scholarly journals Horizontal and Vertical Structures of the Northward-Propagating Intraseasonal Oscillation in the South Asian Monsoon Region Simulated by an Intermediate Model*

2007 ◽  
Vol 20 (16) ◽  
pp. 4278-4286 ◽  
Author(s):  
Hae-Kyung Lee Drbohlav ◽  
Bin Wang
Keyword(s):  
Phase Difference ◽  
Rossby Wave ◽  
3D Model ◽  
Asian Monsoon ◽  
Intraseasonal Oscillation ◽  
Boreal Summer ◽  
Monsoon Region ◽  
Wave Type ◽  
Asian Monsoon Region ◽  
Northward Propagation

Abstract The structures and mechanism of the northward-propagating boreal summer intraseasonal oscillation (BSISO) in the southern Asian monsoon region are simulated and investigated in a three-dimensional intermediate model (3D model). The horizontal structure of the intraseasonal variability in the 3D model depicts the Kelvin–Rossby wave–type disturbance, which may or may not produce the northward-propagating disturbance in the Indian Ocean, depending on the seasonal-mean background winds. Two experiments are conducted in order to identify what characteristic of seasonal-mean background can induce the northwestward-tilted band in the Kelvin–Rossby wave, whose overall eastward movement gives the impression of the northward propagation at a given longitude. When the prescribed boreal summer mean winds are excluded in the first experiment, the phase difference between the barotropic divergence tendency and convection disappears. Consequently, the Rossby wave–type convection forms a zonally elongated band. As a result, the northward propagation of convection at a given longitude disappears. When the easterly vertical shear is introduced in the second experiment, the horizontal and the vertical structures of BSISO become similar to that of the northward-propagating one. The reoccurrence of the northwestward-directed convective band and the phase difference between the barotropic divergence tendency and the convection suggest that the summer mean zonal winds in the boreal Indian summer monsoon region are a critical condition that causes the horizontal and vertical structures of northward-propagating BSISO in the southern Asian monsoon region.

scholarly journals Impact of geographic variations of the convective and dehydration center on stratospheric water vapor over the Asian monsoon region

2016 ◽  
Vol 16 (12) ◽  
pp. 7825-7835 ◽  
Author(s):  
Kai Zhang ◽  
Rong Fu ◽  
Tao Wang ◽  
Yimin Liu
Keyword(s):  
Water Vapor ◽  
Southeast Asia ◽  
Geographic Variation ◽  
Diabatic Heating ◽  
Asian Monsoon ◽  
Boreal Summer ◽  
Monsoon Circulation ◽  
Monsoon Region ◽  
Geographic Variations ◽  
Asian Monsoon Region

Abstract. The Asian monsoon region is the most prominent moisture center of water vapor in the lower stratosphere (LS) during boreal summer. Previous studies have suggested that the transport of water vapor to the Asian monsoon LS is controlled by dehydration temperatures and convection mainly over the Bay of Bengal and Southeast Asia. However, there is a clear geographic variation of convection associated with the seasonal and intra-seasonal variations of the Asian monsoon circulation, and the relative influence of such a geographic variation of convection vs. the variation of local dehydration temperatures on water vapor transport is still not clear. Using satellite observations from the Aura Microwave Limb Sounder (MLS) and a domain-filling forward trajectory model, we show that almost half of the seasonal water vapor increase in the Asian monsoon LS are attributable to geographic variations of convection and resultant variations of the dehydration center, of which the influence is comparable to the influence of the local dehydration temperature increase. In particular, dehydration temperatures are coldest over the southeast and warmest over the northwest Asian monsoon region. Although the convective center is located over Southeast Asia, an anomalous increase of convection over the northwest Asia monsoon region increases local diabatic heating in the tropopause layer and air masses entering the LS are dehydrated at relatively warmer temperatures. Due to warmer dehydration temperatures, anomalously moist air enters the LS and moves eastward along the northern flank of the monsoon anticyclonic flow, leading to wet anomalies in the LS over the Asian monsoon region. Likewise, when convection increases over the Southeast Asia monsoon region, dry anomalies appear in the LS. On a seasonal scale, this feature is associated with the monsoon circulation, convection and diabatic heating marching towards the northwest Asia monsoon region from June to August. The march of convection leads to an increasing fraction of the air mass to be dehydrated at warmer temperatures over the northwest Asia monsoon region. Work presented here confirms the dominant role of temperatures on water vapor variations and emphasizes that further studies should take geographic variations of the dehydration center into consideration when studying water vapor variations in the LS as it is linked to changes of convection and large-scale circulation patterns.

