The temperature of the ocean water plays a crucial role for marine life, including plants like phytoplanktons and animals like zooplanktons. It also has a direct impact on the climate of coastal areas and the plants and animals living there. Understanding both the surface and subsurface temperatures of the sea is really important. We use regular thermometers to measure the surface temperature, and for the subsurface temperature, we rely on reversing thermometers and thermographs. These tools help us get a better grasp of the ocean’s temperature, which, in turn, has a ripple effect on the diverse ecosystems and weather patterns along the coasts.
What is Ocean Temperature?
Ocean Temperature is a measure of the energy due to the motion of molecules in the ocean. Satellites enable measurement of sea surface temperature (SST) from approximately 10 µm below the surface (infrared bands) to 1mm (microwave bands) depths using radiometers. The spatial patterns of SST reveal the structure of the underlying ocean dynamics, such as, ocean fronts, eddies, coastal upwelling and exchanges between the coastal shelf and open ocean. SST is a vital component of the climate system as it exerts a major influence on the exchanges of energy, momentum and gases between the ocean and atmosphere. The heat and moisture exchanges are a main driver of global weather systems and climate patterns.
How is Ocean Temperature Measured?
Before the 1980s, scientists mainly measured sea surface temperature (SST) using tools on land, ships, and buoys. The first attempt at automated data collection involved measuring water entering the ports of ocean-faring ships. While this method provided valuable data, it had its drawbacks. The depth of these ports varied between ships, leading to different temperature readings in a layered ocean. Moreover, the focus on major shipping routes left large parts of the world’s oceans unexplored.
Things took a turn in the 1980s when satellite observations, particularly instruments like MODIS on NASA’s Terra and Aqua satellites, revolutionized our understanding of global SST. These satellites circle the Earth multiple times a day, collecting more SST data in three months than all previous methods combined.
To grasp how this works, think about how charged particles in the ocean emit electromagnetic radiation, covering various wavelengths. Satellites can measure the infrared and microwave radiation emitted by the ocean, with infrared coming from the top 10 microns of the surface and microwave from the top 1-millimeter layer. While infrared sensors offer better spatial resolution, they are prone to cloud interference due to clouds absorbing the emitted infrared energy.
Fast forward to today, and we not only rely on satellites but also on thousands of floats in the oceans measuring temperature and salinity. These floats help validate satellite data and provide valuable insights throughout the water column. The Global Drifter Program’s surface drifters, contributing about 60,000 nighttime SST measurements monthly at a depth of 0.2 meters, have become a major player in global SST, ocean currents, and salinity measurements.
A significant leap in distributing satellite-derived SSTs came with the GHRSST project. This initiative standardized SST datasets, making them easily accessible across various computer platforms. Crucially, to use this data for climate modeling, the GHRSST project ensures that climate data records come with a comprehensive description of the errors associated with each SST value. It’s worth noting that satellites can only measure surface temperature, so additional instruments or models are needed to determine temperatures at depth.
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