Research

The processes controlling the dynamics of tropical rain-bands (ITCZ and monsoon regions where most rainfall occurs) are not completely understood yet. The scientists leading World Climate Research Program have identified this problem as one of the "fundamental puzzle" of climate science. Classically the rain-bands over the ocean (ITCZ) are thought to occur over the warm waters, i.e. the rain producing clouds and convection in the ITCZ is thought to be merely a response to the forcing through warm sea surface temperatures. But recent studies show that the clouds in and around the ITCZ themselves impact the latitudinal extent (the width) and the intensity of the ITCZ through their radiative effects.

To understand this in detail, we designed idealised numerical simulations of the ITCZ using "aqua-planet configuration" (Imaginary Earth covered only with water !!) in a popular climate model (CESM). We developed a new methodology to separate roles of high altitude clouds vs. low altitude clouds in the ITCZ or role of clouds in the ITCZ vs. clouds remote to the ITCZ. The radiative effects of clouds have a complex heating effect at different altitudes in troposphere (within 15km altitude from ground). Our methodology also allowed us to separate the role of these effects.

We found that the heating at low altitude due to high altitude clouds within the ITCZ affects the ITCZ width and its intensity the most. The remote clouds have relatively very small influence on these features of the ITCZ. Diagnostics of changes in these ITCZ features implied that the overturning circulation at the lower altitude strengthens due to these cloud effects which then leads to enhanced moisture import in the ITCZ 'core latitudes' and enhances the precipitation there. In response, the precipitation on the 'edge latitudes' decreases and the ITCZ width reduces. This work was published in Geophysical Research Letters [Link].

All in all, warm waters are important for forming ocean rainbands but clouds induce similar heating effects as the warm sea surface temperatures except that this effect occurs in the lower altitudes in the atmosphere (as opposed to near ground) making them effective in re-enforcing supply of moisture to the ITCZ !

The African Sahara could be the largest art gallery on Earth, showcasing thousands of engravings and cave paintings. Desert today, it was covered with plants around 5,000 years ago during mid-Holocene. This climatic shift had tremendous impact on the African civilization and continues to be a threat to the presently vegetated region south of Sahara which experienced a multi-decadal drought during the later half of the 20th century. Looking at the African geography and local meteorology one wonders if 5,000 years ago African monsoon stretched farther northward to cover the Saharan desert and make it vegetated.

The earth's orbit of revolution around the Sun has a peculiar oscillation of ~24,000 years which brings about the change in amount of Solar energy (heat from the sun-rays) received at different parts of the world over that time-scale. This calculation implies ~5% more solar energy over Saharan desert during African monsoon season. During winter to summer season transition, the energy input almost doubles over the Saharan desert. In this context, can 5% further change cause such a large climatic shift ? A group of Palaeo-modelers have tested this idea. In earth system-climate models, when the earth's orbit is appropriately changed, the African monsoon stretches farther towards Sahara in many models. But it covers not even 20-30% of expected coverage over Sahara and the rainfall increase in those regions is insignificant to affect vegetation. What are these models missing ? This is a perfect problem to work on to improve our understanding of possible climatic shifts that affect the ecosystem in significant way.

We selected one of these models (CESM) to check the sensitivity of monsoonal circulation to mid-Holocene like change in energy input. The idea was to first check the factors limiting the northward extent of African monsoon in the present climate and then to see how / if those factors get modified in response to the energy input. We simulated African climate with present (P simulation) and mid-Holocene (MH simulation) orbital conditions. In parallel, we conducted a series of experiments where we forced the model to undergo a shift from present climate to mid-Holocene vegetated climate by adding energy in the Saharan boundary layer (H simulations). These later experiments allowed us to calibrate the energy source required to produce vegetated climate and compare the energy source with effective energy source in the MH simulation.

