R. S. Ajayamohan

New York University Abu Dhabi · Center for Prototype Climate Modelling

The low-latitudinal cyclones (LLCs, originating between 5°N-10°N) constitute ≈40% of tropical cyclones (TCs) formed in the Bay of Bengal (BoB). We investigate the interannual variability of post-monsoonal (October to December) BoB LLCs and their teleconnection with El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). It is found that...

The dominant interannual SST variability in the eastern equatorial Atlantic is referred to as the Atlantic Zonal Mode (AZM), which peaks in boreal summer impacts global weather patterns. The cold (warm) phase of this ocean-atmospheric coupled phenomenon enhances (weakens) the intensity of the Indian Summer Monsoon Rainfall (ISMR). Observational stu...

Several single‐year‐long (2017) simulations with different configurations of the Weather Research and Forecasting (WRF) model over the United Arab Emirates (UAE) and the Middle‐East at the convective gray‐zone resolution (9km) are evaluated in their ability to capture the temporal and spatial distributions of precipitation. Annual rainfall over the...

Tropical cyclones do not form easily near the equator but can intensify rapidly, leaving little time for preparation. We investigated the number of near-equatorial (originating between 5°N and 11°N) tropical cyclones over the north Indian Ocean during post-monsoon seasons (October to December) over the past 60 years. A marked 43% decline in the num...

The barrier effect of the Maritime Continent (MC) in stalling or modifying the propagation characteristics of MJO is widely accepted. The strong diurnal cycle of convection over the MC is believed to play a dominant role in this regard. This hypothesis is studied here, with the help of a coarse-resolution Atmospheric General Circulation Model (AGCM...

The role of advection of heat and momentum on the evolution of near-surface temperature and wind is evaluated in urban-aware simulations over Houston under dry conditions on a light-wind day. Two sets of experiments, each consisting of four simulations using different planetary boundary layer (PBL) schemes, were conducted over 48 hours using the de...

The central aim of this work is to investigate the characteristics of fog events over the United Arab Emirates (UAE) and identify the underlying physical processes responsible for fog initiation and dissipation. To achieve this, hourly meteorological measurements at eight airport stations, along with ERA-5 reanalysis data (1995-2018), are utilized....

Rain gauge data are routinely recorded and used around the world. However, their sparsity and inhomogeneity make them inadequate for climate model calibration and many other climate change studies. Various algorithms and interpolation techniques have been developed over the years to obtain adequately distributed datasets. Objective interpolation me...

A consensus of the 21 st century climate change in the ocean is surface warming, stratification due to extreme freshening and subsequent weakening of mixing, overturning circulation, and biological production. Counter intuitively, certain parts of tropical ocean may develop a resistance to changes in mixing, where the climate change impacts of atmo...

Due to rapid urbanization, the near-surface meteorological conditions over urban areas are greatly modulated. To capture such modulations, sophisticated urban parameterizations with enhanced hydrological processes have been developed. In this study, we use the single-layer urban canopy model (SLUCM) available within the Weather Research and Forecas...

Atlantic Zonal Mode (AZM) and Indian summer monsoon rainfall (ISMR) are known to have an inverse relationship, which means that the cold (warm) phases of AZM result in strong (weak) ISMR. Here, we report that the inverse relationship between AZM and ISMR has significantly strengthened in recent decades. The cause of this strengthening relationship...

Atlantic Zonal Mode (AZM) and Indian summer monsoon rainfall (ISMR) are known to have an inverse relationship, which means that the cold (warm) phases of AZM result in strong (weak) ISMR. Here, we report that the inverse relationship between AZM and ISMR has significantly strengthened in recent decades. The cause of this strengthening relationship...

Observational and modeling studies have identified an El Niño Southern Oscillation (ENSO) like Ocean-Atmospheric coupled phenomenon in the tropical Atlantic during the boreal summer season, popularly known as Atlantic Zonal Mode (AZM). The atmospheric teleconnection between the AZM and Indian Summer Monsoon Rainfall (ISMR) is significant especia...

Assessing the future projections of the length of rainy season (LRS) has paramount societal impact considering its potential to alter the seasonal mean rainfall over the Indian subcontinent. Here, we explored the projections of LRS using both historical and Representative Concentration Pathways 8.5 (RCP8.5) simulations of the Coupled Model Intercom...

We assess the predictability of the large-scale Monsoon Intraseasonal Oscillations (MISOs) as measured by precipitation. An advanced nonlinear data analysis technique, Nonlinear Laplacian Spectral Analysis (NLSA), is applied to the daily precipitation data that results in two spatial modes associated with the MISO. The large-scale MISO patterns are...

