Nested Character of Arctic Thermohaline Intrusions

During the 1994 Arctic Ocean Section survey (AOS94), Swift et al. (1997) observed an extensive array of thermohaline intrusions extending across the Arctic basin. The intrusions were found to be very uniform in scale (with vertical scales of 40-50m) and remarkably coherent over large horizontal distances, as shown by the fact that temperature-salinity (T/S) anomalies nearly 2000 km apart line up almost perfectly in the T/S plane. The remarkable coherence of the intrusions demonstrates that they are a basin-scale phenomenon, and it has been speculated by Carmack et al. (1998) that they are an important mechanism of Arctic climate change, closely linked to recent thermohaline transitions within the Arctic Ocean. Such intrusions provide an effective means of mixing warm Atlantic water from narrow boundary and ridge currents into the basin interior(s). The Arctic intrusions observed by Swift et al. (1997) exhibit a peculiar ``nested'' structure in T/S space, in which both vertical and horizontal T/S gradients in salt-finger layers have the same T/S slope, and finger layers in adjacent profiles ``line up.'' Thus, the finger layers describe a series of ``mixing lines'' in T/S space. The nested configuration of the intrusions appears to be a very stable one, resistant to disruption by both vertical and horizontal mixing. Here we discuss possible reasons for the nested configuration of the intrusions, and consequences for mixing within the interior of the Arctic ocean.