Figure 1 |  NIOZ jetty: 1860-2021 mean annual values for 8AM daily value: temperature
Annual values (red dots) of 08:00 AM temperature recordings at the NIOZ jetty. The black line represents the 10-year moving average. Annual values up to and including 2018.

Sea water temperature

Since 1860, the seawater temperature of the Marsdiep has been measured daily at 08:00 AM, first at Den Helder and since 1947 from the dike near the jetty of the NIOZ Royal Netherlands Institute for Sea Research. This data series is one of the longest running sets of temperature observations in the Netherlands. Since the year 2000, continuous seawater temperature is also being recorded by electrical sensors. The seawater temperature data show considerable variation. The first 30 years were characterised as a period in which the annual average temperature (figure 1, red dots) continually decreased by a total of 1.5° C.

Since about 1890 up until the last 25 years, average temperature (figure 1, black line) varied without a clear trend. Only the last 25 years clearly show a warming of about 1.5°C. So far, 2014 has been the warmest year, with an average water temperature of 12.5°C. In all, 2018 does not stand out as a particularly warm year: despite the 2 heat waves there was also an extremely cold period. But the highest 08:00 AM temperature recorded (since 1982, when all daily values are available) was 23.8°C, measured on the 4th of August 2018. The overall highest temperature measured (since 2000, when 10 second measurements began) was 26.5°C, recorded on the 25th of July 2018, so during the first heatwave of 2018.

Figure 2 | NIOZ jetty: 1982-2018 annual signals for mean daily values: temperature
Seasonal mean signal of daily mean temperature measured at the NIOZ jetty over the period 1982-2018. The 2018 values are also shown separately (the blue area indicates the normal variation from the mean). Note that the cold period at the start of 2018 could not be measured in its entirety as the temperature probe was removed from the water to prevent ice damage.