Cyclonic atmospheric vortices of varying intensity, collectively known as low-pressure systems (LPS), travel northwest across central India and produce more than half of the precipitation received by that fertile region and its ∼600 million inhabitants. Yet, future changes in LPS activity are poorly understood, due in part to inadequate representat...

The Arabian Gulf (Gulf) and the surrounding regions are centers of intense economic activity. The precipitating weather systems that form over the Gulf are important for this predominantly arid region. It is suggested that El Niño Southern Oscillation (ENSO) influence the Middle East precipitation variability through an equatorward shift of the sub...

An improved index for real-time monitoring and forecast verification of monsoon intraseasonal oscillations (MISOs) is introduced using the recently developed nonlinear Laplacian spectral analysis (NLSA) technique. Using NLSA, a hierarchy of Laplace-Beltrami (LB) eigenfunctions are extracted from unfiltered daily rainfall data from the Global Precip...

It has been recently demonstrated that stratiform heating plays a critical role in the scale-selection of organized tropical convection, in an aquaplanet version of a coarse resolution Atmospheric General Circulation Model coupled to a stochastic multicloud cumulus parameterization scheme. It is shown that Madden-Julian oscillation (MJO)- like orga...

The climate variability on Earth is strongly influenced by the changes in the Sea Surface Temperature (SST) anomalies in the tropical oceans. More specifically, the inter-annual climate variability in the tropics as well as extra-tropical areas has large impact due to the anomalous SSTs in the tropical Pacific coupled with the El Niño Southern Osci...

The oceanic impact of poleward shift in Monsoon Low-Level Jet (MLLJ) is examined using a regional ocean model (ROMS). Two sets of downscaling experiments were conducted using ROMS with boundary and initial conditions from six CMIP5 models. While outputs from the historical run (1981-2000) acts as forcing for the first, the second uses RCP8.5 (2080...

It is widely recognized that stratiform heating contributes significantly to tropical rainfall and to the dynamics of tropical convective systems by inducing a front-to-rear tilt in the heating profile. Precipitating stratiform anvils that form from deep convection play a central role in the dynamics of tropical mesoscale convective systems. The wi...

The north north-west propagating Low Pressure Systems (LPS) are an important component of the Indian Summer Monsoon (ISM). The objective detection and tracking of LPS in reanalysis products and climate model simulations are challenging due to the weak structure of the LPS compared to tropical cyclones. Therefore the skill of reanalysis and climate...

Ph.D Thesis, Indian Institute of Science, Bangalore, India; Submitted November 2001

Almost all climate models in Coupled Model Inter-comparison Project phase five (CMIP5) were found to have a cold bias in Sea Surface Temperature (SST) over the northern Arabian Sea, which is linked to the biases in the Indian Summer Monsoon (ISM). This cold SST bias was attributed to the anomalous cold winds from the north-western part of south Asi...

The skill of the global climate models (GCMs) to realistically simulate the Monsoon Intraseasonal Oscillations (MISOs) is related to the sensitivity of their convective parameterization schemes. Here, we show that by coupling a simple multicloud parameterization to a coarse resolution aquaplanet GCM, realistic MISOs can be simulated. We conduct thr...

The Low Level Jetstream (LLJ) transports moisture from the surrounding Oceans to Indian land mass and hence an important component of the Indian Summer Monsoon. Widening of tropical belts and poleward shifts in mid-latitude jetstreams has been identified as major impacts of global warming on large-scale atmospheric dynamics. A general northward shi...

The study compares the simulated poleward migration characteristics of boreal summer intraseasonal oscillations (BSISO) in a suite of coupled ocean–atmospheric model sensitivity integrations. The sensitivity experiments are designed in such a manner to allow full coupling in specific ocean basins but forced by temporally varying monthly climatologi...

[1] The main mechanisms for the initiation and propagation of the Madden-Julian Oscillation (MJO) are still widely debated. The capacity of operational global climate models (GCMs) to correctly simulate the MJO is hindered by the inadequacy of the underlying cumulus parameterizations. Here we show that a coarse resolution GCM, coupled to a simple m...

The main mechanisms for the initiation and propagation of the Madden-Julian Oscillation (MJO) are still widely debated. The capacity of operational global climate models (GCMs) to correctly simulate the MJO is hindered by the inadequacy of the underlying cumulus parameterizations. Here, we show that a coarse resolution GCM, coupled to a simple mult...

Understanding long-term trends in species abundance and distribution represents an important challenge for future research in the deep sea, particularly as management of human impacts becomes a more important concern. However, until natural higher frequency variability is better understood, it will be difficult to interpret any long-term trends tha...

