However, about six times a decade, this vortex can break down in dramatic fashion. The disturbance in the stratosphere can then be transmitted downward through the atmosphere. If this disturbance reaches the lower levels of the atmosphere it can affect the jet stream, a current of air which normally snakes eastwards around the planet, dividing colder polar air from warmer air to the south.
Where the jet stream crosses the Atlantic it usually points towards the British Isles, but sudden stratospheric warming can lead it to shift towards the equator. As air currents are temporarily rearranged, warmer Atlantic air is replaced by cold air from Siberia or the Arctic, and Europe and Northern Asia may experience unusually cold weather. It can take a number of weeks for the impact of stratospheric warming to reach the surface, or the process may only take a few days.
These events are hard to predict in advance. Rather, stratospheric conditions impose constraints on weather and climate variability, and thus can extend the predictability beyond the day limit, in the same manner as for sea surface temperature or sea-ice cover. Founded in , SPARC has coordinated high-level research activities related to understanding Earth system processes for over two decades. SPARC promotes and facilitates cutting-edge international research activities on how chemical and physical processes in the atmosphere interact with climate and climate change and in particular, takes a lead in organizing a variety of projects focused on many issues related to atmospheric predictability.
Recent advances in the research on stratosphere-troposphere coupled system inspired the drafting of this short summary of how and when the stratosphere provides climate predictability, which should be of interest to a larger audience of readers concerned with adapting to and mitigating the effects of climate change.
In the tropics, the stratosphere starts somewhat higher, at altitudes of about 18 km. The dominant feature of the stratosphere in the winter is a cold circumpolar vortex surrounded by strong westerly winds forming a polar night jet. This jet varies in strength, at times being characterized by anomalously strong winds, while at other times being anomalously weak.
When the polar night jet weakens, the westerly winds can sometimes reverse and even become easterlies. During such periods, the polar vortex warms by several tens of degrees and can move off the pole or even split into smaller pieces. Such periods are called sudden stratospheric warmings.
In the summer, easterly winds prevail and the season is dynamically quiescent with little variability apart from the slow seasonal changes. When the stratospheric polar night jet is anomalously weak, the storm tracks shift towards the Equator. This allows intrusions of cold Arctic and continental air masses into areas with a more temperate climate. In the northern hemisphere, such cold air outbreaks usually occur over northern Europe and eastern United States of America.
In the opposite case, when the polar night jet is anomalously strong, the storm tracks move poleward and bring mild temperatures and moist air to northern Eurasia. Once the winter stratosphere shifts towards anomalous conditions, it may take up to several weeks before it returns to normal.
This persistence of stratospheric anomalies helps to maintain tropospheric circulation and surface climate in anomalous states and thus contributes to enhanced predictability. So, when the winter stratosphere is in an anomalous state, climate conditions in mid-latitudes become more predictable. In extreme cases, such as sudden stratospheric warmings, skilful predictions of mean temperatures and the probability of cold air outbreaks are possible for periods of up to two months.
Extended predictions can be made either by initialized dynamical systems or even by using statistical methods. But what is the cause of these stratospheric anomalies and can we predict them a season or even a year ahead? The Mint off-road race, held just outside Las Vegas, concluded last Saturday evening with a tremendous last minute pass by Las Vegas resident, and second generation off-roader, Rob MacCachren taking home the overall victory, after 15 attempts at the coveted checkered flag.
Rob had won everything else there was to win, but the Mint […]. This is my favorite time of the year, as it is for a lot of us who see spring just around the corner. He was a member of the United States Army Rangers. You can nicely see on the image below, how the average polar cap temperature increased rapidly. It raised well above the long-term average line, showing the typical stratospheric warming temperature spike.
Zonal winds also decreased, indicating a collapse of the Polar Vortex. The winds never fully reversed during the main event, going negative, but the event was strong enough to produce important weather effects in the coming weeks. Now, why was this important? If you look at the image below, you can see a simple schematic, of the global air circulation, between the hemispheres.
There is much more flow and dynamics in the Winter hemisphere, while the Summer hemisphere is calmer generally. In reality, it is far more complex, but this simple schematic shows the main idea of airflow and energy transfers.
Below we have interesting graphics from a study made this year, about the stratospheric warming event in over the South Pole. The first image shows the Ionospheric anomaly over the United States, following the strong stratospheric warming event over the South Pole.
The same, but reversed change was observed over Europe, as the energy wave from the strong Stratospheric warming event already reached the Northern Hemisphere. Not just in the higher atmosphere, but also in the lower atmosphere, direct weather changes were observed following this strong event.
The strong stratospheric warming event in over the South Pole had one more important effect. Ozone is the protective layer in the atmosphere, that shields us from dangerous ultraviolet solar radiation. In the late 20th century, emissions of chemical substances called halocarbons affected the number of ozone molecules in the atmosphere.
It resulted in the creation of dramatic annual ozone reduction over the Antarctic region, known as the Ozone Hole. These chemicals, together with sunlight and very cold temperatures, start a photochemical process, that destroys the Ozone. If one ingredient is missing, the ozone molecules and the ozone layer cannot be destroyed.
Below we have a very cool graph, that shows the area of the ozone hole over Antarctica. Red line shows this year, and we are still t a small size, but at the beginning of the season. The green line shows , and it is a very low level, actually one of the lowest in past decades. Blue line is last year, with a near-record large ozone hole. The Ozone destruction process was limited in , thanks to the strong stratospheric warming event that we discussed above over the south pole.
It limited the cold air available, thus greatly limiting the ozone destruction process. We can see the ozone mass deficit in the image below. It is the amount of ozone destroyed by the chemical process. In green line there was very little ozone destruction, as the stratospheric warming event limited the ozone destruction process.
Looking at the latest conditions, we can see a small ozone hole already developed. The image below shows total atmospheric ozone, with a significant lack of ozone in the middle. That is the infamous ozone hole. But looking ahead at the forecast, we can actually see the ozone hole reducing in size, while it should be growing at this time of year.
0コメント