Trends and Interactions of Atmospheric and Marine Heat Waves in the Gulf of Finland

Atmospheric heat waves (AHW) and marine heat waves (MHW) have both been well studied, however, few studies have considered both phenomena together. In this study, we examine marine and atmospheric heat waves at the coastal Tvärminne Zoological Station, Gulf of Finland, Baltic Sea during the extended warm season (April-September). We quantify the long-term trends in the intensity and duration of both MHWs and AHWs, investigate how they relate to each other, and identify the key atmospheric drivers of the onset and end of the marine heat waves. We utilize the unique 90-year long record of in situ surface and bottom (30 m) water temperature data from Storfjärden, close to Tvärminne, the shorter-term but higher-temporal resolution water temperature data from  Ångbåtsbryggan, and satellite based estimates of sea surface temperatures from 1982 onwards. In-situ 2-m air temperature and wind speed and direction observations are taken from nearby Finnish Meteorological Institute weather stations.

We find that MHWs and AHWs are both becoming more common, longer lasting and more intense. However, the trends are larger for MHWs than for AHWs; the number of AHWs is increasing at a rate of 0.16 per decade whereas MHWs are increasing at 0.54 per decade. We also find that when MHWs and AHWs occur together, they last longer and are more intense than heat waves that only occur in the sea or in the atmosphere. Furthermore, we find that it is more common for a AHW to proceed a MHW than vice versa, but not all MHWs are associated with AHWs.   

Using in-situ wind observations along with ERA5 reanalysis data, we investigate the meso-to-synoptic-scale circulation patterns that occur prior to, at the start of, and at the end of MHWs and AHWs. In the week before the onset of a MHW positive mean sea level pressure (MSLP) anomalies and positive 850-hPa temperature anomalies are present over large parts of Finland, Sweden, Norway, the Baltic countries and western Russia. In contrast, for AHWs no clear anomalies exist one week before onset. For MHWs occurring without an AHW, the synoptic-scale MSLP and 850-hPa temperature anomalies at the start of the MHW are weaker than in MHWs with an associated AHW. Wind observations indicate that wind speed and direction play a major role in the onset and end of MHWs in Tvärminne. Wind speeds have a minimum at MHW onset and wind direction is easterly, favouring downwelling, whereas wind direction is mainly south-south-westerly at the end of MHWs. These changes in wind direction are consistent with the synoptic-scale analysis which shows no high pressure anomaly at the end of MHWs.