Figure 7: (a) CNES-CLS22 MDT, (b) CNES-CLS18 MDT and (c) DTUUH22 MDT in Yermak Plateau area. The white line represents the 1000m isobath and the black line the 200m isobath. Figure 7a shows the bathymetric and geographical features: the Yermak Plateau and Svalbard Archipelago. (d) CNES-CLS22 MDT, (e) CNES-CLS18 MDT and (f) DTUUH22 MDT in St Anna Through area. The white line represents the 1000m isobath and the black line the 200m isobath. Figure 7d shows the geographical features: the St Anna Through, the Franz Josef Land Archipelago, the Novaya Zemlya (NZ), the Barrents Sea (BS) and the Kara Sea (KS).
In the Yermak Plateau area, where the Fram Strait branch of the Atlantic Water inflow enters the Polar Basin, the CNES-CLS22 MDT shows better alignment of the flow with the bathymetry both for the Svalbard branch crossing the plateau and the Yermak branch going around the plateau (see Fig. 1 in Meyer et al. 2017), compared with the CNES-CLS18 MDT and DTUUH22 MDT.
The CNES-CLS22 MDT also shows some improvements in the St. Anna Trough where the Barents Sea branch of the Atlantic Water flow to the Arctic exits the Barents Sea and enters the Polar Basin. Here, the CNES-CLS22 MDT better aligns with the bathymetry in the northeastern Barents Sea and northern Kara Sea compared with the CNES-CLS18 MDT and DTUUH22 MDT. Moreover, several more detailed differences also better align with features of the flow reported in the literature. The main flow from the Barents Sea is clearly aligned along the bathymetry in the southern part of the trough between the Novaya Zemlya and Franz Josef Land archipelagos (Schauer et al. 2002; Lien and Trofimov 2013), and there is a clear indication of cyclonic circulation of the Fram Strait branch within the same trough (Gammelsrød et al. 2009; Lien and Trofimov 2013). There is also indication of a continuous Novaya Zemlya Coastal Current along the northern tip of Novaya Zemlya (Lien and Trofimov 2013), which is not clearly seen in the CNES-CLS18 MDT and DTUUH22 MDT. Further north in the St. Anna Trough the CNES-CLS22 MDT shows better alignment of the flow with the bathymetry (e.g., Dmitrenko et al. 2015) compared with the smoother DTUUH22 MDT, in addition to more distinct features not seen in the CNES-CLS18 MDT or DTUUH22 MDT. At 81N, there is an indication of a cyclonic eddy previously identified by hydrographic observations, likely caused by the Fram Strait branch entering the St. Anna Trough (Osadchiev et al. 2022). Moreover, a bathymetric feature to the east of the Franz Josef Land archipelago likely affects the southward flowing part of the Fram Strait branch and causes an anti-cyclonic flow as shown in the CNES-CLS22 MDT.
4.2.1.2 Mid Atlantic Bight
As seen in section 2.3, the CNES-CLS22 MDT integrates synthetic geostrophic velocities estimated from high-frequency radar velocities in the Mid Atlantic Bight area on the east coast of the USA between Cape Cod and Cape Hatteras. In this section, the CNES-CLS22 MDT is looked at more precisely in the Mid Atlantic Bight, a region just north of the Gulf Stream with recirculation on the slope and circulation on the continental shelf shown in Fig. 8 (b).
Fig. 8 shows the contours of the CNES-CLS22 MDT (d) in comparison with the CNES-CLS18 MDT (c) and a ROMS model MDT (a). The ROMS model MDT (Fig. 8 (a)) is an average from the ROMS model used in a climatological diagnostic configuration to calculate a kinematically (coastline and bathymetry) and dynamically (ROMS nonlinear model physics) consistent circulation constrained by mean observations of the ocean state and forced by mean surface fluxes and river inflows (details given in Wilkin et al. 2022).
Lentz (2008) and Zhang et al. (2011) have advanced arguments as to the magnitude of the along-shelf sea level gradient in the Mid Atlantic Bight necessary to complete a momentum balance. We expect gentle southwestward flow throughout the Mid Atlantic Bight, and certain recirculation features in the Gulf of Maine and Georges Bank that are clearly evident in the ROMS MDT (Fig. 8 (a)). The across-shelf sea level gradient is consistent with observed southwestward mean currents. Furthermore, the known pattern of geostrophic coastal currents requires that the MDT contours be largely parallel to the coast, which is the case with the ROMS MDT and not always the case with the CNES-CLS18 (Fig. 8 (c)) and 22 (Fig. 8 (d)) MDTs.