1. The global XBT network Molly Baringer (1), Gustavo Goni (1), and Dean Roemmich (2) (1) NOAA/AOML, Miami, FL (2) SIO, La Jolla, CA NOAA Climate Program Office Climate Observation Division 8th Annual PI Meeting Annual System Review Meeting Washington, DC June 25-27, 2012

2. XBT Network: Program goals: mesoscale resolving global array of repeated transects, real-time data delivery, providing synergy with other platforms U.S. roles in the XBT Program; Ocean Obs ‘09 recommendations, Strategy for meeting the program’s goals. Users and uses of XBT data : research, operational applications, ocean/climate assessment Challenges for the XBT network, technology improvement

3. The global XBT network OceanObs09 Recommended Transects High Density (HD) – mesoscale resolving, 4 times per year Frequently Repeated (FR) – 100-150 km spacing, 12-18 times per year Low Density – no longer recommended

4. The global XBT network International Collaboration with NOAA France: AX20, AX01, AX02 South Africa: AX25, AX08, AX18 Brazil: AX97 Argentina: AX18 Australia: IX01, IX22 Italy: MX01, MX02, MX4 More than fourteen institutions collaborate on collection, quality control, and science from XBT data including: US (NOAA, SIO), France (IRD, UP), Australia (ABOM, CSIRO), South Africa (UCPT), Japan (JMA), Brazil (FURG, Navy), Italy (ENEA), India (NIO), Germany (BSH), Argentina (SHA)

5. The global XBT network Deployment/Observations 20012002200320042005200620072008200920102011 XBT2523 2925222021 1918 Argo71731477296110121126121126 200120062011 125K 20K XBTs Deployed (Red); Argo Deployed (Blue) Number of profiles on the GTS in units of 1,000

6. Value of the HD (HRX) Network Ocean circulation: – The HD (HRX) Network samples the boundary currents and the ocean interiors at high spatial resolution for transport estimation. – It provides a tool for integrating the observing system. – The combination of XBTs, Argo, and altimetry mitigates the limitations of the individual datasets. Mass, heat, and freshwater budgets: – Estimates of reference velocities, heat storage, vertical advection, and Ekman transport are all much improved in the past decade. – Time mean balances have good confidence. – Time varying balances remain a challenge, but progress is being made.

7. HRX transects are sampling: Kuroshio (3 HRX tracks), Gulf Stream (3 HRX tracks) Agulhas, Brazil Current, East Australian Current (2 HRX tracks) Eastern boundary currents (California Current, Leeuwin Current, …) Low latitude WBCs: Solomon Sea, Indonesian Throughflow Antarctic Circumpolar Current (3 HRX tracks) … AOML status map : Boundary currents sampled by the HRX Network 1.Global in scope (i.e. all 5 subtropical WBCs) 2.Enhanced BC sampling is highest priority, OO’09. 3.Argo provides complementary absolute and/or deep relative reference level velocities. 4.The HD (HRX) Network integrates the BCs and interior. The High Resolution XBT (HRX) Network samples the world’s boundary currents - the subtropical WBCs and EBCs, the low latitude WBCs, and the ACC. Boundary Current Array

8. Goal: The global network Of currents from XBTs Agulhas Current at 28  S: IX21 (1994) Leeuwin Current 32  S: IX15 (1987) Indonesian Throughflow: IX01 (1987) Upstream Kuroshio Current: Upstream: PX44 (since 1991) Downstream: PX05 (2009) East Australian Current: at 27  S: PX30 (1991) at 33  S: PX34 (1991) East Auckland Current and Tasman outflow: PX06 (1986) Solomon Sea current system: PX05 (2009) California Current System: Undercurrent: PX37 (1991) California Current: PX37 (1991) Alaska Current: PX38 (1993) Antarctic Circumpolar Current South of Tasmania: IX28 (1993) Drake Passage: AX22 (1996) South of South Africa: AX25 (2004) Gulf Stream: AX10 (1997), AX32 (1981) Florida Current: AX7 (2000) North Atlantic Drift Current: AX01 (1997) Labrador Current: AX02 (2010) Atlantic Ocean Equatorial Current System: AX08 (2000), AX20 (2010) Brazil Current: AX97 (2004) Brazil/Malvinas Confluence: AX18 (2002) Benguela Current and Agulhas Current Rings: AX18 (2002) and AX08 (2000)

