SST and Combined SST/IST Products Overview: DMI's Contribution to Copernicus Marine and Climate Change Services"

Sea surface temperature (SST) and sea-ice surface temperature (IST) are both essential climate variables (ECVs) and long-term stable observational records of these (and other ECVs) are crucial to monitor, characterize and understand the state of climate as well as its variability and changes. The Copernicus Marine Service (CMS) and Copernicus Climate Change Service (C3S) are responsible for complementary reprocessing activities using satellite ocean observations. CMS encompasses reprocessing at global and regional scales of all satellite observations including all observations available at a given time (reprocessing of Essential Ocean Variables, EOVs). C3S fosters climate reprocessing, typically at global scale, with special focus on the most accurate observations and homogeneous time series (reprocessing of Essential Climate Variables, ECVs).

The Danish Meteorological Institute (DMI) serves as a Production Unit (PU) for the Sea Surface Temperature (SST) and Sea Ice (SI) Thematic Assembly Centers (TAC) of CMS and the SST ECV of C3S. Within both frameworks, a suite of GHRSST-compliant L3S and L4 SST and combined SST/IST products for the Baltic and North Sea (CMS), Pan-Arctic (CMS) and Global Ocean (C3S) are produced. In the beginning of 2025, the new C3S SST/IST global L4 Climate Data Record (1982-2024) was released providing a unique opportunity for assessment of temperature changes over the global ocean including regions with sea-ice cover. Satellite observations from infrared and microwave sensors (independent of in situ measurements) have been blended using an optimal interpolation scheme to provide daily gap-free fields of combined SST and IST on a global 0.05 regular latitude-longitude grid. For consistency with existing L4 SST products, the global C3S SST/IST CDR also includes an estimate of the under-ice water temperature (UISST) in sea-ice covered regions. However, the combination of SST and IST provides a much better and more consistent indicator of climate change and surface temperature trends in the high latitudes, where the coverage of sea ice changes rapidly. The global combined SST and IST has risen 0.5 ^◦C over the period 1982-2024, which is ~25% more than observed in existing global L4 SST products considering the global ocean (using the under-ice SSTs) and the region between 60S and 60N, respectively. This highlights the importance of the combined SST and IST indicator for monitoring the actual surface temperature trends in high latitudes.

In early 2025, a reprocessed version of the Baltic Sea and North Sea SST Reanalysis product (1982-2024) will be released using the latest version of the ESA SST_cci L2P data as input. This product only uses satellite-based SSTs from infra-red sensors covering the North and Baltic Sea basins, at a 0.02 degrees regular latitude-longitude grid. Although it does not directly ingest IST, it uses a new approach for estimating the freezing point temperature of sea water depending on climatological salinity, which is a complex variable in the enclosed Baltic Sea basin. This presentation provides an overview of the existing and new products and their quality, along with a summary of implemented and future improvements.