<p>The Indian monsoon is a seasonal large-scale circulation system with complex dynamical and thermodynamical interactions, the physics of which is not fully understood. In particular, the advance of the monsoon over India, propagating against the mean mid-level wind field, cannot be explained by simple moisture flux arguments.&#160;</p><p>Here we introduce an idealised two-layer model of the moisture dynamics of monsoon onset, with simple and transparent physics,&#160;based on conservation laws applied to a vertical plane (which could represent a transect from northwest to southeast India). The model allows for moisture replenishment in the lower layer (corresponding to evaporation or a moist inflow), a flux of water vapour between the layers (corresponding to convection), and along-transect advection by prescribed upper and lower-layer flows. With idealised parameterisations of replenishment and convection, the model&#160;can be written as a pair of coupled partial differential equations, which permits both analytical and numerical solutions. When an equilibrium solution is perturbed by either a change in replenishment rate, convection strength, or winds, we observe the propagation of moisture fronts in both the upper and lower layers as the solution adjusts to a new equilibrium. When these moisture fronts propagate northwestwards against the upper-layer flow, they can be viewed as the monsoon onset. Taking advantage of the simplicity of the model, which allows a wide parameter regime to be investigated efficiently, we show how the onset&#160;speed depends on the assumed timescales of the parameterised convection and lower-layer replenishment, and that physically plausible parameterisations can lead to realistic onset speeds, even in this highly idealised model.</p>
The Indian monsoon is a seasonal large-scale circulation system with complex dynamical and thermodynamic interactions. The physical processes are not fully understood. In particular, the mechanisms that control the propagation of the monsoon onset across the Indian continent, against the mid-level wind field, are debated. The Indian monsoon is poorly represented in weather and climate models, with persistent systematic errors making it difficult to forecast the Indian monsoon accurately on subseasonal timescales. A two-layer model based on moisture conservation with a parameterised flux representing convection is developed and used to investigate the competition between dry advection in the upper levels, the rate of moisture replenishment at low levels, and the rate of convection from the lower to the upper layers. In a fixed Eulerian frame, the system is initialised at an equilibrium representing pre-onset (May) conditions. Then, changes in the rates of moist inflow and upper-level advection are introduced, triggering a transition to a new equilibrium, which reflects the full monsoon state (July-September). The two-layer model reproduces the Indian monsoon onset and its progression to the northwest, against an imposed 5 m⋅s −1 wind in the upper layer. Increasing the parameter representing moist inflow induces a monsoon onset, defined as a threshold of total column moisture, with clear progression from southeast toward northwest India. A lesser wind speed in the upper layer, signifying a weakening midtropospheric dry intrusion, allows more rapid progression of the monsoon onset. A greater upper-level wind speed, associated with a strengthening dry intrusion, causes the monsoon onset to retreat. We can quantify the nature of the monsoon onset by deriving an onset speed and the time taken for the system to adjust to a new equilibrium, using analytical theory.
Abstract. The South Asian and East Asian summer monsoons are globally significant meteorological features, creating a strongly seasonal pattern of precipitation, with the majority of the annual precipitation falling between June and September. The stability the monsoons is of extreme importance for a vast range of ecosystems and for the livelihoods of a large share of the world's population. Simulations are performed with an intermediate-complexity climate model in order to assess the future response of the South Asian and East Asian monsoons to changing concentrations of aerosols and greenhouse gases. The radiative forcing associated with absorbing aerosol loading consists of a mid-tropospheric warming and a compensating surface cooling, which is applied to India, Southeast Asia, and eastern China both concurrently and independently. The primary effect of increased absorbing aerosol loading is a decrease in summer precipitation in the vicinity of the applied forcing, although the regional responses vary significantly. The decrease in precipitation is not ascribable to a decrease in the precipitable water and instead derives from a reduction in the precipitation efficiency due to changes in the stratification of the atmosphere. When the absorbing aerosol loading is added in all regions simultaneously, precipitation in eastern China is most strongly affected, with a quite distinct transition to a low precipitation regime as the radiative forcing increases beyond 60 W m−2. The response is less abrupt as we move westward, with precipitation in southern India being least affected. By applying the absorbing aerosol loading to each region individually, we are able to explain the mechanism behind the lower sensitivity observed in India and attribute it to remote absorbing aerosol forcing applied over eastern China. Additionally, we note that the effect on precipitation is approximately linear with the forcing. The impact of doubling carbon dioxide levels is to increase precipitation over the region while simultaneously weakening the circulation. When the carbon dioxide and absorbing aerosol forcings are applied at the same time, the carbon dioxide forcing partially offsets the surface cooling and reduction in precipitation associated with the absorbing aerosol response. Assessing the relative contributions of greenhouse gases and aerosols is important for future climate scenarios, as changes in the concentrations of these species has the potential to impact monsoonal precipitation.
<p>The Asian summer monsoons are globally significant meteorological features, creating a strongly seasonal pattern of precipitation, with the majority of the annual precipitation falling between June and September. The stability of such a strongly seasonal hydrological cycle is of extreme importance for a vast range of ecosystems and for the livelihoods of a large share of the world&#8217;s population.</p> <p>&#160;</p> <p>Simulations are performed with an intermediate complexity climate model, PLASIM, in order to assess the future response of the Asian monsoons to changing concentrations of aerosols and greenhouse gases. The radiative forcing associated with aerosol loading consists of a mid-tropospheric warming and a compensating surface cooling, which is applied to India, Southeast Asia and East China, both concurrently and independently. The primary effect of increased aerosol loading is a decrease in summer precipitation in the vicinity of the applied forcing, although the regional responses vary significantly. The decrease in precipitation is only partially ascribable to a decrease in the precipitable water, and instead derives from a reduction of the precipitation efficiency, due to changes in the stratification of the atmosphere.</p> <p>&#160;</p> <p>When the aerosol loading is added in all regions simultaneously, precipitation in East China is most strongly affected, with a quite distinct transition to a low precipitation regime as the radiative forcing increases beyond 60 W/m<sup>2</sup>. The response is less abrupt as we move westward, with precipitation in South India being least affected. By applying the aerosol loading to each region individually, we are able to explain the mechanism behind the lower sensitivity observed in India, and attribute it to aerosol forcing over East China. Additionally, we note that the effect on precipitation is approximately linear with the forcing.</p> <p>&#160;</p> <p>The impact of doubling carbon dioxide levels is to increase precipitation over the region, whilst simultaneously weakening the circulation. When the carbon dioxide and aerosol forcings are applied at the same time, the carbon dioxide forcing partially offsets the surface cooling and reduction in precipitation associated with the aerosol response.</p>
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