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Huang-Hsiung Hsu

Abstract

In this study, SVD analysis is applied to a normalized 200-mb streamfunction/OLR covariance matrix to extract the most recurrent coupled pattern in the northern winter, after the removal of the climatological seasonal cycle and seasonal means for each winter. The first two singular vectors are on an intraseasonal timescale and the combination of the two forms an oscillation. It is characterized by eastward propagation in the Indian Ocean and the western Pacific, standing oscillations in both the Tropics and the extratropics, and the poleward propagation of a zonally symmetric structure.

Although there exists an eastwardly propagating pattern, the phase relationship between geopotential, wind, and streamfunction fields is inconsistent with that of equatorial Kelvin waves. Eastward propagation is most evident in the Tropics of the Southern Hemisphere, while the signals in the Northern Hemisphere are characterized by standing oscillations. The distinct characteristics of the Northern and Southern Hemisphere can be attributed to the different properties of the mean flows, which load to distinct Rossby wave source distributions in the two hemispheres.

Abrupt developments of regional circulation anomalies are found in the exit regions of the Pacific and Atlantic jet streams. The one in the Pacific resembles the Pacific/North American pattern and develops in a process similar to the optimal excitation of the normal mode, by extracting barotropic energy from the mean flow. Similar energy conversion also occurs in the Atlantic. Both analyses of energy conversion and Rossby wave source indicate the occurrence of rigorous extratropical activity in the Northern Hemisphere, that is affected indirectly by tropical beating. The propagating circulations in the Southern Hemisphere, that resemble equatorial Rossby waves, could be the direct response to the tropical heating, while the signals of standing oscillation are the mixed results of direct response to the tropical heating and internal dynamics in the extratropics.

The results show the complexity of the intraseasonal oscillation, involving equatorial wave dynamics, tropical–extratropical interactions, and eddy–mean flow interactions. The phenomenon is global and it is inadequate to treat the problem as either a purely tropical phenomenon or a purely extratropical phenomenon.

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Huang-Hsiung Hsu

Abstract

The local influence of mountains upon large- and synoptic-scale low-level atmospheric circulations is investigated in this study. The sea-level pressure associated with low-frequency fluctuations exhibit phase propagation of monopolar structures around mountains in an anticyclonic sense, while the corresponding 500 mb height patterns are relatively stationary and evolve in a manner consistent with the concept of Rossby wave dispersion on a sphere. The sea-level pressure patterns in the high-pass filtered data exhibit characteristics of synoptic-scale baroclinic waves and are steered around mountain ranges in an anticyclonic sense, while the corresponding 500 mb height patterns propagate nearly parallel to the time-mean flow in the middle troposphere.

It is hypothesized that the phase propagation of lower tropospheric circulation patterns is a reflection of the conservation of potential vorticity in flows over variable terrain. Most of the observations presented in this study are interpreted as the evidence of topographic Rossby waves in the atmosphere. However, the features observed to the north of the Tibetan Plateau exhibit some characteristics of Kelvin waves induced by the blocking effects of the orography on the lower tropospheric flow.

Because of the strong stratification during wintertime, the steering effect of mountains upon atmospheric circulations is restricted to the lower troposphere. Lower tropospheric waveguides exist in the vicinity of the major mountain ranges in the Northern Hemisphere. These regions are located (i) along the eastern slopes of the Rockies (ii) along the west and north coasts of Greenland, (iii) along the eastern slopes of the mountain ranges in Mongolia and northern China, (iv) to the north and east of the Tibetan Plateau, and (v) to the north of the mountains in northern Iran and Afghanistan.

There appear to be only minor differences between the structure and evolution of cyclonic and anticyclonic circulation anomalies, even though the corresponding sequences of synoptic maps may appear quite different. Deviations of the static stability field associated with the anomalies from the climatological mean static stability field are relatively small in comparison to the mean static stability. These observations suggest that the behavior of these features is relatively linear.

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Sho Arakane and Huang-Hsiung Hsu

Abstract

The monsoon trough and subtropical high have long been acknowledged to exert a substantial modulating effect on the genesis and development of TCs in the western North Pacific (WNP). However, the potential upscaling effect of TCs on large-scale circulation remains poorly understood. This study revealed the considerable contributions of TCs to the climate mean state and variability in the WNP between 1958 and 2019, characterized by a strengthened monsoon trough and weakened subtropical anticyclonic circulation in the lower troposphere, enhanced anticyclonic circulation in the upper troposphere, and warming throughout the troposphere. TCs constituted distinct footprints in the long-term mean states of the WNP summer monsoon, and their contributions increased intraseasonal and interannual variance by 50%–70%. The interdecadal variations and long-term trends in intraseasonal variance were mainly due to the year-to-year fluctuations in TC activity. The size of TC footprints was positively correlated with the magnitude of TC activity.