The P simulation showed that in present climate the warm dry air at mid-levels starting from the Saharan desert penetrates the African monsoonal rain-band from the north. In effect, the tug of war between the import of moisture inland from the oceans through monsoonal flow and drying above it through mid-level flow decides the northward limit of rain-band. This implied that if the mid-level flow weakens then the monsoonal circulation can expand further northward and cover a significant part of Sahara. In MH simulation, only a weak northward expansion of monsoons occurred (similar to the other Palaeo-modelers experience described before) because both monsoonal and mid-level flows increased and there wasn't a clear winner in the tug of war. The H simulations showed that the energy input required to produce vegetated climate were not significantly larger than the MH simulation but in those simulations the mid-level flow was significantly reduced and the monsoonal flow extended much farther into the desert.

This has several implications to our understanding of this phenomenon. Firstly, the mid-level flow shows sudden decrease in strength beyond a threshold value of energy input implying that a systematic bias in processes in climate models may underestimate values of effective energy input and hence thereby not allowing the monsoonal flow to stretch well into the desert. Secondly, these changes in flows occur purely through interaction of thermodynamic processes and dynamic response. This suggests that a feedback involving vegetation growth may not be essential process. Furthermore, this mechanism is consistent with the Palaeo-ecologist's notion that a similar "Green Sahara" phenomenon may have happened several times in the past ! This work was published in Journal of Climate [Link].

In summary, the flows controlling exchange of air between deserts and monsoon regions are key to our understanding of climatic shifts.

The south Asian monsoons are unique because there the most severe rainfall occurs at latitudes where otherwise deserts or semiarid climates occur in different parts of the world. Though several theories and explanations involving role of different physical processes are proposed, this mystery is yet to be completely understood. Albeit, it is generally agreed that as a system, the monsoons are manifestation of seasonal excursions of the oceanic rain-bands (ITCZ) during summer from their otherwise preferred location near equatorial Indian ocean towards the south Asian landmass as far north from the equator as the tropic of cancer.

A clear understanding in this important problem is lacking because in this region the interaction of land-atmosphere-ocean with local geography is so complex that it is difficult to understand the involved processes and assess their importance. To understand such complication, a hierarchy of modeling complexity is required. In the hierarchy, one simplified aspect of this complex problem is to simulate the ITCZ in simplified version of climate model and then to test if the dynamics controlling the near equatorial ITCZ is different from the off-equatorial ITCZ.

We imposed a warm sea surface temperature in an aqua-planet version of Climate Model (CESM) to favor a particular location for the formation of deep convection and the ITCZ. In one experiment, the ITCZ was favored near equator and this was contrasted with the other simulation where if was favored away from the equator. We noted that the near equatorial ITCZ is formed by the convergence of trade winds. These winds are forced by a surface pressure gradients and modulated by earth's rotation through the Coriolis force and frictional forces. In contrast, the off-equatorial ITCZ is formed by the deceleration of cross-equatorial 'monsoon like' northward flows. This implies that the location where inter-hemispheric air mixes together is farther northward than the location where maximum rainfall occurs. This asymmetry in the circulation is such that the the inertial effects of the flow are equally important along with the other three forces noted before. These effects add near the off-equatorial location to enhance associated convergence there. This work was published in the Journal of Earth System Science [Link].

The monsoonal winds have a lot of inertia !

The strongest atmospheric flows in the tropics are observed at an altitude of approximately 15Km from the ground. In fact, these flows are a part of gigantic atmospheric overturning that occurs such that mass rises up in the convective rain-bands near the equator, travels towards pole at ~15Km altitude in the overturning branch and starts descending at subtropical latitudes to ultimately return back to the rain-bands near surface. Scientists recently discovered another branch of overturning at low altitudes (4Km). While it is known that the higher branch is forced by pressure gradients generated due to the precipitating tall cumulus clouds, it was not clear as to how the lower branch is generated.

One proposed explanation was that these circulations are analogous to the land and sea breezes observed near coastlines. These breezes occur because during day the land warms quickly whereas during night it cools quickly as compared to adjacent ocean. This explanation argued that a similar circulation happens at larger scale over the oceans when strong warm-cold water contrasts are formed.

