scholarly journals Association between Northward-Moving Tropical Cyclones and Southwesterly Flows Modulated by Intraseasonal Oscillation

2012 ◽  
Vol 25 (14) ◽  
pp. 5072-5087 ◽  
Author(s):  
Jau-Ming Chen ◽  
Ching-Feng Shih
Keyword(s):  
Tropical Cyclones ◽  
Heavy Rainfall ◽  
Large Scale ◽  
Northern South China Sea ◽  
Intraseasonal Oscillation ◽  
Regulatory Processes ◽  
The North ◽  
Intraseasonal Oscillations ◽  
The Tropics ◽  
Northern Taiwan

Abstract Tropical cyclones (TCs) of a particular track type move northward along the open oceans to the east of Taiwan and later pass over or near northern Taiwan. Their northward movement may be associated with intensified monsoon southwesterly flows from the northern South China Sea (SCS) toward Taiwan. Prolonged heavy rainfall then occurs in western Taiwan across the landfall and postlandfall periods, leading to severe floods. Characteristics of this TC–southwesterly flow association and related large-scale regulatory processes of intraseasonal oscillations (ISOs) are studied. For summers from 1958 to 2009, 16 out of 108 TCs affecting Taiwan exhibit the aforementioned northward-moving track. Among them, four TCs (25%) concur with enhanced southwesterly flows. Intensified moisture supplies from the SCS result in strong moisture convergence and prolonged heavy rainfall in western Taiwan. Both 30–60- and 10–24-day ISOs make positive contributions to the TC–southwesterly flow association. Both ISOs exhibit the northward progress of a meridional circulation pair from the tropics toward Taiwan. During landfall and the ensuing few days, Taiwan is surrounded by a cyclonic anomaly to the north and an anticyclonic anomaly to the south of these two ISOs. The appearance of anomalous southwesterly–westerly flows acts to prolong heavy rainfall in western Taiwan after the departure of a TC. The TC–southwesterly flow association tends to occur during the minimum phase of the 30–60-day ISO featuring a cyclonic anomaly in the vicinity of Taiwan but in various phases of the 10–24-day ISO. Rainfall in western Taiwan increases when these two ISOs simultaneously exhibit a cyclonic anomaly to the north of Taiwan.

scholarly journals Interannual Variability of the Intraseasonal Oscillation and Its Impact on the Summertime Wave Patterns and Tropical Cyclones over the Western North Pacific

2017 ◽  
Vol 145 (9) ◽  
pp. 3465-3483 ◽  
Author(s):  
Ken-Chung Ko ◽  
Jyun-Hong Liu
Keyword(s):  
Tropical Cyclones ◽  
Large Scale ◽  
Intraseasonal Oscillation ◽  
Low Frequency ◽  
Mean Flow ◽  
Background Flow ◽  
Wave Patterns ◽  
Intraseasonal Oscillations ◽  
The Mean ◽  
Local Weather

In this study, intraseasonal oscillations (ISOs) and submonthly wave patterns were separated into maximal variance (MaxV) and minimal variance (MinV) years on the basis of ISO variance from July to October. The mean-state 850-hPa streamfunction for submonthly cases indicated that, in the MinV years, tropical cyclones (TCs) formed near areas southeast of those in the MaxV years. ISOs propagated northward in the MaxV years, whereas a weaker westward-propagating tendency was observed in the MinV years. Track analysis of the centers of the submonthly cyclonic anomalies suggested that the background flow dictated the propagation routes of the easterly cyclonic anomalies in the MaxV years. However, the propagation routes of the westerly cyclonic anomalies were barely affected by the background flow. Further analysis of the ISO mean flow patterns showed that in the MaxV years, the propagation routes of the westerly cyclonic anomalies were more likely controlled by the anomalous easterly flow generated by the ISO westerly cyclonic anomalies. Moreover, rainfall was heavier in Taiwan in the MaxV years because the background flow in the MinV years caused the submonthly cyclonic anomaly tracks to shift away from Taiwan. Therefore, low-frequency large-scale circulations can affect smaller-scale phenomena and local weather.

