Hot Tub Time Machine: Part 1
"Heat waves been faking me outtttttt..."
Last Tuesday, the United Kingdom set a record for their hottest temperature ever: it was 40.3°C (aka 104.5°F) in the county of Lincolnshire! Immediately, numerous articles were published by the BBC, Yahoo, & even from outlets as far away as Israel. Extreme heat in an area known to be cold & damp is shocking, yes, but climate change-induced heat waves are not new. We in the West have ignored weather anomalies in South Asia and Africa for all the usual Eurocentric reasons: indeed, some Western media outlets would have you believe that Europe is now more prone to extreme heat than anywhere else in the world. Yet heatwaves in Sub-Saharan Africa have steadily increased in severity and duration since the 1950s. The intensity of African heatwaves goes underreported because there is no routine monitoring of them — at least, not by European research centers. (more on this in a future newsletter — make sure you don’t miss it!)
Climate change is a truly global phenomenon, one that involves all continents AND all environments. As everyone knowns, the oceans make up 71% of the earth’s surface; they are like a giant tub that’s slowly heating and changing our quotidian lives. The ocean functions as a marine time machine: once we understand the sea’s long-term patterns, we can ‘visit’ the future states of our planet by creating accurate climate models. However, the main NYT article about heat waves frustratingly (and unsurprisingly) states that atmosphere-ocean interactions are just “some” of the causes of this extreme heat. Some? Why else is Europe a literal hotspot, post-colonialist karma?
Interactions between the ocean & atmosphere are the main drivers of our long-term climate cycles. Even before climate change flowed into popular culture, scientists were studying various environmental oscillations, the most famous being El Niño. Over the next few weeks, I’ll explain a few weather & climate phenomena and how they affect the world’s oceans & peoples. Some of these patterns occur annually, some cycle every few years, and still others are decadal. Since Europe is ‘in’ this week, let’s look at the North Atlantic Oscillation (NAO).
But first, what’s the difference between “weather” and “climate”?
In my first semester of college, my Weather Climate & Change professor explained how one member of his family kept insisting climate change was fake. This assertion was based on the statement that “the climate changes every day,” to which my professor would respond “no, the weather changes every day.” Weather is the ~umbrella~ term for daily atmospheric patterns; per NOAA’s definition, it is “the state of the atmosphere at a particular location” over a short period of time.
Climate, on the other hand, is the average of weather patterns in a given area; it is measured on longer timescales, usually 30 years or more. The most widely-used climate categorization system is the Koppen climate classification. It divides the earth into five zones (tropical, dry, temperate, continental, & polar) based on average temperatures & precipitation patterns.
“Climate is what you expect, weather is what you get.”
Northwest Europe (the area in lime green) has a type of humid temperate climate known as “oceanic.” These areas are characterized by frequent precipitation, an absence of severe weather, and temperatures ranging from -3°C to 22°C (26-72°F). According to a recent Politico article, most buildings in this area are not designed to withstand heat of greater than 25°C (let alone 40°C!).
Which is more powerful, the atmosphere or the ocean?
The ocean is a powerful long-term influence on our planet’s climate. Our waters absorb most of the sun’s heat; water is darker than most terrestrial environments, so it has a higher albedo. Once heat is reflected into the atmosphere by snow caps, deserts, and forests, the atmosphere helps to trap it. Of the heat retained by this greenhouse effect, 90% of it is absorbed by the oceans.
All this warmth is distributed throughout the world via the global ocean conveyor belt (covered in a previous newsletter). The ocean’s water temperature effectively controls the amount of evaporation & precipitation in different areas of the globe. An interesting example is Namibia, a country on Africa’s southwest coast & the home of the Namib Desert. One would expect a humid or tropical climate in an area so close to the ocean (more access to water & moisture), but the Namibian coast has a hot desert Koppen classification. This is because the water in the southeastern Atlantic is on its way up from Antarctica; since it’s so cold, it does not evaporate. Thus there is no moisture in the air, so rain cannot fall, creating desert conditions.
While measuring atmospheric conditions are useful for predicting weather, understanding oceanic conditions are best for understanding an area’s climate — and how that climate is changing.
What are the typical weather patterns in the North Atlantic?
Ocean currents can be divided into two simple categories: horizontal & vertical. As we learned when discussing vertical (thermohaline) circulation, warm water flows up the Gulf Stream from the tropics, and some of it sinks downwards off the coast of Greenland to form deep water. The water that doesn’t sink, however, continues to cycle around on the surface as part of the North Atlantic Gyre (at the center of this gyre is the Sargasso Sea, if any Jean Rhys fans were wondering). This horizontal movement of water brings warmth to Northern Europe.
