A thermodynamic view of ocean warming

Lead Research Organisation: Imperial College London
Department Name: Dept of Physics

Abstract

The ocean's role in climate: Perspective matters.

Over the last century the ocean has absorbed over 10 times as much heat as all of the other components of the climate system combined (the air, earth, glaciers etc.). The ability for the ocean to absorb so much heat is due mostly to its mass. Heavy things can store a lot more heat that light things. The ocean is close to 200 times heavier than the atmosphere. Since the relative mass of a human to an elephant is about 200 it is no wonder the ocean is often called 'the elephant in the room' when it comes to uncertainty in predicting global warming. Understanding how heat gets from the surface into the many layers of ocean below is what this project is about.

Here are just some of the processes us oceanographers think are important:
1. Pumping of water up and down by the winds. This processes drives for example the sloshing of warm water back and forward across the equator known as 'El Niño'.
2. The cascade of water down continental slopes much like underwater waterfalls.
3. Spinning vortices 10 to 100 miles wide (between the size of a large city and a small country) known as 'eddies'. These are the ocean's equivalent of the storms and low pressure systems one sees in daily weather reports.
4. The breaking of tiny waves like the ones you can see as you pour milk into a cup of coffee.

There are two unique and exciting things about this project: i) in collaboration with researchers from around the globe I will systematically explore, using both observations and computer models, how each of these processes effect ocean warming ii) I will apply a completely new approach to understanding these processes. The new approach involves a simple change in perspective from that of a fixed observer (e.g. an oceanographer staying in one place in the ocean) to a perspective following blobs of water of different temperatures (e.g. moving with the sloshing water across the equator). This may sound abstract - but everybody makes ver similar changes of perspective every day for very practical reasons. Consider this example: You want to explain how to get from Westminster to the Tower of London. A fixed observer would say 'go North for 1 mile then East for 3 miles', if you followed the subject you would say 'follow the Thames then turn left at Tower Bridge'. We all know that the two perspectives can be useful in different circumstances. Throughout the history of science, subtle changes of perspective have led to massive advances such as understanding how an engines, atoms and the climate works.

This new perspective - following water of different temperatures - has allowed me to develop a technique for indicating when and where heat is moved up and down by winds and when it is due to under water cascades. My hypothesis is that these two types of flows are the most important factors in understanding how the ocean will warm up in the future.

Planned Impact

Understanding ocean heat uptake is of fundamental societal importance. Ocean heat uptake can regulate the rate of global warming and is thought to have caused the so-called 'warming hiatus' of the last decade. Ocean warming is estimated to have been the single largest contributor to the rise in sea level over the past half century. Future changes in sea level will influence the habitability of coastal areas and incidence of extreme events.

The hypothesis presented in this study is that ocean heat uptake in response to anthropogenic surface warming can be understood and predicted based on knowledge of the present climate state. Confirmation of this hypothesis will provide an avenue to reduce uncertainty in projecting surface temperatures and sea level. In the short term (1-3 years), this will lead to improvements in projections using simple models and in the medium term (3-6 years) these advancements will be fed into state-of-the-art climate models through interaction with 10 international ocean modelling groups who are contributing data to this project.

Similarly, this study will investigate whether the present spread in ocean heat uptake among state-of-the-art climate models is an accurate representation of the uncertainty. The experiments to be undertaken in this study could potentially indicate that true ocean heat uptake falls outside the spread of existing model projections, which would have fundamental implications for sea level rise projections this century.

In addition, many of the mechanisms at the core of this project are relevant to understanding ocean carbon uptake over the next century. It is expected that an improved representation of this process will be obtained through this project.

Finally, through the Grantham Institute - Climate Change and The Environment and its multiple avenues of communication (some of which will be developed through this project), I will ensure that these advances are accurately and widely communicated.

Publications

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