scholarly journals Precipitation-Top Heights of Heavy Orographic Rainfall in the Asian Monsoon Region

2016 ◽  
Vol 73 (8) ◽  
pp. 3009-3024 ◽  
Author(s):  
Shoichi Shige ◽  
Christian D. Kummerow
Keyword(s):  
Relative Humidity ◽  
Heavy Rainfall ◽  
Asian Monsoon ◽  
The Philippines ◽  
Winter Monsoon ◽  
Boreal Summer ◽  
Monsoon Region ◽  
Asian Monsoon Region ◽  
Low Level ◽  
Orographic Rainfall

Abstract Over coastal mountain ranges of the Asian monsoon region, heavy orographic rainfall is frequently associated with low precipitation-top heights (PTHs). This leads to conspicuous underestimation of rainfall using microwave radiometer algorithms, which conventionally assume that heavy rainfall is associated with high PTHs. Although topographically forced upward motion is important for rainfall occurrence, it does not fully constrain precipitation profiles in this region. This paper focuses on the thermodynamic characteristics of the atmosphere that determine PTHs in tropical coastal mountains of Asia (Western Ghats, Arakan Yoma, Bilauktaung, Cardamom, Annam Range, and the Philippines). PTHs of heavy orographic rainfall generally decrease with enhanced low- and midlevel relative humidity, especially during the summer monsoon. In contrast, PTHs over the Annam Range of the Indochina Peninsula increase with enhanced low-level and midlevel relative humidity during the transition from boreal summer to winter monsoon, demonstrating that convection depth is not simply a function of humidity. Instead, PTHs of heavy orographic rainfall decreased with increasing low-level stability for all monsoon regions considered in this study, as well as the Annam Range during the transition from boreal summer to winter monsoon. Therefore, low-level static stability, which inhibits cloud growth and promotes cloud detrainment, appears to be the most important parameter in determining PTHs of heavy rainfall in the Asian monsoon region.

Intercomparison of Deep Convection over the Tibetan Plateau–Asian Monsoon Region and Subtropical North America in Boreal Summer Using CloudSat/CALIPSO Data

2011 ◽  
Vol 24 (8) ◽  
pp. 2164-2177 ◽  
Author(s):  
Yali Luo ◽  
Renhe Zhang ◽  
Weimiao Qian ◽  
Zhengzhao Luo ◽  
Xin Hu
Keyword(s):  
North America ◽  
Tibetan Plateau ◽  
Asian Monsoon ◽  
Deep Convection ◽  
Local Environment ◽  
Boreal Summer ◽  
The Tibetan Plateau ◽  
Monsoon Region ◽  
Asian Monsoon Region ◽  
Cloud Tops

Abstract Deep convection in the Tibetan Plateau–southern Asian monsoon region (TP–SAMR) is analyzed using CloudSat and Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data for the boreal summer season (June–August) from 2006 to 2009. Three subregions are defined—the TP, the southern slope of the plateau (PSS), and the SAMR—and deep convection properties (such as occurrence frequency, internal vertical structure, system size, and local environment) are compared among these subregions. To cast them in a broader context, four additional regions that bear some similarity to the TP–SAMR are also discussed: East Asia (EA), tropical northwestern Pacific (NWP), and western and eastern North America (WNA and ENA, respectively). The principal findings are as follows: 1) Compared to the other two subregions of the TP–SAMR, deep convection over the TP is shallower, less frequent, and embedded in smaller-size convection systems, but the cloud tops are more densely packed. These characteristics of deep convection over the TP are closely related to the unique local environment, namely, a significantly lower level of neutral buoyancy (LNB) and much drier atmosphere. 2) In a broader context in which all seven regions are brought together, deep convection in the two tropical regions (NWP and SAMR; mostly over ocean) is similar in many regards. A similar conclusion can be drawn among the four subtropical continental regions (TP, EA, WNA, and ENA). However, tropical oceanic and subtropical land regions present some significant contrasts: deep convection in the latter region occurs less frequently, has lower cloud tops but comparable or slightly higher tops of large radar echo (e.g., 0 and 10 dBZ), and is embedded in smaller systems. The cloud tops of the subtropical land regions are generally more densely packed. Hence, the difference between the TP and SAMR is more of a general contrast between subtropical land regions and tropical oceanic regions during the boreal summer. 3) Deep convection over the PSS possesses some uniqueness of its own because of the distinctive terrain (slopes) and moist low-level monsoon flow. 4) Results from a comparison between the daytime (1:30 p.m.) and nighttime (1:30 a.m.) overpasses are largely consistent with researchers’ general understanding of the diurnal variation of tropical and subtropical deep convection.