scholarly journals Heavy Rainfall Induced by Tropical Cyclones across Northern Taiwan and Associated Intraseasonal Oscillation Modulation

2013 ◽  
Vol 26 (20) ◽  
pp. 7992-8007 ◽  
Author(s):  
Jau-Ming Chen ◽  
Pei-Hua Tan ◽  
Ching-Feng Shih
Keyword(s):  
Tropical Cyclones ◽  
Heavy Rainfall ◽  
Intraseasonal Oscillation ◽  
Convergence Result ◽  
Windward Side ◽  
Tropical Western Pacific ◽  
Moisture Supply ◽  
Rainfall Type ◽  
Northern Taiwan

Abstract Tropical cyclones (TCs) moving northwestward/westward across northern Taiwan are defined to have a type-2 track. This study aims to analyze heavy rainfall associated with type-2 TCs in Taiwan, focusing on the modulation processes of the intraseasonal oscillation (ISO). During 1958–2011, 21 summer type-2 TCs are separated into three rainfall types: strong, moderate, and weak. For the strong rainfall type, both 30–60-day and 10–24-day ISOs propagate northwestward across Taiwan. During landfall and the ensuing two days, both ISOs exhibit a cyclonic anomaly centering northwest of Taiwan that causes anomalous westerly flows (or enhance seasonal southwesterly flows) from the South China Sea (SCS) onto Taiwan. Persistent moisture supply and strong moisture convergence result in prolonged heavy rainfall on the windward side over western Taiwan. TCs with weak rainfall are accompanied by a northward-propagating 30–60-day ISO from the tropical western Pacific toward Japan and a westward-propagating 10–24-day ISO along 20°N latitude. During the landfall stage both ISOs have a cyclonic anomaly with its center south of Taiwan. Major anomalous westerly flows are displaced southward across the central SCS, leading to a weak moisture supply and rainfall in Taiwan. The moderate rainfall type features a 30–60-day (10–24 day) ISO resembling that of the weak (strong) rainfall type. The amount of rainfall thus ranges between the strong and weak rainfall types. Major processes regulating the rainfall of type-2 TCs relate to the intensity of the moisture supply associated with anomalous westerly flows from the SCS onto Taiwan, which is jointly modulated by 30–60-day and 10–24-day ISOs.

scholarly journals Lagrangian formation pathways of moist anomalies in the trade-wind region during the dry season: two case studies from EUREC<sup>4</sup>A

2021 ◽  
Author(s):  
Leonie Villiger ◽  
Heini Wernli ◽  
Maxi Boettcher ◽  
Martin Hagen ◽  
Franziska Aemisegger
Keyword(s):  
Long Range ◽  
North Atlantic ◽  
Large Scale ◽  
Cold Front ◽  
Trade Wind ◽  
Long Range Transport ◽  
The North ◽  
Range Transport ◽  
The North Atlantic ◽  
The Tropics

Abstract. Shallow clouds in the trade-wind region over the North Atlantic contribute substantially to the global radiative budget. In the vicinity of the Caribbean island Barbados, they appear in different mesoscale organisation patterns with distinct net cloud radiative effects (CRE). Cloud formation processes in this region are typically controlled by the prevailing large-scale subsidence. However, occasionally weather systems from remote origin cause significant disturbances. This study investigates the complex cloud-circulation interactions during the field campaign EUREC4A (Elucidate the Couplings Between Clouds, Convection and Circulation) from 16 January to 20 February 2020, using a combination of Eulerian and Lagrangian diagnostics. Based on observations and ERA5 reanalyses, we identify the relevant processes and characterise the formation pathways of two moist anomalies above the Barbados Cloud Observatory (BCO), one in the lower (~1000–650 hPa) and one in the middle troposphere (~650–300 hPa). These moist anomalies are associated with strongly negative CRE values and with contrasting long-range transport processes from the extratropics and the tropics, respectively. The low-level moist anomaly is characterised by an unusually thick cloud layer, high precipitation totals and a strongly negative CRE. Its formation is connected to an “extratropical dry intrusion” (EDI) that interacts with a trailing cold front. A quasi-climatological (2010–2020) analysis reveals that EDIs lead to different conditions at the BCO depending on how they interact with the associated cold front. Based on this climatology, we discuss the relevance of the strong large-scale forcing by EDIs for the low-cloud patterns near the BCO and the related CRE. The second case study about the mid-tropospheric moist anomaly is associated with an extended and persistent mixed-phase shelf cloud and the lowest daily CRE value observed during the campaign. Its formation is linked to “tropical mid-level detrainment” (TMD), which refers to detrainment from tropical deep convection near the melting layer. The quasi-climatological analysis shows that TMDs consistently lead to mid-tropospheric moist anomalies over the BCO and that the detrainment height controls the magnitude of the anomaly. However, no systematic relationship was found between the amplitude of this mid-tropospheric moist anomaly and the CRE at the BCO. Overall, this study reveals the important impact of the long-range transport, driven by dynamical processes either in the extratropics or the tropics, on the variability of the vertical structure of moisture and clouds, and on the resulting CRE in the North Atlantic winter trades.

