Observational Evaluation of a Convective Quasi-Equilibrium View of Monsoons

2010 ◽  
Vol 23 (16) ◽  
pp. 4416-4428 ◽  
Author(s):  
Ji Nie ◽  
William R. Boos ◽  
Zhiming Kuang
Keyword(s):  
Boundary Layer ◽  
South Asian ◽  
Asian Monsoon ◽  
Potential Temperature ◽  
South Asian Monsoon ◽  
Quasi Equilibrium ◽  
Tropospheric Temperature ◽  
The South ◽  
Layer Potential ◽  
The North

Abstract Idealized dynamical theories that employ a convective quasi-equilibrium (QE) treatment for the diabatic effects of moist convection have been used to explain the location, intensity, and intraseasonal evolution of monsoons. This paper examines whether observations of the earth’s regional monsoons are consistent with the assumption of QE. It is shown here that in local summer climatologies based on reanalysis data, maxima of free-tropospheric temperature are, indeed, nearly collocated with maxima of subcloud equivalent potential temperature, θeb, in all monsoon regions except the North and South American monsoons. Free-tropospheric temperatures over North Africa also exhibit a strong remote influence from the South Asian monsoon. Consistent with idealized dynamical theories, peak precipitation falls slightly equatorward of the maxima in θeb and free-tropospheric temperature in regions where QE seems to hold. Vertical structures of temperature and wind reveal two types of monsoon circulations. One is the deep, moist baroclinic circulation clearly seen in the South Asian monsoon. The other is of mixed type, with the deep moist circulation superimposed on a shallow dry circulation closely associated with boundary layer temperature gradients. While the existence of a shallow dry circulation has been documented extensively in the North African monsoon, here it is shown to also exist in Australia and southern Africa during the local summer. Analogous to moist QE theories for the deep circulation, the shallow circulation can be viewed in a dry QE framework in which shallow ascent occurs just equatorward of the peak boundary layer potential temperature, θb, providing a unified system where the poleward extents of deep and shallow circulations are bounded by maxima in θeb and θb, respectively.

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

2012 ◽  
Vol 25 (11) ◽  
pp. 3929-3941 ◽  
Author(s):  
Chi-Hua Wu ◽  
Ming-Dah Chou
Keyword(s):  
East Asia ◽  
South Asian ◽  
North Pacific ◽  
Western North Pacific ◽  
Large Scale ◽  
Asian Monsoon ◽  
Monsoon Circulation ◽  
South Asian Monsoon ◽  
The South ◽  
The North

By 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 southwesterly flow strengthens, and the western stretch of the NP high shifts northward ~10° latitude to the south of Japan. This series of changes indicates that the EA-WNP late July monsoon transition is initiated from changes in the upper-tropospheric circulation via the weakening of the EAJS south of ~45°N. The weakening of the EAJS south of ~45°N is related to a reduced gradient of the geopotential height on the northern flank of the Asian high, which is related to the massive inland heating and weakening of the South Asian monsoon circulation. The exact timing of the monsoon onset might be tied to the hypothesized “Silk Road pattern” and/or a strong weakening of the South Asian monsoon circulation.

scholarly journals The South Asian Monsoon Circulation in Moist Isentropic Coordinates

2020 ◽  
Vol 33 (12) ◽  
pp. 5253-5270
Author(s):  
T. P. Sabin ◽  
Olivier M. Pauluis
Keyword(s):  
South Asian ◽  
Asian Monsoon ◽  
Potential Temperature ◽  
Boreal Summer ◽  
South Asian Monsoon ◽  
Equivalent Potential Temperature ◽  
South Asian Summer Monsoon ◽  
The South ◽  
Equivalent Potential ◽  
Isentropic Coordinates

