Potential impacts of wintertime soil moisture anomalies from agricultural irrigation at low latitudes on regional and global climates

 

Abstract

    Anthropogenic water management can change surface energy budgets and the water cycle. In this study, we focused on impacts of Asian low-latitude irrigation on regional and global climates during boreal wintertime. A state-of-the-art Earth system model is used to simulate the land-air interaction processes affected by irrigation and the consequent responses in atmospheric circulation. Perturbed experiments show that wet soil moisture anomalies at low latitudes can reduce the surface temperature on a continental scale through atmospheric feedback. The intensity of prevailing monsoon circulation becomes stronger because of larger land-sea thermal contrast. Furthermore, anomalous upper level convergence over South Asia and midlatitude climatic changes indicate tropical-extratropical teleconnections. The wintertime Aleutian low is deepened and an anomalous warm surface temperature is found in North America. Previous studies have noted this warming but left it unexplained, and we provide plausible mechanisms for these remote impacts coming from the irrigation over Asian low-latitude regions.

Effect of the Arakan Mountains in the Northwestern Indochina Peninsula on the Late-May Asian Monsoon Transition
Chi-Hua Wu1, Huang-Hsiung Hsu1, and Ming-Dah Chou2

1Research center for Environmental Changes, Academia Sinica, Taipei, Taiwan
2Department of Atmospheric Sciences, National Central University, Chung-Li, Taiwan

 

Abstract


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 mid-tropospheric 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 wave-like 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.

Wu, C. H., H. H. Hsu, and M. D. Chou, 2014: Effect of the Arakan Mountains in the northwestern Indochina Peninsula on the late May Asian Monsoon transition. J. Geophys. Res. Atmos. 119, 10769-10779. DOI: 10.1002/2014JD022024

 

Effect of the Arakan Mountains in the Northwestern Indochina Peninsula on the Late-May Asian Monsoon Transition.png

Figure
Schematic diagram of the major thermal and dynamical structure during the late-May Asian summer monsoon transition.

Role of the Indochina Peninsula Narrow Mountains in Modulating the East Asian-Western North Pacific Summer Monsoon
Chi-Hua Wu1 and Huang-Hsiung Hsu1

1Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan

 

Abstract

Unrealistic topographic effects are generally incorporated in the 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 in the coasts of the Indochina Peninsula-in an 1° global climate model, we demonstrate that the proper representation of mesoscale topography over the Indochina Peninsula, is crucial for simulating realistically the seasonality of 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 Meiyu-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 EAWNP suggests a dynamical connection between the midlatitude westerly perturbation and Meiyu-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 intrinsic variability of the summer monsoon in EAWNP.

Wu, C. H., and H. H. Hsu, 2016: Role of the Indochina Peninsula narrow mountains in modulating the East Asia-Western North Pacific summer monsoon. J. Climate, JCLI-D-15-0594. (Published online: 22 March, 2016)

 

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

Figure
Schematic diagram of the atmospheric response (200hPa, 500hPa, and 850hPa) to the mountains for (a) LATE MAY and (b) LATE JULY. Orange color denotes general feature in each period and blue color denotes mountain-induced changes. Abbreviations: H for high pressure and AC/C for anticyclonic/cyclonic circulation anomaly.

Orbital Control of the Western North Pacific Summer Monsoon


Chi-Hua Wu1, John C. H. Chiang2, Huang-Hsiung Hsu1, and Shih-Yu Lee1

1Research center for Environmental Changes, Academia Sinica, Taipei, Taiwan
2Department of Geography and Berkeley Atmospheric Sciences Center, University of California, Berkeley, California

 