scholarly journals Rainfall Mechanisms for One of the Wettest Tropical Cyclones on Record in Australia—Oswald (2013)

2020 ◽  
Vol 148 (6) ◽  
pp. 2503-2525
Author(s):  
Difei Deng ◽  
Elizabeth A. Ritchie
Keyword(s):  
Tropical Cyclones ◽  
Wind Shear ◽  
Heavy Rainfall ◽  
Large Scale ◽  
Vertical Wind Shear ◽  
Monsoon Trough ◽  
Vertical Wind ◽  
Lower Latitude ◽  
Cape York ◽  
Frictional Convergence

Abstract Tropical Cyclone Oswald (2013) is considered to be one of the highest-impact storms to make landfall in northern Australia even though it only reached a maximum category 1 intensity on the Australian category scale. After making landfall on the west coast of Cape York Peninsula, Oswald turned southward, and persisted for more than 7 days moving parallel to the coastline as far south as 30°S. As one of the wettest tropical cyclones (TCs) in Australian history, the favorable configurations of a lower-latitude active monsoon trough and two consecutive midlatitude trough–jet systems generally contributed to the maintenance of the Oswald circulation over land and prolonged rainfall. As a result, Oswald produced widespread heavy rainfall along the east coast with three maximum centers near Weipa, Townsville, and Rockhampton, respectively. Using high-resolution WRF simulations, the mechanisms associated with TC Oswald’s rainfall are analyzed. The results show that the rainfall involved different rainfall mechanisms at each stage. The land–sea surface friction contrast, the vertical wind shear, and monsoon trough were mostly responsible for the intensity and location for the first heavy rainfall center on the Cape York Peninsula. The second torrential rainfall near Townsville was primarily a result of the local topography and land–sea frictional convergence in a conditionally unstable monsoonal environment with frictional convergence due to TC motion modulating some offshore rainfall. The third rainfall area was largely dominated by persistent high vertical wind shear forcing, favorable large-scale quasigeostrophic dynamic lifting from two midlatitude trough–jet systems, and mesoscale frontogenesis lifting.

scholarly journals The Interannual Variability of Tropical Cyclones

2007 ◽  
Vol 135 (10) ◽  
pp. 3587-3598 ◽  
Author(s):  
William M. Frank ◽  
George S. Young
Keyword(s):  
Tropical Cyclone ◽  
Interannual Variability ◽  
Tropical Cyclones ◽  
Large Scale ◽  
Storm Activity ◽  
Modes Of Variability ◽  
The North ◽  
Temporal And Spatial ◽  
Using Data ◽  
The North Atlantic Oscillation

