives to 500-1000m, taking 3-5 hours, and moves forward about 2-4 km.
→ During a dive, it measures the
temperature and salinity. It reports the data by satellite each time it surfaces. 500-1000 m Estimate the currents by the
← glider’s drift: Measured current ← 3 km → Actual motion
20 cm/s Expected motion (3-5 hr) The South Pacific circulation
New Guinea Coastal Current
South Equatorial Current
East Australian Current
About half the SEC transport goes north through the Solomon Sea to the equator. The currents are huge: about 20 million cubic meters per second flows north through the Solomon Sea (roughly 2000 times as large as the Sepik River maximum flood). The 2 glider surveys done so far
Vitiaz St Solomon St New Britain Solomon Islands
Pa pua Ne Gizo
w Guinea
BudiBudi
Honiara Milne Ba y
Yela
Red = Aug-Nov 07 (Yela, PNG to Gizo, Solomon Islands) Green = Nov 07-Feb 08 (Honiara to Gizo via Yela) A first guess at the currents, based on the 2 surveys to date
Vitiaz St Solomon St New Britain Solomon Islands Pa ? pua Ne Gizo
w Guinea
BudiBudi
Honiara Milne Ba y Eddies?
Yela
Red = Aug-Nov 07 (Yela, PNG to Gizo, Solomon Islands) Green = Nov 07-Feb 08 (Honiara to Gizo via Yela)
Ship and glider currents in Milne Bay, PNG 3 straits ~20km wide, 400m deep Land
Kiriwina
Murua D’Entr BudiBudi ecasteaux Is.
PNG
Yela
(Green vectors are from a 1993 ship survey, Blue from the glider) Kiriwina
Murua (Woodlark) BudiBudi
Milne Bay
Kiriwina Murua BudiBudi
4x106m2 6x106m2 9x106m2 10x106m2
6 3 8x10 m /s = 28cm/s average flow through the straits 29x106m2 The climate importance of Solomon Sea currents: El Niño
● The warmest water is not necessarily at the equator! ● There is a roughly 5ºC temperature contrast from west to east. ● Winds blow from the cooler to the warmer water, and converge on the West Pacific Warm Pool. Schematic diagram of the coupled interaction along the equator: The normal situation
Trade Winds Papua New Warm Water heated by the sun Pool Guinea Upwelling South America
Thermocline
Cool lower water W.S.Kessler, NOAA/PMEL Schematic diagram of the coupled interaction along the equator: El Niño El Niño produces a net draining of the warm layer across the Pacific
Persistent westerly wind Trade Winds Papua New Warming Guinea Upwelling South Warm pool America sloshes east
Thermocline deepens in the east and flattens Cool lower water W.S.Kessler, NOAA/PMEL
New Britain rainfall and El Niño New Britain rainfall is about 5-10 cm/month lower during and after an El Niño
This is due to the eastward shifting of the region of warmest water and rainfall. Solomon Sea temperatures and El Niño
La Niña 1988 86-87 97-98 XBT track (Merchant ship)
94-95 El Niño 91-92 Great benefit around the world if these events (and their subsequent effects) could be accurately predicted.
NE Brazil El Niño droughts used to reduce crop yields by 75%. With predictions, famers plant drought-tolerant beans and crop losses are much less. On the other hand, predictions are not always correct (Australia 1997). The Southern Oscillation Index gives a time history of El Niño It is highly irregular and hard to predict!
(The SOI is based on the atmospheric pressure at Tahiti and Darwin, Australia) What does this have to do with the Solomon Sea?
During El Niño, the warm pool drains east, then out Out Warm pool drains east Out
Refilling Westward in subtropics The refilling of the warm pool happens in the far west, a few Refilling months after El Niño’s peak Westward in subtropics
Cannot have another El Niño until the warm pool is refilled South Equatorial Current transport SEC has a strong El Niño signal SEC on the Auckland-Japan XBT track, over10°S-20°S. Demeaned.