Our findings suggest that the full understanding of climate variability and changes cannot be achieved simply on the basis of low-frequency, large-scale circulations. Rather, TCs must be regarded as a crucial component in the climate system, and their interactions with large-scale circulations require thorough exploration. The long-term dataset created in this study provides an opportunity to study the interaction between TCs and TC-free large-scale circulations to advance our understanding of climate variability in the WNP. Our findings also indicate that realistic climate projections must involve the accurate simulations of TCs.

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Sho Arakane and Huang-Hsiung Hsu

Abstract

The monsoon trough and subtropical high have long been acknowledged to exert a substantial modulating effect on the genesis and development of tropical cyclones (TCs) in the western North Pacific (WNP). However, the potential upscaling effect of TCs on large-scale circulation remains poorly understood. This study revealed the considerable contributions of TCs to the climate mean state and variability in the WNP between 1958 and 2019, characterized by a strengthened monsoon trough and weakened subtropical anticyclonic circulation in the lower troposphere, enhanced anticyclonic circulation in the upper troposphere, and warming throughout the troposphere. TCs constituted distinct footprints in the long-term mean states of the WNP summer monsoon, and their contributions increased intraseasonal and interannual variance by 50%–70%. The interdecadal variations and long-term trends in intraseasonal variance were mainly due to the year-to-year fluctuations in TC activity. The size of TC footprints was positively correlated with the magnitude of TC activity. Our findings suggest that the full understanding of climate variability and changes cannot be achieved simply on the basis of low-frequency, large-scale circulations. Rather, TCs must be regarded as a crucial component in the climate system, and their interactions with large-scale circulations require thorough exploration. The long-term dataset created in this study provides an opportunity to study the interaction between TCs and TC-free large-scale circulations to advance our understanding of climate variability in the WNP. Our findings also indicate that realistic climate projections must involve the accurate simulations of TCs.

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Huang-Hsiung Hsu and Chun-Hsiung Weng

Abstract

The convection of the intraseasonal timescale in the western North Pacific during the boreal summer tends to propagate northwestward in the Philippine Sea to near 20°N and then continues propagating westward. The formation of enhanced convection in the western North Pacific is a result of the merging of a convective system moving eastward along the equator and a westward-propagating low-level convergence anomaly, which is located to the east of a vortex in the subtropics. A positive feedback between the anomalous circulation and convection leads to a rapid enhancement of the system. The strengthened southwesterly associated with the vortex enhances evaporation over the oceans (e.g., the eastern Indian Ocean, the Bay of Bengal, and the South China Sea) and transports moisture northeastward. The moisture converges at the northwestern corner of the convection and results in a potentially unstable atmosphere. The result is the northwestward propagation of the coupled circulation–convection system in the western North Pacific. It was found that the ocean–atmosphere interaction plays an important role in supplying energy to sustain the circulation and convection during the course of propagation. The circulation–convection interaction is the key factor in maintaining the system's strength until it reaches the Asian landmass, when the supply of moisture is reduced. The atmosphere seems to play a dominant role during the ocean–atmosphere interaction processes, while the ocean plays a more passive role in response to the atmospheric forcing.

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Ken-Chung Ko and Huang-Hsiung Hsu

Abstract

The impact of tropical perturbation on the extratropical wave activity in the North Pacific in the submonthly time scale is demonstrated here. Previous studies identified a tropical cyclone (TC)/submonthly wave pattern, which propagated north-northwestward in the Philippine Sea and recurved in the oceanic region between Japan and Taiwan. This study found that, after the arrival of the TC/submonthly wave pattern at the recurving region, the eastward-propagating wave activity in the extratropical North Pacific was significantly enhanced. It is suggested that the TC/submonthly wave pattern, which is originated in the tropical western North Pacific, enhances the eastward energy propagation of Rossby wave–like perturbation in the extratropical North Pacific and may have an impact on the long-range weather predictability in the eastern North Pacific and western North America.