Abstract This paper examines the interannual variability of tropical cyclones in each of the earth’s cyclone basins using data from 1985 to 2003. The data are first analyzed using a Monte Carlo technique to investigate the long-standing myth that the global number of tropical cyclones is less variable than would be expected from examination of the variability in each basin. This belief is found to be false. Variations in the global number of all tropical cyclones are indistinguishable from those that would be expected if each basin was examined independently of the others. Furthermore, the global number of the most intense storms (Saffir–Simpson categories 4–5) is actually more variable than would be expected because of an observed tendency for storm activity to be correlated between basins, and this raises important questions as to how and why these correlations arise. Interbasin correlations and factor analysis of patterns of tropical cyclone activity reveal that there are several significant modes of variability. The largest three factors together explain about 70% of the variance, and each of these factors shows significant correlation with ENSO, the North Atlantic Oscillation (NAO), or both, with ENSO producing the largest effects. The results suggest that patterns of tropical cyclone variability are strongly affected by large-scale modes of interannual variability. The temporal and spatial variations in storm activity are quite different for weaker tropical cyclones (tropical storm through category 2 strength) than for stronger storms (categories 3–5). The stronger storms tend to show stronger interbasin correlations and stronger relationships to ENSO and the NAO than do the weaker storms. This suggests that the factors that control tropical cyclone formation differ in important ways from those that ultimately determine storm intensity.

scholarly journals Mesoscale processes for super heavy rainfall of Typhoon Morakot (2009) over Southern Taiwan

2011 ◽  
Vol 11 (1) ◽  
pp. 345-361 ◽  
Author(s):  
C.-Y. Lin ◽  
H.-m. Hsu ◽  
Y.-F. Sheng ◽  
C.-H. Kuo ◽  
Y.-A. Liou
Keyword(s):  
Heavy Rainfall ◽  
Large Scale ◽  
Specific Humidity ◽  
Mountain Range ◽  
Typhoon Morakot ◽  
Data Set ◽  
The North ◽  
Wave Trains ◽  
Convective Cells ◽  
Mesoscale Processes

Abstract. Within 100 h, a record-breaking rainfall, 2855 mm, was brought to Taiwan by typhoon Morakot in August 2009 resulting in devastating landslides and casualties. Analyses and simulations show that under favorable large-scale situations, this unprecedented precipitation was caused first by the convergence of the southerly component of the pre-existing strong southwesterly monsoonal flow and the northerly component of the typhoon circulation. Then the westerly component of southwesterly flow pushed the highly moist air (mean specific humidity >16 g/kg between 950 and 700 hPa from NCEP GFS data set) eastward against the Central Mountain Range, and forced it to lift in the preferred area. From the fine-scale numerical simulation, not only did the convergence itself provide the source of the heavy rainfall when it interacted with the topography, but also convective cells existed within the typhoon's main rainband. The convective cells were in the form of small rainbands perpendicular to the main one, and propagated as wave trains downwind. As the main rainband moved northward and reached the southern CMR, convective cells inside the narrow convergence zone to the south and those to the north as wave trains, both rained heavily as they were lifted by the west-facing mountain slopes. Those mesoscale processes were responsible for the unprecedented heavy rainfall total that accompanied this typhoon.

Interactions between Boreal Summer Intraseasonal Oscillations and Synoptic-Scale Disturbances over the Western North Pacific. Part II: Apparent Heat and Moisture Sources and Eddy Momentum Transport*

2011 ◽  
Vol 24 (3) ◽  
pp. 942-961 ◽  
Author(s):  
Pang-Chi Hsu ◽  
Tim Li
Keyword(s):  
North Pacific ◽  
Western North Pacific ◽  
Wave Train ◽  
Intraseasonal Oscillation ◽  
Momentum Transport ◽  
Boreal Summer ◽  
Synoptic Scale ◽  
Mean Flow ◽  
The North ◽  
Intraseasonal Oscillations

Abstract The interactions between the boreal summer intraseasonal oscillation (ISO) and synoptic-scale variability (SSV) are investigated by diagnosing the atmospheric apparent heat source (Q1), apparent moisture sink (Q2), and eddy momentum transport. It is found that the synoptic Q1 and Q2 heating (cooling) anomalies are in phase with cyclonic (anticyclonic) vorticity disturbances, aligned in a southeast–northwest-oriented wave train pattern over the western North Pacific (WNP). The wave train is well organized and strengthened (loosely organized and weakened) during the ISO active (suppressed) phase. The nonlinearly rectified Q1 and Q2 fields due to the eddy–mean flow interaction account for 10%–30% of the total intraseasonal Q1 and Q2 variabilities over the WNP. During the ISO active (suppressed) phase, the nonlinearly rectified intraseasonal Q1 and Q2 heating (cooling) appear to the northwest of the ISO enhanced (suppressed) convection center, favoring the northwestward propagation of the ISO. A diagnosis of the zonal momentum budget shows that the eddy momentum flux convergence forces an intraseasonal westerly (easterly) tendency to the north of the ISO westerly (easterly) center during the ISO active (suppressed) phase. As a result, the eddy momentum transport may contribute to the northward propagation of the boreal summer ISO over the WNP.

