Interannual variability of the Ocean Circulation in the Atlantic-Arctic Ocean Gateways

Abstract

The northward transport of warm and saline water from the Atlantic Ocean into the Arctic Ocean is a crucial element of the Arctic climate system. On its way north, the Atlantic Water (AW) is either transported through the Barents Sea (BS), a shallow Arctic shelf sea, or with the West Spitsbergen Current through eastern Fram Strait (FS). The temperature and volume of the two-branched AW flow depict pronounced interannual variability, affecting the downstream water properties in the Arctic Ocean and the Arctic sea ice. In the first part of my thesis, I examine a potential positive feedback in the ice-ocean-atmosphere system of the BS. Using satellite data, atmospheric reanalysis, and a special setup of a global ocean and sea ice model I test whether the pronounced local decrease in sea ice in the BS can trigger a local feedback. It was hypothesized that reduced sea ice increases ocean-to-atmosphere heat flux, reduces the air pressure, alters the local wind field, and finally increases the warm water transport into the BS resulting in even less sea ice. The results of my study contradict the existence of this feedback but indicate that air pressure anomalies centered over the north-western BS can modify the Atlantic Water re-circulation in both, the Barents Sea Opening (BSO) and FS. In the second part of my thesis, I investigate the interannual variability of the transport through the BSO and its link to the North Atlantic Oscillation (NAO), using special setups of a global ocean and sea ice model. By combining different atmospheric forcing fields, the transport anomalies are split into local and upstream forced contributions. With the help of the simulations, I show that a previously detected loss of co-variability between the NAO and the BSO AW transport around the year 2000 can be attributed to the upstream forced BSO transport anomalies. These anomalies are strongly affected by atmospheric blocking events in this period. Atmospheric blocking deflects synoptic-scale cyclones that usually maintain the control of the NAO on the BSO AW transport. In the final part of my thesis, I examine the role of the westward-directed AW return-flow in the northern BSO on the Barents-Kara-Sea (BKS) sea ice. Due to insufficient observations, the magnitude and variability of the return-flow are largely unknown. Here, a model-based attempt is made to quantify the transport and temperature of the return-flow and the respective variability. A previously unknown, pronounced co-variation between the return-flow and the BKS sea ice is found. This co-variability is not only evident in the BSO but part of a quasi-simultaneous flow variability along the entire AW pathways through the Barents Sea. In general, my thesis aims at improving our understanding of AW transport variability in the Atlantic-Arctic ocean gateways.