Dr Hu Yang – Understanding Circulation Changes in Earth’s Oceans and Atmosphere

Large-scale circulation patterns can be found throughout Earth’s oceans and atmosphere, and play a crucial role in maintaining the stability of regional climates. As the climate warms, researchers find that these patterns are experiencing a fundamental transformation. Using a combination of satellite observations and computer models, Dr Hu Yang at the Alfred Wegener Institute, identifies and explains the mechanisms of these changes. His study offers crucial insights into how both human populations and natural ecosystems will be affected by these transformations – and how they will need to adapt to cope with them.

Circulation Patterns

Earth’s climate is strongly driven by robust circulation patterns in the atmosphere. At the equator, steady heating from the Sun causes warm, moisture-laden air to rise through the atmosphere to form clouds. This effect leads to high levels of rainfall at low altitudes – and is responsible for sustaining tropical rainforests in both the Amazon and Congo basins, and in Southeast Asia.

As this air cools and loses its moisture, it then migrates to higher latitudes, where it eventually sinks back to Earth’s surface in the subtropics – at latitudes roughly ranging between 20° and 35°. This air is responsible for generating hot, arid subtropical climates, including the Sahara, the Australian Outback, and the Sonoran Desert – shared between the US and Mexico. Finally, the air moves back towards the equator as it picks up warms and picks up moisture, completing a cycle named a ‘Hadley cell’.

This pattern not only plays a fundamental role in sustaining the richly diverse ecosystems of Earth’s tropics and subtropics; it also heavily influences the distribution of human populations. While the tropics are home to many of Earth’s largest and most densely populated cities, the arid areas of the subtropics have always been some of the planet’s most sparsely populated regions.

Circulating Ocean Currents

As well as driving climate variations on land, atmospheric circulation is also strongly connected to circulation patterns in the ocean. Crucially, the subtropical regions, where relatively cool, dry air sinks back to the surface, define the position of subtropical high-pressure systems, and the anticyclonic subtropical ocean circulation, or ‘subtropical ocean gyres’.

At the western boundary of each of these gyres, the western boundary currents are responsible for transporting large amounts of heat from the tropics, creating milder, wetter climates in the mainland regions adjacent to them. Examples of this can be seen in the Gulf Stream in the North Atlantic; the Kuroshio Current in the North Pacific; and the Eastern Australian Current in the South Pacific.

Conversely, currents on the eastern edges of subtropical gyres transport relatively colder water from higher latitudes, creating colder, drier climates on the nearby mainland. Yet at the same time, they can also bring an upwelling of nutrients from the ocean floor, which sustain some of Earth’s richest marine ecosystems. This can be seen in the California Current of the North Pacific, the Peru Current in the South Pacific, and the Benguela Current in the South Atlantic.

All of these circulation patterns contribute to generating the present-day climate zones. Inevitably, however, climate change appears to be driving profound transformations. Satellite observations over the past few decades reveal that the tropical belt surrounding the equator has been expanding towards higher latitudes, while large-scale ocean gyres have been migrating away from the equator.

These changes could have immense consequences – both for natural ecosystems, and for many millions of people. Through their research, Dr Hu Yang and his colleagues at the Alfred Wegener Institute aim to identify and decipher the mechanisms of these changes – and to predict the changes to the oceans and atmosphere they will likely trigger in the future.

Simulations and Satellite Records

To achieve this goal, Dr Yang and his colleagues have two vital tools at their disposal. Firstly, they use observations taken by satellites, which have been collecting consistent, reliable streams of data since the early 1980s. When studying the ocean, one particularly important observation is the Sea Surface Height, which is assessed by measuring the time it takes for radar pulses from satellites to be bounced back by the ocean’s surface.

Although the height variations generated by ocean currents are extremely subtle – typically less than a metre – these measurements provide key clues as to where they are positioned, and how fast they are moving. In addition to Sea Surface Height, Sea Surface Temperature is assessed by measuring the intensity of infrared light emitted from the ocean surface.

The second tool Dr Yang uses to make his assessments is climate modelling, which incorporates the latest physical equations to virtually recreate circulation patterns within computer simulations. By accounting for potential future changes to carbon emissions from human activity, this technique can be used to examine how the ocean gyres and Hadley circulation behave under various amounts of carbon emissions. Subsequently, it can be used to predict the shifts in regional climates that may result from these changes. These independent lines of evidence consistently reveal that the large-scale ocean circulation patterns are drifting towards higher latitudes in response to climate change.

