Adjustment Processes in an Unstructured Ocean Circulation Model at Different Resolutions

Oceanic adjustment processes in response to local changes in atmospheric or buoyancy forcing play a crucial role in understanding how the global climate responds to both short-term variability and long-term changes. Whilst studies show global teleconnections in this context, highly simplified models aiming to explain the fundamental processes at play lack important ingredients in their description. In particular, the effects of continuous stratification, bottom topography, background currents, and realistic coastlines are often neglected. This study aims to provide insight into how the adjustment takes place and which mechanisms are most important, focusing on the timescale of days to weeks. For this purpose, a realistic global ocean general circulation model (FESOM2) is used to apply localized perturbations in temperature, salinity, and freshwater flux in the shelf-slope area of the western North Atlantic and eastern South Pacific. An eddy-permitting mesh (horizontal resolution up to 4 km) is compared with a > 20 km mesh to capture the effect of grid resolution on the modeled adjustment process.

The perturbations are found to generate local anomalies in both salinity and temperature, regardless of the perturbed quantity. Only in limited cases do they propagate as a classical coastal trapped wave of a fixed sign. The strongest anomalies remain close to the perturbation region and are found to propagate advectively after reaching a presumed geostrophic balance. Waves in sea surface height (SSH) originate from the perturbation site and are found to travel at about 1 m s−1 to 6 m s−1 for O(1000 km) along the coast before fading away. In most of the observed cases, the baroclinic response consists of weak coastally confined anomalies with wavelengths of O(100 km) that also propagate with advective speeds after being excited by the waves in SSH. Indications of coastal adjustment through two distinct physical restoring mechanisms are found: Internal Kelvin waves and topographic waves with an offshore sign change.

The high-resolution mesh simulates narrower and faster currents with finer structure resulting in highly complex patterns in the primary (advective) temperature and salinity response. Background currents as well as changing topography along propagation pathways are known to significantly influence propagation velocities. The resolution dependency of adjustment processes in terms of signal velocities is likely to mainly enter through these processes rather than through the direct influence of discretization. Still, high spatial resolution is found to be necessary to resolve baroclinic Poincaré waves.

The observed wide range of different processes involved in adjustment including (complex) advection patterns needs to be considered when seeking explanations for teleconnections in realistic models and observations.