Abstract
Abstract. Atmospheric forcing applied as ocean model boundary conditions can have a critical impact on the quality of ocean forecasts. This paper assesses the sensitivity of an eddy-resolving (1.5 km resolution) regional ocean model of the north-west European Shelf (NWS) to the choice of atmospheric forcing and atmosphere–ocean coupling. The analysis is focused on a month-long simulation experiment for July 2014 and evaluation of simulated sea surface temperature (SST) in a shallow near-coastal region to the south-west of the UK (Celtic Sea and western English Channel). Observations of the ocean and atmosphere are used to evaluate model results, with a particular focus on the L4 ocean buoy from the Western Channel Observatory as a rare example of co-located data above and below the sea surface. The impacts of differences in the atmospheric forcing are illustrated by comparing results from an ocean model run in forcing mode using operational global-scale numerical weather prediction (NWP) data with an ocean model run forced by a convective-scale regional atmosphere model. The value of dynamically representing feedbacks between the atmosphere and ocean state is assessed via the use of these model components within a fully coupled ocean–wave–atmosphere system. Simulated SSTs show considerable sensitivity to atmospheric forcing and to the impact of model coupling in near-coastal areas. A warm ocean bias relative to in situ observations in the simulation forced by global-scale NWP (0.7 K in the model domain) is shown to be reduced (to 0.4 K) via the use of the 1.5 km resolution regional atmospheric forcing. When simulated in coupled mode, this bias is further reduced (by 0.2 K). Results demonstrate much greater variability of both the surface heat budget terms and the near-surface winds in the convective-scale atmosphere model data, as might be expected. Assessment of the surface heat budget and wind forcing over the ocean is challenging due to a scarcity of observations. However, it can be demonstrated that the wind speed over the ocean simulated by the convective-scale atmosphere did not agree as well with the limited number of observations as the global-scale NWP data did. Further partially coupled experiments are discussed to better understand why the degraded wind forcing does not detrimentally impact on SST results.
Highlights
The exchanges of heat and momentum across the air–sea interface are fundamental components of the climate system (e.g. Yu et al, 2012) and can play a significant role in the evolution of natural hazards (e.g. Wada et al, 2018)
Further tuning of the CPL_AOW system may be appropriate, as discussed by Lewis et al (2018c) and Tonani et al (2019). These results demonstrate that sea surface temperature (SST) and temperature profiles through depth are sensitive to the source of atmospheric forcing and to the representation of air–sea interactions across the north-west European Shelf (NWS), with fundamental differences in the vertical structure developing between simulations from a common initial condition over a relatively short period of time
This paper has demonstrated that the simulation of ocean temperature for the NWS is sensitive to the atmospheric forcing at the surface
Summary
The exchanges of heat and momentum across the air–sea interface are fundamental components of the climate system (e.g. Yu et al, 2012) and can play a significant role in the evolution of natural hazards (e.g. Wada et al, 2018). This reflects the challenge of observing these quantities over the ocean compared with on land, and the related limited availability of measurements for evaluation (Drechsel et al, 2012; Banta et al, 2018). This may be a result of operational ocean forecast systems. The evaluation of wind forcing for operational wave models has been more prevalent, given the strong sensitivity of wave predictions to their accuracy (Cavaleri et al, 2009).The development of fully coupled atmosphere–ocean modelling prediction systems provide both motivation and tools with which to better understand the impact of the surface forcing on operational ocean forecasts The study focuses on a nearcoastal region as they represent complex environments where providing accurate predictions can be more challenging due to the strong influence of land–sea contrasts on both atmospheric forcing and ocean models (e.g. Holt et al, 2017; Cavaleri et al, 2018)
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