Large-scale circulation around the Crozet Plateau controls an annual phytoplankton bloom in the Crozet Basin

Abstract

The circulation in the vicinity of the Crozet Plateau in the Southwest Indian Ocean sector of the Southern Ocean is examined using hydrographic sections, Argo floats, surface drifters, and satellite altimetry. All four techniques confirm that a major branch of the Antarctic Circumpolar Current, the SubAntarctic Front (SAF), flows anticyclonically round the Del Cano Rise west of the Crozet Plateau, i.e. eastward to the south of the Del Cano Rise, then northward and sometimes northwestward into the Crozet Basin, before turning back eastward in a combined front with the Agulhas Return Current and the SubTropical Front. This S-bend in the SAF is a permanent feature, controlled by the bathymetry, as has been inferred previously by Pollard and Read [Pollard, R.T., Read, J.F., 2001. Circulation pathways and transports of the Southern Ocean in the vicinity of the Southwest Indian Ridge. Journal of Geophysical Research, 106(C2), 2881–2898]. Similar, but much weaker, anticyclonic flow is found round the Crozet Plateau itself, with no more than 5?10×106 m3 s?1 turning north to the east of the Crozet Islands. Circulation north of the Crozet Plateau, between the Plateau and the S-bend of the SAF, is extremely weak, fed only by anticyclonic meanders breaking off the SAF into the area from the west or north, and occasional input from the northward, partially wind-driven (i.e. Ekman) flow south and east of the islands. In consequence of the weak circulation, dissolved iron from the land or sediments of the Crozet Plateau and Islands can build up during the winter in the Polar Frontal Zone between Crozet and the SAF, which gives rise to an annual bloom in this area. Biological evidence from satellite images, and from phytoplankton and zooplankton distributions, supports the circulation pattern we develop. This pattern confirms that patchiness of productivity in the bloom area results from close juxtaposition of water that has entered the area from the west from the SAF and from the south and east after flowing past the islands. Flow from the south and east has had a chance to entrain iron from the islands and sediments whereas flow from the west has not.