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.

This study investigated the diurnal cycle of convection over Sumatra Island in an active phase of the Madden-Julian Oscillation (MJO) during the Pre-Years of the Maritime Continent (YMC) observation campaign in December 2015 based on in-situ and satellite observations and a convection-permitting numerical model. Observations suggest that before the active phase of the MJO in early December, convection occurred frequently over the island during the afternoon and at midnight. By contrast, during the active phase of the MJO in mid-December, afternoon convection over the island was delayed and suppressed, and midnight convection was suppressed. Numerical experiments also successfully replicated the main features of the observed modulations. In general, during the active phase of the MJO, the troposphere became drier in the Sumatra region. While the clouds reduced the solar radiation over the land, the sea breeze was also found to be delayed and weakened. As a result, the afternoon convection initiation was delayed and weakened. Further analyses suggested that the sea breeze was weakened mainly due to the orographic stagnation effect rather than the slightly reduced land-sea temperature contrast. On the other hand, the increased stratiform-anvil clouds induced the anomalous evaporative cooling in the mid-troposphere and generated island-scale subsidence during the nighttime, which finally led to the suppression of inland convection. Overall, our study reveals the modulation of diurnal convection over Sumatra Island by an active phase of the MJO and also shows the potential role of land-sea interaction in convection initiation and maintenance.

The thermal energy transfer from the sea surface to the atmosphere associated with a cold surge event was investigated with observations from radiosondes on the research vessel “Hakuho-maru” over the Philippine Sea in December 2012. These observations were analyzed, and the results were compared with those obtained from observations over the East China Sea in the Air Mass Transformation Experiment in 1974 (AMTEX ‘74). The horizontal advection of cold and dry air associated with the cold surge dominated at heights below 850 hPa. In spite of this strong advection, the local temporal variations in the temperature and moisture were small, because the advection was balanced by the transfer of heat and moisture from the sea surface, which is qualitatively the same behavior as observed during the cold surge event in AMTEX ‘74. The eddy transport of the total heat energy from the sea surface to the atmosphere was estimated at about 410 W/m2, which is about half of the maximum value of 780 W/m2 observed during AMTEX ‘74. This result shows the existence of considerable heat transfer from the sea surface to the atmosphere over the Philippine Sea, which is the downstream region of the cold surge, after it passed through the East China Sea.

The land-sea surface temperature contrast on the western coast of Sumatra Island was examined using observation data obtained from the pre-Years of the Maritime Continent (YMC) field campaign from November to December 2015. Surface observations showed that, on most days, strong daytime solar radiation caused a pronounced diurnal cycle in surface air temperatures on the island, even during the local active phase of the Madden-Julian Oscillation (MJO). Sudden drops in surface air temperature occurred frequently on the island in the late afternoon and over the sea at nighttime, accompanied by precipitation. Temperatures on the island were higher than those over the sea during the daytime and lower in the night and early morning. Prior to the local active phase of the MJO, dual maxima in the land-sea surface air temperature contrast occurred in the evening and early morning. During the local active phase of the MJO, in spite of cloudy conditions, there were still large land-sea temperature contrasts during the daytime and in the early morning. In addition to the nighttime radiative cooling of the land surface, decreases in air temperature over the land due to precipitation cooling and the lower solar insolation in the MJO active phase caused the larger temperature differences in the morning. These results suggest that the decrease in air temperature caused by precipitation cooling had a substantial effect on the land-sea surface air temperature contrast on the western coast of Sumatra Island, particularly during an active phase of the MJO.

