ENSO and short-term variability of the South Equatorial Current entering the Coral Sea

The South Equatorial Current (SEC) is the westward limb of the South Pacific subtropical gyre, carrying transport and water properties accumulated over a decade or more circulating around the gyre. Arriving at the coast of Australia, the current splits, turning north or south. The northward flowing part is of great interest for the tropical climate because it represents a principal means for the equator to feel the effect of the extratropics, and its effects can be magnified since equatorial air-sea interaction is very sensitive to small anomalies. Much work over the past two decades has been focused on trying to quantify changes of the South Pacific gyre, and to learn to monitor it, either remotely from satellites or with the broad-scale array of temperature/salinity-profiling Argo floats. The present paper mines historical ship data to extend the SEC time series back as far as possible, and to evaluate satellite proxies of its transport.

The full picture is complicated because the SEC is split into three branches by the two large island chains of the western Pacific: New Caledonia near 20°S, and the Solomon Islands near 10°S, each of which block the flow over at least 5° of latitude. The southern branch clearly turns south at Australia, and the northern branch clearly goes to the equator. We are here concerned with the middle branch that flows into the Coral Sea through the 800-km-wide gap between New Caledonia and the Solomons, and splits both north and south.

Developing indices of this flow has been difficult because much of the historical data consists of occasional ship surveys that are hard to put into context, and also differ greatly in their estimates of the transport. We take advantage of an XBT (expendable bathythermograph) line, where volunteer merchant ships have deployed simple temperature probes roughly monthly since the mid-1980s. Although the XBT data is crude by modern standards, it is sufficient to interpret gross changes in the transport, such as those occurring during the seasonal cycle and in response to El Niño and La Niña events. Comparison with the much higher-quality but shorter-duration Argo time series shows that the XBT data are credible for this purpose.

Fluctuations of the SEC due to El Niño and La Niña are shown to be very large: at least ±50% of the background value. The anomalies are concentrated at the equatorial side of the current, consistent with the idea that these changes feed relatively quickly back to the equator. They are also concentrated toward the surface, allowing satellite altimetry (which sees only the surface) to be a useful index of transport variability. Therefore, a transport proxy based on satellite altimetry is developed, and this offers an important advantage because satellites scan the surface weekly and can see the eddies and short time-scale changes that are missed by the broad-scale Argo array. The mining of historical data thus provides confidence that our present-generation tools make it possible to monitor both the transport (from satellites) and the water properties (from Argo) of this key piece of the Pacific climate system.