Nav: Home

Short wind turns with strong cooling effect

July 31, 2020

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.

"We were able to isolate the process behind this strong mixing event, which lasted only for a few days," explains Dr. Hummels. "It is a so-called inertial wave, which is a very short but intense flow event," Hummels continues. Inertial waves are horizontal wave phenomena in which the current at the surface rotates clockwise with time, whereas the movement rapidly decays with increasing depth. The different velocities at the surface and in the layer below cause instabilities and ultimately mixing between the warm water in the surface layer and the colder water below. Such inertial waves can be caused by brief variations in the near-surface winds. Up to now, generally only weak currents have been observed in this region and the rather steady trade winds at this time of year did not suggest particularly strong mixing events. However, wind variations are crucial to trigger these waves in the upper ocean. The winds do not have to be particularly strong, but ideally should rotate the same way the ocean currents do. Since such wind fluctuations are relatively rare and only last a few days, it has not yet been possible to measure such a strong wave phenomenon with the associated strong mixing in this region.

After the discovery of this event during the METEOR cruise in September 2015, the Kiel scientists wanted to know more about the frequency and the actual impact of such events. "Through model-based data analysis, we were able to give a context to the in-situ observations", explains co-author Dr. Willi Rath from the Research Unit Ocean Dynamics at GEOMAR. "Together, we have scanned 20 years of global wind observations looking for similar events triggered by wind fluctuations and described their occurrence in the region and during the course of the year", Dr. Rath adds. This has supported the hypothesis that the temporal and spatial distribution of such events can indeed explain the gap in the heat balance of the upper ocean.

The strong turbulent mixing caused by the inertial waves at the base of the surface layer is also crucial for biology: For example, the cold water that is mixed into the surface layer during such an event also brings nutrients from deeper layers into the upper ocean penetrated by sunlight. "This also explains the hitherto largely unexplained occurrence of chlorophyll blooms in this region, which could now also be attributed to the seasonally increased occurrence of these inertial waves," explains Dr. Florian Schütte, also co-author of the study.

The ship measurements in the tropical Atlantic were carried out in close cooperation with the international PIRATA program. For more than 20 years, the PIRATA surface buoys have been providing valuable data for studies of ocean-atmosphere interaction, which were also used for this study. "Indeed, the intensive mixing measurements resulted from a failure in the hydraulic system of the METEOR, which made other measurements impossible at that time", says Prof. Dr. Peter Brandt, chief scientist of the expedition. Despite buoys and series of ship expeditions to this region, new phenomena are still being discovered - sometimes by chance - which decisively advance our understanding of the tropical climate.

Helmholtz Centre for Ocean Research Kiel (GEOMAR)

Related Climate Articles:

Climate action goes digital
More transparent and accessible to everyone: information and communication technologies bring opportunities for transforming traditional climate diplomacy.
Sub-national 'climate clubs' could offer key to combating climate change
'Climate clubs' offering membership for sub-national states, in addition to just countries, could speed up progress towards a globally harmonized climate change policy, which in turn offers a way to achieve stronger climate policies in all countries.
Review of Chinese atmospheric science research over the past 70 years: Climate and climate change
Over the past 70 years since the foundation of the People's Republic of China, Chinese scientists have made great contributions to various fields in the research of atmospheric sciences, which attracted worldwide attention.
How aerosols affect our climate
Greenhouse gases may get more attention, but aerosols -- from car exhaust to volcanic eruptions -- also have a major impact on the Earth's climate.
Believing in climate change doesn't mean you are preparing for climate change, study finds
Notre Dame researchers found that although coastal homeowners may perceive a worsening of climate change-related hazards, these attitudes are largely unrelated to a homeowner's expectations of actual home damage.
How trees could save the climate
Around 0.9 billion hectares of land worldwide would be suitable for reforestation, which could ultimately capture two thirds of human-made carbon emissions.
Climate undermined by lobbying
For all the evidence that the benefits of reducing greenhouse gases outweigh the costs of regulation, disturbingly few domestic climate change policies have been enacted around the world so far.
Climate education for kids increases climate concerns for parents
A new study from North Carolina State University finds that educating children about climate change increases their parents' concerns about climate change.
Inclusion of a crop model in a climate model to promote climate modeling
A new crop-climate model provides a good tool to investigate the relationship between crop development and climate change for global change studies.
Natural climate solutions are not enough
To stabilize the Earth's climate for people and ecosystems, it is imperative to ramp up natural climate solutions and, at the same time, accelerate mitigation efforts across the energy and industrial sectors, according to a new policy perspective published today in Science.
More Climate News and Climate Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Our Relationship With Water
We need water to live. But with rising seas and so many lacking clean water – water is in crisis and so are we. This hour, TED speakers explore ideas around restoring our relationship with water. Guests on the show include legal scholar Kelsey Leonard, artist LaToya Ruby Frazier, and community organizer Colette Pichon Battle.
Now Playing: Science for the People

#569 Facing Fear
What do you fear? I mean really fear? Well, ok, maybe right now that's tough. We're living in a new age and definition of fear. But what do we do about it? Eva Holland has faced her fears, including trauma and phobia. She lived to tell the tale and write a book: "Nerve: Adventures in the Science of Fear".
Now Playing: Radiolab

First things first: our very own Latif Nasser has an exciting new show on Netflix. He talks to Jad about the hidden forces of the world that connect us all. Then, with an eye on the upcoming election, we take a look back: at two pieces from More Perfect Season 3 about Constitutional amendments that determine who gets to vote. Former Radiolab producer Julia Longoria takes us to Washington, D.C. The capital is at the heart of our democracy, but it's not a state, and it wasn't until the 23rd Amendment that its people got the right to vote for president. But that still left DC without full representation in Congress; D.C. sends a "non-voting delegate" to the House. Julia profiles that delegate, Congresswoman Eleanor Holmes Norton, and her unique approach to fighting for power in a virtually powerless role. Second, Radiolab producer Sarah Qari looks at a current fight to lower the US voting age to 16 that harkens back to the fight for the 26th Amendment in the 1960s. Eighteen-year-olds at the time argued that if they were old enough to be drafted to fight in the War, they were old enough to have a voice in our democracy. But what about today, when even younger Americans are finding themselves at the center of national political debates? Does it mean we should lower the voting age even further? This episode was reported and produced by Julia Longoria and Sarah Qari. Check out Latif Nasser's new Netflix show Connected here. Support Radiolab today at