scholarly journals Impact of geographic variations of convective and dehydration center on stratospheric water vapor over the Asian monsoon region

2016 ◽  
Author(s):  
K. Zhang ◽  
R. Fu ◽  
T. Wang ◽  
Y. Liu
Keyword(s):  
Water Vapor ◽  
Geographic Variation ◽  
Diabatic Heating ◽  
Asian Monsoon ◽  
Boreal Summer ◽  
Monsoon Circulation ◽  
Monsoon Region ◽  
Geographic Variations ◽  
Asian Monsoon Region ◽  
Air Mass

Abstract. The Asian monsoon region is the most prominent moisture center of lower stratospheric (LS) water vapor during boreal summer. Previous studies have suggested that the transport of water vapor to the Asian monsoon LS is controlled by dehydration temperatures and convection mainly over the Bay of Bengal and Southeast Asia. However, there is a clear geographic variation of convection associated with the seasonal and intra-seasonal variations of the Asian monsoon circulation, and the relative influence of such a geographic variation of convection vs. the variation of local dehydration temperatures on water vapor transport is still not clear. Using the Aura Microwave Limb Sounder (MLS) satellite observations and a domain-filling forward trajectory model, we show that almost half of the seasonal water vapor increase in the Asian monsoon LS are attributable to the geographic variations of convection and resultant variations of dehydration center, comparable to the influence of the local dehydration temperature increase. In particular, dehydration temperatures are coldest over the southeast and warmest over the northwest within the Asian monsoon region. Although convective center is located over the southeastern Asia, an anomalous increase of convection over the northwestern Asian monsoon region increases the local diabatic heating in the tropopause layer and air mass entering the LS that is dehydrated at relatively warm er temperatures. The warmer dehydration temperatures allow anomalously moist air enters the LS and then moves eastward along the northern frank of the monsoon anticyclonic flow, leading to wet anomalies in the LS over the Asian monsoon region. Likewise, when convection increases over the southeastern Asian monsoon region, dry anomalies appear in the LS. On seasonal scale, this feature is associated with the march of the monsoon circulation, convection and diabatic heating towards the northwestern Asia monsoon from June to August, leading to an increasing fraction of the air mass to be dehydrated at warmer temperatures over the nort hwestern Asian monsoon region. Work presented here confirms the dominant role of temper atures and also emphasizes that one should take the geographic variations of dehydration center into consideration when studying water vapor variations in the LS, as it is linked to changes of convection and large-scale circulation.

Impact of Subdaily Air–Sea Interaction on Simulating Intraseasonal Oscillations over the Tropical Asian Monsoon Region

2015 ◽  
Vol 28 (3) ◽  
pp. 1057-1073 ◽  
Author(s):  
Wenting Hu ◽  
Anmin Duan ◽  
Guoxiong Wu
Keyword(s):  
General Circulation ◽  
Asian Monsoon ◽  
Intraseasonal Oscillation ◽  
Phase Relationship ◽  
Coupled Model ◽  
Circulation Model ◽  
Boreal Summer ◽  
Lag Phase ◽  
Asian Monsoon Region ◽  
Coupling Frequency

