An extreme rainfall event occurred over western Japan and the adjacent Tokai region mainly in early July, named "the Heavy Rain Event of July 2018", which caused widespread havoc. It was followed by heat wave that persisted in many regions over Japan in setting the highest temperature on record since 1946 over eastern Japan as the July and summertime means. The rain event was attributable to two extremely moist airflows of tropical origins confluent persistently into western Japan and largescale ascent along the stationary Baiu front. The heat wave was attributable to the enhanced surface North Pacific Subtropical High and upper-tropospheric Tibetan High, with a prominent barotropic anticyclonic anomaly around the Korean Peninsula. The consecutive occurrence of these extreme events was related to persistent meandering of the upper-level subtropical jet, indicating remote influence from the upstream. The heat wave can also be influenced by enhanced summertime convective activity around the Philippines and possibly by extremely anomalous warmth over the Northern Hemisphere midlatitude in July 2018. The global warming can also influence not only the heat wave but also the rain event, consistent with a long-term increasing trend in intensity of extreme precipitation observed over Japan.
This study investigates the influence of strong southerly moisture flux on an extreme rainfall event over western Japan in early July 2018, by using a global atmospheric reanalysis dataset. During its peak period from 5 to 7 July, extensive and unprecedented rainfall observed along the well-defined quasi-stationary Baiu front was attributed to two branches of extremely moist inflow from the southern confluence into western Japan. One was a shallow southerly airstream enhanced by the surface North Pacific Subtropical High, and the other was a deeper southwesterly airstream accompanying enhanced convection over the East China Sea. Both the vertically integrated moisture flux from the south and its convergence into western Japan reached the highest levels for 60 years due to an overwhelming contribution from the intensified southerlies. Anomalous diabatic heating associated with the active convection over the East China Sea acted to maintain the southwesterly moisture flux by inducing low-level cyclonic potential vorticity anomalies. During the rainfall event, a strong meander of the upper-level subtropical jet associated with the intensified surface North Pacific Subtropical High accompanied an amplified upper-level trough over the Korean Peninsula, which acted to induce ascent dynamically along the Baiu front.
During a torrential rainfall event in early July 2018, profound enhancement of moisture influx from the south and its convergence occurred over western Japan, which is investigated in this study on the basis of objective analysis and forecast data from the Japan Meteorological Agency Meso-Scale Model. The heavy rainfall over western Japan is found to accompany enhanced oceanic evaporation extensively around Japan, especially around the Kuroshio and entirely over the Sea of Japan. Linear decompositions of the anomalous moisture flux and surface latent heat flux anomalies applied to the high-resolution data reveal that the intensified speed of the low-level southerlies was the primary factor for the pronounced enhancement of both the moisture transport into the heavy rainfall region, especially in its western portion, and evaporation around the Kuroshio into the southerlies. An additional contribution is found from positive sea-surface temperature anomalies to the enhanced southerly moisture inflow into the eastern portion of the rainfall region. These findings have been confirmed through a backward trajectory analysis, which suggests that anomalous moisture supply to air parcels into the rainfall region primarily through the enhanced wind-forced evaporation roughly corresponds to about 10% of the precipitable water anomaly over western Japan.
This study analyzes a Rossby wave-breaking event east of Japan that enhanced the convective activities over the subtropical western North Pacific Ocean. In August 2016, Rossby-wave packets in the upper troposphere above Eurasia reached over and around the seas east of Japan. The wave-breaking event accompanied the amplification of a blocking ridge and the southward intrusion of upper-level high-potential vorticity (PV) south of the ridge. The high PV (i.e., the enhanced mid-Pacific trough) promoted upward motion and enhancement of convective activities over the subtropical western North Pacific Ocean through a quasi-geostrophic balance. In the lower troposphere, large-scale cyclonic circulation anomalies, including tropical disturbances, were observed south and southeast of Japan, and the anomalies caused significant wet climate conditions in the eastern and northern parts of the country. A linear baroclinic model experiment indicates that the lower-level cyclonic circulation anomalies were the Rossby-wave responses to heating anomalies associated with the enhanced convective activities. These results suggest the existence of dynamic interaction between extratropical and tropical circulation over the western North Pacific Ocean and its influence on boreal summer climate in Japan.
We investigated features of the atmosphere and ocean to seek a possible candidate that suppressed the growth of the El Niño event in 2014. In the boreal summer-fall season, equatorially antisymmetric sea surface temperature (SST) anomalies with a positive (negative) sign to the north (south) of the equator prevailed in the central and eastern tropical Pacific. In association with the SST anomalies, cumulus convective activity was enhanced in the region of the climatological Intertropical Convergence Zone (ITCZ). Anomalous southerly surface winds flowing across the equator toward the ITCZ induced upward latent heat flux anomalies and lowered SST in the near-equatorial region. These coherent spatial patterns between SST, wind, and latent heat flux anomalies suggested that the wind-evaporation-SST (WES) feedback sustained the suppression of the El Niño growth. A linear baroclinic model experiment indicated that the enhanced convective heating in the ITCZ also contributed to sustain the anomalous surface southerlies across the equator by the intense meridional atmospheric circulation over the equator. These results indicate that the anomalous southerlies across the equator sustained by the WES feedback and intense convective heating in the ITCZ contributed to the suppression of the El Niño growth.(Citation: Maeda, S., Y. Urabe, K. Takemura, T. Yasuda, and Y. Tanimoto, 2016: Active role of the ITCZ and WES feedback in hampering the growth of the expected full-fledged El Niño in
We investigated the relationship between the atmospheric variability in El Niño conditions and Arctic Oscillation (AO) during the period from late autumn to early spring, focusing on the vertical linkage between the troposphere and the stratosphere, based on a composite analysis. The results of the composite analysis indicate that the vertical linkage is the clearest in late winter to early spring, particularly in the three-month of February -April (FMA). In FMA, the upper tropospheric patterns and upward propagation of planetary waves with zonal wavenumber 1 are enhanced and contribute to a negative phase of the stratospheric Northern Annular Mode (NAM) in El Niño conditions.The results also indicate that the stratospheric negative potential vorticity (PV) anomalies associated with the negative phase of NAM induce a lowered tropopause, vertical compression of the tropospheric column, positive surface pressure anomalies in the polar region and hence the negative phase of the AO. This vertical linkage and the impact of sea level pressure near the pole are consistent with a quantitative estimation based on geostrophic and hydrostatic adjustment associated with the stratospheric PV anomalies.(Citation: Takemura, K., and S. Maeda, 2016: Influence of enhanced variability with zonal wavenumber 1 in El Niño conditions on Arctic Oscillation in late winter to early spring. SOLA, 12, 159−164,
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