Ozonesonde data collected over the western Pacific in the boreal summer of 2021, which is the first continuous, direct ozone observation over the region, were used to investigate the material transport and dynamical variations associated with the summer monsoon and western Pacific convection. The data revealed ozone transport under convectively calm conditions over the western Pacific during the development phase of the Asian summer monsoon. Combining atmospheric reanalysis data and satellite cloud data, the transport process of a high-ozone air mass is described. Near the date line, the air mass intruded from the midlatitude stratosphere into the tropical upper troposphere due to Rossby wave breakings, then moved westward over the Pacific Ocean along 20°N, experiencing nearly adiabatic transport. Subsequently, as it approached the convectively active region of Southeast Asia, it underwent non-conservative mixing with the tropospheric air due to convective processes. The climatological situation in the summer Asia-Pacific region can facilitate frequent occurrences of this type of transport, which may play a role in determining ozone concentration in the tropical upper troposphere.

Coastal upwelling along the southwestern coast of Sumatra is a seasonal upwelling that occurs in areas of high sea surface temperature and abundant precipitation at the southeastern edge of the Indian Ocean warm pool. Based on observations from two Argo floats that drifted and stayed around Sumatra, we investigated ocean temperature and salinity variations during several coastal upwelling events observed in 2013--2017. The Argo floats observed the vertical structure of temperature and salinity every 10 days within 100 km from the southwestern coast of Sumatra. The observation data show intraseasonal-scale subsurface temperature cooling events with significant upward displacements of the thermocline and high-salinity water, led by anomalous local southwesterly winds and equatorial easterly winds. During the coastal upwelling events, salinity stratification and a thick barrier layer related to local precipitation were also observed. Surface mixed layer temperature cooling were relatively small in contrast to the significant subsurface anomalies. It was found that during the coastal upwelling events, subsurface cold-water upwelling signals did not necessarily reach the mixed layer when salinity stratification and a thick barrier layer were present. The implications of these observational results for understanding the local atmosphere ocean interaction, and hence the occurrence of the Indian Ocean Dipole, are discussed.

Coastal upwelling along the southern coast of Java brings cold and nutrient-rich subsurface water to the surface. We explored whether the upwelling could trigger the onset of the Indian Ocean Dipole (IOD) by supplying cold water to the southeastern tropical Indian Ocean. We used satellite-based daily chlorophyll-a concentration (Chl-a) data during 2003-2020 as a proxy of the coastal upwelling. We focused on first Ch-a blooming that occurred in April–June, the onset phase of the positive IOD (pIOD). We found that the timing and strength of the upwelling signals were significantly correlated with the subsequent IOD peaks. We diagnosed processes associated with the upwelling affecting sea surface temperature (SST) in the southeastern Indian Ocean. Results indicate that after the cold-water upwelling south of Java, westward surface temperature advection plays a role in anomalously cooling the SST in the southeastern Indian Ocean and setting a favorable condition for the subsequent pIOD development.

Recently, Uchida et al. [2016, Science] revealed the periodic changes in the interplate locking in the northeast Japan subduction zone based on the activity of small repeating earthquakes and terrestrial crustal deformation data. They found that slow slip on the plate interface has occurred repeatedly at intervals of from 2 to 6 years, depending on the location. In the northern part of the Japan Trench, a tsunami earthquake occurred with rupturing the shallow plate boundary in 1896, but it is not well understood whether the coseismic slip fully released the interseismic slip deficit, and whether periodic slow slip events occur near the trench or not. In order to investigate the interplate locking state in this region, we have just started a research project titled “Head and tail of massive earthquakes: Mechanism arresting growth of interplate earthquakes” (JSPS KAKENHI Grant Number JP19H05596), under which GNSS-Acoustic observation to detect the seafloor crustal deformation will be performed more frequently than ever before. To accomplish frequent observations, we have been developing automatic GNSS-A data acquisition system using an unmanned surface vehicle, the Wave Glider. As a first observation, we have performed GNSS-A observation at a seafloor station off Aomori Prefecture in this July. The Wave Glider (SV3-240) was equipped with 2 GNSS antennas, acoustic transducer, MEMS gyro, and their control and logging units. The data acquisition from these sensors and the autonomous activation of the seafloor transponders were successfully executed only with turning the power supply to the payload on/off from land via a satellite communication. The Wave Glider rarely strayed off the configured course, and the solar panels generated enough power to perform the observations although the weather was mostly cloudy during the operation. Now, we are processing the obtained data, and the results will be presented at the meeting.