The Pacific–South American modes and their downstream effects

Abstract

There are two pervasive modes of atmospheric variability in the Southern Hemisphere (SH) that influence circulation and rainfall anomalies over South America. They appear as leading empirical orthogonal functions (EOFs) of 500-hPa height or 200-hPa streamfunction anomalies and are found from intraseasonal to decadal time scales. Both patterns exhibit wave 3 hemispheric patterns in mid to high latitudes, and a well-defined wave train with large amplitude in the Pacific-South American (PSA) sector. Therefore, they are referred to as the PSA modes (PSA1 and PSA2).PSA1 is related to sea surface temperature anomalies (SSTAs) over the central and eastern Pacific at decadal scales, and it is the response to El Niñ o-Southern Oscillation (ENSO) in the interannual band. The associated rainfall summer pattern shows rainfall deficits over northeastern Brazil and enhanced rainfall over southeastern South America similar to rainfall anomalies during ENSO. PSA2 is associated with the quasi-biennial component of ENSO, with a period of 22-28 months and the strongest connections occur during the austral spring. The associated rainfall pattern shows a dipole pattern with anomalies out of phase between the South Atlantic Convergence Zone (SACZ) extending from central South America into the Atlantic and the subtropical plains centred at 35°S.These two modes are also apparent in tropical intraseasonal oscillations for both summer and winter. Eastward propagation of enhanced convection from the Indian Ocean through the western Pacific to the central Pacific is accompanied by a wave train that appears to originate in the convective regions. The positive PSA1 pattern is associated with enhanced convection over the Pacific from 150°E to the date line. The convection pattern associated with PSA2 is in quadrature with that of PSA1. Both PSA modes are influenced by the Madden Julian Oscillation and influence rainfall over South America.

Variability in the characteristics of cut‐off low pressure systems over subtropical southern Africa

“…Strong ENSO events were defined in terms of the Southern Oscillation Index and central and eastern equatorial Pacific sea surface temperature (SST) as in Allan et al (2003).The SAM was defined as the leading Empirical Orthogonal Function (EOF) of the anomalies in 700 hPa geopotential height over the Southern Hemisphere south of 20°S (similar to the US Climate Prediction Center definition). The second EOF calculated in this way reflects the PSA (Mo and Paegle, 2001) and its time series is used as the PSA index below. Following van Loon (1967), the semi-annual oscillation SAO index was defined as the difference of the zonal mean 500 hPa temperature between 50°S and 65°S.…”

“…Such interannual variability prompted an investigation of associations of cut-off lows with large-scale interannual modes that are known to influence the climate of the region such as the ENSO (Lindesay, 1988;Reason et al, 2000), the SAM (Reason and Rouault, 2005), and the PSA pattern (Mo and Paegle, 2001). ENSO is known to project strongly over the southern African and South Atlantic regions, particularly in JJA of the onset year, and the following December, January, February (DJF) of the mature phase year (Lindesay, 1988;Reason et al, 2000;Colberg et al, 2004).…”

Interannual variability in rainy season characteristics over the Limpopo region of southern Africa

“…Figure 6 suggests that early onset over Limpopo is associated with an anticyclonic (cyclonic) anomaly to the southeast of South Africa (in the southwest Atlantic). These features are suggestive of a Rossby wave train that extends from the tropical central Pacific across the South East Pacific and mid-latitude South Atlantic Oceans and into the South West Indian Ocean and are reminiscent of the Pacific South America (PSA) pattern associated with ENSO (Mo and Paegle, 2001;Colberg et al, 2004). Indeed, the SST anomalies show an El Niño-like warming in the tropical Pacific with the east-west zonal SST anomaly contrast across the tropical Indian Ocean that typically occurs during the austral spring of an El Niño event (Reason et al, 2000).…”

Interannual relations between South American rainfall and tropical sea surface temperature anomalies before and after 1976

“…These anomalous circulation features are indeed a part of the southern branch of the Rossby-wave train induced by the tropical heating associated with El Niño. This branch is located in the Pacific-South American sector and closely relates to anomalous rainfall in southern South America (Mo and Nogués-Paegle, 2001). The wet conditions over subtropical South America are also favoured by the stronger than normal low-level southward flux of moisture east of the Andes (Nogués-Paegle et al, 2002).…”