Satellite observations provide valuable information on the rapidly increasing marine heatwaves over the Arctic Ocean, yet current assessments of these events lack reliability because the satellite-based temperature in ice-covered regions heavily relies on ice conditions. Here we compared the marine heatwaves derived from two commonly used satellite products to evaluate the potential influences of sea ice in the pan-Arctic. We found inconsistent behaviors of marine heatwaves in both mean fields and temporal variations on interannual and seasonal timescales using two datasets, which became even larger in recent years when they showed opposite tendencies over regions with higher ice concentrations (>50%). Our findings revealed our knowledge of marine heatwaves in the Arctic is facing challenges due to the large influences of sea ice and datasets that were ignored in previous studies, highlighting an urgent need for in situ observations and consistent and reliable long-time datasets for the warming Arctic.

Prolonged extremely warm ocean temperatures have great impacts on both natural ecosystems and human communities. These phenomena (i.e., marine heatwaves) could be easily monitored globally by satellite-based sea surface temperatures; however, the choice of datasets may lead to potential uncertainties in the marine heatwave assessment. Here we compared the marine heatwaves using three commonly used satellite products to illustrate the uncertainties over Asia and the Indo-Pacific. Distinct differences were found in the occurrence, duration, and long-term trend of marine heatwaves over both coastal and open oceans, while some discrepancies could become comparable with the obtained metrics themselves. Although differences in mean sea surface temperatures or their variances among datasets could not explain the abovementioned discrepancies, different sensors, procedures, and sea ice treatments in each dataset may contribute partially. Overall, our study suggests that the use of multiple datasets is crucial for evaluations of extreme events.

In August 2022, the Tohoku region of Japan suffered three major heavy rainfall events, while one was related to an anticyclone, and the others were induced by a ‘Baiu-like’ stationary front. By using the Lagrangian backward trajectories, we found the moisture sources of the three events varied much due to the rapidly changing atmospheric conditions. In general, moisture from the Sea of Japan contributed the most in all events, while the western Pacific also played a certain role. Surprisingly, moisture from tropical storms and the East China Sea did not contribute much due to the along-transport precipitation. But, based on forward-traced trajectories, we confirmed that tropical storms did enhance moisture transport from subtropical regions via their southerly wind. Overall, this study highlights the role of the moisture gains nearby and the outer flows of tropical storms during heavy rainfalls in a northern region.

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 midtroposphere 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 transport and accumulation of moisture played an essential role in the extremely heavy rainfall of July 2020 in Japan. To better understand this event in terms of moisture sources and transport routes, backward particle trajectory analysis was conducted. We found two major moisture sources: transport from the tropics and uptake from the subtropics. A narrow moisture channel was found along the edge of the western Pacific Subtropical High (WPSH), transporting the moisture to the Baiu front. However, most moisture from the tropics was lost due to precipitation, and their contributions were reduced to about 15%. In contrast, the subtropical regions contributed over 80% moisture via evaporation and lower tropospheric convection. Among those regions, the western Pacific contributed the most (> 33 %). This study highlights the role of WPSH in moisture transport and demonstrated the importance of moisture uptake during transport.