Ninety years of coastal monitoring reveals baseline and extreme ocean temperatures are increasing off the Finnish coast

Global marine heatwave assessments often rely on satellite-derived sea
surface temperature. However, these data have low accuracy in coastal
areas, are unable to measure sub-surface temperatures and have only been
available since the 1980s. Here, we analyse 90 years of in situ surface
and bottom (30 m) water temperature data from a Finnish coastal monitoring
site. Water temperatures were significantly higher between 1991–2020 than
1931–1960 and 1961–1990. We find strong differences between
satellite-derived and in situ temperatures, with in situ temperatures
being lower in autumn and winter and higher in spring. Measurements at the
seafloor indicate marine heatwaves occurred during all seasons between
2016 and 2020, with intensities and durations exceeding previous records.
Since the 1990s, we find an upward shift of the baseline temperature and
increasingly frequent occurrences of temperatures previously considered as
an extreme. Our findings highlight the importance of long-term in situ
data and choice of climatological reference periods for assessing change.

全球海洋热浪评估通常依赖卫星反演海表温度。然而，此类数据在近岸区域精度较低，无法探测次表层水温，且仅自1980年代起才有观测记录。本研究针对芬兰某近岸监测站点90年的原位海表及海底（30米水深）水温数据展开分析。1991-2020年的水温显著高于1931-1960年及1961-1990年两个基准时段。研究发现卫星反演水温与原位水温存在显著差异：原位水温在秋、冬两季偏低，春季则偏高。海底测温数据显示，2016-2020年间四季均发生了海洋热浪，其强度与持续时长均突破既往纪录。自1990年代以来，水温基准线呈上升趋势，此前被认定为极端值的水温出现频次亦不断增加。本研究结果凸显了长期原位观测数据以及气候学参考时段选择在海洋变化评估中的重要性。