Upper Arctic Ocean velocity structure from in-situ observations

Supervisor: Ursula Schauer (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven)
The gross circulation of the upper and intermediate layers of the Arctic Ocean has been inferred from water mass properties: the mixed layer, containing fresh water from the shelf seas, travels from Siberia towards the Atlantic sector, and the saline and warm layer of Atlantic origin below, follows cyclonic pathways along topographic features. Direct observations of the ow below the sea ice are, however, di_cult to obtain and thus poor. This research presents the analysis of a unique time series/section of in situ velocity measure- ments obtained by an ice drifting platform. Two instruments were used to obtain in situ measurements of velocity, temperature, salinity and pressure: an Ice-Tethered Acoustic Current pro_ler (ITAC) and an Ice-Tethered Pro_ler (ITP). Both systems were deployed in the Amundsen basin, during the Arctic Ocean expedition ARK XXII/2 of the German Research Vessel Polarstern in September 2007. The systems transmitted measurements from the 14th of September to the 29th of November 2007 and the best depth range covered was from 23 to 400 m. In addition to these two measuring systems, shipboard Acoustic Doppler Current Pro_ler (ADCP) data from a Polarstern cruise in 2011 is also used in the analysis. As well as wind and ice concentration satellite reanalysis products. This work presents an overview of the upper and intermediate circulation along the Lomonosov Ridge. Daily averages and Latitude and Longitude averages of velocities obtained with the ITAC unit showed that the direction that the system followed, and the main direction of ice and surface velocities are consistent with the Transpolar Drift Current. Ekman transport velocities were calculated from ice drift and surface water velocities. This analysis showed that in some parts of the transect, the Ekman transport inuenced velocities at higher depths (e.g. 39 m depth) than the Ekman layer; calculated to be 28 m deep for the ITAC track. The analysis of velocities made in this thesis focuses in one region around the geographic North. In this region two water masses were identi_ed by looking at temperature and salinity distribu- tions: the halocline layer and the warm Atlantic Water. Throughout this section of the track a horizontal increase of velocity is observed. This suggests the existence of a current along the Eurasian side of the Lomonosov Ridge. Observed before only by tracer studies and potential temperature and salinity distributions but never measured directly before this study. Relative geostrophic velocities were also calculated for this section from hydrographic data measured by the ITP unit. The analysis showed that the ow had a distinct geostrophic baroclinic compo- nent from 200 m to 400 m, mostly at the end of the section track (alongside of the Lomonosov Ridge). The direction of this ow is also consistent with the circulation of the surface Atlantic water layer presented in previous studies.