Chi-Hua Wu

Asian Summer Monsoon in CMIP5 Projections: A Link between the Change in Extreme Precipitation and Monsoon Dynamics

AbstractChange in extreme events in climate projections is a major concern. If the frequency of dry events is expected to increase in a warmer climate (thus, the overall number of wet days will decrease), heavy and extreme precipitation are also expected to increase because of a shift of the precipitation spectrum. However, the forecasts exhibit numerous uncertainties.This study focuses on the Asian region, separated into the following three subregions: the East Asian region, the Indian region, and western North Pacific region, where the summer monsoon can bring heavy rainfall. Particularly emphasized herein is the reliability of the projection, using data from a large ensemble of 30 models from phase 5 of the Coupled Model Intercomparison Project. The scattering of the ensemble enables obtaining an optimal estimate of the uncertainties, and it is used to compute the correlation between projected changes of extreme events and circulation changes.The results show clear spatial and temporal variations in th...

Upper-Tropospheric Forcing on Late July Monsoon Transition in East Asia and the Western North Pacific

AbstractBy investigating the large-scale circulation in the upper troposphere, it is demonstrated that the rapid late July summer monsoon transition in the East Asia and western North Pacific (EA-WNP) is associated with a weakened westerly at the exit of the East Asian jet stream (EAJS). Even in a normally stable atmosphere under the influence of the North Pacific (NP) high in late July, convection rapidly develops over the midoceanic region of the western NP (15°–25°N, 150°–170°E). Prior to the rapid transition, the EAJS weakens and shifts northward, which induces a series of changes in downstream regions; the northeastern stretch of the Asian high weakens, upper-tropospheric divergence in the region southwest of the mid-NP trough increases, and convection is enhanced. At the monsoon transition, upper-level high potential vorticity intrudes southward and westward, convection expand from the mid NP westward to cover the entire subtropical western NP, the lower-tropospheric monsoon trough deepens, surface ...

Large-Scale Control of Summer Precipitation in Taiwan

AbstractTaiwan is located at the western stretch of the North Pacific high pressure (NP high) ridge in boreal summer, and its climate is highly sensitive to the NP high. By grouping years of anomalously high and low summer precipitation in Taiwan, this study investigated the large-scale atmospheric circulation and the land–sea temperature contrast during these two groups of years and identified the control of summer precipitation in Taiwan. It is found that in years when summer precipitation in Taiwan is anomalously high, the western stretch of the NP high weakens. Weakening of the western stretch of the NP high induces strengthened southerly wind and enhanced vertical motion in East Asia and the western NP (EA–WNP) region, which is essentially an invigorated summer monsoon circulation. Corresponding to the invigorated circulation, precipitation increases in the southern section of the EA–WNP but decreases in the midlatitude section of the EA–WNP. It is further found that in those wet years, the land–sea ...

Orbital control of the western North Pacific summer monsoon

Orbital forcing exerts a strong influence on global monsoon systems, with higher summer insolation leading to stronger summer monsoons in the Northern Hemisphere. However, the associated regional and seasonal changes, particularly the interaction between regional monsoon systems, remain unclear. Simulations using the Community Earth System Model demonstrate that the western North Pacific (WNP) summer monsoon responds to orbital forcing opposite to that of other major Northern Hemisphere monsoon systems. Compared with its current climate state, the simulated WNP monsoon and associated lower-tropospheric trough is absent in the early Holocene when the precession-modulated Northern Hemisphere summer insolation is higher, whereas the summer monsoons in South and East Asia are stronger and shift farther northward. We attribute the weaker WNP monsoon to the stronger diabatic heating of the summer Asian monsoon—in particular over the southern Tibetan Plateau and Maritime Continent—that in turn strengthens the North Pacific subtropical high through atmospheric teleconnections. By contrast, the impact of the midlatitude circulation changes on the WNP monsoon is weaker when the solar insolation is higher. Prior to the present WNP monsoon onset, the upper-tropospheric East Asian jet stream weakens and shifts northward; the monsoon onset is highly affected by the jet-induced high potential vorticity intrusion. In the instance of the extreme perihelion-summer, the WNP monsoon is suppressed despite a stronger midlatitude precursor than present-day, and the midlatitude circulation response to the enhanced South Asian precipitation is considerable. These conditions indicate internal monsoon interactions of an orbital scale, implying a potential mechanistic control of the WNP monsoon.

Effect of the Arakan Mountains in the northwestern Indochina Peninsula on the late May Asian monsoon transition

By simulations using a global climate model with and without the Arakan Mountains in the northwest of Myanmar, we demonstrated that this mesoscale meridionally elongated mountain range has a substantial effect on anchoring and enhancing precipitation in the region during the Bay of Bengal (BoB) monsoon onset in late May. In this period, the presence of the Arakan Mountains in the model significantly improves the simulation of the thermal and dynamical atmospheric structure of the monsoon, by substantially enhancing precipitation, deepening the midtropospheric trough and the southwesterly flow over the BoB and strengthening the upper tropospheric anticyclone atop the trough. These mountain-induced changes essentially improve the simulation of the late May Asian summer monsoon transition. Prior to the BoB monsoon onset, the blocking and deflecting effects of the Arakan Mountains on the low-level flow are marked, which enhance the moisture convergence and also probably influence the oceanic forcing over the BoB. As well as the mountain effect on the moisture convergence, inclusion of the Arakan Mountains apparently initiates an upstream troughing effect, which in turn enhances the synoptic and large-scale circulation during the monsoon transition. Furthermore, the presence of the Arakan Mountains induces a large-scale wavelike perturbation, which likely leads to an improvement of Meiyu/Baiu simulation. This study reveals that a narrow mountain like the Arakan Mountains likely contributes markedly to the characteristics of the Asian summer monsoon during its early seasonal march. Such effects need to be reasonably resolved in the models.

