There are large uncertainties looming over the status and fate of the South Asian summer monsoon, with several studies debating whether the monsoon is weakening or strengthening in a changing climate. Our analysis using multiple observed datasets demonstrates a significant weakening trend in summer rainfall during 1901-2012 over the central-east and northern regions of India, along the Ganges-Brahmaputra-Meghna basins and the Himalayan foothills, where agriculture is still largely rain-fed. Earlier studies have suggested an increase in moisture availability and land-sea thermal gradient in the tropics due to anthropogenic warming, favouring an increase in tropical rainfall. Here we show that the land-sea thermal gradient over South Asia has been decreasing, due to rapid warming in the Indian Ocean and a relatively subdued warming over the subcontinent. Using long-term observations and coupled model experiments, we provide compelling evidence that the enhanced Indian Ocean warming potentially weakens the land-sea thermal contrast, dampens the summer monsoon Hadley circulation, and thereby reduces the rainfall over parts of South Asia.

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Reliable age models are fundamental for any palaeoclimate reconstruction. Available interpolation procedures between age control points are often inadequately reported, and very few translate age uncertainties to proxy uncertainties. Most available modeling algorithms do not allow incorporation of layer counted intervals to improve the confidence limits of the age model in question. <br><br> We present a framework that allows detection and interactive handling of age reversals and hiatuses, depth-age modeling, and proxy-record reconstruction. Monte Carlo simulation and a translation procedure are used to assign a precise time scale to climate proxies and to translate dating uncertainties to uncertainties in the proxy values. The presented framework allows integration of incremental relative dating information to improve the final age model. The free software package COPRA1.0 facilitates easy interactive usage

Analysis of the microphysical structure of deep convective clouds using in situ measurements during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment (CAIPEEX) over the Indian peninsular region is presented. It is shown that droplet size distributions (DSDs) in highly polluted premonsoon clouds are substantially narrower than DSDs in less polluted monsoon clouds. High values of DSD dispersion (0.3–0.6) and its vertical variation in the transient and monsoon clouds are related largely to the existence of small cloud droplets with diameters less than 10 μm, which were found at nearly all levels. This finding indicates the existence of a continuous generation of the smallest droplets at different heights. In some cases this generation of small droplets leads to the formation of bimodal and even multimodal DSDs. The formation of bimodal DSDs is especially pronounced in monsoon clouds. Observational evidence is presented to suggest that in-cloud nucleation at elevated layers is a fundamental mechanism for producing multimodal drop size distribution in monsoon clouds as well as in most deep convective clouds. These findings indicate that inclusion of continued nucleation away from the cloud base into numerical models should be considered to predict microphysics and precipitation of clouds in monsoons and other cloud-related phenomena.

[1] Aircraft measurements of cloud condensation nuclei (CCN) and microphysics of clouds at various altitudes were conducted over India during CAIPEEX (Cloud Aerosol Interaction and Precipitation Enhancement Experiment) phase I and II in 2009 and 2010 respectively. As expected, greater CCN concentrations gave rise to clouds with smaller drops with greater number concentrations (N c ). The cloud drop effective radius (r e ) increased with distance above cloud base (D). Warm rain became detectable, i.e., rain water content >0.01 g/Kg, at the tops of growing convective clouds when r e exceeded 12 mm. The r e is determined by the number of activated CCN, N ad , and D. The N ad can be approximated by the maximum measured values of N c . Higher N c resulted in greater D for reaching the r e threshold for onset of warm rain, r e *, denoted as D*. In extreme cases of highly polluted and moist air that formed the monsoon clouds over the Indo-Gangetic plains, D* exceeded 6 km, well above the 0 C isotherm level. The precipitation particles were initiated there as supercooled raindrops at a temperature of À8 C. Giant CCN reduced r e * and D*, by initiating raindrops closer to cloud base. This effect was found mainly in dusty air masses over the Arabian Sea. Besides, the aerosol effect on D*, D* was found to decrease with increase in cloud water path.

