How ocean salt patterns affect global weather

The study investigates the relationship between salinity, ocean currents and climate.

Researchers from Dalhousie University in Canada, the GEOMAR Helmholtz Center for Ocean Research in Kiel, the Alfred Wegener Institute, the Helmholtz Center for Polar and Marine Research (AWI), and the Marom Center for Marine Ecology at the University of Bremen have revealed the vital role played by the marine environment. The distribution of salt by ocean currents plays a role in regulating global climate.

Their new study looked at natural climate anomalies, such as the Little Ice Age, a period from the 15th century to the mid-19th century that led to disastrous consequences such as poor harvests, famine and disease in Europe. Although this period has been well researched, the climatic mechanisms underlying it are still a matter of debate.

“Given recent natural climate anomalies, it helps to understand the processes and mechanisms that human-induced global warming may lead to,” says Dr. Anastasia Zhuravleva, lead author of the study. She was a PhD student at GEOMAR and won the Annette Barthelt Prize for her dissertation in 2019. She subsequently worked as a postdoctoral researcher at GEOMAR and Dalhousie University, where the study was completed.

The importance of the tropical Atlantic Ocean

“Researchers often consider increased sea ice extent and desalination in the subarctic North Atlantic to be potential triggers of past cold spells, but processes in the tropical Atlantic appear to be just as important,” says Dr. Jouravleva. “In fact, in contrast to the northern and mid-latitudes, there is little information about these recent climate events from the subtropical Atlantic and their impact on regions in the Northern Hemisphere,” adds Dr. Henning Bausch, a paleoclimatologist at AWI and GEOMAR. , co-initiator and co-author of the study. “This is where our research comes in.”

So, what happened in the tropical Atlantic during historical climate anomalies, and how might potential changes there affect ocean circulation and climate to the north? To answer these questions, the team worked on a sediment profile from the southern Caribbean Sea and reconstructed the salinity and temperature of surface waters over the past 1,700 years. Among other things, the researchers determined the isotopic and elemental composition of the calcareous shells of plankton.

Climate anomalies and their impact

The results show a cooling of about 1°C during the Little Ice Age. “It’s a significant temperature change for this region,” says Dr. Mahyar Mohtadi, co-author of the study and head of the Low-Latitude Climate Variability Group at MARUM. “Particularly noteworthy was another pronounced cooling in the 8th and 9th centuries. Cooling temperatures in the warm equatorial ocean led to a decline in precipitation at a regional level, which coincided with severe drought in the Yucatan Peninsula and the decline of Classic Maya culture.”

In addition, the researchers found that cold climate anomalies in the subarctic North Atlantic and Europe were accompanied by weaker ocean circulation and increased salinity in the Caribbean. “Advection, or the movement of tropical salt to high northern latitudes, is essential for maintaining high surface densities in the subarctic North Atlantic. This is a prerequisite for the overall stability of large-scale ocean motion, including the transport of warm Gulf Stream waters, which are responsible for Moderate temperatures in Europe.

Data on the historical past thus allow for the reconstruction of trans-North Atlantic connectivity. The initial cooling could be caused by volcanic eruptions, decreased solar activity and feedbacks between sea ice and ocean in the north. The new study provides evidence that reduced salt movement to high northern latitudes will amplify and prolong these climate events. Conversely, the slow movement of positive salinity anomalies from the tropics will eventually lead to increased density at the subarctic North Atlantic surface. This may cause heat to be transported northward by ocean currents, resulting in milder temperatures over Europe and North America.

“These salinity responses are known from models and have been assumed for the Little Ice Age. However, in the absence of tropical ocean data, these assumptions were based on less direct records of precipitation.”

There is evidence to suggest that the Gulf Stream is weakening and that human-induced global warming is a likely cause. What is certain is that the consequences of this change will be global. The extent to which different climate mechanisms interact has been an open question. This study now confirms that salt transport from south to north is a major factor in the processes involved.

Reference: “Caribbean salinity anomalies contributed to North Atlantic circulation and climate change during the Common Era” by Anastasia Zhuravleva, Henning A. Bausch, Mehyar Mohtadi, Kirsten Vale, and Markus Kienast, November 3, 2023, Advancement of science.
doi: 10.1126/sciadv.adg2639