La Niña = Weak SEC /s) 3 m 6 t (10 SOI ranspor T
El Niño = Strong SEC
South Equatorial The Southern Oscillation Index Current flow (time history of El Niños) Where are we now? A La Niña began in late 2007. We would expect a weak NGCC. The biggest picture is the flow around Australia-PNG
N. Pacific Ocean
Indian Ocean S. Pacific Ocean
Shallower Deeper Future plans Funded (NOAA/Scripps/IRD) for 2 more deployments this year ➙ Redeploy in Mar 08, then May 08. Recover in Aug 08 after sampling a complete annual cycle...... Digest results, then propose ongoing monitoring. Explore further north? What would be useful to PNG? Looking for guidance and collaborators.
Vitiaz St Solomon St Bougain New Britain Solomon Islands
ville
V
Pa V V pua Ne Late 2008 to 2009? V The Slot w Guinea
V
Guadalcanal
V Milne Ba V y Mar and May 2008 V
V ● Still experimental! But proof of concept that the glider can measure the NGCC. ● The Solomon Sea is a crucial area whose currents may give important predictability for the turns of the El Niño cycle and other climate signals. These currents will be difficult to monitor except by instruments that can control their position. Extra Figures Follow ... A B C D E F G
D E C F B G
A
ORCA model circulation at surface, thermocline and below
Above 100m: Flow through Sol. St. is southward. (Consistent w/ obs).
Thermocline level: Sol. St. flow is northward (Pacific inflow ~1/2 total).
Below Vitiaz St: Entire WBC exits Solomon Sea via Sol. St. (No Pacific inflow). Shortland Vector absolute The Slot current above 500m (Tide-filtered) Ranongga
50 Raw v 40 Filter BudiBudi 30 ed
20
cm/s Remainder 10
0 Milne
-10 Bay
237 238 239 240 Day of 2007
Tide-filtering by a Gaussian objective mapping on time with a time-scale of 1.5 days. Yela
(Every other vector plotted) OGCM meridional current at 10°S with and without an ITF:
(Difference = effect of closing ITF)
Lee et al (2002) MIT OGCM The glider observations show that the NGCC is weakening
Vitiaz St Solomon St New Britain Solomon Islands
Pa pua Ne Aug 07 Jan 08 Gizo w Guinea
BudiBudi
Honiara Milne Ba y
Yela
The transport of the NGCC declined by about 60% between August 2007 and January 2008 as the La Niña grew Shipboard and glider section between Guadalcanal and New Caledonia A coordinated experiment July-October 2005, glider repeated Nov 06-Mar 07 (Gourdeau et al, in press, JPO)
PNG Solomon Solomon Is. Sea G ?
NVJ NFJ Coral South Sea Equatorial NCJ Vanuatu Current Fiji SFJ Australia NC
200m isobath shown Glider track Cruise track (profiles) Schematic diagram of the coupled interaction along the equator: El Niño onset
Opposing wind Trade Winds Papua New Warm Water heated by the sun Guinea Pool Upwelling South America
Thermocline
Cool lower water W.S.Kessler, NOAA/PMEL First mission: A coordinated experiment to study the South Equatorial Current NVJ Jul-Oct 2005 V an uatu ● A shipboard section made 14 profiles to 2000m. ● A glider section made dense profiles to 600m. ● An Argo float drifted NCJ through the NCJ.
Strong near-coastal circulation Salinity and Oxygen on Sigma-theta = 27 CARS data. Overlay streamlines on isopycnal
At sigma 27 (~6-800m), the sparse available data suggests that a
low-S, high-O2 tongue penetrates out of the Solomon Sea into the equatorial Pacific via the Australian WBCs. Glider monitoring of the Solomon Sea Funded for 4 deployments starting Aug 07
Vitiaz St Solomon St Bougain New Britain Solomon Islands ville
Pa pua Ne The Slot w Guinea
Guadalcanal Milne Ba y
BudiBudi Is Yela Is Shortland Is Ranongga Is Honiara Crosstrack transport accumulated from Rossel Is.
A D
C B (8.0 Sv) B
A
C(15.5 Sv) D (18.2 Sv)