Winter storms over Labrador Sea influence Gulf Stream system

The Gulf Stream, which brings warm water from the Gulf of Mexico to Europe and keeps the climate mild, is only part of a larger system of oceanic currents called the Atlantic Meridional Overturning Circulation, or AMOC for short. It runs through the Atlantic like a giant climate machine: as warm water from the tropics is transported northwards at the surface, the current reverses in the North Atlantic -- the water cools, becomes heavier and flows south at depth.

Where exactly these sinking processes take place is the subject of current research, and recent measurement programmes have located them to the east of Greenland. A team of scientists from the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, has now conducted a modelling study focusing on the Labrador Sea southwest of Greenland. In their study, now published in the journal Nature Communications, the researchers used complex computer simulations to show that fluctuations in the Labrador Sea can have a significant influence on the strength of sinking processes east of Greenland. An important link is a little-noticed system of deep currents that ensures rapid spreading of Labrador Sea water into the deep-sea basin between Greenland and Iceland.

"We oceanographers have long had our eyes on the Labrador Sea between Canada and Greenland," says Professor Dr Claus Böning, who led the study. "Winter storms with icy air cool the ocean temperatures to such an extent that the surface water becomes heavier than the water below. The result is deep winter mixing of the water column, whereby the volume and density of the resulting water mass can vary greatly from year to year."

In the model simulations of the past 60 years, the years 1990 to 1994 stood out, when the Labrador Sea cooled particularly strongly. "The unusually large volume of very dense Labrador Sea water that formed following extremely harsh winters led to significantly increased sinking between Greenland and Iceland in the following years," explains Claus Böning. As a result, the model simulations calculated an increase in Atlantic overturning transport of more than 20%, peaking in the late 1990s. The measurements of the circulation in the North Atlantic, which have only been carried out continuously since 2004, would then fall exactly in the decay phase of the simulated transport maximum.

"According to our model results, the observed weakening of the Atlantic circulation during this period can therefore be interpreted, at least in part, as an aftereffect of the extreme Labrador Sea winters of the 1990s," summarises Professor Dr Arne Biastoch, head of the Ocean Dynamics Research Unit at GEOMAR and co-author of the study. However, he clarifies: "Although we cannot yet say whether a longer-term weakening of the overturning is already occurring, all climate models predict a weakening as a result of human-induced climate change as 'very likely' for the future.

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Potential threat to heart health from extreme weather

An analysis in nearly 2.3 million Europeans has found detrimental associations between cold weather and deaths from heart disease, particularly in poor neighbourhoods. The late-breaking research is presented at ESC Congress 2022.1 Hot weather was linked with excess deaths from heart disease and stroke in patients with heart conditions.

Study author Professor Stefan Agewall of the University of Oslo, Norway said: "Climate change is leading to a rise in the average global temperature but also extreme cold in some regions. More than 70,000 excess deaths occurred across Europe during the summer of 2003 due to intense heatwaves.2 Cold weather also accounts for excess deaths and hospital admissions.3,4 Previously studies on the cardiovascular effects of heat and cold mainly used aggregated data, such as daily deaths in a city. The EXHAUSTION project used individual data, enabling us to identify vulnerable subgroups for protective interventions, thereby increasing resilience for future weather events."

The analysis included 2.28 million adults from five cohort studies conducted in Italy, Germany, the UK, Norway, and Sweden between 1994 and 2010. The average age ranged from 49.7 years to 71.7 years and the proportion of women ranged from 36.0% to 54.5%. Participants with and without cardiovascular disease at baseline were included. Data on mortality and new-onset disease were collected through death and disease registries and follow up surveys. Daily average air temperatures at participants' home addresses were collected from local weather stations or estimated using modelling of temperature data from weather stations

The relationships between temperature and cardiovascular conditions and death were analysed for all participants and in subgroups with particular characteristics. A time-stratified case-crossover study design was used where for each participant, the researchers compared the temperature on the day of the week an adverse event occurred (e.g. Monday) with the temperature on the same day of the week without an adverse event (e.g. all remaining Mondays) within the same month. Using within-participant comparisons between days in the same month eliminated the potential confounding effects of participant characteristics and time trends.

The analysis found increased risks of death from cardiovascular disease overall and ischaemic heart disease in particular, as well as an elevated risk of new-onset ischaemic heart disease, associated with cold weather. With an approximately 10°C temperature drop, from 5°C to -5°C, there was a 19% greater risk of death from cardiovascular disease (relative risk [RR] 1.19; 95% confidence interval [CI] 1.04-1.36) and a 22% elevated likelihood of death from ischaemic heart disease (RR 1.22; 95% CI 1.07-1.38). There was a 4% higher risk of new-onset ischaemic heart disease associated with an approximately 11°C temperature drop, from 2°C to -9°C (RR 1.04; 95% CI: 1.01-1.08).

Professor Agewall said: "The relationships between cold temperatures and deaths were more pronounced in men and people living in neighbourhoods with a low socioeconomic status. The links between cold and new-onset ischaemic heart disease were stronger among women and people older than 65 years."

Heat was not related to detrimental effects in the overall study population. However, temperature rises from 15°C to 24°C were associated with 25% (RR 1.25; 95% CI 1.12-1.39) and 30% (RR 1.30; 95% CI 1.10-1.53) elevated risks of death from cardiovascular disease and stroke, respectively, in people with heart disease at baseline.

Professor Agewall said: "Clinicians can use this information to provide tailored advice to those most at risk of adverse health outcomes during hot and cold days. Patients with heart conditions should stay hydrated in hot weather and adhere to advice from their cardiologist on medication use. We can all check the news for extreme heat and cold alerts and follow safety tips from local authorities."

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