Friday, September 30, 2011
Monday, September 26, 2011
Besides the scales imposed by the planet itself and its motions in the solar system, the atmosphere and ocean can generate their own scales. A characteristic of fluids is that their motions can, at least in principle, have any scale that their total dimensions allow. In any turbulent fluid motion – for example, a smoke plume billowing from a smokestack - there are whirls or “eddies” of many different sizes, from the total size taken up by the fluid (say, the total width of the smokestack plume) down to something much smaller. This multiplicity of scales is almost a working definition of turbulence, in fact, and the atmosphere and ocean are turbulent in this sense.
In a turbulent fluid, like in other physical systems (e.g., atomic gases) we can compute a “spectrum”, in either space or time. If we have enough measurements of the fluid motion, we can compute a spatial spectrum which tells us how much energy is in eddies of each size, or a temporal spectrum which tells us how much energy is in fluctuations of each frequency. The idea and the math involved are exactly the same as for the wavelength and frequency spectra of atomic gases, but the results are completely different. In a fluid undergoing pure turbulent motion, there are no distinct frequencies or wavelengths which have either much more or less energy than those nearby – no spectral lines. Rather, the spectrum is smooth, but "red". This has nothing to do with the color red now, but just means there is more energy in large scales than small. The bigger and slower the eddy, the more energy it has, but no specific size or speed is favored - there are no particular natural scales that emerge from the dynamics.
The atmosphere and ocean are turbulent, but they are not pure turbulence. On top of the overall continuous red spectrum, there are a few “peaks” – not precise frequencies as in atomic gases, but distinct frequency ranges (just like the range of heights in a group of human beings, different but all not too far from a typical value) – with which the fluids oscillate preferentially. These correspond to particular weather or climate phenomena, and the task of atmosphere and ocean science is to explain these phenomena. Just as in other sciences, any successful explanation must explain the scales.
One example is the “synoptic” low and high pressure systems that produce most of the weather in the middle and high latitudes. These have time scales of a few days – most of the time, we know that’s about how long the weather we’re experiencing at the moment is likely to last – and spatial scales of a few thousand km (say, the size of the United States, or some good fraction of it; look at the weather map today, and see how big the region is that is under roughly the same weather pattern as where you live). The space and time scales are related by a speed; if you know how far away the next weather change is, and you know how fast it is moving, you can figure out how soon it will arrive. That speed is also an important scale of the system, a velocity scale. For the midlatitude weather, it’s a few meters per second; this happens to be (not entirely by accident) similar to the scale of the actual wind velocity near the surface of the earth.
The scales of midlatitude synoptic weather systems were successfully explained by the theory of baroclinic instability developed by Jule Charney and Eric Eady in the 1940s, building on earlier work of Carl-Gustav Rossby and others. The enterprise of modern numerical weather prediction – the practice of using computers to solve equations derived from physics in order to produce the weather forecasts that we all rely on – grew out of that theory. The scales are ultimately related to the natural scales of the system – the size and rotation rate of the earth, etc. – but not in a simple way. The inherent dynamics of the fluid system plays an essential role in setting the scales of the weather systems we observe.
Saturday, September 17, 2011
The MJO is a distinct, coherent mode of variability, with well-defined space and time scales. There are not many such modes in the climate system, and it behooves us to understand the few that there are very well. They provide most of the useful predictability that we can use in making forecasts, and are also fundamental to our understanding of the system as a whole.
When we say the MJO is a distinct, coherent mode of variability, what do we mean by that? We mean that while the weather tends to fluctuate in ways that are chaotic and difficult to predict, certain patterns are there which give a certain degree of order to the chaos. These patterns are distinguished by particular scales in space and time – how big they are and how long they last. In this post I will give a little more context to this idea, before going on to discuss the MJO in particular.
In the sciences, an essential part of explaining anything is explaining its scales, meaning the magnitudes of key quantities expressed in physical units. For example, some of the scales of a biological species – say, homo sapiens - are its physical size (height and weight), its lifetime, the mass of its brain relative to its total body mass, etc. All these quantities vary from one individual to another, but they are well bounded within a fairly narrow range – no adult human being is two inches tall or 20 feet tall, for example. Any theory of human evolution which doesn’t include some explanation of these scales (about two meters for height, a few decades to maybe a century for lifetime, etc.) would be pretty poor.
