The Role of the Ocean Surface

The sea surface forms the interface between the ocean and the atmosphere. Sea Surface Temperature (SST) helps scientists understand how energy is exchanged between these two systems, revealing how oceans influence weather patterns and climate, and how climate in turn affects the oceans. Direct ship-based SST measurements date back to the mid-19th century, making them among the longest instrumental climate records available. Since the early 1980s, satellites and ocean buoys have also been used to collect continuous observations.

SST distribution plays a major role in key components of the climate system, including atmospheric circulation, rainfall patterns, and the development of tropical cyclones. The upper few meters of the ocean can store as much heat energy as the entire atmosphere, highlighting the importance of SST monitoring. These datasets are also essential for estimating global temperature trends and supporting forecasts ranging from short-term weather prediction to long-term climate projections. Features such as marine heatwaves, ocean fronts, and upwelling zones can all be tracked using SST observations.

Long-Term Changes in Sea Surface Temperature

Since 1850, average SST across the extratropical oceans (60°S–60°N) has shown a clear upward trend. Conditions were relatively stable in the late 19th century, followed by a cooling period in the early 20th century. Warming occurred during the 1930s and 1940s, after which temperatures remained relatively steady from the 1950s through the mid-1970s. From the late 1970s onward, a sustained warming trend has been observed.

Comparing periods, SST has increased by about 0.9°C between 1880–1900 and 2019–2023, and by roughly 0.6°C since 1980. The most recent five-year average is around 0.3°C above the 1991–2020 reference period. The dominant source of year-to-year variability is the El Niño–Southern Oscillation (ENSO), which alternates between warmer El Niño and cooler La Niña conditions in the tropical Pacific. El Niño events temporarily raise global mean SST, while La Niña events reduce it.

Earlier SST records contain larger uncertainties due to sparse coverage and changing measurement techniques. In the 19th century, sea temperature was often measured by collecting seawater in wooden or canvas buckets lifted onto ships, which could introduce cooling errors. Later, improved insulated buckets were used. From the 1930s, engine intake measurements became common, followed by hull-mounted sensors in modern ships. Since the late 1970s, moored and drifting buoys have further improved accuracy. These methodological changes require careful correction when constructing long-term datasets.

Regional Differences in Ocean Warming

Over the past three decades, most ocean regions have warmed, although not at the same rate everywhere. Temperature trends show clear spatial variability, and ocean warming is rarely linear over time.

The fastest warming regions include parts of the Arctic Ocean such as the Barents and Kara Seas, as well as the Baltic Sea, the Black Sea, and parts of the North Pacific. In contrast, a cooling region has been observed in the North Atlantic south of Greenland and Iceland, consistent with longer-term observations. Cooling in the eastern tropical Pacific is linked to extended La Niña conditions between 2020 and early 2023.

Changes in the European Ocean Region

Sea surface temperatures in the WMO European marine domain (Region VI) have increased since 1850, but the rate of warming has varied over time. Rapid warming occurred between the 1920s–1940s and again from the 1990s to the early 2000s, while slower warming or cooling occurred from the 1940s–1980s and the early 2000s to the late 2010s.

In 2023, SST reached a record high, significantly exceeding recent decades. These variations broadly mirror patterns seen in land surface temperatures and reflect multidecadal variability in the North Atlantic, often associated with the Atlantic Multidecadal Oscillation (AMO). This variability is thought to result from internal climate fluctuations and possible changes in aerosol concentrations.

For 2019–2023, European SST was about 1.1°C higher than the 1880–1900 average and approximately 0.4°C above the 1991–2020 baseline. Since 1980, the region has warmed by roughly 1.1°C.

ENSO in the Tropical Pacific

In the tropical Pacific, wind patterns and ocean currents drive upwelling along the eastern Pacific and the west coast of the Americas. When trade winds strengthen, upwelling increases, leading to cooler surface waters and La Niña conditions. When winds weaken, upwelling decreases and surface waters warm, producing El Niño conditions.

ENSO (El Niño–Southern Oscillation) strongly influences global weather patterns. Scientists monitor SST anomalies in regions such as Niño 1+2, Niño 3, Niño 3.4, and Niño 4, with Niño 3.4 being the most widely used index in climate research due to its representation of key variability.

La Niña events often occur in consecutive years more frequently than El Niño events, and ENSO does not occur every year. After a strong 2015–2016 El Niño, La Niña conditions dominated from late 2016 through early 2023. In 2023, conditions shifted to El Niño, though this event was weaker than major historical events such as those in 1982–83, 1997–98, and 2015–16.