Abstract
A dominant paradigm for mid-latitude air-sea interaction identifies the synoptic-scale atmospheric “noise” as the main driver for the observed ocean surface variability. While this conceptual model successfully holds over most of the mid-latitude ocean surface, its soundness over frontal zones (including western boundary currents; WBC) characterized by intense mesoscale activity, has been questioned in a number of studies suggesting a driving role for the small scale ocean dynamics (mesoscale oceanic eddies) in the modulation of air-sea interaction. In this context, climate models provide a powerful experimental device to inspect the emerging scale-dependent nature of mid-latitude air-sea interaction. This study assesses the impact of model resolution on the representation of air-sea interaction over the Gulf Stream region, in a multi-model ensemble of present-climate simulations performed using a common experimental design. Lead-lag correlation and covariance patterns between sea surface temperature (SST) and turbulent heat flux (THF) are diagnosed to identify the leading regimes of air-sea interaction in a region encompassing both the Gulf Stream system and the North Atlantic subtropical basin. Based on these statistical metrics it is found that coupled models based on “laminar” (eddy-parameterised) and eddy-permitting oceans are able to discriminate between an ocean-driven regime, dominating the region controlled by the Gulf Stream dynamics, and an atmosphere-driven regime, typical of the open ocean regions. However, the increase of model resolution leads to a better representation of SST and THF cross-covariance patterns and functional forms, and the major improvements can be largely ascribed to a refinement of the oceanic model component.
Highlights
Nowadays the ocean is no longer seen as a passive agent being modulated by the chaotic, high-frequency atmospheric variability, but as an integrator that feedbacks onto the atmosphere and provides memory (andUniversity of Reading, Reading, UK 10 Department of Physics and Astronomy, University of Bologna, Bologna, Italy1 3 Vol.:(0123456789)predictability) to the system thanks to its longer persistence
The impact of model resolution on the representation of air-sea interaction in the mid-latitude North Atlantic has been systematically examined in a multi-model set of present climate simulations, performed following the CMIP6 HighResMIP common experimental protocol in the framework of the EU-H2020 PRIMAVERA project
Based on a set of statistical metrics it was possible to assess that coupled GCMs based on either
Summary
Nowadays the ocean is no longer seen as a passive agent being modulated by the chaotic, high-frequency atmospheric variability, but as an integrator that feedbacks onto the atmosphere and provides memory Small et al (2019) extended the analysis of B17 to two configurations of the CESM1 climate model, at standard and high resolution, coupling the same 0.25° atmosphere to a 1° and 0.1° ocean model, respectively Their findings are consistent with Kirtman et al (2012) results, showing that over the WBC regions the standard resolution model is dominated by an atmosphere-driven regime, while the highresolution (ocean eddy-resolving) model is dominated by an ocean-driven regime. These analyses point to (either fully or partly resolved) mesoscale geostrophic turbulence in the ocean as a key feature in model simulations to properly reproduce the observed correlation between SST anomalies and turbulent heat fluxes at the air-sea interface These results suggest that model resolution ( for the ocean component) can play a crucial role in the representation of coupled ocean–atmosphere processes over oceanic regions characterized by baroclinically unstable current systems.
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