Atmospheric cycles coming together have been indicted for frequent heat waves, says S.Ananthanarayanan.
Close on the heels of the brutal May-June in the plains of northern India and Pakistan, heat is raging in Europe, with temperature over 38°C. The death toll in the Indian subcontinent has been in thousands, with the heat wave in Pakistan reckoned the 8th worst in the history of the world and the one in India as the 5th worst. These events extend the series during the past decade, last in Russia in 2010 and the US in 2012 and studies show that the frequency of heat extremes would double by 2020 and quadruple by 2040
While man-made global warming explains gradual increase in spells of severe heat, the intensity and duration of events that we are seeing cannot be understood as a result of just warming of the globe. “The large number of recent high-impact extreme weather events has struck and puzzled us,” says Dim Coumou, lead author of a study at the Potsdam Institute for Climate Impact Research (Institut für Klimafolgenforschung, or PIK), published in the Proceedings of the US National Academy of Science (PNAS). The team, Dim Coumou, Vladimir Petoukhov, Stefan Rahmstorf, Stefan Petri, and Hans Joachim Schellnhuber, of PIK and the Santa Fe Institute, analysed massive sets of weather data from around the world and find that episodes of extreme heat are related to wave forms in the circulation patterns in the atmosphere acting together.
Planetary waves
The fact that the earth is surrounded by extensive oceans and a massive atmosphere, coupled with the rotation of the earth, leads to complex and very high energy circulation patterns. First, the fact of rotation leads to days and nights, and alternate warming and cooling of the sea and the atmosphere. Next, the tilt of the axis of the earth results in unequal warming of the hemispheres and low and high latitudes. Added to this is the fact of rotation itself, which brings about an eastward driver, called the Coriolis force, of air which is moving towards the poles. This force varies when the moving air rises or descends and there are conditions when the force on the mass of air can be westwards, creating counter-currents There is one current of warm air moving towards the poles and another of cool air coming down from the poles. With opposing Coriolis forces, the mixing of the two streams leads to an eastward stream, called the Jet Stream, at an altitude of 9 km, which airlines use to save fuel while flying east. Given the scale – viz, the mass and volume of air, the atmosphere weighs five billion million tonnes, and the distances in hundreds of kilometers, these effects of warming and the rotation of the earth, which is twice the speed of sound at the equator, lead to effects that are beyond the capacity of human control
The stream of the air mass also veers and strays in its flow, and returns, due to restoring forces, to veer the other way, and so on, which leads to meandering, periodic, wavy motion. These kinds of wavy motion are known as Rossby waves, after Carl-Gustaf Arvid Rossby, who explained how they came about. The wave formation in the cold circulation around the poles can become so strong, that a portion of it could break free to form a cyclone or an anticyclone to cause extremes of cold at lower latitudes. And there are Rossby waves that move northwards from the equator, bringing heat from the tropics to Russia, Europe or the US. The PIK study has shown that at times of extreme weather, the wave which brings great heat from the low to high latitudes could begin to move very slowly, so that the heat stays without dissipating for a long time, says a press release from PIK.
“Behind this, there is a subtle resonance mechanism that traps waves in the mid-latitudes and amplifies them strongly,” says Stefan Rahmstorf, co-author of the study. The study cites a proposal by an earlier group, Petoukhov et al, that combination of the right conditions could lead to the wave energy being trapped. The group had worked out the years, in July-Aug of the period from 1980 to 2011, in which these conditions, called resonance conditions, were fulfilled. The group used wind speed and other data and computed the timing of different components of the wave movement and found that there were 19 instances in 1980 to 2011, of the different components acting together, or a resonance, and these corresponded to events of extreme weather. A resonance happens when the frequency of a wave motion matches the native frequency of another system, which absorbs the energy of the wave and saves it from being dissipated. At such an event, the extent of up or down motion of both systems add up and the swing can be very wide. It is like a particular violin string that may be set vibrating by the voice of a singer. The listener then hears the singer as well as the violin string and the sound is louder.
The PIK group notes that in recent years there has been a cluster of resonance events and this rise coincides with the rapid warming of the Arctic. The PIK press release observes that since the year 2000, the Arctic has been warming nearly twice as fast as the rest of the globe. The reason is that for this warming is again global warming and the loss of Arctic sea ice. Loss of ice reduces the proportion of heat reflected back to space and a darker sea warms faster. And the rise in the frequency of resonance effects in planetary waves since 2000, with a rise in Arctic temperature by about 1°C, indicates how closely factors like temperature differences, which drive the weather, are linked with events like extreme climate.
The study, in fact, puts a finger on a mechanism by which marginal changes in global temperature act as a switch to turn on forces that have drastic consequences. “The planetary waves topic illustrates how delicately interlinked components in the Earth system are, and it shows how disproportionately the system might react to our perturbations,” says Joachim Schellnhuber, co-author of the study. If climate change continues unabated, severe heat extremes may affect 85% of the land area of the earth by the year 2100 and 60% would face extremes that are unknown today, the press note says.
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