Tropical Atlantic Climate Experiment (TACE) TACE “White Paper”:

1. Tropical Atlantic Climate Experiment (TACE) TACE “White Paper”: http://www.clivar.org/science/atlantic.htm Authors: F. Schott, J. Carton, W. Hazeleger, W. Johns, Y. Kushnir, C. Reason, S-P. Xie Contributors: B. Bourles, D. Dommenget, M. Latif, P. Malanotte-Rizzoli, C. Zhang TACE Synopsis: http://www.clivar.org/science/atlantic.htm#NEWS

2. History leading up to TACE: • COSTA Workshop (May 1999, Miami) • “Subtropical Cells” Workshop (Oct. 2000, Venice) • CLIVAR Workshop on TAV (Sep. 2001, Paris) • Tropical Atlantic Workshop (Aug. 2002, Kiel) • TAV/AMMA meeting (ad hoc, Mar 2003, Miami) • CLIVAR Atlantic Panel (Apr 2003) – subgroup charged with drafting a “TACE” plan • TAV/TACE meeting (June 2004, de Bilt) • Review of TACE by CLIVAR Atlantic Panel (Oct. 2004); Comments received from WGSIP • TACE Implementation Workshop (Feb. 2005, Miami)

3. Key Climate Processes in the Tropical Atlantic: • The SST “Meridional Gradient” Mode • The Atlantic “Nino” • The Benguela “Nino” • Western South Atlantic (SACZ,SITCZ) Variability

4. SST Gradient Mode Atlantic Nino Mode First EOF (33%) of the March-April rainfall from GPCP 1979-2001 (contours in mm/day). March-April SST anomaly (colors, in °C & white contours, every 0.2°) and surface wind anomaly (vector, in m/sec) are determined by regression on the time series of the rainfall EOF. First EOF (23%) of the June-August rainfall from GPCP 1979-2001 (contours in mm/day). June-August SST anomaly (colors, in °C & white contours, every 0.2°) and surface wind anomaly (vector, in m/sec) are determined by regression on the time series of the rainfall EOF.

5. The Benguela “Nino” The temporal history of sea surface temperature anomalies (°C) (left hand panel) spatially averaged from 10 to 20°S, and from 8°E to the coast of south-western Africa. The right hand panel shows the standard deviation of OI-SST anomalies from 1982 to 1999 (°C). This area corresponds to the location of sea surface temperature anomalies associated with Benguela Niños (Florenchie et al., 2003).

6. Large-scale Ocean Circulation and its role in TAV • Subtropical Cells (STC’s): • Supply of subducted waters to upwelling zones (EUC,NEUC,SEUC) • Southern hemisphere branch is dominant • Interannual variability - impacts on the equatorial thermocline?

7. TACE Objectives: • Enhance existing observing system to provide the data needed for research and operations. • Improve coupled predictive systems, ocean synthesis, and transfer from science to operations. • TACE is planned as ~6 year (2007-2012) enhanced monitoring study, ultimately leading to specification of the sustained observations network in the tropical Atlantic needed to meet CLIVAR goals

8. From “Proceedings of the CLIVAR Workshop on Atlantic Climate Predictability, April 2004”: Overarching Challenge: To realize fully the potential of seasonal predictions for the tropical Atlantic. “The potential skill and value of seasonal forecasts is highest in the tropical Atlantic. The challenge is to build a seasonal climate prediction system for the tropical Atlantic region that is comparable (in terms of data coverage, model fidelity, and -subject to physical limits- forecast skill) to that in the tropical Pacific.”

9. Observational foci for TACE: • For seasonal predictions: • (i) in-situ surface and subsurface current, temperature and salinity measurements along the Atlantic equator, • (ii) surface and subsurface temperature and salinity measurements in the eastern off-equatorial portions of the basin between 20ºS to 20ºN, including the coastal upwelling regions off NW Africa and SW Africa, • (iii) in-situ surface measurements of temperature, air temperature, air humidity, radiative fluxes and winds north and south of the equator through the regions of dominant SST variability. Additionally, satellite measurements of SST (including microwave SST), sea surface height, wind stress, cloudiness, precipitation, and ocean colour are needed throughout the TA region.

10. Observational foci for TACE: For interannual variability and longer time scales (in addition to above): (i) upper ocean temperature and salinity throughout the tropical Atlantic, (ii) moored time series sections in the central/eastern equatorial band (~ 5ºS to 5ºN) to monitor the equatorial wave system and the supply of water masses to the eastern equatorial thermocline and the Angola and Guinea domes, (iii) surface drifters to monitor the divergence at the equator from upwelling regions to subduction regions, (iv) tracer observations to monitor changes in ventilation rates to the equatorial thermocline, and (v) atmospheric soundings to study boundary layer and air-sea interaction processes occurring under the varying SST regimes.

11. Modeling foci for TACE: • Determine oceanic processes important in regulating SST in the tropical Atlantic and associated atmospheric responses • Improve SST forecasts on seasonal to interannual time scales in the tropical Atlantic • Provide parameterizations and model improvements to global and regional prediction centers • Investigate response of tropical Atlantic region to global warming, including teleconnection patterns • Promote synergy between research-mode models and operational centers

12. Observing System Components

13. Findings/Recommendations from the TACE Workshop (1) Participants endorsed the TACE initiative as timely and relevant. (2) A critical mass of people and resources are focused on the problem. Much of the observational plan is already subscribed. (3) The overall observational plan is suitable but needs enhancements: Recommended Observational Enhancements: - More focus on western equatorial region to improve equatorial SST predictions (recommended: additional moorings across the equatorial waveguide at 35ºW) - Address the data void in the southwestern tropical Atlantic (recommended: extend the 23ºW array to at least 10ºS) - Better observational coverage is needed of the eastern Gulf of Guinea, incl. south of the equator (recommended: enhance profiling float coverage; probably the best alternative due to vandalism problems with PIRATA moorings in the region)

14. Findings/Recommendations from the TACE Workshop (cont’): (4) A more focused effort on model development and intercomparison for the tropical Atlantic (and Pacific) is needed. The Clivar WGOMD charter is focused on mid-to-high latitudes; therefore the WGSIP may be the logical place to coordinate this activity. An initial step would be to organize a comparison experiment of currently available high-resolution data assimilation models of the region (MERCATOR, HYCOM/GODAE, etc.) (5) A TACE data archive (distributed mode) needs to be created to facilitate and encourage full use of TACE observations in synthesis experiments. (6) Two TACE Working Groups should be established: - A TACE “Observations Working Group” (coordinate observational logistics, evaluate effectiveness of the observational network, develop needed enhancements) - A TACE “Modeling and Synthesis Working Group” (work with WGOMD and WGSIP to coordinate modeling efforts, encourage collaboration between research modelers and operational centers, provide “transitions” to operational centers)

15. TACE: Next Steps • Submit revised TACE plan (incorporating workshop recommendations) to CLIVAR Atlantic Implementation panel for final comment and endorsement. • Draft Terms of Reference for TACE Working Groups; appoint membership • Follow-up at Tropical Atlantic Workshop (October 2005) • Implementation