MJO impacts on rainfall variability over South America

MARIANO S. ALVAREZ; CAROLINA S. VERA; GEORGE N. KILADIS; BRANT LIEBMANN

Montevideo

Congreso; WCRP Conference for Latin America and Caribbean: Developing, linking and applying climate knowledge; 2014

Facultad de Ciencias, Universidad de la República

Madden-Julian Oscillation (MJO) regional influence is described in order to provide fundamental basis for MJO monitoring in South America (SA). Monitoring and prediction of the intraseasonal variability and in particular of the MJO is very important for decision making of many socio-economic sectors that need climate information on weeks in advance. MJO was characterized by the Real-Time Multivariate MJO index, which classifies it in 8 phases according to the position of MJO associated convection. The probability of weekly-averaged rainfall on exceeding the median was used to compute probability maps for all seasons (December-February, March-May, June-August and September-November), so as to detect regions in which enhanced rainfall might be favored or disfavored due to MJO activity.Central Brazil, SESA and Argentinean Mesopotamia were the more MJO-influenced regions across seasons and phases. Circulation composites were also performed so as to dynamically explain the regions of rainfall enhancement. They can be related to divergent circulation changes at tropical regions (evaluated here through velocity potential anomaly composites) when MJO propagates across the continent. Moreover, circulation changes mainly at subtropical and extratropical latitudes of the continent can occur in association to MJO-induced Rossby wavetrains (RWT) emanating from tropical Indian and Pacific regions (diagnosed here through geopotential height anomaly composites). Both mechanisms are observed in the four seasons, though with some differences. DJF anomalous upper-level tropical convergence progresses from western tropical Pacific in phase 2 eastwards to tropical SA by phases 4 and 5, inhibiting upward motion in the SAMS region. On the other hand between phases 6 and 1, the anomalous upper-level tropical divergence progresses from central tropical Pacific into tropical SA by phases 7 to 1, where it favors anomalous upward motion and thus positive rainfall anomalies over the region. The latter in turn inhibits upward motion at subtropical regions and thus inhibiting convection. MAM and SON tropical circulation is quite similar to DJF, and also JJA. However, the latter season presents on SA mean dry conditions so changes in the tropical divergent circulation have the weakest impacts. On DJF, RWT emanate towards Bellinghausen - Amudsen seas, reflecting equatorward along South America, where a cyclonic anomaly develops, contributing to promote anomalous upward motion over SESA that explain the large chance of enhanced rainfall observed there between phases 2 and 5. During MAM, RWT have the weakest amplitude and reflects at high latitudes farther west than on DJF. On JJA and SON, the extratropical RWT is very coherent, but on JJA a subtropical RWT is also observed. In addition, while in JJA RWT propagate equatorward along South America, in SON the extratropical RWT does it along the southwestern Atlantic Ocean, influencing less the circulation anomaly in the continent.