Abstract


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 adjustments made between regional monsoons, remain unclear. Simulations using the Community Earth System Model have demonstrated that the Western North Pacific (WNP) summer monsoon responds to orbital forcing in a sense opposite to that of other major Northern Hemisphere monsoon systems. Compared with its current climate state, the WNP monsoon was likely absent without a lower-level monsoon trough in the early Holocene when summer insolation was higher because of precessional changes, whereas the summer monsoons in South Asia and East Asia were stronger and shifted farther northward. The stronger Asian monsoon is suggested to be responsible for the weaker WNP monsoon, as though the former’s effect on increasing the North Pacific subtropical high.
The disappearance of the WNP monsoon trough is ascribed to the perihelion-induced high summer insolation and the teleconnected influence of enhanced precipitation over the southern Tibetan Plateau and Maritime Continent. By contrast, the impact of the midlatitude circulation changes on the WNP monsoon onset was weaker when the solar insolation was higher. In the instance of the extreme perihelion summer, the WNP monsoon was suppressed despite a stronger midlatitude precursor than present-day, and the midlatitude circulation response to the enhanced precipitation of South Asia was considerable. These conditions indicate internal monsoon interactions of an orbital scale, implying a potential mechanistic control of the WNP monsoon.

Wu, C. H., J. C. H. Chiang, H. H. Hsu, and S. Y. Lee, 2016: Orbital control of the western North Pacific summer monsoon. Climate Dynamics, 46(3), 897-911. DOI:10.1007/s00382-015-2620-3

 

Orbital Control of the Western North Pacific Summer Monsoon.png

Figure
Schematic diagram of the atmospheric response to the 11 ka BP solar forcing. Results are based on the 11K (light colors and dashed contours) and present-day (dark colors and solid contours) simulations from 25 July to 13 August. Contours denote zonal wind speeds (m/s). Blue shadings denote area in 11 ka BP where precipitation was enhanced relative to the present-day condition.

Tibetan Plateau Westerly Forcing on the Cloud Amount over Sichuan Basin and the Early Asian Summer Monsoon


Chi-Hua Wu1 and Ming-Dah Chou2

 

1Research center for Environmental Changes, Academia Sinica, Taipei, Taiwan
2Department of Atmospheric Sciences, National Central University, Chung-Li, Taiwan

 

Abstract


Data show that a sizeable mechanical effect of the Tibetan Plateau (TP) on the westerly, referred to as TP westerly forcing, is related to a sizeable mid-level cloud amount over Sichuan Basin (SB; centered near 30°N and 105°E) to the east of the TP. An investigation of the interannual variability of clouds over SB demonstrated that the TP westerly forcing closely correlates to the early Asian summer monsoon (ASM). When the monsoon commences in mid-May, the East Asian upper tropospheric jet stream moves northward with a southwest-northeast tilt; the tilt of the jet considerably influences the TP westerly forcing. Because of the stationary wave pattern superimposed on the westerly jet, the early ASM is substantially distinct by year. In the years the mid-level cloud amount in SB is sizeable (which corresponds to a strong TP westerly forcing), the tilt of the upper-level jet increases, and the ascending southerly east of SB in 110°E−125°E strengthens. The warming of the troposphere in 110°E−125°E centered near 35°N corresponds to the westerly and southerly changes. Further downstream in 125°E−140°E region, the ascending southerly weakens. Correspondingly, an anomalous anticyclonic circulation occurs between SB and Japan, leading to a westward extension of the North Pacific high pressure in the lower-troposphere and a reduction in precipitation of the early Mei-Yu season. Coincidentally, the mid-tropospheric monsoon trough south of the TP weakens, implying a weak monsoon in both East Asia and South Asia. Furthermore, the weak monsoon continues through the early summer.

Wu, C. H., and M. D. Chou, 2013: Tibetan Plateau westerly forcing on the cloud amount over Sichuan Basin and the Early Asian summer monsoon. J. Geophys. Res. Atmos., 118, 7558-7568. DOI: 10.1002/jgrd.50580

 

Tibetan Plateau Westerly Forcing on the Cloud Amount over Sichuan Basin and the Early Asian Summer Monsoon.png

Figure
The schematic of years with a strong TP westerly forcing (M+ years). When the mid-level cloud amount in SB east of the TP is large, corresponding to a strong TP westerly forcing, the southwest-northeast tilt of the upper-level jet stream increases and the ascending southerly east of SB between 110°E and 125°E strengthens. Further downstream to the region between 125°E and 140°E, a strong TP westerly forcing is related to a weak ascending southerly. Correspondingly, an anomalous anticyclonic circulation between SB and Japan leads to a strong western North Pacific high pressure in the lower-troposphere. The monsoons in East Asia and South Asia are both weak, corresponding to a strong TP westerly forcing.

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