Abstract The off-equatorial boreal summer intraseasonal oscillation (ISO) is closely linked to the onset, active, and break phases of the tropical Asian monsoon, but the accurate simulation of the eastward-propagating low-frequency ISO by current models remains a challenge. In this study, an atmospheric general circulation model (AGCM)–ocean mixed layer coupled model with high (10 min) coupling frequency (DC_10m) shows improved skill in simulating the ISO signal in terms of period, intensity, and propagation direction, compared with the coupled runs with low (1 and 12 h) coupling frequency and a stand-alone AGCM driven by the daily sea surface temperature (SST) fields. In particular, only the DC_10m is able to recreate the observed lead–lag phase relationship between SST (SST tendency) and precipitation at intraseasonal time scales, indicating that the ISO signal is closely linked to the subdaily air–sea interaction. During the ISO life cycle, air–sea interaction reduces the SST underlying the convection via wind–evaporation and cloud–radiation feedbacks, as well as wind-induced oceanic mixing, which in turn restrains convection. However, to the east of the convection, the heat-induced atmospheric Gill-type response leads to downward motion and a reduced surface westerly background flow because of the easterly anomalies. The resultant decreased oceanic mixing, together with the increased shortwave flux, tends to warm the SST and subsequently trigger convection. Therefore, the eastward-propagating ISO may result from an asymmetric east–west change in SST induced mainly by multiscale air–sea interactions.

scholarly journals What controls the stable isotope composition of precipitation in the Asian monsoon region?

2017 ◽  
Author(s):  
Le Duy Nguyen ◽  
Ingo Heidbüchel ◽  
Hanno Meyer ◽  
Bruno Merz ◽  
Heiko Apel
Keyword(s):  
Isotopic Composition ◽  
Asian Monsoon ◽  
Precipitation Amount ◽  
Climatic Conditions ◽  
Monsoon Region ◽  
Asian Monsoon Region ◽  
Air Mass ◽  
Local Factors ◽  
Regional Factors ◽  
Isotopic Records

Abstract. This study analyzes the influence of local and regional climatic factors on the stable isotopic composition of rainfall in the Vietnamese Mekong Delta as part of the Asian monsoon region. It is based on 1.5 years of weekly rainfall samples. Their isotopic content is analyzed by Local Meteoric Water Lines (LMWL) and single-factor regressions. Additionally, the contribution of several regional and local factors is quantified by multiple linear regressions (MLR) of all possible factor combinations and by relative importance analysis, a novel approach for the interpretation of isotopic records. The local factors are extracted from local climate records, while the regional factors are derived from atmospheric backward trajectories of water particles. The regional factors, i.e. precipitation, temperature, relative humidity and moving distance of the backward trajectories, are combined with equivalent local climatic parameters to predict the response variables δ18O, δ2H, and d-excess of precipitation at the station of measurement. The results indicate that (i) MLR can much better explain the isotopic variation of precipitation (R2 = 0.8) compared to single-factor linear regression (R2 = 0.3); (ii) the isotopic variation in precipitation is controlled dominantly by regional moisture regimes (~ 70 %) compared to local climatic conditions (~ 30 %); (iii) the most important climatic parameter during the early rainy season is the precipitation amount along the trajectories of air mass movements; (iv) the influence of local precipitation amount and temperature is not significant during the early rainy season, unlike the regional precipitation amount effect; (v) secondary fractionation processes (e.g. sub-cloud evaporation) take place mainly in the dry season, either locally for δ18O and δ2H, or along the air mass trajectories for d-excess. The analysis shows that regional and local factors vary in importance over the seasons, and that the source regions and transport pathways, and in particular the climatic conditions along the pathways, have a large influence on the isotopic composition of rainfall. The proposed methods thus proved to be valuable for the interpretation of the isotopic records in rainfall and the factors controlling it. The results illustrate that the interpretation of the isotopic composition in precipitation as a recorder of local climatic conditions, as for example performed for paleo records of water isotopes, may not be adequate in the Southern part of the Indochinese Peninsula, and likely also not in other regions affected by monsoon processes. However, the presented approach could open a pathway towards better and seasonally differentiated reconstruction of paleoclimates based on isotopic records.

Temperature signals of ice core and speleothem isotopic records from Asian monsoon region as indicated by precipitation δ18O

2021 ◽  
Vol 554 ◽  
pp. 116665
Author(s):  
Wusheng Yu ◽  
Tandong Yao ◽  
Lonnie G. Thompson ◽  
Jean Jouzel ◽  
Huabiao Zhao ◽  
...  
Keyword(s):  
Asian Monsoon ◽  
Ice Core ◽  
Monsoon Region ◽  
Asian Monsoon Region ◽  
Isotopic Records ◽  
Precipitation Δ18o
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