Impacts of CENS and MJO phenomena on diurnal cycle of local convection, moisture convergence, and rainfall over land, coast, and sea areas in the western part of Java Island

This study aims to investigate the effect of large-scale phenomena, particularly Cross Equatorial Northerly Surge (CENS) and Madden–Julian Oscillation (MJO), on the diurnal cycle of local convection, moisture convergence, and rainfall over the land, coast, and sea in the western part of Java Island. The data used in this study were the CENS and MJO indices, Convective Available Potential Energy (CAPE), convective inhibition (CINH), Vertically Integrated Moisture Flux Convergence (VIMFC), and rainfall from atmospheric reanalysis of the global climate (ERA-5) for 18 years (2001–2018). The average diurnal profiles of CAPE, CINH, VIMFC, and rainfall over five different locations (mountain, coastal land, coast, coastal sea, non-coastal sea) were plotted to see their patterns under five different large-scale conditions: CENS, Active MJO (AMJO), CENS + Active MJO (CAMJO), CENS + Inactive MJO (CIMJO), and the mean condition. The results showed a distinctive and regular character of diurnal profiles of CAPE, CINH, VIMFC, and rainfall in the five locations caused by the specific responses of different underlying surface properties to the diurnal cycle of incoming solar radiation. Furthermore, CENS tended to reduce CAPE (~ 33% reduction, averaged over all locations), which might suppress free (natural) convection. CENS also tended to reduce CINH (~ 41%), which might facilitate convection. Conversely, CENS tended to increase VIMFC significantly (~ 174%), probably through its interaction with the land breezes, which could induce forced convection. As a result, rainfall significantly increased (~ 63%) during CENS. AMJO showed similar effects to CENS but with much weaker influences. On average, AMJO reduced CAPE (~ 14%), slightly reduced CINH (~ 2%), increased VIMFC (~ 28%), and slightly reduced rainfall (~ 3%). However, the effects were the strongest when CENS and AMJO coincided (CAMJO), during which CAPE reduced (~ 37%), CINH reduced (~ 31%), VIMFC increased (~ 334%), and rainfall increased (~ 123%). The effect of CENS and AMJO was strongest over the coastal area, where rainfall increased by ~ 215%. Therefore, one of the most important results from this study was that the increase in rainfall during CENS and AMJO was not caused primarily by the increase of free convection (indicated by the CAPE), which tended to decrease. The increase of rainfall during CENS and AMJO was found mainly due to the increase of moisture convergence (indicated by the VIMFC), which might induce forced convection. We also found that a relatively high value of CINH tended to coincide with a lower value of rainfall, indicating a possible role of CINH in preventing convection. The results of the analysis of the effects of CENS and MJO on the diurnal cycle of CAPE, CINH, and VIMFC have given a better understanding of the detailed mechanisms that produce rainfall increase, which have not been much discussed in previous studies. During CENS/MJO, CAPE decreased, so its role was reducing free convection. CINH also generally decreased, so its role was increasing the potential for convective initiation. VIMFC tended to increase significantly, especially around the coast, so its role was increasing the potential for convection. The role of VIMFC tended to be dominant in increasing rainfall. Conversely, the role of CAPE was less important since it was reduced. The effects of MJO were similar to but weaker than CENS in reducing CAPE, reducing CINH, and increasing VIMFC.