weather and climate: flux versus transport

For many, many years I have been taught that the climate is the “average weather.” But thinking about that, it’s not really true. There are a couple of reasons. One, there are many features that are part of “climate” that are not weather. For example there are short waves in the atmosphere caused by flow over the mountains, that are very important to climate, but they drive weather forecasters crazy. They’re gravity waves, if you want to look them up.

The role of weather in the climate system is to transport stuff. Ultimately, the weather needs to transport an excess of energy at the equator to the poles. The sun places more energy at the equator at the poles, but in an approximate sense, the Earth emits energy to space about the same at the poles as at the equator. The atmosphere and the ocean carry that energy, heat, to the poles. The weather also transports water and momentum. I’ve studied transport for years, and if there is one thing that I have learned, it is that if you want to calculate how much stuff is transported by the atmosphere, you do not make that calculation by averaging. A lot of the motions in the atmosphere slosh stuff back and forth. For transport to occur there has to be something that interrupts this sloshing. Something has to happen in order for there to be irreversible transport. Something like wave breaking – there has to be something that dissipates the motion so that it doesn’t simply return to where it started from. What’s really important to climate is - how do the weather systems dissipate? How do these transport events accumulate? Climate is really more of the accumulation of weather than the average of weather.

Here’s a figure. It’s complex and full of information. It may take several blogs to talk about it. At least two. This is a wavelet analysis of the north-south flux of water vapor at San Antonio, Texas. Some meteorologists call flux, “transport” but it is not. Flux is how much stuff goes by you. It might come back the other direction – flux is positive and negative.


Figure 1: Wavelet analysis of north-south moisture flux at San Antonio, Texas.

Back to the Figure. On the horizontal axis is time. On the vertical axis is period in days, as in the period of a wave (Labeled on the right hand side).

First you see the sloshing. The flux is first to the north (positive, red), then to south (negative, blue). It is also organized into periods. Through the month of May, there is a lot of activity in the 4-8 day period. These are the ordinary high and low pressure systems that we normally associate with weather, and take 4-8 days to cross Texas. Then starting in June this period stops, and there is the start of this short period, 1 day, oscillation. This is the flux associated with the low level jet in the Great Plains. Getting back to climate – these mechanisms, synoptic waves and the low level jet are important for carrying moisture to the continent, especially from the Gulf of Mexico. This is an important climate parameter. The mechanism of the transport changes with the season, and the seasonal transition is definitive. Seasonal transitions are part of the climate of a region. These mechanisms which are local in nature determine the regional climate and it is important to understand how these mechanisms will change with time.

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