STATE OF THE CLIMATE IN 2009

For 2009 as a whole, the TRMM data suggestsrainfall was between 50% and 150% of normal overmost of the equatorial and tropical Pacific (except inthe far eastern sector south of the Equator), reflectingthe influence of opposite ENSO phases duringthe year.Fig. 4.31. (a) Atlantic ITCZ position inferred from outgoinglongwave radiation during May 2009. The coloredthin lines indicate the approximate position for thesix pentads of May 2009. The black thick line indicatesthe Atlantic ITCZ climatological position. The SSTanomalies (Reynolds et al. 2002) for May 2009 basedon the 1982-2009 climatology are shaded. (b) SST timeseries averaged over the Atlantic warm pool (5°W to30°W, 0°S to 5°S) for the period 1982–2009. The solidhorizontal central line indicates the climatologicalmean. The other solid horizontal lines represent theupper and lower terciles. The dashed horizontal lineputs the May 2009 record value in climate perspective.equatorward of its normal position and often moreintense (Fig. 4.29). In the OND quarter the SPCZtypically reintensifies, but this year the SPCZ wasweak, with suppressed convection in the central portionof the southwest Pacific. Rainfall in the islandswas very variable, but a number of islands (NewCaledonia, Vanuatu, Fiji, Tonga, and French Polynesia)experienced some months of very low rainfallduring this period. By contrast, enhanced convectionoccurred along the Equator west of the Dateline(Western Kiribati). Fig. 4.30 summarizes this typicalEl Niño rainfall pattern; the increased rainfall alonga line from 5°S, 150°W to 20°S, 120°W appears verysignificant, but this band of stronger convection is atthe very northeast edge of the SPCZ and the rainfallamounts are not very large.2) Atlantic—A. B. Pezza and C. A. S. Coelho(i)DescriptionThe Atlantic ITCZ is a well-organized convectiveband that oscillates approximately between 5°N–12°N during July–November and 5°N–5°S duringJanuary–May (Waliser and Gautier 1993; Nobreand Shukla 1996). As equatorial Kelvin waves canmodulate its interannual variability, ENSO is alsoknown to influence it on a seasonal scale (Münnichand Neelin 2005). In 2009, the Atlantic ITCZ wasinfluenced by ENSO and the Atlantic SST gradientbetween the Northern and the Southern Hemisphere.A transition occurred from a weak La Niña (January–March)to El Niño towards the end of the year,but the dynamic signature in the Atlantic was alsovery marked with a strong SST gradient towards theSouthern Hemisphere during the first half of the year(warmer towards the south, Fig. 4.31a) changing intoa reversed gradient pattern in the second half. As aresult the ITCZ reached its southernmost positionand maximum intensification in April and May,rapidly migrating back to the north from mid-June.The maximum intensification in May was accompaniedby outbursts of severe convection facilitatedby the ITCZ migrating into the core of the ‘Atlanticwarm pool’ with SSTs of the order of 30°C (Fig. 4.31b)and high values of CAPE producing positive rainfallanomalies over a large area of northeastern SouthAmerica and adjacent ocean near the equator (Fig.4.32a and b).(ii) Air France 447 disaster: did the Atlantic warmpool play a role?These formations are not rare but are within thevariability of an enhanced pattern intrinsic to yearsof very active ITCZ. It was unfortunate that a routinecommercial flight between Rio de Janeiro and Parisstarting its trip on the evening of 31 May (Braziliantime) crossed overnight an area of strong cumulustowers (cold tops) under development at about 3°N(Fig. 4.33), crashing into the ocean at about 0200UTC on 1 June. The accident made headlines internationallyand raised the question as to whether it issafe to fly over the ITCZ during very active periods.We note that there is no definite proof of causalityS102 | juNE 2010