In an attempt to develop a better simulation of the climatology of monsoon precipitation in climate models, this paper investigates the impacts of different convective closures on systematic biases of an Indian monsoon precipitation climatology in a high-resolution regional climate model. For this purpose, the Weather Research Forecast (WRF) model is run at 45-and 15-km (two-way nested) resolution with three convective parameterization schemes, namely the Grell-Devenyi (GD), the Betts-Miller-Janjić (BMJ), and the Kain-Fritsch (KF), for the period 1 May-31 October 2001-07. The model is forced with the NCEP-NCAR reanalysis data as the initial and boundary conditions. The simulated June-September (JJAS) mean monsoon rainfall with the three convective schemes is compared with the observations. KF is found to have a high moist bias over the central and western coastal Indian region while GD shows the opposite. Among the three, BMJ is able to produce a reasonable mean monsoon pattern. In an attempt to get further insight into the seasonal bias and its evolution, the probability distribution function (PDF) of different rain-rate categories and their percentage contribution to the seasonal total are computed. BMJ and KF underestimate the observations for lighter rain rates and overestimate for rain-rate categories of more than 10 mm day 21 . GD shows an overestimation for lighter rain and an underestimation of PDF for moderate categories. The seasonal patterns of evolution of PDF plots of three rain-rate categories are analyzed to determine whether the convective schemes show any systematic bias throughout the season or if they have problems during certain phases of the monsoon. This shows that the GD systematically overestimates the lighter rain rate and underestimates the moderate rain rate throughout the season, whereas BMJ and KF have problems in the initial stages. The heavy rain category is systematically overestimated by the KF compared to the other two. To further evaluate the proportionate contribution of each rain-rate bin to the total rain, the percentage contribution of each rain rate to the seasonal total is computed. Analyzing all the rain-rate simulations produced by the three schemes, it is found that KF has a moist bias and GD has a dry bias in the spatiotemporal distribution of the monsoon precipitation. Further, this paper investigates the causes behind the mean monsoon precipitation bias. It is shown that GD produces a model climate where the vertical velocity is less than that of the observations up to 500 hPa and the vertically integrated moist instability is also weaker. KF, on the other hand, shows a higher than the observed vertical velocity and a stronger moist instability. Along with this, the vertical profile of heating suggests a warmer middle level in the KF case and significantly reduced midlevel heating for GD. Thus, KF (GD) has produced a model atmosphere that has a stronger (weaker) convective instability to produce the observed bias in the model precipitation. BMJ is found to s...

During summer, the northern Indian Ocean exhibits significant atmospheric intraseasonal variability associated with active and break phases of the monsoon in the 30-90 days band. In this paper, we investigate mechanisms of the Sea Surface Temperature (SST) signature of this atmospheric variability, using a combination of observational datasets and Ocean General Circulation Model sensitivity experiments. In addition to the previously-reported intraseasonal SST signature in the Bay of Bengal, observations show clear SST signals in the Arabian Sea related to the active/break cycle of the monsoon. As the atmospheric intraseasonal oscillation moves northward, SST variations appear first at the southern tip of India (day 0), then in the Somali upwelling region (day 10), northern Bay of Bengal (day 19) and finally in the Oman upwelling region (day 23). The Bay of Bengal and Oman signals are most clearly associated with the monsoon active/break index, whereas the relationship with signals near Somali upwelling and the southern tip of India is weaker. In agreement with previous studies, we find that heat flux variations drive most of the intraseasonal SST variability in the Bay of Bengal, both in our model (regression coefficient, 0.9, against *0.25 for wind stress) and in observations (0.8 regression coefficient); *60% of the heat flux variation is due do shortwave radiation and *40% due to latent heat flux. On the other hand, both observations and model results indicate a prominent role of dynamical oceanic processes in the Arabian Sea. Wind-stress variations force about 70-100% of SST intraseasonal variations in the Arabian Sea, through modulation of oceanic processes (entrainment, mixing, Ekman pumping, lateral advection). Our *100 km resolution model suggests that internal oceanic variability (i.e. eddies) contributes substantially to intraseasonal variability at small-scale in the Somali upwelling region, but does not contribute to largescale intraseasonal SST variability due to its small spatial scale and random phase relation to the active-break monsoon cycle. The effect of oceanic eddies; however, remains to be explored at a higher spatial resolution.

The presence of a low- to mid-latitude interhemispheric hydrologic seesaw is apparent over orbital and glacial-interglacial timescales, but its existence over the most recent past remains unclear. Here we investigate, based on climate proxy reconstructions from both hemispheres, the inter-hemispherical phasing of the Intertropical Convergence Zone (ITCZ) and the low- to mid-latitude teleconnections in the Northern Hemisphere over the past 2000 years. A clear feature is a persistent southward shift of the ITCZ during the Little Ice Age until the beginning of the 19th Century. Strong covariation between our new composite ITCZ-stack and North Atlantic Oscillation (NAO) records reveals a tight coupling between these two synoptic weather and climate phenomena over decadal-to-centennial timescales. This relationship becomes most apparent when comparing two precisely dated, high-resolution paleorainfall records from Belize and Scotland, indicating that the low- to mid-latitude teleconnection was also active over annual-decadal timescales. It is likely a combination of external forcing, i.e., solar and volcanic, and internal feedbacks, that drives the synchronous ITCZ and NAO shifts via energy flux perturbations in the tropics.