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.

Combined effects of anthropogenic aerosols and global warming on the South Asian Monsoon

2020 ◽  
Author(s):  
Ayantika Dey Choudhury ◽  
Krishnan Raghavan ◽  
Manmeet Singh ◽  
Swapna Panickal ◽  
Sandeep Narayansetti ◽  
...  
Keyword(s):  
Global Warming ◽  
South Asian ◽  
Large Scale ◽  
Asian Monsoon ◽  
Monsoon Circulation ◽  
South Asian Monsoon ◽  
Combined Effects ◽  
The South ◽  
Anthropogenic Aerosols ◽  
Aerosol Forcing

<p>The South Asian monsoon (SAM) precipitation has been generally regarded to exhibit contrasting responses to greenhouse gas (GHG) and anthropogenic aerosol forcing, although it is not adequately clear as to how it might respond to the combined influence of GHG and aerosol forcing.  The present study examines the individual and combined effects of global warming and anthropogenic aerosols on the SAM based on a suite of numerical experiments conducted using the IITM Earth System Model version2 (IITM-ESMv2). Four sets of 50-year model integrations are performed using IITM-ESMv2 with different anthropogenic forcings 1) Pre-Industrial control, 2) anthropogenic aerosols of 2005 3) CO2 concentrations of 2005 4) anthropogenic aerosols and CO2 of 2005. In the experiment with the elevated CO2 level of 2005, an intensification of SAM precipitation and strengthening of large-scale monsoon cross-equatorial flow is noted relative to the PI-CTL run. In contrast, the experiment with elevated anthropogenic aerosols of 2005 shows a decrease of SAM precipitation and weakening of monsoon circulation relative to the PI-CTL run. A striking result emerging from this study is the strong suppression of SAM precipitation, pronounced weakening of the monsoon circulation and suppression of organized convection in response to the combined radiative effects of elevated CO2 and anthropogenic aerosols relative to the PI-CTL run. By diagnosing the model simulations it is noted that the radiative effects in the combined forcing experiment lead to a pronounced summer-time cooling of the NH as compared to the equatorial and southern oceans which are predominantly influenced by global warming, thereby creating a north-south differential radiative forcing over the Indian longitudes.  Additionally, the influence of absorbing aerosols over South and East Asia creates a surface radiation deficit over the region, stabilizes the lower troposphere, slows down the monsoon winds and reduces surface evaporation.  Although the anticyclones over the subtropical Indian Ocean intensify in the combined forcing experiment, the model simulation shows that much of the precipitation enhancement occurs to the south of the equator over the Indian Ocean whereas the moisture transport and convergence to the north of the equator is substantially reduced. Furthermore, the combined forcing experiment shows that anomalous large-scale descent over the subcontinent reinforces the suppression of organized convection giving rise to more intense breaks and weaker active spells in the southwest monsoon on sub-seasonal time-scales. This study hints that future decreases in NH aerosol emissions could potentially reverse the ongoing decreasing trend of the observed SAM precipitation since 1950s in a purely global warming environment.</p>

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 Dominant patterns of interaction between the tropics and mid-latitudes in boreal summer: causal relationships and the role of timescales

2020 ◽  
Vol 1 (2) ◽  
pp. 519-539
Author(s):  
Giorgia Di Capua ◽  
Jakob Runge ◽  
Reik V. Donner ◽  
Bart van den Hurk ◽  
Andrew G. Turner ◽  
...  
Keyword(s):  
South Asian ◽  
North Pacific ◽  
Asian Monsoon ◽  
Tropical Convection ◽  
Boreal Summer ◽  
South Asian Monsoon ◽  
Convective Activity ◽  
The South ◽  
The Tropics ◽  
Patterns Of Interaction

Abstract. Tropical convective activity represents a source of predictability for mid-latitude weather in the Northern Hemisphere. In winter, the El Niño–Southern Oscillation (ENSO) is the dominant source of predictability in the tropics and extratropics, but its role in summer is much less pronounced and the exact teleconnection pathways are not well understood. Here, we assess how tropical convection interacts with mid-latitude summer circulation at different intra-seasonal timescales and how ENSO affects these interactions. First, we apply maximum covariance analysis (MCA) between tropical convective activity and mid-latitude geopotential height fields to identify the dominant modes of interaction. The first MCA mode connects the South Asian monsoon with the mid-latitude circumglobal teleconnection pattern. The second MCA mode connects the western North Pacific summer monsoon in the tropics with a wave-5 pattern centred over the North Pacific High in the mid-latitudes. We show that the MCA patterns are fairly insensitive to the selected intra-seasonal timescale from weekly to 4-weekly data. To study the potential causal interdependencies between these modes and with other atmospheric fields, we apply the causal discovery method PCMCI at different timescales. PCMCI extends standard correlation analysis by removing the confounding effects of autocorrelation, indirect links and common drivers. In general, there is a two-way causal interaction between the tropics and mid-latitudes, but the strength and sometimes sign of the causal link are timescale dependent. We introduce causal maps that show the regionally specific causal effect from each MCA mode. Those maps confirm the dominant patterns of interaction and in addition highlight specific mid-latitude regions that are most strongly connected to tropical convection. In general, the identified causal teleconnection patterns are only mildly affected by ENSO and the tropical mid-latitude linkages remain similar. Still, La Niña strengthens the South Asian monsoon generating a stronger response in the mid-latitudes, while during El Niño years the Pacific pattern is reinforced. This study paves the way for process-based validation of boreal summer teleconnections in (sub-)seasonal forecast models and climate models and therefore works towards improved sub-seasonal predictions and climate projections.

scholarly journals The influence of vegetation on the ITCZ and South Asian Monsoon in HadCM3

2012 ◽  
Vol 3 (1) ◽  
pp. 91-111 ◽  
Author(s):  
M. P. McCarthy ◽  
J. Sanjay ◽  
B. B. B. Booth ◽  
K. Krishna Kumar ◽  
R. A. Betts
Keyword(s):  
Northern Hemisphere ◽  
South Asian ◽  
Vegetation Cover ◽  
Large Scale ◽  
Asian Monsoon ◽  
Global Dynamics ◽  
South Asian Monsoon ◽  
The South ◽  
Tropical Vegetation

Abstract. The role of extra-tropical vegetation on the large-scale tropical circulation is examined in the version 3 Hadley Centre Climate Model (HadCM3). Alternative representations of present day vegetation from observations and a dynamic vegetation model were used as the land-cover component for a number of HadCM3 experiments under a nominal present day climate state, and are shown to induce perturbations to the simulated global dynamics. This results in a shift in the location of the Inter Tropical Convergence Zone (ITCZ) and changes in the South Asian monsoon circulation. This has a significant impact on the Indian land precipitation compared to the standard configuration of HadCM3. This large-scale forcing is consistent with documented mechanisms relating to temperature and snow perturbations in the Northern Hemisphere extra-tropics. This analysis demonstrates that uncertainties in the representation of present day vegetation cover can result in significant perturbations to the simulated climate. The role of the Northern Hemisphere extra-tropics is further demonstrated with a fourth representation of vegetation cover produced by imposing simulated changes in Northern Hemisphere extra-tropical vegetation from the end of the 21st century on the present day climate. This experiment shows that through similar processes extra-tropical vegetation changes in the future contribute to a strengthening of the South Asian monsoon in this model, with a particular influence on the monsoon onset. These findings provide renewed motivation to give careful consideration to the role of global scale vegetation feedbacks when looking at climate change and its impact on the tropics and South Asian monsoon in the latest generation of Earth System models.

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.

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