Masters Thesis Defence by Francesco Emanuele Bucci – Niels Bohr Institutet - Københavns Universitet

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Masters Thesis Defence by Francesco Emanuele Bucci

The North Atlantic Ocean presents different modes of natural variability. This variability does not concern only the ocean but it relates with atmospherical variability and atmospheri- cal phenomenon. A recent study by Hakkinen2011 suggests a mechanism which relates the atmospheric blocking effect over the northern North Atlantic to the variability over the whole basin. It is proposed that winters with high blocking activity could affect the North Atlantic Ocean circulation for several years. The blocking effect causes anomalies in the jet stream which affects the wind forcing on the ocean. It is hypothesised that frequent blocking during winter corresponds to weak subpolar gyre circulation allowing influxes of warm water masses from the South which could relate to the variability of the SST, described for example by the Atlantic Meridional Oscillation index.
A sensitivity study is presented here, performed with the NEMO ocean model, which simulates three experiments with a different annual atmospheric forcing repeated for 20 years. The forcing fields are obtained from the NCEP/NCAR Reanalysis and the three periods considered here are centred on the winters of the 1961, 1963 and 1964. The winter of the 1961 presents low atmospheric blocking, the winter of the 1963 presents high blocking centred northeast of Iceland and the winter of the 1964 shows a broader high blocking over the ocean between Iceland and Norway.
A decrease of the meridional volume transport by 15% and a decrease of the meridional heat transport by about 20 % are found at 60°N across the whole basin in the situation with blocking. In presence of frequent winter blocking, warmer water masses are found at subpolar latitudes, to the South of Greenland: warmer SST up to 2°C in the subpolar gyre, and a 1000m of depth influx of warm waters up to 2.5°C are reproduced by the simulations between 50°N and 60°N, from 20°W to 40°W.
Farther North, at 65°N between Iceland and Norway, the meridional transport responds differently depending on the location of the blocking. A reduction of the transport is found at 65°N in presence of frequent blocking above Iceland, while in presence of the blocking pattern corresponding to the winter of the 1963 the meridional transport is more similar to the situation with low blocking. At 30°N meridional volume transport and meridional heat transport of the Gulf Stream decrease in presence of high blocking.
Furthermore, in presence of high blocking larger seasonal variability of the meridional volume transport and of the meridional heat transport, up to 100 %, are shown at 60°N, 30°N and 65°N. At 60°N an increase in the meridional transport within one month is found during the winters in presence of blocking while a correspond decrease of meridional transport is seen at 65°N. The results support the mechanism hypothesised by Hakkinen2011 which connects the atmospherical phenomenon of the blocking effect to the natural variability of the North Atlantic Ocean. The sensitivity study presented here quantifies the response of the ocean to to the winter blocking effect.