Sensitivity of Marine Heatwave Metrics to Ocean Model Resolution

Abstract

Sustained extreme temperature events in the ocean, referred to as marine heatwaves (MHWs), generate substantial ecological, social, and economic impacts. Ocean models provide insights to the drivers, persistence, and dissipation of MHWs. However, the sensitivity of MHW metrics to ocean model resolution is unknown. Here, we analyze global MHW metrics in three configurations of a global ocean-sea ice model at coarse (1 • ), eddy-permitting (0.25 • ), and eddy-rich (0.1 • ) resolutions. We show that all configurations qualitatively represent broad-scale global patterns of MHWs. These simulated MHWs are, however, weaker, longer-lasting, and less frequent than in observations. The 0.1 • configuration, despite local biases, performs best both globally and regionally. Based on these results, model projections of future MHW metrics using coarse-resolution models are expected to be biased toward weaker and less frequent MHWs, when compared with results using an eddy-rich model. Plain Language SummaryMarine heatwaves (MHWs) are persistent extreme temperatures in the ocean. They have a negative impact on marine life, fisheries, and tourism, and are becoming more frequent and more intense. One way to understand how MHWs form, intensify, and decay is by analyzing results from computer simulations of the ocean. However, these simulations are a simplification of reality, and depending on how they are designed they represent different aspects of the ocean circulation. It is still unknown how much the resolution of an ocean simulation matters when representing MHWs. In this work, we compare the performance of three ocean simulations-with low, medium, and high resolutions-when representing MHWs. We find that, regardless of their resolution, all simulations have weaker, longer, and less-frequent MHWs, when compared with the real world. Despite these differences, we find that simulations with medium and high-resolutions realistically represent global spatial patterns of MHWs. However, the ocean simulation with high resolution is preferable when studying regional patterns of MHWs. These results show how simulated MHWs differ from the real world, helping us to improve ocean simulations to be more realistic. In addition, we now better understand how computer simulations of future oceans, under climate change conditions, represent these extreme events. Key Points:• Marine heatwaves are weaker, longer-lasting, and less frequent in models than in observations • The higher the model resolution, the less biased the marine heatwave metrics • Eddy-permitting models can be used for global marine heatwave analyses, but eddy-rich models are optimal for regional analyses

Characterizing Marine Heatwaves in the Kerguelen Plateau Region

“…The model's frontal structure and filaments of the ACC, climatological winter mixed layer, and vertical velocities have been shown to be in good agreement with observations (Langlais et al, 2017). OFAM's horizontal resolution (i.e., 1/10 • ) resolves mesoscale processes in highly energetic regions, and is therefore suitable for regional MHW studies (Pilo et al, 2019).…”

“…Globally, modeled MHWs have been described to be less frequent, longer-lasting, and weaker than observed MHWs (Pilo et al, 2019). To the north of the KP, in contrast to the results in Pilo et al (2019), modeled MHWs are more intense than observed MHWs. To the west of the KP and on the plateau itself, however, results corroborate with those from the global analysis.…”

“…To the west of the KP and on the plateau itself, however, results corroborate with those from the global analysis. The higher frequency of modeled MHWs might relate to the difference between the thickness of the model's surface level, and the thickness of the surface layer represented in observation-based products (Pilo et al, 2019). While OFAM's surface represents the top 2.5 m of the water column, the NOAA-OI product represents just the top 0.5 m of the ocean.…”

Insights into projected changes in marine heatwaves from a high-resolution ocean circulation model

“…Western boundary currents are regions of fast and variable currents, where a substantial fraction of MHWs are generated by internal variability arising from local forcing and mesoscale processes, rather than large-scale climate modes 28 . Higher spatial resolution models outperform their lower resolution counterpart in simulating MHWs of recent decades 14 because they simulate realistic mean flow and mesoscale variability 13 . Our study demonstrates that coarse-resolution global climate models simulate less intense MHWs in western boundary current regions due to the lack of strong internal variability.…”

“…In particular, western boundary currents are regions of intense eddy activity where high-resolution models simulate the historical mean state and variability better than the coarse-resolution models 13 . This includes the simulation of MHWs 14 . Therefore, the projected changes in MHWs in western boundary current regions may be better represented in higher-resolution models.…”