Hot Tub Time Machine: Part 2

A thermal expansion of the pool party analogy

Feb 13, 2024

Over the years I’ve spent a lot of time sailing, swimming, and exploring the environment of Cape Cod in Massachusetts, USA. Because of the Cape’s unique geography, its environments change every year: dunes erode, sand bars shift, forests become swamps and swamps meld with the sea. When I was younger the ocean waters were bitterly cold: rare was the day that the lifeguards measured a balmy temperature of 59°F/15°C. But during my last visit to the beach, the waters were 61°F and eventually reached a high that summer of the 68°F/20°C! How could seawater temperatures have increased so drastically, both over the course of that summer and in recent decades?

Even though the summer solstice occurs on June 21st, water temperatures peak in late August. This is because water has a high specific heat capacity and can absorb a lot of heat before its temperature increases. Ocean water takes around two months longer than land to adapt to the summer heat. Therefore, there’s a lag time between peak heat in the atmosphere (the sunniest day of the year) and peak heat in the ocean (warmest SSTs of the year). As I’ve mentioned before, climate change is a truly global phenomenon, one that involves all continents and environments. The ocean, which makes up 71% of the earth’s surface, acts as a giant tub that’s slowly heating and changing the climate. It functions like a marine time machine, and once we understand the sea’s long-term patterns, we can ‘visit’ the future states of our planet by creating accurate climate models. In this series, I hope to explain a few naturally-occurring climate change phenomena and how they affected the world’s oceans.

Hot Tub Time Machine: Part 1

Diana Haemer

July 25, 2022

Read full story

But first: you’re invited!

Interactions between the ocean and the atmosphere distribute heat throughout the world and dictate the abundance and location of water vapor (i.e. clouds), which affect the earth’s total energy budget. These systems are complex with lots of moving parts (literally), so to aid our comprehension, let’s think of it as a pool party — this is the Hot Tub Time Machine series, after all:

Pretend this tub covers 71% of Earth’s surface.

Heat is transported by individual water molecules — or plastic cups, for the sake of this metaphor. In our planet-sized hot tub, there are an infinite number of floating drink holders, or climate systems. Each of these floaties rests on the water’s surface and holds a certain number of cups. The floaties are kept in roughly the same area of the hot tub by a couple of factors, including the water jets on the tub’s sides and bottom.

Let’s use our imagination to scale up this floatie a few billion times. That way, the water molecules will be the same size as these red cups!

Now, individual cups are frequently moved between floaties and into different areas of the hot tub by the partygoers (aka winds). Everyone knows it’s not sanitary to swim during a huge pool party, so these people sit on the side and dip their legs into the water, only disturbing the upper 100 meters of our hot tub. When their drinks become flavorless due to the tub’s residual heat, the partygoers get out of the pool and ask the bar (aka atmosphere) for a couple of ice cubes. They then return poolside with their refreshed drink and place it back into the floatie. Thus the cups cycle from the hot tub to the bar and back again, mimicking the movement of water vapor between the sea and air.

If you look closely, you can see all the Paul Bunyan-sized pool partiers getting their refreshments.

Sometimes the partiers overload on the ice, so the heavier drink cup upsets the balance of the floatie, causing it to drift to a different spot in the hot tub before eventually being propelled back by the jets. Other times, only one drink in the floatie has been refreshed, so the sudden influx of cold ice cubes affects the temperature of all other drinks nearby. Occasionally, partygoers become disgusted by the warmth of their drinks and push their floatie towards another group of people, who also don’t want them, initiating an ongoing back-and-forth (some people are too lazy to refill their own cups, I guess). Due to the rowdy nature of this event, some floaties seem to roam aimlessly around the tub and can disappear from sight, only to bump into the same group of partiers hours later. The party hosts (aka scientists) continue to study certain groups of floaties to try to figure out 1) any patterns in their individual movements and 2) whether and how those movements affect the rest of the hot tub.

The floatie we’ll be looking at today is commonly called the Atlantic Multidecadal Oscillation (AMO).

So what is the AMO, and how does it differ from the NAO?

The AMO is a unique variation in the Atlantic Ocean’s temperature that takes around 60-90 years to complete. Because its nature and origin is still being studied, scientists were unsure for years if it was a phenomenon in its own right or an aspect of the North Atlantic Oscillation (remember, the NAO controls seasonal rainfall in Europe). In the first decade of this century, scientists definitively determined that sea surface temperature variations caused by the AMO drive “climate and precipitation patterns over North America, droughts in the Sahel region of Africa, variability in Northeast Brazilian rainfall, and tropical hurricane frequency and intensity.” (Knudsen et. al.) Essentially, the AMO is a measurement of the ocean’s heat content, which is measured by looking at long-term trends in sea surface temperatures (SSTs). The phenomenon either causes SSTs to remain warmer than usual or cooler than usual, and its effects have been most visible since the Industrial Revolution in 1870.