9. Zonal currents in the Tropical Atlantic NECC NEUC SEUC SECC NEC nSEC cSEC sSEC Goni and Baringer, 2002

10. PX37S Line 90 PX37S Line 90 PX37S Argo Transport PX37S The “real” boundary current is the northward California Undercurrent, not the southward California Current. Argo Steric Height 0/2000 Integrating the ocean observing system: HD (HRX), Argo, CalCOFI Courtesy of D. Roemmich

11. Geostrophic volume transport in subtropical Pacific Integrated transport: Black: Argo RG high resolution Red: HRX during Argo era (29 cruises) Dark blue: HRX, all Hong Kong (44 cruises) Argo era: 29 cruises; mean -12.9 Sv; σ = 3.66 Sv; Std error = 0.7 Sv Differences between PX37 and Argo: At high spatial resolution Argo has larger errors in the temporal mean Argo misses the northward EBC Courtesy D. Roemmich and J. Gilson

12. Example: Northward Heat Transport in SA (AX18) Garzoli and Baringer (2007) Baringer and Garzoli (2007) Both geostrophic and Ekman transports experience annual cycles, but they are out of phase.  Geostrophic transport controls the total northward heat transport.  Geostrophic and Ekman transports experience comparable variability Total = 0.51  0.15 PW Geos. = 0.40  0.16 PW Ekman = 0.11  0.16 PW …now using altimetry …e.g. Brazil current

13. On average 23 publications a year are published using XBTs as the primary data source. The global XBT network Scientific Publications

14. SIOAOML NOAA Role in the global XBT network NOAA funds approximately 60% of XBTs, while international partners aid in the actual deployments E.g. of the 11 HD transects done by AOML, international partners deploy XBTs on 9 lines.

15. The Future of XBTs: 15.Create an international science panel for upper ocean thermal observations to support and evaluate recommendations of the integration of the different platforms, including XBTs (XBT Science Team created) Gouretski and Reseghetti, 2010 1.Fully implement and maintain the XBT network as recommended in OceanObs99 (phase out of LD, increases HD) 2.Expand transects to include interior and marginal seas, such as the Mediterranean Sea and the Gulf of Mexico (Med Sea expansion, no Gulf of Mexico) 8.Support technological improvements (underway) 9.Implement XBT calibrations based on CTDs (underway) 13.Continue XBT data analysis for scientific studies and increase its operational applications

16. The global XBT network New technology Climate quality XBT with two pressure switches, which trigger signal at predetermined depth (By Sippican) First test with improved thermal sensor was carried out in 2012 on the Western Boundary Time Series cruise. < 1 m error

17. Challenges Resources: Near level funded has forced a more rapid transition to HD with a reduction in FR (and all LD). Spatial Coverage: Deployment opportunities are limited. For example transects in the Indian Ocean are extremely difficult, Ax18/Ax18* in the South Atlantic. Technical failures (e.g. variable fall rate): Quick detection and correction is essential. Important to have synergy of multiple platforms with different, enhancing goals. System Integration: Many other platforms require XBT program for logistical support (e.g. Argo float deployments, drifter deployments, pCO2/TSG calibrations and maintenance, weather service Met messages and US Coast Guard Amver alert system).

18. The global XBT network summary 1.XBT network provides 20,000 T(z) profiles each year globally. 2.Network has transitioned away from LD and FR towards HD. 3.Science emphasizes HD transects: Monitoring currents Monitoring heat budgets, transport, regional balances 4.In the future: HD (HRX) network forms the backbone of a boundary current observing system FRE studies refine corrections for historical XBT data Probe improvements, T sensors, pressure switches produce climate quality measurements 5.First XBT Science Workshop (Australia, 2011) Highlight scientific accomplishments XBT Science Steering Team