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https://www.klimarealistene.com/2022/12/07/naturlige-klimaprosesser-intertropisk-konvergenssone/
Naturlige klimaprosesser – Intertropisk konvergenssone
- Kategorier: Atmosfære, Klimamodeller, Vitenskap
Klimanytt nr. 332. Redaktør: Ole Henrik Ellestad.
Forfattet av Ole Henrik Ellestad.
Noen titalls kjente, naturlige variasjoner påvirker atmosfæren, været og på lengre sikt klima. I den intertropiske konvergenssone (ITCZ) påvirkes vinder, temperatur, fuktighet, nedbør, tørke og dermed vegetasjon. Den flytter seg over tid med betydelige effekter, men merkverdig nok uten bidrag i IPCCs klimamodeller.
Den intertropiske konvergenssone (ITCZ)
Når bakken og atmosfæren varmes opp av sola dannes oppadstigende luftstrømmer (konveksjon) markert med røde tynne piler ut fra overflaten (bildet). De er spesielt kraftige i ekvatoriale områder der solens innstråling er mest intens. Lavtrykksområdet som dannes, genererer overflate-luftstrømmer fra omliggende områder mot ekvator. De bøyes av vestover langs breddegradene slik de røde tykke pileviser (jordrotasjon og Corioliskrefter). Strømmene fra nord og sør (passaten) møtes og danner et belte på +/- 5 breddegrader – den intertropiske konvergenssone. Dette påvirker også andre komplekse strømningsforhold i atmosfæren vist i bildet, men de omtales ikke her.
De oppadgående varme, fuktige luftstrømmer blir avkjølt og avgir store mengder fuktighet med kraftig skydannelse og store nedbørsmengder. Neste bilde viser tydelig skybeltet i ekvatoriale sone i Stillehavet/Mellom-Amerika med til dels betydelig avstand mellom luftstrømmene fra sørlig og nordlig hemisfære.
I de områdene passatvindene passerer ekvator, vil jordrotasjonen (Corioliskreftene) kunne snu de vestlige vindene i østligere retning. Svakere vinder og sterke vertikale strømninger er karakteristiske for konvergenssonen. Dagen kan starte med blå himmel. Skydannelsen tiltar ut over dagen, etter hvert med tordenskyer og kraftig energitransport oppover i atmosfæren, spesielt over landområdene, mindre over hav. Nedbøren kommer ut på ettermiddagen over land, men primært tidlig på morgenen (kaldest) over hav. Den ‘dype konveksjonen’ med vanndamp bryter gjennom tropopausen og bringer vanndamp helt opp i nedre del av stratosfæren (18 km høyde).
ITCZ danner den våteste sonen på jorden med store regnmengder i over 200 dager i året og bidrar til å forme klodens regnskoger samtidig som passatens utgangspunkt nær vendekretsene bidrar til dannelse av klodens dominerende ørkenområder.
ITCZ dannes i områder der det er varmest og følger i hovedtrekk solens veksling mellom vendekretsene. Fordampningen er kraftig, særlig over hav, og reduserer oppvarmingen og dermed intensiteten i konveksjonen. Landområder blir kraftigst oppvarmet og påvirker posisjonen til ITCZ (neste bilde) som gjenspeiler at det er mer hav på sydlige halvkule. Derfor ligger posisjonen i juli (rød, varmest på nordlig halvkule) noe nord for ekvator, mens januar (blå) varierer noe i det ekvatoriale beltet.
Vindretning, nedbør og temperatur påvirker landområdene når posisjonen endres og resulterer i værforandringer. ITCZ er en nøkkelfaktor i det hydrologiske kretsløp og atmosfærens sirkulasjonssystem i tropisk og subtropiske sone. Når dette varierer over tiår endres også klima.
Væreffekter
Med variasjoner i ITCZs posisjon varierer våtere og tørrere sesonger som med dramatiske endringer over lengre tidsrom kan utvikle seg til tørke- eller flom-perioder i berørte landområder. ‘Monsoon trough’ er en del av den vestlige Stillehavets ITCZ og gir bidrag til mye av klodens regnskoger. Også dannelsen av tropiske sykloner påvirkes.
Under det kjente værfenomenet El Niño (KN 120) i det ekvatoriale Stillehavet vil varmt vann som har hopet seg opp mot de vestlige øyene, bre seg østover mot Sør-Amerikas kyster og varme opp havoverflaten i enorme områder. Da kan passatvindene som bidrar til ITCZ, ofte forsvinne, og vestlige vinder vil blåse mot Perus og omliggende lands kyster. Været vil da skifte fra relativt tørt til fuktig.
Luftstrømmene som går nordover høyt oppe og gradvis synker ned vil influere transport av luftmasser mot polene og kunne påvirke været også i subtropiske områder.
Klima
Sesongvariasjoner som skaper værendringer og som forskyves systematisk over flere tiår medfører klimaendringer. Bildet over viser endringer av ITCZ i løpet av året med forskyvning mot nord i juli (rød) relativt til januar (blå). Under Den lille istid (LIA) ble hele systemet forskjøvet systematisk sørover som markert med sort pil (høyre), slik studier i en rekke områder i Afrika, Asia og karibiske øyer viser med våtere områder (blå sirkler betegnet W0-W4), mens de gule firkanter (05-013) ble tørrere.
Endringer i ITCZ er observert over tusenvis av år og er ikke relatert til CO2. ITCZ endrer seg med Milancowich-syklusene og endrer Hadley-luftsirkulasjoene helt til ca. 30 grader. I området rundt Iran er det påvist en ca. 60-års syklus – samme intervaller som påvirker Arktis (AMO) og nordlige Stillehavet (PDO). Påvirkning av havets omrøring (oppvelling) endrer også pH-verdiene i havet.
Konklusjon
ITCZ er en av klodens viktigste naturfenomener som former klimaet i tropene med følgeeffekter mot polene. Når den endrer seg på sesong-, års-, flerårs- og flere tusen års basis endrer også vær og klimaet seg. ITCZ påvirkes ikke av endret CO2 og er med sine to ‘bånd og ‘vandringer’ et meget komplekst system som ikke kan beregnes – en av årsakene til at naturlige variasjoner ikke markerer seg i IPCCs beregningsmodeller som derved blir feilaktige. Mer om klimabidragene i et senere KN.
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