Does the MJO impact dry spells in the Central Peruvian Andes?

The Mantaro basin (MB) is located in central Peruvian Andes. Rainfall in the MB has a well-defined annual cycle and its maximum is observed during austral summer (December-March). Occasional, dry spells occur in MB during this season. However, there is no previous research about teleconnections between these events and any large-scale phenomenon over South America (SA). This study investigates the association between dry spells in MB and the Madden-Julian Oscillation (MJO).

To identify dry spells in MB, we used daily rainfall data from 14 climate stations for austral summer for period 1965-2002. Here, we defined dry spells to include daily rainfall ≤ 30% percentile for at least 5 consecutive days. To identify MJO phase associated with these events, we used the Real-time Multivariate MJO index (RMM1, RMM2) available since 1974. To characterize the large-scale circulation and convective activity, we used daily winds and specific humidity from the NCEP-NCAR reanalysis while daily outgoing longwave radiation (OLR) data from NCAR/NOAA for same period.

Dry spells in MB are associated with positive OLR anomalies, which extend over much of the tropical Andes, indicating the large-scale nature of these events. At 200 hPa anomalous westerly zonal winds aloft accompany dry spells. Our composite analysis of dry spells reveals significant contemporaneous humidity anomalies of opposite sign over northeastern Brazil, which suggest that intraseasonal humidity variability over the two regions may be dynamically linked. Upper-tropospheric circulation anomalies over the central Peruvian Andes extend across South America and are tied by weakening of Bolivian high-Nordeste low (BH-NL) system.

Dry episodes tend to occur during RMM Phases 1, 7, and 8, and RMM Phases 2 and 3. RMM phases 1, 7 and 8 represent 46.9 % of total dry spells in MB while RMM Phases 1 and 2 represent 18.4%. These RMM phases are associated with dry spells in the MB because they are associated with reduced convective activity over western Pacific Ocean, which weakens BH-NL system over SA. Whereas, RMM phases 4, 5, and 6 (18.4%) inhibit these events.