Abstract. Characteristics of marine heatwaves (MHWs) and cold spells (MCSs) in the Northwest Atlantic during 1993–2023 are derived from a global ocean reanalysis product of the European Union Copernicus Marine Service. For surface parameters, the quantification using the reanalysis data is more advantageous than using the satellite remote sensing data in regions with the presence of seasonal sea ice and strong eddies. At the sea bottom, the reanalysis data reproduces well the observed rising trend and sharp increase in bottom temperature around 2012 on the Scotian Shelf and associated changes in MHW/MCS parameters. The 31 years of reanalysis data enable the quantification of spatial variations, interannual variations, and long-term trends in MHW/MCS parameters in the water column in our study region. The corresponding parameters of surface MHWs and MCSs are overall similar due to the nearly symmetrical probability distribution of sea surface temperature (SST) anomalies around the mean. On the Scotian Shelf, the MHW parameters present layered structures in the water column, influenced by the heat flux in the upper layer and the different water mass compositions in the deeper layer. During 1993–2023, the surface MHW (MCS) total days show increasing (decreasing) trends corresponding to the gradually increasing SST, and the MHW total days reached a peak value of 215 d in 2012 corresponding to the highest annual SST. The bottom temperature shows a stronger increasing trend than the SST and a regime shift around 2012, resulting in the increasing (decreasing) trend and regime shift in bottom MHW (MCS) total days. In 2012, the bottom MHW total days experienced a sharp increase and the entire water column was warmer than the climatology. Opposite conditions presented in 1998, with the longest bottom MCS total days of ∼ 300 near the coast. The quantification of the extreme conditions in 2012 and 1998 supports the results of previous studies on the impacts of these conditions on several marine life species.
Zhai et al. (Tue,) studied this question.