23 Jun 2021

23 Jun 2021

Review status: a revised version of this preprint is currently under review for the journal OS.

Refined estimates of water transport through the Åland Sea, Baltic Sea

Antti Westerlund1, Elina Miettunen2, Laura Tuomi1, and Pekka Alenius1 Antti Westerlund et al.
  • 1Finnish Meteorological Institute, Marine Research, Erik Palménin aukio 1, P.O.Box 503, FI-00101 Helsinki, Finland
  • 2Finnish Environment Institute SYKE, Marine Research Centre, Latokartanonkaari 11, FI-00790 Helsinki, Finland

Abstract. Water exchange through the Åland Sea, Baltic Sea, greatly affects the environmental conditions in the neighbouring Gulf of Bothnia. Recently observed changes in the eutrophication status of the Gulf of Bothnia may be connected to changing nutrient fluxes through the Åland Sea. Pathways and variability of sub-halocline northward-bound flows towards the Bothnian Sea are important for these studies. While the general nature of the water exchange is known, that knowledge is based on only a few studies that are somewhat limited in details. Notably, no high-resolution modelling studies of water exchange in the Åland Sea area have been published. In this study, we present a configuration of the NEMO 3D hydrodynamic model for the Åland Sea-Archipelago Sea area at around 500 m horizontal resolution. We then use it to study the water exchange in the Åland Sea. We first ran the model for the years 2013–2017 and validated the results, with a focus on the simulated current fields. We found that the model reproduced current direction distributions and layered structure of currents in the water column with reasonably good accuracy. Next, we used the model to calculate volume transports across several transects in the Åland Sea. These calculations provided new detail of water transport in the area. Time series of monthly mean volume transports showed a consistent northward transport in the deep layer. In the surface layer there was more variability: while net transport was towards the south, in several years some months in late summer or early autumn showed net transport to the north. Furthermore, based on our model calculations, it seems that dynamics in the Lågskär Deep are more complex than has been previously understood. While Lågskär Deep is the primary route of deep water exchange, still a significant volume of deep water enters the Åland Sea through the depression west of the Lågskär Deep. Better spatial and temporal coverage of current measurements is needed to further refine the understanding of water exchange in the area.

Antti Westerlund et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2021-56', Anonymous Referee #1, 28 Jun 2021
    • AC1: 'Reply on RC1', Antti Westerlund, 23 Sep 2021
  • RC2: 'Comment on os-2021-56', J. H. Reißmann, 02 Aug 2021
    • AC2: 'Reply on RC2', Antti Westerlund, 23 Sep 2021
  • RC3: 'Comment on os-2021-56', Anonymous Referee #3, 07 Aug 2021
    • AC3: 'Reply on RC3', Antti Westerlund, 23 Sep 2021

Antti Westerlund et al.

Model code and software

Archipelago Sea - Åland Sea NEMO ocean model configuration files Antti Westerlund, Elina Miettunen

Antti Westerlund et al.


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Short summary
Water exchange through the Åland Sea, Baltic Sea, affects the conditions in the neighbouring Gulf of Bothnia. Pathways and variability of flows were studied with a high-resolution hydrodynamic model. Our analysis showed a northward transport in the deep layer and net transport towards south in the surface layer. While on the southern edge of the Åland Sea the primary route of deep water exchange is through Lågskär Deep, still some deep water bypasses it to the Åland Sea.