Just as ocean currents distribute heat throughout the ocean, winds distribute heat throughout the atmosphere. If we remember from elementary school earth science, lines of latitude run horizontally around the earth and range from 0-90° north and south. Air alternatively rises and sinks at four different points of latitude: 0°, 30°, 60°, and 90° — think of this as the vertical version of ocean gyres. This topic truly deserves its own newsletter, but for the sake of simplicity let’s just look at mid-latitude cells (aka Ferrell cells), which range from 30-60° latitude. Warm air that’s coming off the Gulf Stream & North Atlantic Drift rises at 60°, is transported southwards as it cools, and sinks back to the surface at around 30°. It then flows northwards across the continent of Europe & northern Atlantic Ocean to replace the evaporated air, and so begins the cycle again.
When water molecules meet an air mass that’s ~cooler~ than them, they evaporate and immediately condense, forming clouds and increasing the temperature & humidity of the surrounding air. And when there is less air on the surface due to evaporation, the atmospheric pressure is lower and there is increased precipitation. This is why some parts of northwestern Europe (i.e., Iceland and the British Isles) are perpetually damp: the warm water coming off the Gulf Stream is interacting with cooler air flowing down from the poles, so it evaporates and forms rain clouds. In fact, there is an area of perpetual low pressure hanging over Iceland; while I have never been to the country, a former resident told me that it’s always humid and moist, as if the air is spitting on them.
Newton’s Third Law states that ‘all actions have an equal & opposite reaction’. So if there are areas of the world with perpetual low pressure, there must be one of perpetual high pressure! Looking at the Ferrell cell, we can see that the risen air moves south and sinks at 30° latitude. This buildup creates a larger-than-normal airmass at this line of latitude, which happens to be above the Azores Islands. Scientists creatively call this the ‘Azores High.’ High pressure systems typically bring dry, warm, & calm weather — and the Koppen classification of the Azores is a Mediterranean climate, which is characterized by hot, dry summers and low precipitation. Fancy that!
What is the NAO, and how does it affect European weather?
As established above, because of wind patterns & ocean currents there will always be low pressure over Iceland and high pressure over the Azores. The strength of these two weather patterns affects conditions on the entire continent of Europe. Climate scientists have come up with a name for this measurement: the North Atlantic Oscillation (NAO). Each year, depending on the warmth of the Gulf Stream, ice melt in the Arctic, and other still-unknown factors, the Icelandic low and Azores high are either stronger or weaker. In the NAO’s weaker or negative phase, Northern Europe is cooler and drier while Southern Europe is the recipient of the Jet Stream’s warmth & precipitation. In the stronger or positive phase, the pressure differential between the Low & High is larger. This means that the rotating pressure systems are powerful enough to funnel the Jet Stream directly at Northern Europe. This causes warmer, stormier conditions in Northern Europe and cooler, drier ones in Southern Europe.
The NAO’s phases oscillate between negative & positive, sometimes changing multiple times per year. Research has shown that in the past few decades, the NAO has stayed in its positive for longer periods of time before switching back to negative. NOAA scientists calculate the strength of the oscillation each month: this year’s data shows the NAO as mostly positive throughout last winter and into this summer.
The NAO usually has the strongest influence on Europe’s climate during the wintertime, but a warmer-than-normal winter has lasting consequences. The NAO+ brings extra heat and moisture, so the UK & Ireland’s physical land is more saturated with water and not as cool. Thus there is a smaller temperature differential between the atmosphere and the earth; when this temperature differential is lower, less water evaporates. So when summer rolls around and the Jet Stream reaches the British Isles, less of the water from the moist earth evaporates. This causes the land to increase in temperature as liquids absorb heat faster than solids (so the water retained in the earth is causing it to heat up more quickly). Additionally, the water that does evaporate into the air increases the humidity and makes the summer heat more oppressive. In fact, scientists have discovered that the NAO accounts for 31% of the Norther hemisphere’s “inter-annual variance” — meaning that it’s a major cause of much of the observed regional surface warming in Europe.
Finally, will things get worse as our climate changes?
Although the North Atlantic Oscillation is a naturally occurring phenomenon, scientists worry that more greenhouse gases will cause the NAO to trend more positively. Both the atmosphere and the ocean’s Gulf Stream are warming, which could lead to less evaporation and more extreme heat. And when seawater absorbs carbon dioxide, it both becomes more acidic AND increases in temperature. Dissolving a greenhouse gas into a liquid does not cause it to lose its heat-trapping abilities; in fact, too much CO2 makes ice more fragile & prone to melting, which would dilute the ocean and slow down global overturning circulation (read more here).
Phew! I hope you now understand why I took offense at the NYT’s assertion that ocean-atmosphere interactions were only partially responsible for European heat waves. Predicting when the Icelandic low & Azores high will weaken — and by how much — would be a great tool for governments worldwide to be able to adapt to climate change. But due to the complexity of ocean-atmosphere interactions, scientists have many theories about the NAO, and much more research needs to be done to understand the oscillation. The sooner (and better) we can understand the ocean’s influence on our weather, the easier it will be to adapt to climate change. Otherwise, we’ll be stuck in our hot tub as its jet stream weakens and its waters rapidly warm.