ALBERT

Description: Major rainfall (≥60%) in the northern part of the South China Sea (between North Vietnam and Taiwan) during May–June (the mei-yu season—the first phase of the Southeast–East Asian monsoon) is produced by rainstorms originating over the northern Vietnam–southwestern China region and the northern part of the South China Sea. As observed in this study, the occurrence frequency of rainstorms and rainfall contribution by these rainstorms undergoes a distinct interannual variation, in-phase with those of monsoon westerlies in northern Indochina and sea surface temperature (SST) anomalies over the NOAA Niño-3.4 region ΔSST (Niño-3.4). This in-phase relationship between monsoon westerlies and the ΔSST (Niño-3.4) anomalies is a result of the filling (deepening) of the subtropical Asian continental thermal low in response to the ΔSST (Niño-3.4) warm (cold) anomalies. Accompanied with this response is a slight southward (northward) shift of the North Pacific convergence zone (NPCZ), which extends from southern China to the North Pacific east of Japan. Thus, a favorable environment that meets the Charney–Stern instability criterion in initiating rainstorm genesis is enhanced (suppressed) by the intensification (weakening) of the monsoon shear flow formed by the midtropospheric northwesterly flow around the northeast periphery of the Tibetan Plateau and the monsoon westerlies. The meridional shift of the NPCZ established an elongated anomalous convergence (divergence) zone of water vapor flux along rainstorm tracks to increase (reduce) the rain-producing efficiency of rainstorms. Consequently, this interannual rainfall variation between northern Vietnam and Taiwan is primarily caused by rainstorm genesis and rain-producing efficiency.

Description: Summer stationary waves in the Northern Hemisphere are separated by a midlatitude transition zone into the subtropical monsoon regime with a vertical phase reversal and the subarctic regime with a vertically uniform structure. The dynamics and maintenance mechanism of the subtropical stationary waves have been investigated in the context of monsoon circulation. Depicted in terms of streamfunction with 40-yr ECMWF Re-Analysis (ERA-40), the dynamic characteristics of stationary waves in the transition zone and the subarctic region are thus the focus of this study. The dynamics and maintenance mechanism of these waves were explored with the streamfunction budget and the velocity potential maintenance equations. Stationary waves across the transition region consist of anticyclonic shear zones over the North Pacific and North Atlantic and a cyclonic shear zone in east Eurasia. These transition elements are linked to subtropical oceanic anticyclones and continental thermal lows. At high latitudes, a three-wave structure emerges with a weak central Eurasian trough aligned with two deep oceanic troughs. A longitudinal phase change occurs across the transition zone, but the direction of the east–west circulation associated with the transitional anticyclonic (cyclonic) zone is the same as that of the subtropical trough (high). This phase change is caused by the dynamics transition from the Sverdrup regime to the Rossby regime because of the increasing importance of relative vorticity advection. At high latitudes, relative vorticity advection becomes the dominant dynamic process in the upper atmosphere, but is negligible in the lower troposphere. This subarctic dynamic regime results in the vertically uniform structure of stationary waves. These waves are maintained by in situ diabatic heating (cooling) ahead of three subarctic troughs (ridges). Thus, the structure of the east–west circulation of subarctic stationary waves is opposite to that of subtropical stationary waves. These findings not only disclose more detailed structure and dynamics of summer stationary waves, but also provide a more complete basis to validate summer climate simulations and to search for the cause of interannual variation in summer climate.

