TOP: WHOI climate scientist Kristopher Karnauskas examined this global satellite map of chlorophyll in surface waters, indicating the growth of phytoplankton. Higher chlorophyll is shown in aqua, green, yellow, and red; lower in blue and purple. To the left of South America, a line of chlorophyll extends westward toward the left side of the map.
MIDDLE: When Karnauskas moved the map to look at the whole Pacific basin and focus on the equator (black line), he saw higher chlorophyll values (top map) toward the west at the Gilbert Islands. At the right are the Galapagos Islands, west of South America, where the Equatorial Undercurrent (EUC) carries nutrients and cooler water toward the surface around the islands. As a result, the islands have higher chlorophyll levels (red) than surrounding areas.
BOTTOM LEFT: The ECU also hits the Gilbert Islands, which have localized regions of higher chlorophyll. In a close-up map of sea surface temperatures (SST) near the Gilberts (part of the nation of Kiribati), dark blue on the islands’ west sides indicates cool water upwelling from the Equatorial Undercurrent.
BOTTOM RIGHT: The Gilbert Islands lie in a line across the equator, and the Equatorial Undercurrent spans the area from about 2 degrees (138 miles) north to 2 degrees south of the equator. Chlorophyll values, shown in shades of green, decline from east to west but increase again around the islands(Images courtesy of NASA)
TOP (Present): At present at the equatorial Pacific Gilbert Islands, east-to-west trade winds produce a surface current along the equator. The Equatorial Undercurrent (EUC) flows in the opposite direction at 100 to 200 meters depth. When it hits a barrier (such as an island), the cooler, higher-nutrient water in the EUC flows up toward the surface. The waters near the island exhibit both cooler temperatures (shown as blue) and higher productivity of chlorophyll-containing marine phytoplankton, both of which diminish to the west.
BOTTOM (Future): In the future, global climate models predict a sea surface temperature rise of nearly 3 degrees C (5.4 degrees F) by the end of the century. According to Karnauskas’s and Cohen’s fine-scale model, equatorial trade winds will weaken, causing a weakening of the surface current. In turn, the frictional drag on the EUC will lessen, and the EUC will strengthen, carrying more cool, nutrient-enriched water to the surface around the Gilbert Islands. The result will be enhanced productivity close to the islands, and slower warming during the coming century than neighboring islands not in the EUC’s path. The slower warming may allow corals to adapt and survive, making the Gilberts a refuge for coral reef ecosystems. (Amy Caracappa-Qubeck, Woods Hole Oceanographic Institution)
The Gilbert Islands, part of the nation of Kiribati, are coral atolls, strips of land surrounding shallow lagoons. Seen from space, the central lagoons are aqua; deeper water appears black; and low-lying land, composed of broken coral skeleton, forms a bright, narrow border along the lagoons. (Image courtesy of NASA)