Understanding the impact of Anthropogenic Aerosol emissions on North Atlantic Multi-decadal Variability

Over the past 150-years North Atlantic temperatures have swung between periods, many decades long, which were either
anomalously warm or cold. This phenomenon has become known as Atlantic Multi-decadal Variability, or AMV, and it has
been linked to a wide range of important changes in regional climate. For example, when the North Atlantic is cooler than
average, there are less Atlantic Hurricanes, but the risk of severe drought in the Sahel region or Northern Europe is
increased. Many studies have shown that by predicting these multi-decadal changes in Atlantic surface temperature we
can improve predictions of regional climate years to decades in advance.

Climate simulations with computer models suggest that multi-decadal changes in Atlantic surface temperatures is a natural
phenomenon, and is related to changes in the strength of the Atlantic Ocean circulation. However, recent research led by
the UK Met Office has challenged this paradigm by suggesting that changes in particulate pollution - known as aerosols -
that are emitted from heavy industry in Europe and North America was a key factor in changing Atlantic surface
temperatures. This is because the aerosols can modify the reflectiveness of clouds in the North Atlantic. Thus, more
aerosols act to cool the North Atlantic by making clouds reflect more sunlight, and, thus, multi-decadal changes in the
emissions of aerosol pollution could modulate the temperatures of the Atlantic.

So far, the evaluation of the changes in particulate pollution has focused on simple measures (e.g. average surface
temperatures). Furthermore, the proposed mechanisms are at odds with evidence that other important changes in ocean
and atmospheric circulation have occurred, which we would also expect to change Atlantic temperatures.

Unfortunately, our understanding of how this aerosol pollution will affect the North Atlantic is limited by weaknesses in our
previous climate simulations. Climate simulations often have problems at simulating the North Atlantic, and the important
aerosol pollution processes. Many different processes or external influences are also occurring at the same time.
Therefore, detailed analysis of simplified experiments with state-of-the-art models is needed to make progress.
This project will use state-of-the-art climate simulation experiments to understand how the North Atlantic Ocean and
atmosphere respond to simplified changes in aerosol pollution emissions from North America and Europe. These
experiments will use the latest version of the UK Met Office climate model, which has a much improved simulation of the
North Atlantic Climate, and has been shown to have unprecedented skill at predicting changes in atmospheric circulation,
ocean temperatures and precipitation in the Atlantic Sector. The model also has a significantly improved simulation of
aerosol pollution, and its interaction with clouds.

The student will focus on describing the impact of aerosol pollution on the North Atlantic and clarifying the relative
importance of the different processes that shape the temporal and spatial pattern of changes that result. The student will
also evaluate how the changes in the North Atlantic impact on other aspects of regional climate, such as hurricane
numbers, or rainfall. By comparing with available observations, the student will assess the realism of the simulated
response by comparing the experiments with the observed changes. Finally, the student will assess the implications for
climate predictions over a decadal time-scale.

Improving the understanding of how aerosols affect these complex phenomena will bring benefits through development of
climate models, and ultimately, improved confidence in climate predictions of the North Atlantic up to a decade in advance.