Relative Roles Of Background Moisture And Vertical Shear In Regulating Inter-annual Variability Of Intraseasonal Oscillation Intensity

The inter-annual variability of the boreal summer and winter intra-seasonal oscillation is investigated using observed outgoing long wave radiation (OLR) and ERA-I reanalysis data for the period of 1980-2012. It is found that the interannual variability of the boreal summer intra-seasonal oscillation (BSISO) intensity is much stronger in the tropical western Pacific (TWP) than the tropical Indian Ocean (TIO). A BSISO intensity index is defined based on a Multi-variate EOF analysis in TWP. It is found that strong BSISO years are associated with El Nino-like sea surface temperature anomalies in the tropical Pacific, anomalous easterly shear and enhanced background moisture condition in the region. Using a 2.5-layer atmospheric model with a specified idealized background mean state, we further examine the relative roles of background moisture and vertical shear fields in modulating the BSISO intensity. Sensitivity numerical experiments indicate that the background moisture change is most important in regulating the BSISO intensity, whereas the background vertical shear change also plays a role.The standard deviation of 20-80-day filtered OLR anomaly is used to measure the MJO intensity. The dominant spatial structure of the inter-annual variability is revealed by an EOF analysis of the MJO intensity field. It is found that the leading mode is associated with eastern Pacific (EP) type ENSO, whereas the second mode is related to central Pacific (CP) type ENSO. A simple atmospheric model is used to investigate the relative roles of background moisture and wind changes in affecting the overall strength of MJO. The numerical experiments also indicate that the background moisture effect is dominant while the background wind change has a minor effect.