The cross-equatorial northerly surges (CENSs) often induce severe rainfalls in the Java region; however, the role of CENSs in the formation and moisture supply of these rainfalls remains unclear. To evaluate this, we analyzed the CENS-related heavy rainfalls over the western Java region in mid-February 2021 based on observations, ERA5 reanalysis, and a set of numerical experiments using atmosphere-ocean coupled and atmosphere-only models. Results show that convection over the western Java region was enhanced due to the southward moisture transport by CENSs and related moisture convergence, except for the most severe event on 18 February. By focusing on this event, we found that the joint efforts of CENS and an anomalous southerly wind played the key role, while the latter also helped the merging of the convection south of Java and the diurnal convection over Java Island that eventually induced the long-lasting heavy rains. To quantitatively evaluate the role of CENS, we performed a backward trajectory experiment and found that about 40% of the accumulated moisture over the Java region was contributed by CENSs, with a dominant proportion from the surface evaporation over the oceans nearby, instead of remote regions. Moreover, based on our experiments, we demonstrated that the air-sea interactions enhanced the surface evaporation and helped in reproducing more realistic moisture supplies and convective activities during CENSs. In addition, our results also indicated that ERA5 underestimated the severe events, which was likely due to the poorly represented land-sea breezes and reanalysis-related unrealistic moisture variations.