Observed Subseasonal Variability of Oceanic Barrier and Compensated Layers

Abstract

Abstract A monthly gridded analysis of barrier-layer and compensated-layer width based on observed vertical profiles of temperature and salinity and covering the period 1960–2007 is explored for evidence of subseasonal variability and its causes. In the subtropics and midlatitudes this variability is mostly evident during the local cold season when barrier layers and compensated layers are present. There is significant variability of anomalous (nonseasonal) barrier-layer and compensated-layer width on interannual periods, while in the North Pacific longer-term changes are also detectable. In the winter North Pacific a salinity-stratified barrier layer exists at subpolar latitudes. Farther south along the Kuroshio Extension a compensated layer exists. The width of the barrier layer varies from year to year by up to 60 m while compensated-layer width varies by half as much. During the observation period the barrier-layer width decreased in response to a strengthening of the Aleutian low pressure system, the resulting strengthening of dry northerly winds, and a decrease of precipitation. In contrast, the compensated-layer width increased in response to this pressure system strengthening and related amplification of the midlatitude westerly winds, the resulting increase of net surface heat loss, and its effect on the temperature and salinity of the upper-ocean water masses. The tropical Pacific, Atlantic, and Indian Oceans all have permanent barrier layers. Their interannual variability is less than 20 m but is comparable in magnitude to the time mean barrier-layer width in these areas. In the tropical Pacific west of 160°E and in the eastern tropical Indian Ocean, the barrier-layer width changes by approximately 5 m in response to a 10-unit change in the Southern Oscillation index. It thickens during La Niñas as a result of the presence of abundant rainfall and thins during dry El Niños. Interannual variations of barrier-layer width in the equatorial Pacific are weak east of 160°E with an exception of the area surrounding the eastern edge of the warm pool. Here subduction of salty water contributes to locally stronger variations of barrier-layer width.