3.7.3 The Americas
The North American Monsoon System (NAMS) is characterised by ocean-land contrasts including summer heating of higher-elevation mountain and plateau regions of Mexico and the southwestern USA, a large-scale upper-level anticyclonic circulation, a lower-level thermal low and a strong subsidence region to the west in the cool stratus regime of the eastern North Pacific (Vera et al., 2006). The NAMS contains a strong seasonal structure (Higgins and Shi, 2000), with rapid onset of monsoon rains in southwestern Mexico in June, a later northward progression into the southwest USA during its mature phase in July and August and a gradual decay in September and October.
Timing of the start of the northern portion of the NAMS has varied considerably, with some years starting as early as mid-June and others starting as late as early August (Higgins and Shi, 2000). Since part of NAMS variability is governed by larger-scale climate conditions, it is susceptible to interannual and multi-decadal variations. Higgins and Shi (2000) further suggested that the northern portion of the NAMS may be affected by the PDO, wherein anomalous winter precipitation over western North America is correlated with North American monsoon conditions in the subsequent summer.
The South American Monsoon System (SAMS) is evident over South America in the austral summer (Barros et al., 2002; Nogués-Paegle et al., 2002; Vera et al., 2006). It is a key factor for the warm season precipitation regime. In northern Brazil, different precipitation trends (see Figure 3.14 for the Amazon and southern South America regions) have been observed over northern and southern Amazonia, showing a dipole structure (Marengo, 2004) that suggests a southward shift of the SAMS. This is consistent with Rusticucci and Penalba (2000), who found a significant positive trend in the amplitude of the annual precipitation cycle, indicating a long-term climate change of the monsoon regime over the semi-arid region of the La Plata Basin. In addition, the mean wind speed of the low-level jet, a component of the SAMS that transports moisture from the Amazon to the south and southwest, showed a positive trend (Marengo et al., 2004). Positive SST anomalies in the western subtropical South Atlantic are associated with positive rainfall anomalies over the SACZ region (Doyle and Barros, 2002; Robertson et al., 2003). Barros et al. (2000b) found that, during summer, the SACZ was displaced northward (southward) and was more intense (weaker) with cold (warm) SST anomalies to its south. The convergence zone is modulated in part by surface features, including the gradient of SST over the equatorial Atlantic (Chang et al., 1999; Nogués-Paegle et al., 2002), and it modulates the interannual variability of seasonal rainfall over eastern Amazonia and northeastern Brazil (Nobre and Shukla, 1996; Folland et al., 2001).