Hydrologic impact of cold episodes in the Central Peruvian Andes over South America during the Austral Summer (1958–2009)

The Mantaro valley (MV) is located in the central Peruvian Andes. Cloud cover over the MV has a well-defined annual cycle and its maximum is observed during austral summer (January-March). Occasionally, cold episodes are observed in this season, which can lead to significant crop damage. However, there is no existing literature, which has analyzed the causes and impacts of such cold episodes in the MV. The main goal of this study is thus to characterize cold episodes in the MV and assess their large-scale hydrologic impacts over South America (SA) during austral summer.

To identify cold episodes in the MV we used daily minimum temperature from Huayao station, which is located within the MV for the period 1958-2009. We defined a cold episode as the period when daily minimum temperature drops below the 10-percentile for at least one day. To characterize the large-scale circulation and cloud cover associated with these events, we used wind, geopotential height and outgoing longwave radiation (OLR) data from NCEP/NCAR and ERA40 reanalysis and NCAR/NOAA for the same period. To characterize rainfall anomalies over SA during these events, we used daily rainfall data from Huayao station and daily gridded rainfall data for eastern SA.

Our results show that cold episodes in the MV are associated with positive OLR anomalies, which extend over much of the central Andes, indicating the large-scale nature of these events. Both reanalysis show anomalous westerly zonal winds aloft accompanying cold events at 200 hPa; while at low levels (850 hPa) an anomalous cyclonic circulation with negative geopotential height anomalies develops near 30°S, 50°W.

Furthermore, the dry conditions observed during cold episodes in the MV tend to coincide with significant contemporaneous wet anomalies over northeastern Brazil (NEB). Negative OLR anomalies over NEB are caused by a displacement of the South Atlantic Convergence Zone (SACZ) toward the northeast of its climatologic position. The Peruvian Andes – NEB OLR dipole is corroborated by a composite analysis during the previous days, showing the establishment of this rainfall dipole on day -1 and day -2. Upper-tropospheric circulation anomalies over the central Peruvian Andes extend across South America and are tied to changes of the opposite sign over NEB through a weakening of the Bolivian High - Nordeste Low (BH-NL) system.

In conclusion, cold episodes in the MV appear to be caused by radiative cooling associated with reduced cloudiness, rather than cold air advection. The reduced cloud cover in turn results from a robust large-scale pattern of westerly wind anomalies over central Peruvian Andes, inhibiting moisture influx, convective activity and hence cloud formation. At the same time NEB registers strong convective activity and enhanced cloud cover. This dipole is caused by a weakening of the BH-NL system at upper levels, which is associated with a low-level migratory high-pressure center, propagating from mid- to low latitudes as part of an extratropical Rossby wave train.