Abstract
The growing attention on mechanisms that can provide predictability on interannual-to-decadal time scales, makes it necessary to identify how well climate models represent such mechanisms. In this study we use a high (0.25° horizontal grid) and a medium (1°) resolution version of a forced global ocean-sea ice model, utilising the Norwegian Earth System Model, to assess the impact of increased ocean resolution. Our target is the simulation of temperature and salinity anomalies along the pathway of warm Atlantic water in the subpolar North Atlantic and the Nordic Seas. Although the high resolution version has larger biases in general at the ocean surface, the poleward propagation of thermohaline anomalies is better resolved in this version, i.e., the time for an anomaly to travel northward is more similar to observation based estimates. The extent of these anomalies can be rather large in both model versions, as also seen in observations, e.g., stretching from Scotland to northern Norway. The easternmost branch into the Nordic and Barents Seas, carrying warm Atlantic water, is also improved by higher resolution, both in terms of mean heat transport and variability in thermohaline properties. A more detailed assessment of the link between the North Atlantic Ocean circulation and the thermohaline anomalies at the entrance of the Nordic Seas reveals that the high resolution is more consistent with mechanisms that are previously published. This suggests better dynamics and variability in the subpolar region and the Nordic Seas in the high resolution compared to the medium resolution. This is most likely due a better representation of the mean circulation in the studied region when using higher resolution. As the poleward propagation of ocean heat anomalies is considered to be a key source of climate predictability, we recommend that similar methodology presented herein should be performed on coupled climate models that are used for climate prediction.
Highlights
The major warm water pathway in the North Atlantic Ocean, the North Atlantic Current, and its extension into the Nordic Seas, keep the eastern North Atlantic warm compared to the western part (e.g., Brambilla and Talley 2008)
We focus on variables and mechanisms that are relevant for predictability in the subpolar North Atlantic and the Nordic Seas, such as heat transport and hydrographic anomalies along the pathway of Atlantic water (Fig. 1)
We focus on the surface signature of the warm Atlantic water pathway when we investigate sensitivity to changes in horizontal model resolution
Summary
The major warm water pathway in the North Atlantic Ocean, the North Atlantic Current, and its extension into the Nordic Seas, keep the eastern North Atlantic warm compared to the western part (e.g., Brambilla and Talley 2008). Warm and cold anomalies occurring along the pathway of the North Atlantic Current are brought northwards and eventually into the Nordic Seas (Chepurin and Carton 2012; Holliday et al 2008; Yashayaev and Seidov 2015). How such anomalous heat is formed and expressed on its way northward is still not fully understood (e.g., Krahmann et al 2001; Årthun and Eldevik 2016). These warm packets, seen as anomalous temperature at the ocean surface, occur about every 14 years and take approximately 10 years to travel from the subpolar North Atlantic and to the northern end of the Nordic Seas, the Fram Strait (Årthun et al 2017)
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