Description: The heavy rainfall/flood (HRF) event in central Vietnam usually occurs in October–November, the maximum rainfall season. This rainfall maximum undergoes a distinct interannual variation, opposite the interannual variation of sea surface temperature (SST) anomalies averaged over the NOAA Niño-3.4 area—ΔSST(Niño-3.4)—but coincident with the intensification (weakening) of the low-level easterlies at 15°N and westerlies at 5°N. The changes of low-level zonal winds reflect the strengthening (weakening) of the tropical cyclonic shear flow in tropical South/Southeast Asia in response to the tropical Pacific SST anomalies. Because the rainfall maximum in central Vietnam is primarily produced by the HRF cyclone, the interannual rainfall variation in this region should be attributed to the HRF cyclone activity—a new perspective of the climate change in precipitation. On average, one HRF cyclone occurs in each cold late fall. The population of the HRF cyclone may not be an important factor causing the interannual rainfall variation in central Vietnam. During the cold late fall, the rain-producing efficiency of the individual HRF cyclone is statistically almost twice those during warm and normal late falls and the most crucial factor leading to the interannual rainfall variation in central Vietnam. It is shown by further hydrological analysis that the increase (decrease) of the HRF cyclone’s rain-producing efficiency is determined by the large-scale environmental flow through the enhancement (weakening) of the regional convergence of water vapor flux.

Description: The north–south semiannual oscillation (SAO) of the North Pacific jet stream, part of the atmospheric SAO in the Northern Hemisphere, can be well depicted by the semiannual component of the monthly-mean eddy streamfunction. Expressed by the semiannual eddy streamfunction budget, the dynamic processes develop and maintain the SAO, including the adjustment between vorticity advection and convergence of vorticity flux of the monthly-mean mode and the convergence of transient vorticity flux. An empirical orthogonal function analysis of these dynamic processes shows an east–west elongated cyclonic (anticyclonic) cell of the semiannual eddy streamfunction anomaly, which appears in January and July (October and April) south of the Siberia–Alaska landmass. The maximum (minimum) adjustment processes by the monthly-mean mode and the maximum (minimum) feedback impact of transient activity on the SAO occur in December and June (September and March), a month ahead of the maximum (minimum) north–south SAO of the North Pacific jet stream. Because vorticity is supplied by the convergence of vorticity flux associated with divergent flow, the SAO for the rotational flow is established by diabatic heat and heat transport through the divergent circulation over the North Pacific Ocean, and by precipitation maintained by convergence of water vapor flux along the oceanic storm track. Additionally, the feedback impact of the modulated transient activity affects the SAO development of the atmospheric rotational and divergent circulations, and the hydrological cycle.

Description: Heavy rainfall/flood (HRF) cyclones contribute close to two-thirds of the total rainfall PT in both parts of Malaysia [peninsular Malaysia (M) and west Borneo (B)]. Judging by the rainfall variance produced by these cyclones and its correlation (~0.9) with the interannual PT variation, this variation is caused primarily by HRF cyclones through two factors: 1) their westward propagation properties and 2) their rain-producing efficiency. The former is regulated by the change of the cyclonic shear flow around the near-equator trough, while the latter is determined by the change of the convergence of water vapor flux toward tropical Southeast Asia. During November–December of cold (warm) ENSO phases, the westward propagation of the cyclone's parent cold surge vortices (CSVs) from the Philippine (P) vicinity (Borneo) to peninsular Malaysia CSVPMs (CSVBMs) and intensified (weakened) convergence of water vapor flux toward tropical South/Southeast Asia act to enhance (reduce) the rain-producing efficiency of HRFPM (HRFBM) cyclones. During winter cold (warm) phases, the deepening (filling) of the near-equator trough crossing west Borneo allows some CSVs formed/trapped in Borneo CSVBBs to develop into HRFBB (HRFBBM) cyclones (to propagate westward to peninsular Malaysia). The rain-producing efficiency of HRFBB and HRFBBM cyclones is also increased (reduced) by the intensified (weakened) convergence of water flux toward tropical South/Southeast Asia. Interannual variations of both PT(M) and PT(B) caused by the impacts of the circulation pattern changes on occurrences of HRFPM and HRFBB/HRFBBM cyclones, respectively, and their rain-producing efficiency may pose a new challenge to simulate the weather–climate relationship in climate modeling.

