Atmospheric centres of action associated with the Atlantic ITCZ position

摘要

Relationships between Atlantic extra-tropical features and Intertropical Convergence Zone (ITCZ) position close to Northeast Brazil are discussed, on the basis of atmosphere and ocean patterns in DJF and April. Composites for three situations were analysed. In the first composite, years with extreme positive North Atlantic Oscillation Index (NAOI) in DJF and the ITCZ displaced southward in April were selected. The other two composites were related only to anomalous ITCZ positions - either south or north of the equator - in April. The physical mechanism for the relation between the NAOI and the ITCZ is the influence of the southern North Atlantic Oscillation (NAO) centre on the North Atlantic subtropical high, the sea surface temperature (SST) and the intensity of the trade winds; however, this situation occurred in only a few years. Analysis of the other situations indicated the influence of a dominant mode of variability over the North and South Atlantic Oceans that affects the ITCZ position. Over the North Atlantic, this mode was related to a shifting of the NAO pattern, which modified the position of the North Atlantic subtropical high. Over the South Atlantic, this mode indicated an extra-tropical centre of action associated with displacement of the South Atlantic subtropical high. A proposed index for monitoring ITCZ position, named the North Atlantic Index (NAI), was calculated from sea level pressure (SLP) anomalies in the two centres of action of the North Atlantic in DJF, which are related to the shifting of the NAO. Although the NAO index is related to atmospheric/oceanic features associated with ITCZ displacement, the new index presented higher and more organized correlations with atmospheric and oceanic conditions linked to ITCZ behaviour and can be useful even when the NAO is not extreme.