Investigating the Use of a Genesis Potential Index for Tropical Cyclones in the North Atlantic Basin

2012 ◽  
Vol 25 (24) ◽  
pp. 8611-8626 ◽  
Author(s):  
Cindy L. Bruyère ◽  
Greg J. Holland ◽  
Erin Towler
Keyword(s):  
Tropical Cyclones ◽  
North Atlantic ◽  
Large Scale ◽  
Vertical Shear ◽  
Genesis Potential Index ◽  
The North ◽  
Potential Index ◽  
Cyclone Genesis ◽  
Future Work ◽  
Cyclone Frequency

Abstract Large-scale environmental variables known to be linked to the formation of tropical cyclones have previously been used to develop empirical indices as proxies for assessing cyclone frequency from large-scale analyses or model simulations. Here the authors examine the ability of two recent indices, the genesis potential (GP) and the genesis potential index, to reproduce observed North Atlantic cyclone annual frequency variations and trends. These skillfully estimate the mean seasonal variation of observed cyclones, but they struggle with reproducing interannual frequency variability and change. Examination of the independent contributions by the four terms that make up the indices finds that potential intensity and shear have significant skill, while moisture and vorticity either do not contribute to or degrade the indices’ capacity to reproduce observed interannual variability. It is also found that for assessing basinwide cyclone frequency, averaging indices over the whole basin is less skillful than its application to the general area off the coast of Africa broadly covering the main development region (MDR). These results point to a revised index, the cyclone genesis index (CGI), which comprises only potential intensity and vertical shear. Application of the CGI averaged over the MDR demonstrates high and significant skill at reproducing interannual variations and trends in all-basin cyclones across both reanalyses. The CGI also provides a more accurate reproduction of seasonal variations than the original GP. Future work applying the CGI to other tropical cyclone basins and to the downscaling of relatively course climate simulations is briefly addressed.

Extratropical Forcing of Submonthly Variations of Rainfall in Vietnam

2019 ◽  
Vol 32 (8) ◽  
pp. 2329-2348 ◽  
Author(s):  
Bui Minh Tuan
Keyword(s):  
Heavy Rainfall ◽  
Wave Train ◽  
Intraseasonal Oscillation ◽  
Power Spectral Analysis ◽  
Red River Delta ◽  
The North ◽  
Power Spectral ◽  
Precipitation Anomalies ◽  
Tropical Depressions ◽  
Extratropical Forcing

Abstract An EOF analysis is applied to high-resolution Vietnam Gridded Precipitation anomalies to support the notion that the characteristics of intraseasonal oscillation (ISO) of rainfall in Vietnam are distinct from location to location and highly affected by topography. Power spectral analysis reveals that the ISO of rainfall in Vietnam is dominated by submonthly-scale ISO (SISO), which is most active in September–October. The rainfall SISO shows remarkable relationships with heavy rainfall days in the Red River Delta and Mid-Central and Central Highlands but relatively weak correlations with heavy rainfall days in the Northeast and Southern Plain. A composite technique applied to filtered OLR and ERA-Interim shows that the first four principal components (PCs) of the rainfall SISO involve four different processes that closely relate to extratropical systems. The rainfall SISO in the PC1 is governed by interaction between the pressure surge induced by the submonthly amplifications of the Siberian high and tropical depressions (TDs). Rainfall SISO in PC2 is modulated by the convergence of the southward excursion of the polar air mass and TD-type waves. Rainfall SISO in PC3 is generated by the quasigeostrophic lifting of the extratropical wave train associated with TD-type waves. The effect of upstream development of the wave train from the North Pacific and TD-type wave is the key process inducing the rainfall SISO in PC4.

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