Decadal and multi-decadal variability of Labrador Sea Water in the north-western North Atlantic Ocean derived from tracer distributions: Heat budget, ventilation, and advection

• Published in: Deep-sea research. Part I, Oceanographic research papers Oceanographic research papers, 2011-05, Vol.58 (5), p.505-523
• Main Authors: van Aken, Hendrik M., Femke de Jong, M., Yashayaev, Igor
• Format: Article
• Language: English
• Subjects: Atlantic Ocean; Convection; Decadal variability; Dissolution; Dynamics of the ocean (upper and deep oceans); Earth, ocean, space; Exact sciences and technology; External geophysics; Heat budget; Homogenizing; Labrador; Labrador Sea Water; Marine; Ocean temperature; Oceanography; Physics of the oceans; Radioisotopes; Salinity; Sea water; Time series; Tracers; Ventilation; Vorticity
• ISSN: 0967-0637; 1879-0119
• DOI: 10.1016/j.dsr.2011.02.008

Time series of profiles of potential temperature, salinity, dissolved oxygen, and planetary potential vorticity at intermediate depths in the Labrador Sea, the Irminger Sea, and the Iceland Basin have been constructed by combining the hydrographic sections crossing the sub-arctic gyre of the North Atlantic Ocean from the coast of Labrador to Europe, occupied nearly annually since 1990, and historic hydrographic data from the preceding years since 1950. The temperature data of the last 60 years mainly reflect a multi-decadal variability, with a characteristic time scale of about 50 years. With the use of a highly simplified heat budget model it was shown that this long-term temperature variability in the Labrador Sea mainly reflects the long-term variation of the net heat flux to the atmosphere. However, the analysis of the data on dissolved oxygen and planetary potential vorticity show that convective ventilation events, during which successive classes of Labrador Sea Water (LSW) are formed, occurring on decadal or shorter time scales. These convective ventilation events have performed the role of vertical mixing in the heat budget model, homogenising the properties of the intermediate layers (e.g. temperature) for significant periods of time. Both the long-term and the near-decadal temperature signals at a pressure of 1500 dbar are connected with successive deep LSW classes, emphasising the leading role of Labrador Sea convection in running the variability of the intermediate depth layers of the North Atlantic. These signals are advected to the neighbouring Irminger Sea and Iceland Basin. Advection time scales, estimated from the 60 year time series, are slightly shorter or of the same order as most earlier estimates, which were mainly based on the feature tracking of the spreading of the LSW 94 class formed in the period 1989–1994 in the Labrador Sea. ► Time series of hydrographic profiles, from the 1950s until present, have been constructed for the central Labrador Sea, the Irminger Sea, and the Iceland Basin. ► Deep convective mixing events, which form Labrador Sea Water, are highlighted by coinciding maxima in oxygen concentration and minima in potential vorticity. ► Convective ventilation events, which form Labrador Sea Water classes, occur at a decadal time scale. ► Succession of deep (>1500 dbar) LSW classes and their characteristic tracer variations since 1950 can be followed from the Labrador Sea to the neighbouring Irminger Sea and Iceland Basin