AbstractThe atmospheric circulation during the South Asian summer monsoon season is analyzed in moist isentropic coordinates. The horizontal mass transport is sorted in terms of its equivalent potential temperature and is separated into the upper- and lower-tropospheric contributions. This technique makes it possible to trace the transport of air parcels over long distances, identify regions of convective motion in the tropics, and assess the impacts of diabatic processes. The goal here is to assess the thermodynamic characteristics of the atmospheric overturning associated with the South Asian monsoon and to connect this thermodynamic structure to horizontal transport. The monsoon is associated with a low-level inflow of warm and moist air, compensated by an upper-tropospheric outflow at high potential temperature. The South Asian monsoon differs, however, from other monsoonal systems in two important ways. First, the ascending air exhibits an unusually high equivalent potential temperature, which results in global lifting of the tropopause during the boreal summer. Second, on a seasonal basis the main monsoon regions appear to be shielded from dry air intrusion from the extratropical regions.

scholarly journals Synoptic Preconditions for Extreme Flooding during the Summer Asian Monsoon in the Mumbai Area

2014 ◽  
Vol 15 (1) ◽  
pp. 229-242 ◽  
Author(s):  
Marco Lomazzi ◽  
Dara Entekhabi ◽  
Joaquim G. Pinto ◽  
Giorgio Roth ◽  
Roberto Rudari
Keyword(s):  
Time Series ◽  
South Asian ◽  
Asian Monsoon ◽  
Indian Subcontinent ◽  
Monsoon Season ◽  
Value Added ◽  
South Asian Monsoon ◽  
Flood Events ◽  
The South ◽  
Synoptic Conditions

Abstract The summer monsoon season is an important hydrometeorological feature of the Indian subcontinent and it has significant socioeconomic impacts. This study is aimed at understanding the processes associated with the occurrence of catastrophic flood events. The study has two novel features that add to the existing body of knowledge about the South Asian monsoon: 1) it combines traditional hydrometeorological observations (rain gauge measurements) with unconventional data (media and state historical records of reported flooding) to produce value-added century-long time series of potential flood events and 2) it identifies the larger regional synoptic conditions leading to days with flood potential in the time series. The promise of mining unconventional data to extend hydrometeorological records is demonstrated in this study. The synoptic evolution of flooding events in the western-central coast of India and the densely populated Mumbai area are shown to correspond to active monsoon periods with embedded low pressure centers and have far-upstream influences from the western edge of the Indian Ocean basin. The coastal processes along the Arabian Peninsula where the currents interact with the continental shelf are found to be key features of extremes during the South Asian monsoon.

scholarly journals The remote response of the South Asian Monsoon to reduced dust emissions and Sahara greening during the middle Holocene

2021 ◽  
Vol 17 (3) ◽  
pp. 1243-1271
Author(s):  
Francesco S. R. Pausata ◽  
Gabriele Messori ◽  
Jayoung Yun ◽  
Chetankumar A. Jalihal ◽  
Massimo A. Bollasina ◽  
...  
Keyword(s):  
South Asian ◽  
Asian Monsoon ◽  
Monsoon Season ◽  
Dust Emission ◽  
Northern Africa ◽  
South Asian Monsoon ◽  
The South ◽  
Airborne Dust ◽  
Orbital Parameters ◽  
Humid Period

Abstract. Previous studies based on multiple paleoclimate archives suggested a prominent intensification of the South Asian Monsoon (SAM) during the mid-Holocene (MH, ∼6000 years before present). The main forcing that contributed to this intensification is related to changes in the Earth's orbital parameters. Nonetheless, other key factors likely played important roles, including remote changes in vegetation cover and airborne dust emission. In particular, northern Africa also experienced much wetter conditions and a more mesic landscape than today during the MH (the so-called African Humid Period), leading to a large decrease in airborne dust globally. However, most modeling studies investigating the SAM changes during the Holocene overlooked the potential impacts of the vegetation and dust emission changes that took place over northern Africa. Here, we use a set of simulations for the MH climate, in which vegetation over the Sahara and reduced dust concentrations are considered. Our results show that SAM rainfall is strongly affected by Saharan vegetation and dust concentrations, with a large increase in particular over northwestern India and a lengthening of the monsoon season. We propose that this remote influence is mediated by anomalies in Indian Ocean sea surface temperatures and may have shaped the evolution of the SAM during the termination of the African Humid Period.

