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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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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
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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Huang-Hsiung Hsu
and
Shih-Hsun Lin

Abstract

Teleconnections of the streamfunction in the global domain based on ECMWF 250-mb winds for the 11 northern winters from 1978/79 through 1988/89 are documented in this study. A zonal structure with a node near the equator, indicating an out-of-phase relationship between the streamfunctions in the Northern and Southern hemisphere, appears to mask the fluctuations of the asymmetric components of streamfunction. After removing zonal means, a global pattern emerges as the dominant structure in the low-frequency band. This pattern consists of several dipoles straddling either the exit region of midlatitude jets or the equator, indicating the existence of teleconnections not only between the midlatitudes and the tropics but also between the Northern and Southern hemispheres.

Teleconnection patterns in the intermediate-frequency band are predominantly wavelike. Seven waveguides are identified based on the one-point lag-correlation maps for base points near the maximum teleconnectivity. Among them are three waveguides that have not been identified in previous studies. One originates in Europe, skirts the southern Eurasian continent, and spreads into the western Pacific. The other two originate in the northern central Pacific and the North American continent, respectively, and cross the equatorial regions of the westerlies into the Southern Hemisphere. The existence of cross-equatorial waveguides indicates the possibility of interhemispheric interaction and is in agreement with the hypothesis of Webster and Holton. Squared refractive indices are calculated based on the climatological flow and are found to be consistent with the existence of waveguides.

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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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Huang-Hsiung Hsu
and
John M. Wallace

Abstract

Orthogonal rotated principal component analysis of the wintertime, Northern Hemisphere, 5-day mean sea level pressure field yielded five modes which are of some dynamical interest. One can be identified with the well-known North Atlantic Oscillation and another with the Pacific/North American pattern. Three of the other modes are highly baroclinic in the sense that their sea level pressure patterns and their associated 500 mb height patterns are different in shape and opposite in polarity over substantial areas. These more baroclinic patterns attain their largest amplitudes in the vicinity of the Himalayas and Rockies. Their spatial patterns evolve very differently in the lower and middle troposphere: the sea level pressure patterns exhibit a distinctive eastward and/or equatorward phase propagation, parallel to contours of surface elevation, along the northern and/or eastern side of the mountain ranges, while the corresponding 500 mb patterns evolve in a manner consistent with the concept of Rossby wave dispersion. It is hypothesized that the phase propagation of the sea level pressure pattern is due, in part, to the equivalent-beta effect responsible for the terrain slope.

These highly baroclinic patterns appear to be associated with the low-temporal correlations between 1000 and 500 mb height and for the deep equatorward penetration of wintertime cold air outbreaks observed along the lee slopes of the major mountain ranges.

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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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Huang-Hsiung Hsu
and
Shih-Ming Lin

Abstract

This study investigates the tripole rainfall pattern in East Asia during the northern summer. The tripole pattern is characterized by a zonally elongated and meridionally banded structure with signs changing alternatively from 20° to 50°N along the East Asian coast. The positive (negative) phase of the pattern is characterized by more (less) rainfall in central-eastern China, Japan, and South Korea, and less (more) rainfall in northern and southern China. Asymmetry between the positive and negative phases is one of the key findings of this study. The tripole pattern is closely associated with two wavelike patterns: the Pacific–Japan pattern and the Silk Road pattern. The former, which emanates from the tropical western Pacific to extratropical East Asia, is more evident in the positive phase, while the latter, emanating across the Eurasian continent, is more evident in the negative phase. The positive phase appears to have a stronger tropical connection, while the negative phase has a stronger extratropical connection. The positive and negative phases are associated with the positive and negative SSTA in the equatorial eastern Pacific, respectively. It is suggested that in the positive phase the zonally oriented overturning circulation driven by the positive SSTA in the equatorial eastern Pacific induces heating anomalies in the tropical western Pacific, which in turn triggers a wavelike pattern emanating northward toward extratropical East Asia. This indirect SSTA effect is not evident in the negative phase, which is predominantly affected by the extratropical Eurasian wavelike perturbations. On the other hand, anomalous heating over the eastern Tibetan Plateau seems to induce the eastward-propagating wavelike structure in both phases. It is suggested that the tripole pattern is a result of the amplification of an intrinsic dynamic mode that can be triggered by various factors despite their different origins.