To take an example from physics, it was discovered in the 19th century that atomic gases have distinct spectra, meaning that when stimulated they emit light only of very specific colors. Since light is a wave, a color corresponds to a particular wavelength as well as a specific frequency. Hydrogen, say, emits light at a precise set of frequencies, but not at any others in between those. Why is that, and what determines which frequencies they are? Quantum mechanics emerged in the early 20th century in part to answer these questions.
In the climate system, there are a few natural space and time scales that are imposed by external factors. The size of the earth itself – its radius and surface area - determine the horizontal size of the atmosphere and oceans, since obviously those have to fit on the planet. That gives us at least one inherent spatial scale.
What about time scales? The length of the day is set by the rate of rotation of the earth on its axis, and the length of the year is set by the orbit of the earth around the sun. These two astronomical rotation periods drive the most regular, and thus most predictable variations in the weather and climate. We know without a doubt that night will be darker, and usually cooler compared to day; and (outside the tropics at least) that winter will be colder compared to summer. We take for granted that we know and understand these very large and important modes of variability – the diurnal and annual cycles – very well, and that all the effort we put into predicting weather and climate is about predicting just the differences from those well-known cycles. It’s worth keeping in mind, though, that their real causes weren’t always so well understood.
Monday, September 5, 2011
MJO as a fluctuation of the monsoon
Over the wettest parts of the tropics, the climate tends to be monsoonal. This means two things:
1. There is a wet season and a dry season,
2. The wind blows in different directions in the two seasons. Westerly (from the west to the east) in the rainy season, and easterly (from the east to the west) in the dry season.
In monsoonal climates, most of the rain that falls in the entire year falls in the wet season; the dry season tends to be almost completely dry. Temperature, on the other hand, doesn’t differ all that much between the seasons. This is the opposite of what happens in some midlatitude climates. For example, in New York City, the temperature is tremendously different in summer and winter, but the amount of precipitation is pretty similar in the two seasons (though some of it is snow in the winter). Another difference is when it is hottest or coldest. In monsoon climates, the rainy season comes at what would be the peak of summer, when the sun is high and you would think it would be hottest; but all the clouds and rain cool things down. The hottest time is actually the late dry season, just before the monsoon, which would be “spring” at higher latitudes.
We think of the monsoons as occurring over land. India in particular is the most famous monsoon region. The Indian monsoon is not just India’s, but extends into southeast Asia as well. Northern Australia and northern South America also have monsoons, as does West Africa, and (arguably) southwestern north America. But the land monsoons are not the whole story. Actually the climate is monsoonal over large regions of tropical ocean also. The tropical Indian ocean experiences the monsoon just as much as India itself does, and so does the tropical western Pacific (say, from Asia to the date line, within 10 or 20 degrees of the equator). By this we mean that over these parts of the oceans, there is a season when it rains a lot and a season when it doesn’t, and the wind shifts from easterly in the dry season to westerly in the rainy season.
Just like summers and winters at higher latitudes, monsoon seasons aren’t exactly the same from one year to the next. Some wet seasons are wetter and some drier than others. Some start earlier, or later; or end earlier or later. Once the rains do start, they may keep going for a long time, but on the other hand they may stop and start. Sometimes there are “breaks” in the middle of monsoon season, during which it may rain little or not at all.
While rainy periods and breaks don’t come and go with complete regularity, there is some rhythm to them. The weather tends to switch from rainy to dry a few times over the course of a rainy season. This is what we mean by “intraseasonal” variability. The period of “intraseasonal oscillations” – the time for one complete cycle to complete itself - is sometimes quoted as 40-50 or 30-60 days, but it’s just as well to think of it as “a few times in a season”. These intraseasonal oscillations affect the land monsoon regions, but are actually strongest over the oceans.
What Roland Madden and Paul Julian discovered in the early 1970s is not just that these active and break periods occur over the oceans (to people living on the islands or adjacent continents this surely was known already) but that there are connections between all the active and break cycles occurring in different places. The activeness or breakness – the “phase”, in jargon – occupies a large chunk of real estate at any given time, but moves at a leisurely pace from west to east. (In northern hemisphere summer, the Indian and southeast Asian monsoon season, it can also move from south to north.) This is the MJO – the eastward-propagating, intraseasonal variability of the monsoons, especially (but not only) over the oceans.