FY 2007

Southwest Pacific Ocean Circulation and Climate Experiment (SPICE)—Part I. Scientific Background

Ganachaud, A., W. Kessler, S. Wijffels, K. Ridgway, W. Cai, N. Holbrook, M. Bowen, P. Sutton, B. Qiu, A. Timmermann, D. Roemmich, J. Sprintall, S. Cravatte, L. Gourdeau, and T. Aung

International CLIVAR Project Office, CLIVAR Publication Series No. 111, NOAA OAR Special Report, NTIS: PB2007-111469, 37 pp (2007)

1. Rationale

South Pacific thermocline waters are transported in the westward flowing South Equatorial Current from the subtropical gyre center toward the southwestern Pacific Ocean—a major circulation pathway that redistributes water from the subtropics to the equator and to the southern ocean. The transit in the Coral Sea is potentially of great importance to tropical climate prediction because changes in either the temperature or the amount of water arriving at the equator have the capability to modulate the El Niño-Southern Oscillation (ENSO; glossary of acronyms at the end of the document) cycle and thereby produce basin-scale climate feedbacks. The southern fate of thermocline waters is, comparably, of major influence on Australia and New Zealand’s climate; its seasonal and interannual evolution influences air-sea heat flux and atmospheric conditions, and it participates in the combined south Indian and Pacific Ocean "supergyre." Substantial changes of this circulation have been observed over the past 50 years, and are continuing in global climate projections. The subtropical gyre has been spinning up in recent years with possible consequences for ENSO modulation and for the East Australian Current (EAC), whose influence has moved south, dramatically affecting the climate and biodiversity of Tasmania.

Despite its apparent importance to the climate system, few observations are available to diagnose the processes and pathways of transport through the complicated geography of the southwest Pacific. The South Pacific Convergence Zone is poorly documented; the region is remote, and the large temporal variability and strong narrow currents in a complex bathymetry pose serious challenges to an observing system. Numerical model results are sensitive to parameter choices and forcing, and the results are uncertain because of the lack of in situ data for validation. The existing observational network (Argo, VOS XBT sampling, and satellite winds and altimetry) is beginning to provide a large-scale picture, but the complex circulation and western boundary currents require further dedicated study. This document lays out the scientific background and identifies the open issues in the southwest Pacific Ocean. Its purpose is to set the basis of a regionally coordinated experiment, the Southwest PacIfic Ocean Circulation and Climate Experiment (SPICE, http://www.ird. nc/UR65/SPICE) under the umbrella of the International CLIVAR program (Climate Variability and Prediction, http://www.clivar.org). The corresponding implementation plan will be developed in a second report, integrating both observational and modeling analysis to provide a more complete description of the mean and variable circulation and climate in the southwest Pacific Ocean.