Re : Wetenschappers dachten in de jaren 90 dat de opwarming
Bericht van: Ben (Lelystad) , 29-10-2009 19:39[...]
Dat denkt men nog maar de opwarming is daarvoor niet ver genoeg gevorderd, zo simpel is het.
Daar is men feitelijk nog enigszins over verdeeld, maar de signalen wijzen vooral richting een kleiner aantal stormen als gevolg van afgenomen barocliniteit. Dat is weer een gevolg van de verwachte sterkere opwarming van de polen dan de gebieden rond de evenaar. Over extratropische cyclonen (depressies, stormen) op het noordelijk halfrond citeer ik Geng et. al., 2003:
,,1) The total cyclone density (number of cyclones in a 4.5° × 4.5° area per season) tends to decrease significantly in the midlatitudes of both of the Northern and Southern Hemispheres during the December–January–February (DJF) and June–July–August (JJA) seasons. The decrease of cyclone density in the midlatitudes of both of the Northern and Southern Hemispheres in the DJF season is about 7%. In the JJA season, the decreases of cyclone density in the Northern and Southern Hemispheres' midlatitudes are about 3% and 10%, respectively. 2) Although weak and medium-strength cyclones decrease, the density of strong cyclones increases by more than 20% in the Northern Hemisphere in JJA and in the Southern Hemisphere in both DJF and JJA. 3) The density of strong cyclones in the Northern Hemisphere summer (JJA) increases over the eastern coasts of Asia and North America.''
Samengevat: het aantal stormen neemt af, maar de stormen die nog wel optreden pakken heftiger uit.
Waardoor komen die veranderingen precies? Het onderzoek: ,,It is shown that the changes in the extratropical cyclone activity are closely linked to the changes in the baroclinicity in the lower troposphere, which are mainly related to the changes in the horizontal and vertical temperature distributions in the atmosphere due to enhanced greenhouse gases and sulfate aerosols. It is shown that, in the Northern Hemisphere midlatitudes, the decrease of baroclinicity is mainly caused by the decrease of meridional temperature gradient, while in the Southern Hemisphere midlatitudes, the decrease of baroclinicity is mainly caused by the increase of static stability caused by the enhanced greenhouse gases and sulfate aerosols.''
Gr. Ben
Bronnen:
Geng, Q., and M. Sugi, 2003: Possible Change of Extratropical Cyclone Activity due to Enhanced Greenhouse Gases and Sulfate Aerosols—Study with a High-Resolution AGCM. J. Climate, 16, 2262–2274.
