Short Wind Turns With Strong Cooling Effect


Why is the sea surface temperature of the northern tropics in the summer months often lower than expected? 

Why is the sea surface temperature of the northern tropics in the summer months often lower than expected? This question was investigated by a German-American team of scientists led by the GEOMAR Helmholtz Centre for Ocean Research Kiel. Their results, which have now been published in the international journal Nature Communications, show that a short-term, wind-driven wave phenomenon provides very efficient vertical mixing and cooling of the upper water layer.

Sea surface temperatures in the tropics have a major influence on the climate in the tropics and the adjacent continents. For example, they determine the position of the Intertropical Convergence Zone and the beginning and strength of the West African monsoon. Therefore, it is important to understand the variability of sea surface temperatures for climate predictions. Until now, the seasonal cycle of sea surface temperature in the tropical North Atlantic could not be sufficiently explained. "More precisely, the sea surface is colder than predicted by the combination of previous direct observations of solar radiation, currents and mixing, especially in the summer months from July to September", explains Dr. Rebecca Hummels from the GEOMAR Helmholtz Centre for Ocean Research Kiel and first author of a study now published in Nature Communications.

Ship-based observations with the German research vessel METEOR in September 2015 provided first measurements of a strong turbulent mixing event below the sea surface, where mixing was up to a factor of 100 higher than previously observed at this location. "When we noticed the greatly enhanced turbulence in the water column during data processing, we at first suspected a malfunction of our sensors," says Dr. Marcus Dengler, co-author of the study. "But when we also noticed strong currents at the ocean surface, we became curious". Precisely such events can explain the lower temperatures at the ocean surface.

Read more at Helmholtz Centre for Ocean Research Kiel (GEOMAR)

Image: Microstructure probe at the stern of the Meteor when launching with the instrument's own winch. The fast fading of the orange Kevlar cable allows the turbulence measurements to be carried out almost in free fall of the probe through the water. (Photo credit: Marcus Dengler)