We tested this hypothesis in simplified aqua-planet version of the climate model. This allowed us to focus on processes relevant to surface warming contrast, convection and associated circulations. We ran two simulations, one similar to equinoctial conditions when the ITCZ forms near the equator and the associated overturning circulation is symmetric in both the northern and southern hemispheric troposphere. In this, strongest wam-cold surface temperature gradients were imposed to south and north of the equator to favour the ITCZ location near the equator. The other one was similar to solisticial conditions, when the ITCZ forms away from the equator and the associated overturning is asymmetric both with respect to the equator and also with respect to the ITCZ. In this simulation, the surface temperature gradients were weak to the south and strong to the north of the ITCZ. If the proposed explanation was correct, then strongest circulations should form in the first ITCZ case when surface temperature gradients were strongest and weakest circulations should form to the south of the ITCZ in second case. Interestingly, we noticed exactly opposite !

We did further diagnostics to understand this mystery and noticed that the latitude of ITCZ formation and associated hemispheric asymmetry plays important role in the strength of circulation. We did force balance analysis to notice that the balance driving low altitude circulation is different in these two cases and is more dominant effect than the surface temperature contrast effect. We noticed that the development of low altitude circulation was arrested by wavy-motion of flows (eddies) when ITCZ was near equator, in contrast such eddies were weakest to the south of the off-equatorial ITCZ allowing for development of such circulation. This study was published in the journal of Meteorology and Atmospheric Physics [Link].

We learned that convection and associated low altitude circulation is strongly modulated by earth's rotational effects.

A simple linear model was proposed by Jiang et al. (2004) to highlight the mechanism of scale selection during the northward propagation of cloud bands over Bay of Bengal. The easterly shear in zonal winds was shown to be an essential parameter in scale selection. This model was criticized for the use of unrealistic values of the friction and diffusion parameters. The present study extends this model by adding baroclinic vertical shear in the meridional mean winds. The correct rate of propagation is obtained with reasonable values of friction and diffusion parameters. In the present model, the direction of propagation is essentially determined by easterly vertical shear of zonal winds, while vertical shear of meridional wind also contributes to the observed propagation phase speed and instability. The correct phase speed is obtained for southerly mean meridional vertical shear.

We published this work in the Geophysical research letters [Link].

The Initiation of Gill type response to the elevated heat source in LMDZ

I worked on "The Initiation of Gill type response to the elevated heat source in LMDZ" while doing an intern-ship at LMD, Paris with Dr. J P Duvel. I performed a series of simulations to examine the response of the dynamics in LMDZ to the prescribed steady and transient, large scale, elevated heat source. The simulations were set up according to the proposal for the inter-comparison of the dynamical cores of atmospheric general circulation models by Held and Suarez. A typical Gill type response was set up in the simulations characterized by a strong westerly jet in the middle troposphere. The cyclonic Rossby gyres with westerly wind burst to the west and easterly flow to the east were observed in the lower troposphere. The upper level flow was characterized by easterly (westerly) flow to the west (east) of the heating. Sensitivity studies were carried out to study the effect of changes in spatial and temporal structure of elevated heating on the Gill type response.

The structure and dynamics of Inter-tropical convergence zone

For my PhD thesis, I am studying various aspects of structure and dynamics of ITCZ. I use zonally symmetric "Aqua planet" (earth covered with water !!) simulations as a tool to simulate these aspects. The "aqua-planets" are not as complex as an Earth GCM which involves the complexity due to Land, Orography and Ocean processes while they are not as simple as a Shallow water model or a single column model. It retains the complexity associated with the atmospheric convection and associated non-linear processes but simplifies the lower boundary conditions. Hence they are best suited to study processes controlling structure and Dynamics of ITCZ. I calculate budgets of mass, momentum and energy as a diagnostic tool to identify the important processes which govern various aspects of ITCZ.

I presented a poster about "The sensitivity of location of ITCZ to SST gradients and convective relaxation time scale" in OCHAMP conference in IITM, Pune in 2012.

I investigated processes controlling abrupt transition of ITCZ in an Aqua planet simulation. I presented a poster in EGU 2014.