Introduction
Motivation
Seasonal changes are only plotted where the annual harmonic accounts for at least 85% of the total variance. Any location where the first harmonic accounts for less than 50% of the total variance for precipitation is not shaded.
Approach
Using CMIP5 models forced with increased greenhouse gases, Dwyer et al. 2014) performed an annual mode analysis of the seasonal cycle of precipitation to check for trends in the amplitude and phase (Fig. 1.3). To study shifts in the position of the ITCZ and the coupled tropical circulation, we use the energetic framework.
Science Questions
The energy divergence is mainly carried out in the upper branches of the Hadley circulation, as the higher MSE is maintained in the upper atmosphere. Consequently, the region of maximum NEI is located along the ascending branch of the Hadley circulation.
Thesis Outline
The delay in the onset of the monsoon with warming is also noticeable in the changes in the time switch of the dynamic regime. Interseasonal differences in changes in the tendency component of the latent energy MSE tendency,.
Response of Monsoon Rainfall to Changes in the Latitude of the
Abstract
For continents extending to tropical latitudes, the simulated monsoon has a rapid migration of the convergence zone over the continent, similar to what is seen in observed monsoons. We show that the absence of land at tropical latitudes prevents the rapid shift to the subtropics of the maximum in lower-level moist static energy and thus the establishment of an overturning circulation with a subtropical convergence zone that can rapidly transition to an angular momentum-conserving monsoon regime.
Introduction
Recent studies have shown that the rapid onset of the monsoon can be interpreted as a switch in the tropical circulation, which can occur even in the absence of land-sea contrast, from a dynamic regime governed by eddy momentum currents to a monsoon regime. more directly controlled by energetic limitations. Here we investigate how one aspect of continental geometry, namely the position of the equatorward coastal boundary, affects such transitions.
Methods and Tools
Seasonal Cycle
While several ITCZ metrics exist in the literature, here we identify the ITCZ as the location of the precipitation maximum. Therefore, the hemispheric asymmetry in
Role of Dynamics
In the 10◦simulation, during NH summer the poleward magnitude of the Hadley cell increases rapidly in winter, starting around June 21 (highlighted), and gradually retreats after September 11 (highlighted) around late September (Fig. 2.4). The magnitude of the mean current advection is especially pronounced in the SH range of the winter Hadley cell. The divergent flow in the ascending branch of the Hadley cell instead contributes to a negative tendency for vortex stretching.
This is also evident from the very smooth evolution of the zero contour of the absolute vorticity (Fig. 2.11f).
Discussion
Having more land or areas of low thermal inertia in the tropics allows the distribution of the surface MSE to evolve rapidly, and the nonlinear mechanisms described above help keep its maximum far enough away from the equator to the subtropics to push for circulation. to grow rapidly in strength and size. Evidence of a slight temporal asymmetry between onset and retreat is also seen in the land-wide (not shown) and ocean-wide aquaplanet simulations (Fig. 2.2). However, it is clear that a large hemispheric asymmetry in the thermal inertia at the lower limit gives rise to a more pronounced asymmetry in the temporal evolution of the monsoon: in the presence of a continent at tropical latitudes, the lower level MSE can adapt quickly as the circulation transitions to the monsoon regime.
We believe that the hemispheric asymmetry in the lower limit thermal inertia may be a more relevant mechanism than WISHE feedback.
Conclusion
More targeted simulations, in which for example the WISHE reaction is disabled, will shed more light on these open questions. Sikan, 1991; Peyrillé et al., 2016). Other limitations include the lack of important radiative feedbacks, such as those associated with water vapor and cloud feedbacks, and the lack of albedo contrast between land and ocean, and any zonal asymmetry (Maroon & Frierson, 2016; Maroon et al ., 2016). Furthermore, we describe a zonally symmetric OHT, which neglects possible seasonality in the amplitude and direction of the OHT in response to surface wind variation (Kang et al., 2018; Lutsko et al., 2019).
Acknowledgements
Furthermore, we find that in extremely hot climates, the compensatory effect of the energy storage is limited due to complex changes in the surface temperature seasonality. As the climate warms to extreme temperatures, due to complex seasonal changes in the surface energy balance, the surface temperature trend may cause unexpected changes to the seasonal cycle of the energy storage. With warming, due to the compensatory effect of the latent energy storage in the spring, the circulation's response to thermal forcing was slowed.
