Sea surface temperature

Sea surface temperature, often called SST, is the temperature of ocean water near the top.

While the term "surface" can vary, it generally refers to the water at or just below the ocean's surface. Scientists use different methods to measure this layer. Some focus on the "skin temperature," which is only about 20 micrometres thick. Others measure the "subsurface bulk temperature," which looks at the top few metres of the ocean. These measurements are vital because the ocean surface interacts constantly with the atmosphere. This interaction influences weather patterns, such as the formation of sea breezes and sea fog.

The depth of the surface layer is not a fixed number. It depends on how much the water mixes with the deeper ocean. In tropical regions, the warm surface layer is about 100 metres deep and stays quite stable. This layer does not mix much with the colder water below. Near the poles, however, winter cooling and storms make the surface water denser. This causes the water to mix to much greater depths before it settles again in the summer. This heated layer is often related to the photic depth, which is the zone where light reaches. In the open ocean, this depth can reach around 200 metres.

Many different forces cause the temperature of the surface to change. On a daily scale, SST has a diurnal range, meaning it changes between day and night. This change is similar to the air temperature, but the ocean changes less because of its thermal inertia. Winds also play a massive role in moving water. Offshore winds can cause a process called Ekman transport. This moves warm surface water away from the coast and replaces it with cooler water from below. This process is known as upwelling. Upwelling is important because it brings nutrient-rich water to the surface, which helps marine life grow.

Large-scale weather events can also cause sudden temperature shifts. Tropical cyclones, such as hurricanes, can create a "cool wake" in the ocean. As the storm passes, turbulent mixing pulls cooler water up to the surface. This results in a visible drop in SST. Other factors like Saharan dust outbreaks can also reduce temperatures. For example, dust in the North Atlantic can lower SST by 0.2 °C to 0.4 °C. Even biological activity can matter, as concentrated phytoplankton blooms can cause short-term fluctuations in temperature.

In the Pacific Ocean, massive climate patterns called El Niño and La Niña occur regularly. El Niño is defined by a warming or cooling of at least 0.5 °C in the east-central tropical Pacific. These events usually happen every two to seven years and can last between nine months and two years. During an El Niño, warm water spreads from the western Pacific and the Indian Ocean toward the east. This warm water replaces the cold, nutrient-rich Humboldt Current. This shift can cause heavy rain in normally dry areas and droughts in the western Pacific.

History shows us how our understanding of the ocean has grown. Benjamin Franklin was a pioneer in this field during the late eighteenth century. He used a mercury thermometer suspended from a ship to study the Gulf Stream. Later, scientists used buckets to collect water samples from the surface. By 1963, ships began using automated systems to measure water from engine intake ports. However, these early automated methods had a "warm bias" because of the heat from the engines. Today, we use much more precise tools like fixed weather buoys and satellites to track these changes globally.

Recent data shows that the global mean sea surface temperature is rising due to climate change. It is estimated that global SST increased by 0.88 °C between 1850–1900 and 2011–2020. Most of this warming, about 0.60 °C, happened between 1980 and 2020. The ocean is a massive heat sink, absorbing about 90% of the excess heat generated by global warming. While land temperatures are rising faster, the ocean's warming is a critical part of the Earth's changing climate.

Scientists project that all ocean regions will continue to warm by 2050.