ENSO-related moisture and temperature anomalies over South America derived from GPS radio occultation profiles

Abstract

El Nino Southern Oscillation- (ENSO) related anomaly patterns of monthly specific humidity (q), precipitable water content (PW) and temperature (T) over South America are analysed using the FORMOSAT-3/COSMIC mission data (August 2006–December 2013). The Global Positioning System radio occultation (GPS RO) technique provides high vertical resolution profiles of atmospheric properties (refractivity, density, temperature and water vapour pressure) with a global coverage under any weather condition. During the ENSO warm phase (or El Nino), positive PW anomalies are found over the equatorial Pacific Ocean, southeastern Brazil, Bolivia, Paraguay and northern Argentina. Negative anomalies are found in the northern part of South America, but covering a smaller area. During the ENSO cold phase (or La Nina), positive anomalies are found in the northern part of the continent whereas negative anomalies are found over the Amazon Basin region. A Morlet continuous wavelet transform analysis is applied to deseasonalized PW anomaly time series. Over the areas showing PW anomalies related to ENSO, main modes of oscillation between 35 and 38 months are found, similar to the main oscillation mode of the Oceanic Nino index. Consistent with previous studies, PW anomalies in almost all these areas show a general behaviour towards opposite signals during the opposite ENSO phases. q patterns show a linear (in phase with Oceanic Nino Index) response to ENSO along the equator below 8 km height. The Andes mountains separate positive (west) from negative (east) anomalies during El Nino (EN) events (reversing during LN). The higher q positive anomalies are found over the equatorial Pacific Ocean and over 60°–50°W, 35°–20°S during EN months. T patterns show positive anomalies in the whole tropical troposphere during EN months, with higher values over the equatorial Pacific, west of the Andes between 7 and 15 km height.