During the Monsoon Asian Hydro−Atmosphere Scientific Research and Prediction Initiative (MAHASRI; 2006–16), we carried out two projects over the Indonesian maritime continent (IMC), constructing the Hydrometeorological Array for Intraseasonal Variation−Monsoon Automonitoring (HARIMAU; 2005–10) radar network and setting up a prototype institute for climate studies, the Maritime Continent Center of Excellence (MCCOE; 2009–14). Here, we review the climatological features of the world’s largest “regional” rainfall over the IMC studied in these projects. The fundamental mode of atmospheric variability over the IMC is the diurnal cycle generated along coastlines by land−sea temperature contrast: afternoon land becomes hotter than sea by clear-sky insolation before noon, with the opposite contrast before sunrise caused by evening rainfall-induced “sprinkler”-like land cooling (different from the extratropical infrared cooling on clear nights). Thus, unlike the extratropics, the diurnal cycle over the IMC is more important in the rainy season. The intraseasonal, seasonal to annual, and interannual climate variabilities appear as amplitude modulations of the diurnal cycle. For example, in Jawa and Bali the rainy season is the southern hemispheric summer, because land heating in the clear morning and water vapor transport by afternoon sea breeze is strongest in the season of maximum insolation. During El Niño, cooler sea water surrounding the IMC makes morning maritime convection and rainfall weaker than normal. Because the diurnal cycle is almost the only mechanism generating convective clouds systematically near the equator with little cyclone activity, the local annual rainfall amount in the tropics is a steeply decreasing function of coastal distance ( e -folding scale 100–300 km), and regional annual rainfall is an increasing function of “coastline density” (coastal length/land area) measured at a horizontal resolution of 100 km. The coastline density effect explains why rainfall and latent heating over the IMC are twice the global mean for an area that makes up only 4% of the Earth’s surface. The diurnal cycles appearing almost synchronously over the whole IMC generate teleconnections between the IMC convection and the global climate. Thus, high-resolution (<< 100 km; << 1 day) observations and models over the IMC are essential to improve both local disaster prevention and global climate prediction.

This paper presents an overview of the HARIMAU2010 campaign focusing on convective activity with the diurnal rainfall meridional march (DRMM) over Jakarta, which is located on the northern coast of Jawa Island of the Indonesian maritime continent (IMC), based on 1-month intensive observations by a C-band Doppler radar and multi-point atmospheric sounding array conducted during 16 January–14 February 2010. The campaign period corresponded to a phase after large-scale Madden–Julian oscillation (MJO) active convections passed over Jakarta (MJO inactive phase). The cross-equatorial northerly surge (CENS) intruded into the Jawa Sea with a cold tongue (CT) of sea surface temperature (SST) in the beginning of the period (CENS active period: 16–26 January), and then, it started to retreat (transition period: 27 January–05 February); afterward, only a few signs of it were apparent (CENS inactive period: 06–14 February). The observational results showed that (1) rainfall over Jakarta has the nature of DRMM during the MJO inactive phase at least, (2) the DRMM is likely driven primarily by “land-breeze”-like local meridional circulation, and (3) the meridional spatiotemporal variation of rainfall over Jakarta is thus controlled by activities of both the CENS and CT over the Jawa Sea.

This study examined the impact of an active phase of the Madden-Julian Oscillation (MJO) on a torrential rain event that occurred on the western coast of Sumatra Island on 12 December 2015, using surface meteorological observations, meteorological radar observations, and balloon sounding data obtained from the pre-Years of the Maritime Continent field campaign. Strong MJO activity took place in mid-December 2015 into January 2016. Radar observations revealed that a convergence and convective cloud merger of mesoscale convective systems from an eastward propagating MJO and westward moving diurnal convection over the western coast of the island was the immediate cause of the torrential rain. An investigation of the occurrence of convection over the island showed that both westward moving diurnal convection from the mountains and eastward propagating convection from the Indian Ocean occurred on 12 December, because the westerly winds in the lower troposphere associated with the MJO were only just initiated and were weak on the day. The results suggest that the leading edge of the MJO westerly wind bursts provided favorable conditions for an active phase of the MJO to work with the westward moving diurnal convection and cause torrential rain on the western coast of Sumatra Island.