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Ming-Ying Lee and Huang-Hsiung Hsu

Abstract

A multidecadal geopotential height pattern in the upper troposphere of the extratropical Northern Hemisphere (NH) is identified in this study. This pattern is characterized by the nearly zonal symmetry of geopotential height and temperature between 35° and 65°N and the equivalent barotropic vertical structure with the largest amplitude in the upper troposphere. This pattern is named the Eurasian–Pacific multidecadal oscillation (EAPMO) to describe its multidecadal time scale and the largest amplitudes over Eurasia and the North Pacific. Although nearly extending over the entire extratropics, the EAPMO exhibits larger amplitudes over western Europe, East Asia, and the North Pacific with a zonal scale equivalent to zonal wavenumbers 4 and 5. The zonally asymmetric perturbation tends to amplify over the major mountain ranges in the region, suggesting a significant topographic influence. The EAPMO has fluctuated concurrently with the Atlantic multidecadal oscillation (AMO) at least since the beginning of the twentieth century. The numerical simulation results suggest that the EAPMO could be induced by the AMO-like sea surface temperature anomaly and strengthened regionally by topography, especially over the Asian highland region, although the amplitude was undersimulated.

This study found that the multidecadal variability of the upper-tropospheric geopotential height in the extratropical NH is much more complicated than in the tropics and the Southern Hemisphere (SH). It takes both first (warming trend) and second (multidecadal) EOFs to explain the multidecadal variability in the extratropical NH, while only the first EOF, which exhibited a warming trend, is sufficient for the tropics and SH.

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Ken-Chung Ko and Huang-Hsiung Hsu

Abstract

This study demonstrates the multiscale nature, from synoptic to intraseasonal time scales, of the atmospheric flow in the tropical western North Pacific. The multiscale features include intraseasonal oscillations (ISO), northwestward-propagating submonthly wave patterns, and recurving tropical cyclones (TCs). In the ISO westerly phase, the wave pattern was better organized and the TCs were clustered near the cyclonic circulation of the wave pattern during the genesis, development, and propagation. On the other hand, the wave pattern and TCs were weak and poorly organized in the ISO easterly phase. The distinct characteristics between the westerly and easterly phases could be attributed to the ISO modulation on the monsoon trough and the subtropical anticyclonic ridge. The ISO in the westerly phase provided a favorable background (e.g., enhanced monsoon trough and moisture confluent zone) for the wave–TC pattern development, while the ISO in the easterly phase provided a less favorable environment.

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Chih-wen Hung and Huang-Hsiung Hsu

Abstract

This study reveals the close relationship between the first transition of the Asian summer monsoon (ASM), the tropical intraseasonal oscillation (TISO), and the mei-yu in Taiwan, which occurs climatologically between mid-May and mid-June. For about half of the years in 1958–2002, the first transition of the Asian summer monsoon can be classified as a sharp onset, which is characterized by an abrupt reversal of the monsoon flow from northeasterly to southwesterly. The evolution of the large-scale monsoon circulation and convection in the sharp-onset years is characterized by an eastward-propagating TISO from eastern Africa and the western Indian Ocean to the Maritime Continent. Upon the arrival of the TISO in the Maritime Continent, a sharp onset of the ASM occurs, and a channel supplying moist air in the lower troposphere is well established across the Indian Ocean to the South China Sea (SCS). This channel consists of the Somali jet, transporting the moisture from the Southern Hemisphere to the Northern Hemisphere, and the southwesterly monsoon, delivering the moisture across the Indian Ocean to the SCS and the western North Pacific. This efficient and persistent transport of moisture to the SCS and surrounding areas presumably provides a favorable condition for the maintenance of the mei-yu front and the development of convective systems. This also marks the onset of the Taiwan mei-yu season. Because a strong TISO signal, which tends to occur concurrently with the sharp onset of the ASM, is often observed prior to the onset of the first transition and Taiwan mei-yu, a close monitoring of the TISO can be informative for the weather forecasters in Taiwan to project the initiation of the Taiwan mei-yu.

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John M. Wallace and Huang-Hsiung Hsu

Abstract

The characteristics of planetary wave dispersion in the wintertime troposphere are investigated on the basis of 5 day mean 500 mb height data for 30 winters, making use of simple analysis techniques involving lag-correlation maps for individual gridpoints and for Fourier coefficients of zonal wavenumbers 1 and 2 on 50°N. It is shown that the time evolution of the planetary-waves is dominated by energy dispersion through longitudinally localized wavetrains with “great circle route” orientations, revealed most clearly by the lag-correlation maps for individual gridpoints. When the polarity of these localized patterns is such that large anomalies of like (opposing) sign appear in the Atlantic and Pacific sectors near 50°N, a strong zonal wavenumber 2 (1) pattern results. These wavenumber 1 and 2 patterns do not retain their identity from one 5 day period to the next as distinctly as the localized wavetrains do.

The conceptual model of Rossby-wave propagation along latitude circles still appears to be valid for the wintertime stratosphere, where the waves have the same two-dimensional scale as the polar vortex itself, and for external Rossby-modes such as those described by Madden (1978). It may also be valid at times in the Southern Hemisphere troposphere.

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