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Huang-Hsiung Hsu
and
Ming-Ying Lee

Abstract

This study investigates the relationship between deep convection (and heating anomaly) in the Madden–Julian oscillation (MJO) and the tropical topography. The eastward propagation of the deep heating anomalies is confined to two regions: the Indian Ocean and the western Pacific warm pool. Superimposed on the eastward propagation is a series of quasi-stationary deep heating anomalies that occur sequentially and discretely downstream in a leapfrog manner in the central Indian Ocean, the Maritime Continent, tropical South America, and tropical Africa.

The deep heating anomaly, usually preceded by near-surface moisture convergence and shallow heating anomalies, tends to occur on the windward side of the tropical topography in these regions (except the central Indian Ocean) under the prevailing surface easterly anomaly of the MJO. It is suggested that the lifting and frictional effects of the tropical topography and landmass induce the near-surface moisture convergence anomaly, which in turn triggers the deep heating anomaly. Subsequently, the old heating anomaly located to the west of the tropical topography weakens and the new heating anomaly east of the topography develops because of the eastward shift in the major moisture convergence center to the east of the mountains. Therefore, the deep heating anomaly shifts eastward from one region to another. The equatorial Kelvin wave, which is forced by the tropical heating anomaly and propagates quickly across the ocean basins in the lower troposphere, plays an important role by helping to strengthen the easterly anomaly and lowering the surface pressure.

This process is proposed to further our understanding of the shift in the deep convection from the Indian Ocean to the western Pacific, the reappearance of the deep convection in tropical South America, and the initiation of the MJO in the western Indian Ocean. It is suggested that the fast eastward propagation and the slow development of quasi-stationary convection together determine the quasi-periodicity of the MJO.

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Huang-Hsiung Hsu
and
Ying-Ting Chen

Abstract

Torrential rainfall occurring along the North American northeast coast (NANC) in summer and autumn is accompanied by strong atmospheric rivers (ARs), which efficiently transport abundant moisture along a narrow-stretched path associated with a low pressure system. In this study, an autodetection method was used to identify ARs that reached the NANC, based on the 6-hourly data of the ERA-Interim reanalysis conducted by the European Centre for Medium-Range Weather Forecasts, in summer and autumn from 1979 to 2016. Stronger ARs tended to occur in the eastern flank of a cyclonic anomaly that covered the entire North American east coast from Florida to Newfoundland, with a positive precipitation anomaly over the NANC. The cyclonic anomalies and precipitation in autumn were stronger but less frequent than those in summer. Cyclonic anomalies were parts of westward-tilting wavelike circulation perturbations moving into North America from the extratropical North Pacific and moving continuously eastward, reaching the east coast in approximately five days. The Geophysical Fluid Dynamics Laboratory (GFDL) High-Resolution Atmospheric Model (HiRAM), which realistically simulates the occurrence frequency and key characteristics of ARs in current climatic conditions, was used to project the AR activity and corresponding circulations in the future warmer climate under the representative concentration pathway 8.5 scenario. The HiRAM that was driven by sea surface temperature changes projected an overall increase in the occurrence of stronger ARs in both summer and autumn and the precipitation strength in autumn along the NANC by the end of the twenty-first century. This projected enhancement was contributed to by two processes—a smaller contribution was from the weakened basin-scale North Atlantic anticyclone but with higher moisture content, and a larger contribution was from the enhancement in anomalous circulation during AR events with integrated vapor transport exceeding the 75th percentile. These results suggest that the influence of strong ARs on the NANC may increase in the warmer future due to the combination of increased water vapor in the large-scale environment (thermodynamic effect) and enhanced anomalous circulations (dynamic effect). The AR-associated circulations in autumn were also projected to have a stronger tropical connection in the warmer future.

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