Description: Malaysia is geographically separated into Peninsular Malaysia and west Borneo. The rainfall maximum in the former region occurs during November–December, whereas that in the latter region occurs during December–February. This difference of maximum rainfall period indicates that the formation mechanism is different for the rainfall centers in these two parts of Malaysia. Since rainfall is primarily produced by severe weather systems, the formation of a climatological rainfall center is explored through synoptic activity and the rainfall amount of this center is estimated through contributions by rain-producing disturbances. The major cause of the rainfall maximum of Peninsular Malaysia is cold surge vortices (CSVs) and heavy rainfall/flood (HRF) events propagating from the Philippine area and Borneo. In contrast, the major cause of the rainfall maximum of Borneo is these rain-producing disturbances trapped in Borneo. Disturbances of the former group are formed by the cold surge flows of the Philippine Sea type, whereas disturbances of the latter group are formed by cold surge flows of the South China Sea (SCS) type. The population of HRF events is about one-fourth of the rain-producing disturbances in both Peninsular Malaysia and Borneo, but they produce less than ~60% rainfall for these two regions. It is revealed from the synoptic and dynamic analyses that the major Borneo rain-producing disturbances propagate westward before December by strong tropical easterlies, but they are trapped after December by strong northeasterlies of the SCS-type cold surge flow.

Description: The formations of heavy rainfall/flood (HRF) events in Vietnam are studied from diagnostic analyses of 31 events during the period 1979–2009. HRF events develop from the cold surge vortices formed around the Philippines. These vortices’ speed, size, and rainfall, which evolve into HRF events, are enhanced distinguishably from non-HRF vortices, as they reach Vietnam. The HRF cyclone, the North Pacific anticyclone, and the northwestern Pacific explosive cyclone simultaneously reach their maximum intensities when the HRF event occurs. An HRF cyclone attains its maximum intensity by the in-phase constructive interference of three monsoon (30–60, 12–24, and 5 days) modes identified by the spectral analysis of zonal winds. The rainfall center of an HRF event is formed and maintained by the in-phase constructive interference of rainfall and convergence of water vapor flux anomalies, respectively, from three monsoon modes. Forecast times of regional models are dependent and constrained on the scale of the limited domain. For 5-day forecasts, a global or at least a hemispheric model is necessary. Using the salient features described above, a 5-day forecast advisory is introduced to supplement forecasts of HRF events made by the global model. Non-HRF vortices are filtered by threshold values for the deepening rate of explosive cyclones and basic characteristics of the HRF events predicted by the global model. A necessary condition for an HRF event is the in-phase superposition of the three monsoon modes. One-week forecasts for 12 HRF events issued by the NCEP Global Forecast System are tested. Results demonstrate the feasibility of the forecast advisory to predict the occurrence dates of HRF events.

Description: Stretched from Indochina, across the South China Sea, to the Philippine Sea, a monsoon cyclonic shear flow was formed by easterlies of the cold surge-like flow in the north and monsoon westerlies in the south before the onset of the tropical Southeast Asian monsoon on 12 May 2008. On this date, two named tropical cyclones (Halong and Matmo) evolved with a 12-h lag from a closed vortex adjacent to the coast of central Vietnam and another closed vortex near Palawan Island (Philippines) within this shear flow. These two cyclones, named the twin Philippine tropical cyclones, moved almost on the same track, along the anomalous shear line (departure from the climatological one) across the Philippines, and turned northeastward to the ocean south of Japan. It was revealed from synoptic analysis that the cold surge-like flow was coupled with the midlatitude eastward-propagating short wave in northeast Asia, and part of the monsoon westerlies were fed by the cross-equatorial flow, the downstream flow of easterlies around the northern rim of the Southern Hemisphere subtropical high. The environment favorable for the formation of the twin cyclones was developed from the tropics–midlatitude interaction between synoptic systems in these two latitudinal zones. Formations of these cyclones were a result of drastic spinups of the two closed vortices (within the monsoon shear flow) following the surge of monsoon westerlies, which coincided with those of easterlies of the cold surge-like flow, and the cross-equatorial flow originating from easterlies between the Southern Hemisphere subtropical high and the Southern Hemisphere shear flow.