Response of monsoon rainfall to changes in latitude of the equatorward coastline of a zonally symmetric continent.
Response of Monsoon Onset Timing to Greenhouse Warming in
Abstract
GCMs in the CMIP5 archive strongly project a delay in monsoon onset time with warming. Using the angular momentum budget, we find evidence of a delay in the speed and timing of the transition of the tropical circulation regime from an eddy-driven cell to a more angular momentum-preserving Hadley cell. Analyzing the static energy budget with atmospheric moisture, we demonstrate that as the climate warms, atmospheric energy storage increases significantly during spring and is able to compensate for changes in thermal forcing.
As a result, the circulation is able to respond to thermal forcing later in the season and monsoon onset is delayed.
Introduction
Several studies have also looked more specifically at changes in the amount of summer monsoon rainfall with warming. While the GCMs in the CMIP5 and CMIP6 archives robustly project a delay in the timing of monsoon onset with warming (Biasutti & Sobel, 2009; Dwyer et al., 2014;. As higher MSE is maintained in the upper atmosphere, the upper divergent branches depending on the HC, the energy diverges from the tropics towards the pole.
Using this energetic framework, recent work has related interseasonal changes with warming in NEI and effective atmospheric heat capacity 𝐶𝐴 between NH spring and summer to changes in the timing of monsoon onset (Song et al., 2018;.
Methods and Tools
The extreme range of simulated climates also shows detectable changes in the monsoon circulation strength, defined as the extreme value of the cross-equatorial winter Hadley cell in the solstice season. In the limit of 𝑅𝑜 → 0, the strength of the mean meridional flow (𝑣) is directly and linearly linked to the EMFD. In Section 3.4, we use the AM budget analysis to examine whether changes in monsoon onset timing are related to observed.
Importantly, when considering seasonal transitions, the change in shelf life plays a fundamental role.
Seasonal Cycle
Since both remain unchanged, the timing of the near-surface temperature gradient reversal in the equatorial region also remains largely unchanged. A delay in the timing of reversals in surface zonal winds can be observed in Figs. We investigate whether the monsoon onset delay is observable as a delay in the regime transition timing between a flow coupled to the EMFD.
There are striking differences in the circulation structure, strength and transition timing across the climates.
Energetic Framework Perspective
As expected by the Clausius-Clapeyron relationship, as the climate warms we see a noticeable increase in energy storage. What could cause latent energy storage to decline earlier in the season in the warmest climates? As the climate warms, we see a change in the dominant terms in the surface energy balance.
However, as the climate warms, the magnitudes of the SH surface flux changes are small compared to the LH surface flux changes.
Discussion and Conclusion
The analysis of the surface energy budget reveals interesting changes that occur as the climate warms. In our simulations, we observed a significant delay in the timing of monsoon onset with warming and observed changes in the double-peak structure and poleward magnitude of tropical precipitation. In contrast, in warmer climates, the seasonal transitions of the ITCZ were delayed, resulting in the tropical circulation reaching the monsoon dynamic AMC regime later in the season.
Furthermore, as the climate warms, the poleward extent of the summer excursion of the ITCZ increases, especially in the simulations with the 30◦ continent.
Conclusion
Thesis Summary and Concluding Remarks
With the AM budget, we find evidence of an initial delay in changes in the timing of the dynamical regime switch from the eddy-driven to the AMC monsoon regime. In this regime, the upper Hadley branch is in the angular momentum conservation (AMC) limit, where its streamlines are parallel to the AM contours. We found that both frameworks can be linked to the poleward excursion of the ITCZ.
Once the ITCZ was able to move far enough from the equator due to changes in the atmospheric energy balance, dynamical feedbacks were initiated that allowed rapid seasonal migration of the ITCZ due to dynamical regime switching, as understood by the AM budget .
Motivation for Future Work
Within our simulations, the energy framework helped us understand the position of the ITCZ throughout the season. Although highly idealized, our work contributes to deepening our fundamental understanding of the underlying mechanisms that determine the monsoon in the current climate and the future response to climate warming. Our work specifically highlights the impact of not only increases in magnitude but also changes in the seasonality of atmospheric energy storage on shifts in the timing of the future monsoon.
Baroclinic eddies and the extent of the hadley circulation: An idealized GCM study. Journal of the Atmospheric Sciences.
