An analysis of the 2023 summer and fall marine heat waves on the Newfoundland and Labrador Shelf

Abstract

Abstract. In this study, we investigated a series of moderate to severe surface marine heat waves (MHWs) impacting the Newfoundland and Labrador (NL) Shelf during the summer and fall of 2023. Using a combination of ocean model reanalysis data, in situ data collected under the Atlantic Zone Monitoring Program (AZMP), and atmospheric reanalysis data, we explored several factors that contributed to the intensity of these MHWs. We concluded that, firstly, due to an unusually cold spring and abnormally fresh conditions advected from upstream, the water column was highly stratified. Secondly, atmospheric conditions were calm and anomalously warm, and wind speeds were unusually low for prolonged periods in the summer. The combination of increased stratification and lower wind speeds caused a reduction in vertical mixing, limiting the exchange of warm surface waters with colder waters below and amplifying the retention of heat near the surface. However, by the late fall, the signature of the surface heat wave had vanished when the cooler subsurface waters were mixed vertically due to increased winds, storms, and surface cooling. During the most intense MHW in July 2023, we found that this event was confined to the surface as demonstrated by temperature anomalies along several standard transects which showed a thin layer of warm anomalies in the upper 10 m and cold anomalies below. Consequently, the vertical extent and distribution of MHWs are important considerations when exploring ecosystem impacts because not all elements of the ecosystem are equally sensitive to surface conditions. Finally, these results suggest that ocean model nowcast and reanalysis products can complement observational methods for studying MHWs in near real-time over large geographic areas and at multiple depths.