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Sea Surface Temperature & Chlorophyll
At the base of the ocean food web are single-celled algae and other plant-like organisms known as phytoplankton. Like plants on land, phytoplankton use chlorophyll and other light-harvesting pigments to carry out photosynthesis. Where phytoplankton grow depends on available sunlight, temperature, and nutrient levels. Because cold waters tend to have more nutrients than warm waters, phytoplankton tend to be more plentiful where waters ware cold.
These sea surface temperature maps are based on observations by the MODIS sensors on NASA's Terra and Aqua satellites. The satellites measure the temperature of the top millimeter of the ocean surface. In this map, the coolest waters appear in blue (approximately -2 degrees Celsius), and the warmest temperatures appear in pink-yellow (45 degrees Celsius). Landmasses and the large area of sea ice around Antarctica appear in shades of gray, indicating no data were collected.
The chlorophyll maps show milligrams of chlorophyll per cubic meter of seawater each month. Places where chlorophyll amounts were very low, indicating very low numbers of phytoplankton are blue. Places where chlorophyll concentrations were high, meaning many phytoplankton were growing, are yellow. The observations come from the MODIS sensor on NASA's Aqua satellite. Land is dark gray, and places where MODIS could not collect data (reasons include sea ice, polar darkness, or clouds) are light gray.
The highest chlorophyll concentrations, where tiny surface-dwelling ocean plants are thriving, are in cold polar waters or in places where ocean currents bring cold water to the surface, such as around the equator and along the shores of continents. The polar–equator difference intensifies in the hemisphere experiencing summer. For example, the difference in chlorophyll concentration between the Southern Ocean and equatorial ocean areas is bigger in December than September. In polar waters, nutrients accumulate in surface waters during the dark winter months when plants can’t grow. When sunlight returns in the spring and summer, the plants flourish in high concentrations.
Also, when surface waters are cold, it is easier for deeper water to rise to the surface, bringing nutrients to sunlit areas where phytoplankton can use them. When surface water is warm, cooler, nutrient-rich water is trapped below. Because the vertical layers of the ocean aren’t mixing, nutrients that have built up in deep waters can’t reach the surface. In places where ocean currents cause upwelling, sea surface temperatures are often cooler than nearby waters, and chlorophyll concentrations are higher. This connection is evident in multiple places. A band of cool, plant-rich waters circles the globe at the Equator, with the strongest signal in the Atlantic Ocean and the open waters of the Pacific Ocean. In many coastal areas, the rising slope of the sea floor pushes cold water from the lowest layers of the ocean to the surface. The rising, or upwelling water carries iron and other nutrients from the ocean floor. Cold coastal upwelling and subsequent phytoplankton growth are most evident along the west coasts of North and South America and southern Africa.