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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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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.

Open access
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.

Open access
Chi-Hua Wu
and
Huang-Hsiung Hsu

Abstract

Unrealistic topographic effects are generally incorporated in global climate simulations and may contribute significantly to model biases in the Asian monsoon region. By artificially implementing the Arakan Yoma and Annamese Cordillera—two south–north-oriented high mountain ranges on the coasts of the Indochina Peninsula—in a 1° global climate model, it is demonstrated that the proper representation of mesoscale topography over the Indochina Peninsula is crucial for realistically simulating the seasonality of the East Asian–western North Pacific (EAWNP) summer monsoon.

Presence of the Arakan Yoma and Annamese Cordillera helps simulate the vertical coupling of atmospheric circulation over the mountain regions. In late May, the existence of the Arakan Yoma enhances the vertically deep southwesterly flow originating from the trough over the Bay of Bengal. The ascending southwesterly flow converges with the midlatitude jet stream downstream in the southeast of the Tibetan Plateau and transports moisture across the Indochina Peninsula to East Asia. The existence of the Annamese Cordillera helps the northward lower-tropospheric moisture transport over the South China Sea into the mei-yu–baiu system, and the leeside troughing effect of the mountains likely contributes to the enhancement of the subtropical high to the east. Moreover, the eastward propagation of wave energy from central Asia to the EAWNP suggests a dynamical connection between the midlatitude westerly perturbation and mei-yu–baiu. Including the Annamese Cordillera also strengthens a Pacific–Japan (PJ) pattern–like perturbation in late July by enhancing the cyclonic circulation (i.e., monsoon trough) in the lower-tropospheric western North Pacific. This suggests the contribution of the mountain effects to the intrinsic variability of the summer monsoon in the EAWNP.

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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

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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