Deep Water Formation and Spreading Dynamics in the subpolar North Atlantic from Observations and high-resolution Ocean Models
The subpolar North Atlantic (SPNA) circulation is comprised of a complex interplay between the wind- driven gyre circulation and the buoyancy driven meridional overturning circulation (MOC). As the Atlantic MOC (AMOC) plays an essential role in our climate system due to the associated meridional transport of heat, mass and freshwater it is of fundamental importance to understand its forcing mechanisms, variability and impacts on various different time scales. Due to its role in the formation of North Atlantic Deep Water (NADW), the SPNA is of crucial importance to the understanding of the AMOC. This thesis presents se- lected aspects of the SPNA circulation dynamics, based on various observational data sets in combination with two high-resolution ocean general circulation models (OGCMs; VIKING20, VIKING20X). In order to understand observations in correspondence with OGCM output, the model fidelity in comparison to observed quantities has to be secured. These quantities should be available for sufficiently long time scales and should be determined similarly in the OGCM and the observations. Using observational data in the vicinity of 53◦N in the Labrador Sea and the ocean model VIKING20, the following comparable robust integral quantities were defined: the magnitude and spatial and temporal variability of integral circulation elements on the regional scale (NADW transport at 53◦N; 33 Sv model, 31 Sv observations), the horizontal and vertical extend of the March Mixed Layer Depth in the Labrador Sea and the gyre scale baroclinicity. The models’ boundary current system is more barotropic and indicates stronger monthly to interannual transport variability compared to the observations. Furthermore, during periods of enhanced deep convec- tion an increased correlation between different components of NADW is found in the model, which is found to be the result of a complex modulation of wind stress and buoyancy forcing on regional and basin wide scale. Apart from the challenging to measure AMOC strength, these above mentioned regional and basin scale quantities were found to be suitable for model verification. The export routes of deep water from the Labrador Sea with a specific focus on the connection to the Irminger Sea are further investigated using different Lagrangian particle tracking experiments based on both the Argo observations as well as the VIKING20X model output. The connection between the Labrador- and Irminger Sea on the Labrador Sea water (LSW) depth is evaluated with pure advective trajectory integra- tion as well with a simple additional diffusion parametrization. Advective experiments with the temporarily varying model output and seeding in the central Labrador Sea and the advective-diffusive experiments with the Argo based Eulerian velocity fields resulted in ∼ 20% of the total particles connecting the two regions v within 1.5 - 2.5 years, with shorter transit times in the model (∼ 1.5 years). The DWBC export of LSW towards the south was found to be strongly decreased with the applied diffusion parameterization. A rela- tively simple method reproduced a similar connection of the two regions as derived from various observations. The relation between the SPNA AMOC and the horizontal circulation of the subpolar gyre was then subject to a model only study with VIKING20. Current transports and the AMOC strength are evaluated along the “Overturning in the Subpolar North Atlantic Program” (OSNAP) array and a section spanning the Greenland-Scotland Ridge (GSR), where for both recently observational estimates became available. The AMOC strengths (GSR 6.6 ± 0.9 Sv, OSNAP eastern leg 17.2 ± 3.0 Sv) are comparable with observational values. However, due to the limited time series of observations, the variability of the AMOC can only be determined with the model on longer than interannual time scales. In VIKING20 all SPNA currents, the AMOC and the subpolar gyre index exhibit strong interannual to decadal variability. Using a simple box model, water mass transformation in the SPNA showed similar formation rates in the Labrador Sea (7.7 ± 3.0 Sv) compared to the Irminger Sea (1.6 ± 2.8 Sv) and Iceland basin (5.1 ± 1.2 Sv) combined. During periods of deep convection (1990’s) the transport of the boundary currents and the North Atlantic current import through the Iceland basin is particularly enhanced suggesting a similar forcing mechanism.
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