Prominent renewal of Weddell Sea Deep Water from a remote source

Abstract

Three transient tracer sections of CFC-11 across the Weddell Sea are presented, collected during Polarstern cruises ANT X/4 (July 1992), ANT XIII/4 (May 1996) and ANT XV/4 (April 1998). The corresponding sections of silicate, a quasi-steady state tracer, are displayed for comparison and as a supplement. Two distinct CFC-11 maximum layers are found in the deep water, one centred near 2200 m and another near 3500 m. These layers, noticed previously also by other investigators, represent recently ventilated Weddell Sea Deep Water. The deeper, more pronounced core occurs along the southern continental slope, whereas the shallower core occurs in the northern Weddell Sea. The deeper CFC-11 maximum layer coincides with a pronounced silicate minimum layer. Quantitatively, the deeper core constitutes a ventilation route for the Weddell Sea of utmost importance, the amount of ventilated surface water involved being 2.7 ± 0.9 Sv. Most of the deep interior Weddell Sea appears to be ventilated by this external source. The ventilation rate of the Weddell Sea due to the inflow from the east is at least as high as that from the local southern and western sources that produce bottom water. Associated with the deep CFC-11 maximum core are discontinuities in the potential temperature-property diagrams of silicate, oxygen, total carbon dioxide, nitrate and salinity. The recently ventilated deep water is characterized by low concentrations of silicate, total carbon dioxide and nitrate, and by high oxygen content and salinity as compared to the deep water at the same potential temperature formed by mixing of Warm Deep Water and Weddell Sea Bottom Water.