The impact of the radiosonde observations of cold surge over the Philippine Sea on the tropical region and the Southern Hemisphere has been investigated by the assimilation of radiosonde data obtained during the R/V Hakuho Maru cruise KH-12-6 in late December 2012. After assimilating the observation data, the modified surface winds of the cold surge were generally stronger than those before the assimilation. In addition, cyclonic rotations around the 4 developing tropical cyclones in the Northern and Southern Hemispheres were more intensified. Furthermore, the analysis errors over the Indian Ocean and the Pacific Ocean in the Northern and Southern Hemispheres were reduced by 1 to 10%. The impacts of the additional radiosonde observations in the cold surge immediately propagated up to the updraft region near the equator and to the mid-latitude downdraft regions through the local Hadley circulation. After the impact spread in the lower troposphere, large impacts were deepened around the tropical cyclones and depressions within 2 days. The propagation process of the additional observation impact over the Philippine Sea suggested that the cold surge could affect large-scale circulation, including typhoons and tropical depressions in the tropics and the mid-latitude regions.

1. Department of Coupled Ocean-Atmosphere-Land Processes Research, Japan Agency for Marine-Earth Science and Technology; Professor Emeritus, Kobe University, 2. Department of Coupled Ocean-Atmosphere-Land Processes Research, Japan Agency for Marine-Earth Science and Technology, 3. Department of Geography, Tokyo Metropolitan University, 4. Research and Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, 5. Agency for the Assessment and Application of Technology

Lightning frequency over Indonesian Maritime Continent (MC) is quite high (Petersen and Rutledge 2001, Christian et al. 2003, Takayabu 2006, etc). In particular, Bogor (south of Jakarta, west Jawa) had 322 days of lightning in one year (Guinness Book in 1988). Lightning causes serious damage on nature and society over the MC; forest fore, power outage, inrush/surge currents on many kinds of electronics. Lightning climatology and meso-scale characteristics of thunderstorm over the MC, in particular over Jakarta, where social damage is quite serious, were examined.

We investigated the role of Sumatra Island convection over the maritime continent during the preconditioning stage of the Madden-Julian Oscillation (MJO) using intensive observations of CINDY2011/DYNAMO and HARIMAU2011. CINDY2011/DYNAMO and HARIMAU2011 were conducted over the Indian Ocean from October 2011 to January 2012 and Sumatra Island, Indonesia in December 2011. Both observation datasets covered the preconditioning stage of the MJO in December 2011. We found that convection was activated over the Sumatra Island with diurnal cycle associated with the moist air mass, which originated from a tropical depression generated in the South China Sea. Then, two-day period disturbances that propagated westward to the central Indian Ocean were coupled with the diurnal cycle of convection over the Sumatra Island. The structure of the two-day period disturbances was consistent with that of westward propagating inertio-gravity waves. When the westward propagating disturbances arrived over the central Indian Ocean, low-level moisture advection was excited. Moistening process was promoted in Gan Island over the central Indian Ocean, which had a two-day period. After the favorable condition of large-scale convection was established, the MJO was activated in the central Indian Ocean. The two-day period westward disturbances were organized when large-scale moisture convergence became positive in Sumatra Island and continued until a strong low-level westerly wind of the active phase of the MJO was formed.

Long-term changes of the frequency of heavy precipitation occurrence along the eastern coast of the Indochina Peninsula were analyzed using daily data from six Vietnamese meteorological stations for the period September–November of 1961–2010. The heavy precipitation days were defined by the 50 and 100 mm/day threshold values. The frequency of the coastal heavy precipitation days were decomposed into tropical cyclone (TC)-induced heavy precipitation days and non-TC heavy precipitation days, and their contribution to a recent increase in the coastal precipitation was examined. Over the 50-yr period, heavy precipitation occurrence indices show a significant increasing trend that is linked to an increasing trend in seasonal amount of the coastal precipitation. A rapid increase in the coastal heavy precipitation days was found from the mid-1990s through the 2000s. This marked increase is basically due to non-TC heavy precipitation events, suggesting that TC passages do not play a role in the recent increase in the seasonal precipitation amount and the heavy precipitation events. A role of tropical synoptic-scale disturbances (TSDs) as non-developing disturbances for TC formation in the non-TC heavy precipitation events was also explored. About 70% of the non-TC heavy precipitation events are associated with TSDs originated from the western North Pacific–South China Sea region. TSD passages are responsible for the recent increase in non-TC heavy precipitation events.