Characteristics of Precipitating Convective Systems in the South Asian Monsoon

2011 ◽  
Vol 12 (1) ◽  
pp. 3-26 ◽  
Author(s):  
Ulrike Romatschke ◽  
Robert A. Houze
Keyword(s):  
South Asian ◽  
Diurnal Cycle ◽  
Bay Of Bengal ◽  
Arabian Sea ◽  
Asian Monsoon ◽  
South Asian Monsoon ◽  
Western Himalayas ◽  
The South ◽  
Convective Systems ◽  
Large Systems

Abstract Eight years of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) data show how convective systems of different types contribute to precipitation of the South Asian monsoon. The main factor determining the amount of precipitation coming from a specific system is its horizontal size. Convective intensity and/or number of embedded convective cells further enhance its precipitation production. The precipitation of the monsoon is concentrated in three mountainous regions: the Himalayas and coastal ranges of western India and Myanmar. Along the western Himalayas, precipitation falls mainly from small, but highly convective systems. Farther east along the foothills, systems are more stratiform. These small and medium systems form during the day, as the monsoon flow is forced upslope. Nighttime cooling leads to downslope flow and triggers medium-sized systems at lower elevations. At the mountainous western coasts of India and Myanmar, small and medium systems are present throughout the day, as an orographic response to the southwesterly flow, with a slight superimposed diurnal cycle. Medium systems are favored over the eastern parts of the Arabian Sea and large systems are favored over the Bay of Bengal when an enhanced midlevel cyclonic circulation occurs over the northern parts of these regions. The systems forming upstream of coastal mountains over the Bay of Bengal are larger than those over the Arabian Sea, probably because of the moister conditions over the bay. The large systems over the bay exhibit a pronounced diurnal cycle, with systems forming near midnight and maximizing in midday.

Recent dust variability over South Asia controlled by North Atlantic SST anomalies

2021 ◽  
Author(s):  
Priyanka Banerjee ◽  
Sreedharan Krishnakumari Satheesh ◽  
Krishnaswamy Krishna Moorthy
Keyword(s):  
South Asia ◽  
South Asian ◽  
North Atlantic ◽  
Asian Monsoon ◽  
Positive Phase ◽  
South Asian Monsoon ◽  
The North ◽  
Sst Anomaly ◽  
The North Atlantic ◽  
High Dust

<p>Several studies have associated high dust years over South Asia to warming of the central or eastern equatorial Pacific Ocean (El Nino conditions) and the resulting weakening of the summer monsoon. Using satellite aerosol data for 2001-2018, we show that there has been a departure from this relation since the second decade of the 21st century with the North Atlantic Ocean emerging as a major driver of interannual variability of dust over South Asia. This change in relation coincides with the end of the global warming hiatus and a shift towards persistent positive phase of the winter North Atlantic Oscillation (NAO). Positive phase of the NAO induces cold phase of the spring/summer North Atlantic sea surface temperature (SST) tripole pattern. We show here that high dust activity during 2011-2018 is associated with positive SST anomaly over the mid-latitude North Atlantic and negative SST anomaly over the sub-tropical North Atlantic: the two southern arms of the SST tripole pattern. Interestingly, the relation between NAO and these two southern arms of the SST tripole has undergone changes in recent years, which has impacted the South Asian monsoon. The result is general drying over South Asia and an increase in the strength of the dust-carrying northwesterlies. Simulations with the Community Earth System Model (CESM) shows that SST tripole-like anomalies recorded during 2011-2018 over the North Atlantic can generate mid-latitude wave train that weakens the South Asian monsoon circulation, leads to surface high pressure anomalies and increase in dust emission and transport over northwest India and Pakistan. Most of the increase in the dust load can be attributed to enhanced transport at 800 hPa pressure level during May-June, which can lead to ~40-50% increase in dust concentrations at this level.</p>

Climate response of the south Asian monsoon system to West Asia, Tibetan Plateau and local dust emissions

2019 ◽  
Vol 53 (9-10) ◽  
pp. 6245-6264 ◽  
Author(s):  
Charu Singh ◽  
Dilip Ganguly ◽  
Puneet Sharma ◽  
Shiwansha Mishra
Keyword(s):  
Tibetan Plateau ◽  
South Asian ◽  
Asian Monsoon ◽  
South Asian Monsoon ◽  
Climate Response ◽  
The South ◽  
Dust Emissions ◽  
Monsoon System ◽  
Local Dust
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