As you can see from this graph (Knudsen et. al. 2011), the periods of abnormally hot or cold temperatures in the N. Atlantic correspond to whether the AMO was in a positive (hot) or negative (cold) phase.

Broadly speaking, changes in sea surface temperatures affect both where and how much water evaporates. During its positive phase — when SSTs are increasingly warmer than usual — the AMO causes warm, dry conditions in North America; more precipitation in a region of sub-Saharan African known as the Sahel; and long, wet, warm summers coupled with anomalously cold winters in western Europe. The AMO’s negative phase (when SSTs remain cooler than usual) has the opposite effect (cooler summers and increased year-round precipitation in North America, drier conditions in the Sahel, etc.).

According to our favorite Koppen Climate Classification system, the Sahel region has a tropical semi-arid climate (BSh). The heat and dryness, however, are less extreme than those of the Sahara Desert on the Sahel’s northern border because of the intense rainfall it receives during the monsoon season.

While the NAO is primarily an atmospheric climate phenomenon, the AMO is an oceanic one. As established above, all climate variations depend on ocean-atmosphere interactions, but each yields a different end result. Many recent studies concern whether changing ocean circulation patterns will push the AMO into longer positive or negative phases and how that will impact different areas of the globe. Although permanently cooler Atlantic SSTs sound nice given that sea ice typically expands during these decades, anomalously cooler or drier conditions in one part of the world would have a ripple effect around the globe, both environmentally and sociologically. For example, both of the 20th-century time periods when the AMO was negative saw extreme droughts and years-long famines in the Sahel. And if we think back to the pool party, we’d remember that any changes made to one drink floatie might cause might have larger-than-intended consequences.

So the AMO doesn’t just affect the Atlantic?

Nope! Knudsen et. al. also wrote that the AMO can influence large-scale climate trends in places as far away as the Tibetan Plateau and India “through changes in the inter-hemispheric redistribution of heat.” A few years later, more scientists determined that positive AMO phases interact with Pacific Ocean climate phenomena in addition to affecting the East Asian winter monsoons. This is because warmer waters leads to stronger storms: the more a parcel of water is heated, the likelier it will evaporate. Water vapor holds a lot of heat, which is a form of energy; just like the warm drinks that pool partiers push away, this heat gets distributed around the ocean-atmosphere system. So, during an AMO-positive phase when SSTs in the North Atlantic are hotter than usual, more water evaporates and moves around the globe, leading to longer, more intense periods of rainstorms in the western Pacific.

Although a bit oversimplified, this graphic shows how Earth’s jet streams distribute heat and moisture around the globe. Their patterns vary frequently, leading to changes in weather — but not quite changes in climate!

Just as drinks at the party are constantly moved between floaties as different groups of partiers interact, a change in the strength or pattern of one climate phenomena will eventually affect them all.

Is the AMO affected by climate change?

Actually, the AMO affects climate change by enhancing the warming of our oceans. Knudsen et. al. noticed that the AMO exerted a stronger influence on regional climate during periods of warm temperature anomalies rather than colder ones. As we can see from the first graph, the AMO switched into its positive phase during the 1990s, so scientists believe this is accentuating global warming. Alternatively, as Knudsen wrote in 2011, “A return from a warm to a cold AMO phase could temporarily mask the effects of anthropogenic global warming, and thus lead to possible underestimation of future warming if the variability of the AMO is not taken into account.” In other words, when the next negative AMO phase arrives, it could appear as though sea surface temperatures are leveling off or decreasing, when in reality they are predicted to steadily increase as this century goes on.

And finally, could a positive-phase AMO explain the warmer SSTs off Cape Cod?

The existence of the AMO is something we should definitely keep in mind when reading about Atlantic SSTs. Many recent studies of the Gulf Stream (also known as the Atlantic Meridional Overturning Circulation) have predicted that the AMO will be pushed further into a negative phase in the decades to come. One theory is that the warm-water Gulf Stream will be slowed down or even halted because of a large influx of cold water from Greenland’s melted glaciers. This would cause a decrease in SSTs, meaning that the AMO will be unable to continue its natural fluctuations between warm and cold.

Climate change’s affect on ocean circulation is often mentioned by science writers and journalists who consider themselves to be informed. Yet these articles never mention the natural variations of the Atlantic Ocean’s temperature, even though the AMO is a well-established phenomenon and considered important by those who study the Gulf Stream. Perhaps the writers worry that average readers won’t be able to grasp the ‘complexities’ of science. The real problem, however, is that any masking of ocean warming would lend surface-level credibility to climate deniers, who will view the AMO as an excuse rather than a reason as to why ‘global warming’ isn’t as severe as the scientists predicted. By taking the time to look at the full picture now, we are preparing ourselves a time when understanding the intricacies of our ocean will be even more vital.

Thank you for reading In Circulation. This post is public so feel free to share it with a friend :)

In Circulation