Subseasonal Variations of Rainfall in South America in the Vicinity of the Low-Level Jet East of the Andes and Comparison to Those in the South Atlantic Convergence Zone

Abstract

Regional and large-scale circulation anomalies associated with variations in rainfall downstream of the South American low-level jet are identified and compared to those in the South Atlantic convergence zone (SACZ). Composites of precipitation associated with strong jets reveal an approximate doubling of the quantities one would expect from climatology, with an evolution of the rainfall pattern from south to north. The occurrence of extreme precipitation events follows a similar pattern. Meridional cross sections of composite wind reveal a distinct low-level jet near 20ЊS and a baroclinic development farther south that appears to force the jet. Geopotential height, temperature, and large-scale wind composites suggest that this developing disturbance is tied to a wave train that originates in the midlatitude Pacific and turns equatorward as it crosses the Andes Mountains. Similar composites based on SACZ rainfall reveal similar features, but of opposite sign, suggesting that the phase of the wave as it crosses the Andes Mountains determines whether rainfall will be enhanced downstream of the jet or in the SACZ. The alternate suppression or enhancement of rainfall in these adjacent regions results in a precipitation ''dipole.'' Many previous studies have found a similar out-of-phase relationship over many time scales. The phase of the Madden-Julian oscillation (MJO) is composited relative to anomalous precipitation events, revealing statistically relevant amplitudes associated with rainfall both downstream of the jet and in the SACZ. The MJO is a particularly interesting intraseasonal oscillation because it has some predictability. It is speculated that the slowly varying dipole that has been observed is a consequence of the preferred phasing of synoptic waves due to variations of the planetary-scale basic-state flow, which is at times associated with the MJO.

Moisture transport and intraseasonal variability in the South America monsoon system

“…The propagation of tropical and extratropical intraseasonal disturbances causes significant changes in regional atmospheric circulations (Jones and Carvalho 2002;Carvalho et al 2002a;Liebmann et al 2004) and in moisture transport (Herdies et al 2002) over SA that can persist from one up to five pentads (Muza et al 2009). The objective of the present study is to evaluate the transport of when convection increases over central SA during active phases of the monsoon with no evident organization of the SACZ over the ocean (LISAM IS events).…”

A Case Study of a Chaco Low-Level Jet Event

“…These data sets have also been widely used to describe SALLJ role in the South American climate -particularly in the warm season -and its impact on the hydrological cycle of the La Plata Basin (James and Anderson 1984, Sugahara et al 1994, Rasmusson and Mo, 1996, Nogués-Paegle and Mo, 1997, Nicolini et al 2002, Marengo et al 2004, Liebmann et al 2004, among others).…”

Observational evidences on the modulation of the South American Low Level Jet east of the Andes according the ENSO variability

“…For example, when moisture flux into southeastern South America via the low-level jet is strong, SACZ convection is weaker than normal (Sugahara et al, 1994;Liebmann et al, 1999). Moreover, Liebmann et al (2004) have shown the relationship among the SALLJ and extreme precipitation events over subtropical South America during austral summer. Using lead and lag composites of circulation for rainfall anomalies associated with the jet, they suggested that on a daily time scale, a preference for rain (no rain) in the SACZ may agree with a weak (strong) jet and dry (wet) conditions downstream of it.…”