Warming in the Subtropics

In previous studies, researchers have linked a poleward shift in both oceanic and atmospheric circulation patterns with complex atmospheric processes. Human-induced changes in atmospheric concentrations of greenhouse gases, aerosols and ozone are believed to contribute to the circulation shift. A number of theories have been put forward to explain why these shifts are happening. Some researchers have concluded that they may not be driven by greenhouse gas emissions after all, but are instead influenced by natural variabilities in climate.

Through their research, Dr Yang and his colleagues show that this is not likely to be the case. They propose that a relatively faster warming subtropical ocean is the key driver for the circulation displacement. This subtropical ocean warming is a robust response to human-induced global warming. To explain why, Dr Yang uses the analogy of plastic pollution. Because of the influence of large-scale subtropical gyres, the distribution of this pollution isn’t uniform, but collects in certain areas, such as the Great Pacific garbage patch. The positions of these accumulations aren’t a coincidence; rather, they occur at ‘convergence zones’, which arise at the boundary between prevailing westerly winds and the trade winds.

As Dr Yang shows, this same behaviour is also driving disproportionate warming in the subtropics, by transporting excess heat generated closer to the poles and equator into subtropical latitudes. The effect has important consequences for the ocean’s temperature distribution – increasing the temperature difference between subtropical and mid-latitude regions, while decreasing the difference between the tropics and subtropics. The changes in the temperature gradient profile, which is the fundamental ‘fuel’ of planetary circulation, eventually causes the large-scale circulation pattern to migrate.

An Expanding Tropical Belt

For the atmosphere, the most striking consequence of this trend will be a wholesale expansion of the tropics. As the subtropics become warmer, air within the higher-altitude branch of the Hadley cell will not cool down as quickly, allowing it to reach higher latitudes before sinking back to Earth’s surface. In turn, there will be a poleward shift in the arid conditions currently only found in the subtropics.

If this hypothesis is correct, it could lead to hotter, drier conditions in regions including the Mediterranean, California, and Australia – which are already recording an increase in the frequency and severity of droughts and wildfires. In the decades to come, such a shift towards Sahara-like conditions in these regions could completely reshape both the natural ecosystems of these regions, and the distributions of human populations.

To accommodate the expanding tropics, weather patterns currently found at higher latitudes would also need to shift towards the poles. This could bring about widespread changes to regional climates – affecting behaviours including precipitation and cloud formation, and the paths and shapes of fast air currents called jet streams.

Poleward Circulation Shifts

In the oceans, Dr Yang’s results point to a poleward shift in the centres and boundaries of five subtropical gyres. This is already leading to the transformation of some marine ecosystems, like the collapse of local cod populations in the Gulf of Maine, resulting from shifting flows in the Gulf Stream. In the future, such shifts could bring about broad changes to the regional climates of the lands adjacent to these currents.

Similar patterns can be clearly seen if we turn back to previous periods of warming, in Earth’s more distant past. From analysis of sediments on the ocean floor, researchers have determined that during previous ice ages, occurring within the past 800,000 years, the Agulhas current (the western boundary of the southwest Indian Ocean) lay roughly 750 kilometres closer to the equator. Conversely, around 100 million years ago, Earth was warm enough for Antarctica to remain ice-free. While the continent today receives very little precipitation, fossil records show that it was at least partially covered by temperate rainforests during this time.

From this evidence, it becomes clear that the effects of climate change on ocean circulation may transform the face of our planet, if human emissions aren’t dramatically reduced in the near future. By drawing on climate models, future studies could build on Dr Yang’s results to gain important insights into how these changes might unfold.

Understanding Future Changes

Dr Yang points out that all of human civilisation has developed over a period of relatively stable climate. By growing accustomed to robust circulation patterns in the oceans and atmosphere, billions of people have managed to thrive in a diverse array of regional climates, and continue to depend on reliable patterns in precipitation, temperature, wind, and ocean currents.

Dr Yang’s results paint a stark picture of how this stability is being threatened by climate change. Shifting circulation patterns are not only bringing extreme weather events to regions that have very rarely experienced them before; they are also triggering fundamental shifts in regional climates – threatening both natural ecosystems and human populations.

In this fast-changing landscape, Dr Yang hopes that his results will enable researchers to better understand how shifting circulation patterns are affecting our planet. In turn, this may provide communities with the vital guidance they need to effectively prepare for and mitigate the changes to come.