Zanimljiv tekst o zagrevanju stratosfere i njegovom uticaju na vreme, ima dosta strucnih termina, pa ako bi neko strucniji mogao da procita i da priblizi to svima ostalima na forumu. Tekst je na engleskom ali verujem da to nece nikome biti problem, i izvinjavam se za off, ali ipak je ovo povezano sa onim sto se trenutno desava, bilo bi dobro da se jos po nesto nauci. Evo tekst:
Sudden stratospheric warmingAsudden stratospheric warming(SSW) is an event where thepolar vortexof westerly winds in the winter hemisphere slows down or even reverses direction over the course of afew days. The change is accompaniedby a rise of stratospheric temperature by several tens ofkelvins.HistoryThe first continuous measurements ofthe stratosphere were taken by Richard Scherhag in 1951. He usedradiosondesto take reliable temperature readings in the upperstratosphere(~40 km). It was his persistence which led him to witness the first ever observed stratospheric warming on 27 January 1952. After his discovery, Scherhag assembled a team ofmeteorologistsspecifically to study the stratosphere at theFree University of Berlin. This group continued to map the northern-hemisphere stratospheric temperatureandgeopotential heightfor many years usingradiosondesandrocketsondes. In 1979 when thesatelliteera began, meteorological measurements became far more frequent. Althoughsatelliteswere primarily used for thetropospherethey also recorded data for the stratosphere. Today bothsatellitesand stratosphericradiosondesare used to take measurements of thestratosphere.Classification and descriptionTypically stratosphericmeteorologistsclassify the vortex breakdown into three categories: major, minor, and final.Sometimes a fourth category, the Canadian warming, is included because of its unique and distinguishing structure and evolution.MajorThese occur when the westerly winds at 60N and 10hPa (geopotential height) reverse, i.e. become easterly. A complete disruption of thepolar vortexis observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole.According to theWorld MeteorologicalOrganization's Commission for Atmospheric Sciences (Mclnturff, 1978):a stratospheric warming can be said to be major if 10 mb or below the latitudinal mean temperature increases poleward from 60 degree latitude and an associated circulation reversal is observed (that is, the prevailing mean westerly winds poleward of 60 latitude are succeededby mean easterlies in the same area).MinorMinor warmings are similar to major warmings however they are less dramatic, the westerly winds are slowed, however do not reverse. Therefore a breakdown of the vortex isnever observed.Mclnturff states:a stratospheric warming is called minor if a significant temperature increase is observed (that is, at least 25 degrees in a period of week or less) at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly to easterly is less extensive.FinalThe radiative cycle in thestratospheremeans that during winter the mean flow is westerly and during summer it is easterly (westward). A final warming occurs on this transition, so that thepolar vortexwinds change direction for the warming, however do not change back until the following winter. This is because thestratospherehas entered the summer easterly phase. It is final because another warming cannot occur over the summer, so it is the final warming of the current winter.CanadianCanadian warmings occur in early winter in the stratosphere of the Northern Hemisphere, typically from mid November to early December. They have no counterpart in the southern hemisphere.DynamicsIn a usual northern-hemisphere winter, several minor warming events occur, with a major event occurring roughly every two years. One reason for major stratospheric warmings to occur in the Northern hemisphere is becauseorographyand land-sea temperature contrasts are responsiblefor the generation of long (wavenumber1 or 2)Rossby wavesin thetroposphere. These waves travel upward to thestratosphereand are dissipated there, decelerating the winds and warming the Arctic. This is the reason that major warmings are only observed in the northern-hemisphere, with one exception. In 2002 a southern-hemisphere major warming was observed.[1]This event to date is not fully understood.At an initial time ablocking-type circulation patternestablishes in the troposphere. This blocking pattern causes[clarification needed]Rossby waves with zonal wavenumber 1 and/or 2[clarification needed]to grow to unusually large amplitudes. The growing wave propagates into thestratosphere and decelerates the mean zonal winds.[clarification needed]Thus thepolar night jetweakens and simultaneously becomes distorted by the growing planetary waves. Because the wave amplitude increases with decreasing density this easterly acceleration process is not effective at fairly high levels.[why?]If the waves are sufficiently strong the mean zonal flow may decelerate sufficiently so that the winter westerlies turn easterly. At this point planetary wavesmay no longer penetrate into the stratosphere[2][clarification needed]). Hence further upward transfer of energy is completely blocked and a very rapid easterly deceleration and the polar warming occur at this critical level, which must then move downward until eventually the warming and zonal wind reversal affect the entire polar stratosphere.There exists a link between sudden stratospheric warmings and thequasi-biennial oscillation: If the QBO is in its easterly (westward) phase, theatmospheric waveguideis modified in such a way that upward-propagatingRossby wavesare focused on thepolar vortex, intensifying their interaction with the mean flow. Thus, there exists a statistically significant imbalance between the frequency of sudden stratospheric warmings if these events are grouped according tothe QBO phase (easterly or westerly).Weather effectsAlthough sudden stratospheric warmings are mainly forced by planetary scale waves which propagate up from the lower atmosphere, there is also a subsequent return effect of sudden stratospheric warmings on surface weather. Following a sudden stratospheric warming, the high altitude winds reverse to flow eastward instead of their usual westward. The eastward winds progress down through the atmosphere and weaken the jet stream, often giving westward winds near the surface and resulting in dramatic reductions in temperature in the Eastern U.S. and Europe.
