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

Abstract

a b s t r a c tTime 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

Changes in column inventories of carbon and oxygen in the Atlantic Ocean

“…For this region, with active deep water formation and significant advection of overflow water, the deeper parts might indeed be important and is a source of error in this comparison. Significant temporal variations in T , S and dissolved O 2 occurred in the SPNA during the last ∼ 60 yr are also reported by van Aken et al (2011) who conclude that the long-term variations of the intermediate water mass properties in the SPNA are related to meteorological forcing of the Labrador Sea. Significant changes in the SPNA salinity balance has been observed during the last half century (Curry and Mauritzen, 2005) which can be consistent with varying dominance of different water masses.…”

“…This is similar to the factor "a" in the TrOCA method (0.78) found by Touratier et al (2007) that also considers mineralization of organic matter in the alkalinity budget. The uncertainty in the C/−O 2 ratio introduces uncertainty in the calculation of DIC abio , particularly for areas where we find large storage rates of AOU, see Sect.…”

Evaluation of a Finite-Element Sea-Ice Ocean Model (FESOM) set-up to study the interannual to decadal variability in the deep-water formation rates

“…It should be mentioned that because here we use a transient SSS forcing, we can not clearly separate for this model run how much of the GSA events is model skill and how much is related to the salinity forcing. Nevertheless, the comparison of the modelled and observed temperature and salinity time series of van Aken et al (2011), of the 1500 dbar pressure level, which is not directly influenced by the sea surface forcing, reveals that the model is able to reproduce the deep signal of GSA events. An analogous model simulation where the SSS is restored to climatology (not shown) indicates that the model has the skill to also reproduce the surface signal of a GSA event.…”

From climate research to climate compatible development: experiences and progress in the Netherlands

“…Apart from providing society with estimates of future climate conditions, it is necessary to remain alert about the many unknowns in the models, the observations, and the generated scenarios for temperature (van den Hurk et al 2007), precipitation (van Haren et al 2012) and sea level rise (Katsman et al 2011). To give emphasis to this awareness, CcSP (and to a lesser extent KfC) followed a dual-course strategy in climate scenario research: one track aimed at generating tailored climate scenarios for a wide range of users, as exemplified by the paper about bias correction (Bakker et al 2013) in this special issue.…”

“…The subsurface temperatures in the climate models that were used are higher compared to the observations in the North Atlantic (De Jong et al 2009). A large natural variability at all water mass horizons could be described from daily to multi-decadal time scales (van Aken et al 2011). Because of that natural variability, one may expect that the West European climate that depends on heat flux from the Atlantic Ocean will be more variable than currently assumed in climate models used for climate scenarios simulations.…”