1) The length of day has steadily increased throughout the earth’s history. Geological evidence1 suggests that it was only 21.9 hours 620 million years ago (Mya). In other words, the earth rotated nearly 10%faster 620 Mya. If we assume the earth’s atmosphere had same temperature distribution then as it does now (which it certainly did not), what changes in the primordial wind speeds (faster or slower) would expect relative to today conditions over the middle latitudes? Use concepts of chapter 6 of Ahrens to defend your answer.
The temperatures of the earth have increased over time. Taking an assumption that the temperature distribution is the same with time, the primordial wind speeds have become faster in the mid-latitudes due to increased variation in the temperature gradient between different layers of the atmosphere. An increase in the length of day on the other hand creates a disparity in the temperature difference between day and night therefore increasing the temperature gradient and consequently the wind speeds in the middle latitudes.
The most recent assessment from International Panel on Climate Change (IPCC 2103) states that during the next 20 years (2016-2036; see Fig. 11-10 of AR5-WG1), the average surface temperatures over high latitudes of the wintertime Northern Hemisphere will warm by 3˚C, which is more than any other region of the world. On the other hand, the tropics are expected to warm the least, 1˚C or less. If the IPCC surface temperature projections materialize, it follows that features of the global circulation during northern hemisphere winter could change too. Answer the last two questions based on the IPCC projections.
2) What changes in the intensity (i.e. average speed) of the polar jet stream (increase or decrease) would you expect if the polar troposphere warms 2˚C more than the tropics? Explain your answer using concepts of module 2. Material in chapters 6 and 7 of Ahrens should prove especially useful.
Polar stream jets are powerful winds that occur at the boundary of the troposphere and the stratosphere. They travel on narrow strips while covering larger extent of latitudes therefore causing a lift in air masses. When the troposphere temperatures increase more than that of the tropics a partial vacuum is created which increases the average speed of the polar jets. An increase in the temperature difference between the troposphere and the stratosphere increases the temperature gradient therefore increasing the intensity of the polar jet stream. Therefore the average speed of winds would increase with an increase in the temperature gradient.
- What changes in the intensity and frequency of extratropical cyclones during winter would you expect if average tropics-to-pole temperature difference decreases by 2˚C? Again, explain your answer using concepts of module 2. Material in chapter 8 of Ahrens should prove quite valuable.
Tropics to pole are characterized by lower temperatures due to the orientation and distance from the sun among other factors as compared to the latitudes around the equator. Further decrease in the temperature in the tropics to pole latitudes by two degrees centigrade increases the temperature gradient, that is the, temperature difference with the equatorial and tropics latitudes. This increased temperature gradient increases the intensity and frequency of the cyclones and strong winds between these two regions. Therefore, the occurrence of cyclones will be more frequent while on the other hand the cyclones will be of high magnitude and speed during winters.