Role of the Indochina Peninsula Narrow Mountains in Modulating the East Asian–Western North Pacific Summer Monsoon

AbstractUnrealistic 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 moistur...

Impact of the Himalayas on the Meiyu–Baiu migration

The subseasonal migrations of the East Asian summer monsoon are nearly identical to that of the South Asian summer monsoon. In mid-May to mid-June (Phase 1), the South Asian westerly strengthens, the center of the South Asian high moves northwestward, and the East Asian frontal system coupled to and located north of the Western North Pacific (WNP) high moves northward, with all of these actions occurring rapidly. In mid-June to late July (Phase 2), the strength of the South Asian westerly reaches a maximum, the center of the South Asian high remains at approximately 30°N, and the northward propagation of the WNP high becomes stagnant. The speed of the northward movement of the WNP high in Phase 2 is only half the speed of that in Phase 1. By late June, the South Asian monsoon has reached northern India and been blocked by the Himalayas. This indicates that the Himalayas have an effect of constraining the speed of the northward movement of the South Asian high and, in turn, the WNP high. Climate model experiments further reveal that the near-stationary nature of the East Asian frontal system in Phase 2 is related to the blocking of the South Asian summer monsoon by the Himalayas. We suggest that the evolution of the WNP high is constrained by the evolution of the South Asian high, and the Meiyu–Baiu is connected to the South Asian summer monsoon through the impact of the South Asian monsoon heating on the upper tropospheric circulation.

The influence of obliquity in the early Holocene Asian summer monsoon

The early Holocene climatic optimum is associated with perihelion precession and high obliquity, though most studies emphasize the former over the latter. Asian monsoon proxy records only decisively show the precessional impact. To explore the obliquity effect, four climate simulations are conducted by fixing orbital parameters of present-day (0K), early Holocene (11K), the lowest obliquity (31K), and 11Ks precession and eccentricity with 31Ks obliquity (11Kp31Ko). We show that high obliquity significantly augments the precessional impact by shifting the Asian monsoon farther north than present. By contrast, the present-day monsoon seasonality can still be identified in the simulations with low obliquity. We argue that the upper tropospheric (South Asian) and lower tropospheric (North Pacific) high-pressure systems are affected by the subtropical atmospheric heating changes responding to obliquity. As a consequence, associated with the changes in meridional gradients of geopotential height and temperature made by the heating, midlatitude transient eddies and monsoon-midlatitude interactions are modulated.

Large-scale control of the Arabian Sea monsoon inversion in August

The summer monsoon inversion in the Arabian Sea is characterized by a large amount of low clouds and August as the peak season. Atmospheric stratification associated with the monsoon inversion has been considered a local system influenced by the advancement of the India–Pakistan monsoon. Empirical and numerical evidence from this study suggests that the Arabian Sea monsoon inversion is linked to a broader-scale monsoon evolution across the African Sahel, South Asia, and East Asia–Western North Pacific (WNP), rather than being a mere byproduct of the India–Pakistan monsoon progression. In August, the upper-tropospheric anticyclone in South Asia extends sideways corresponding with the enhanced precipitation in the subtropical WNP, equatorial Indian Ocean, and African Sahel while the middle part of this anticyclone weakens over the Arabian Sea. The increased heating in the adjacent monsoon systems creates a suppression effect on the Arabian Sea, suggesting an apparent competition among the Africa–Asia–WNP monsoon subsystems. The peak Sahel rainfall in August, together with enhanced heating in the equatorial Indian Ocean, produces a critical effect on strengthening the Arabian Sea thermal inversion. By contrast, the WNP monsoon onset which signifies the eastward expansion of the subtropical Asian monsoon heating might play a secondary or opposite role in the Arabian Sea monsoon inversion.

Thermodynamic and dynamic influences in the Far East‐Okhotsk region on stagnant Meiyu‐Baiu

Westerly perturbation is enlarged over the Far East–Okhotsk region in late June and early July, and is associated with the largest land-sea heating contrast surrounding the Sea of Okhotsk. The corresponding characteristics in the lower troposphere are southward deepening of the cold low over northeastern China, and intensification of the Okhotsk high. Coincidentally, the Meiyu–Baiu coupled with the western North Pacific (WNP) subtropical high is nearly stagnant during this period. By simulations using a global climate model with an intensified Okhotsk surface high in response to cooling Sea of Okhotsk, it is suggested that the enhanced thermal contrast over the Far East–Okhotsk region can generate an obstacle to Meiyu–Baiu poleward migration. Corresponding to the intensified Okhotsk high, the WNP subtropical high is strengthened by the high ridge over Taiwan. The East Asian midlatitude westerly jet stream in the northern flank of the WNP subtropical high also strengthens; the consequently enhanced midtropospheric warm temperature advection can regulate the latitude position of the Meiyu–Baiu. The wave source generated in the upper troposphere is located over the midlatitude WNP (anomalous cyclone) bordering the Sea of Okhotsk, whereas that in the lower troposphere is related to the strengthened westerly in the northern WNP subtropical high. The wave activity propagation consistently indicates the strengthening and equatorward confinement of the westerly jet. Therefore, the intensified Okhotsk high and enlarged westerly perturbation beginning in late June are suggested as an inherently geographic limit of the Far East–Okhotsk region in regard to Meiyu–Baiu migration.