Simulation of the MJO and its influence on South American intraseasonal variability by the CFSv2 model

CAROLINA S. VERA; MARIANO S. ALVAREZ

Qingdao

Conferencia; CLIVAR OSC 2016; 2016

CLIVAR

Recent studies have shown the influence of the Madden-Julian Oscillation (MJO) in the climate of South America (SA). This influence is greater during austral spring and summer, that is, during the onset and the mature phase of the South American Monsoon System. The accurate representation of the features associated to the MJO evolution is therefore crucial in order to produce skillful subseasonal forecasts. This works aims to study how a global model that runs subseasonal forecasts operatively represents the MJO and its influence on intraseasonal (IS, 10-90 days) variability in SA. The CFSv2 model of the NCEP was selected for this study, and 123 day-long reforecasts during 1999-2010 for DJF and JJA, the target periods, were used. For each year, 10 runs were selected to study the variability of the model, which were considered as simulations.Spectral and spatial properties of the tropical convective anomalies associated to the MJO were studied in the CFSv2 and compared to those observed. Zonal wave number-frequency spectra were used to detect the representation of the MJO in the model. The signal is present in the simulations during DJF, though it maximizes in longer wave numbers than those observed. During JJA the MJO is weaker than observed. The spatial representation of the MJO by the CFSv2 was studied using Filtered-MJO OLR (FMO) patterns, which showed that convective anomalies associated with the MJO are weaker and located more to the east (west) during DJF (JJA) than observed.The ability of the CFSv2 model in representing the main seasonal features associated with the IS variability in SA was also assessed. They are described by the leading pattern (EOF1) resulting from an EOF analysis of bandpass filtered OLR anomalies (FOLR). When the South Atlantic Convergence Zone (SACZ) is in average active (warm season, October to April), EOF1 structure is a dipole one with centers of action of opposite sign located over the SESA and SACZ regions. Its variability is associated to that of the tropical convection in the Indian and Pacific oceans and to the MJO. During the cold season, when the SACZ is absent, EOF1 pattern is a monopole over SESA. The associated PC1 time series was used to indicate the level of activity of the pattern, and it was previously proved to be related to the occurrence of wet spells in SESA. The leading patterns for both, DJF and JJA, are well represented by the model, though with some differences in the tilting of the centers during summer and a region of higher simulated variability over northern Argentina during austral winter. The PC1 was used to compute lagged correlation maps against simulated OLR and upper-level geopotential height anomalies, and those were compared to the observations. OLR evolution showed indications of MJO-like anomalies propagating along the tropics, which are less intense during JJA for the model and observations. The associated wavetrains are better represented during winter, while in summer the differences in tilting and location might explain the differences observed in the convective centers.