Léon Chafik (Department of Meteorology, Stockholm University, Sweden); Kristofer Döös (Department of Meteorology, Stockholm University, Sweden)

Ocean observations on a global scale and with reasonable time resolution are expensive, as a result global ocean observations are sparse. An exception are satellite observations, which cover nearly the entire ocean surface. Unfortunately, the surface is the only part of the ocean which can be directly observed using remote sensing, nonetheless, the value of these observations is undeniable. Utilizing the temporal and spatial consistency, we explore within a Lagrangian framework the possibilities of geostrophic currents as they are derived from satellite observations. We focus on shelf seas, an ocean system that can be considered a weakness of exactly this particular observational product. Nonetheless, our main interest lies exactly in the shelf seas which connect the eastern North Atlantic, an important climate component, to the North Sea, an important economic region. Shelf seas in general, including those mentioned, are driven by both the state of the local atmosphere and furthermore the adjacent major oceans. Unfortunately, the restriction to surface observations makes it difficult to directly identify processes which drive change on shelf seas. Moreover, the ocean processes become more complex where major ocean currents hit shallower waters, making it especially difficult to observe. Satellite altimetry, regardless of the constraints, always yields a projection of the underlying ocean dynamics onto the surface, which we capitalize on by treating the observed currents as a tracer of the underlying dynamics. Doing so, we avoid the main issues of currents from satellite-altimetry, an unidentifiable error in both strength and direction for one, and on top general ignorance regarding the vertical extent. Aiming to identify the responses of the shelf seas between the eastern North Atlantic and the North Sea to both atmospheric and oceanic climate indices, we track virtual particles. We show seasonal and decadal variability of the Lagrangian properties in the shelf seas and furthermore show that satellite-geostrophy can be used to approximate in particular the subpolar gyre index, the dominant principal component of sea surface height in the central subpolar gyre. Finally, probing the limits of satellite-geostrophy we show that across the shelf, physical tracers like temperature and salinity cannot be followed assuming advection by these theoretical current pathways originating from satellite observations.