The Canadian Seasonal to Interannual Prediction System (CanSIPS) became operational at Environment Canada's Canadian Meteorological Centre (CMC) in December 2011, replacing CMC's previous two-tier system. CanSIPS is a two-model forecasting system that combines ensemble forecasts from the Canadian Centre for Climate Modeling and Analysis (CCCma) Cou...

In the deep sea and along the continental slope, benthic observations have often been limited to seasonal or longer time scales, conducted at irregular and intermittent intervals. The recent development of cabled observatories now permits continuous high-frequency studies of the ecology of deep environments, and will bring greater temporal resoluti...

A clear shift in the withdrawal dates of the Indian Summer Monsoon is observed in the long term time series of rainfall data. Prior (posterior) to the 1976/1977 climate shift most of the withdrawal dates are associated with a late (an early) withdrawal. As a result, the length of the rainy season (LRS) over the Indian land mass has also undergone s...

A tendency for an early withdrawal date and shortening of the length of rainy season (LRS) is observed during the recent decades. Prior (Posterior) to the 1976/77 climate shift majority of the years showed a late (an early) withdrawal of the monsoon. But there is no long term trend in the onset date. So the shortening of the LRS is mainly determine...

The south Asian summer monsoon precipitation shows no long term trend over the last several decades despite the rise in global surface temperature. However, few recent studies highlights the importance of the timing and variability of the withdrawal phase of monsoon as it is intrinsically linked with the length of the summer monsoon season. The sea...

The tropical disturbances formed in the Bay of Bengal and the Arabian Sea and over land points in central India, known as low pressure systems (LPSs), are shown to contribute significantly to the seasonal monsoon rainfall over India. Analyses of daily rainfall over India and statistics of the LPSs for the period of 1901–2003 show that the rainfall...

Role of internal processes in maintaining boreal summer intraseasonalvariability

1] Using a new data set we demonstrate the variability of upper troposphere humidity (UTH) associated with the Indian Summer Monsoon (ISM). The main advantage of the new data set is its all‐sky representation which is essential to capture the full variability of humidity even in cloudy areas. We show that UTH undergoes significant variations during...

ABSTRACT The nature of the increasing frequency of extreme rainfall events or ERE in central India is investigated by relating their occurrence to synoptic activity. Using a long record of the paths and intensities of monsoon synoptic disturbances, a Synoptic Activity Index (SAI) is defined,whose interannual variation correlates strongly with that...

1] The role of Indian Ocean Dipole (IOD) on poleward propagation of boreal summer intraseasonal oscillations (BSISO) is examined using long simulation of a coupled ocean-atmosphere general circulation model. The model (SINTEX-F1) simulates the salient features of BSISO realistically. It is found that coherent (incoherent) poleward propagation of pr...

The influence of the Indian Ocean dipole (IOD) on the poleward propagation of boreal summer intraseasonal oscillations (BSISOs) is examined using observed datasets. This study finds that coherent (incoherent) poleward propagation of precipitation anomalies from 5°S to 25°N are observed during negative (positive) IOD years. Disorganized poleward pro...

The tropical disturbances formed in the Bay of Bengal and the Arabian Sea and over land points in central India, known as low pressure systems (LPSs), are shown to contribute significantly to the seasonal monsoon rainfall over India. Analyses of daily rainfall over India and statistics of the LPSs for the period 1901-2003 show that the rainfall pat...

This study shows that high resolution rainfall data indicate an increase in number of extreme rainfall events over India in the last few decades. Unravelling the mechanism behind this increasing number of extreme rainfall events in central India is important as these intense rainfall events cause floods and related damage to the life and property o...

The link between realism in simulation of the seasonal mean precipitation and summer tropical intraseasonal oscillations and their dependence on cumulus parameterization schemes is investigated using the Florida State University Global Spectral Model (FSUGSM). Forty-member model ensemble simulations of the northern summer season are generated for t...

Major characteristics of Indian summer monsoon climate are analyzed using simulations from the upgraded version of Florida State University Global Spectral Model (FSUGSM). The Indian monsoon has been studied in terms of mean precipitation and low-level and upper-level circulation patterns and compared with observations. In addition, the model's fid...

Following the seminal work of Charney and Shukla (1981), the tropical climate is recognised to be more predictable than extra tropical climate as it is largely forced by ‘external’ slowly varying forcing and less sensitive to initial conditions. However, the Indian summer monsoon is an exception within the tropics where ‘internal’ low frequency (LF...

The potential predictability of the monthly and seasonal means during the Northern Hemisphere summer and winter is studied by estimating the signal-to-noise ratio. Based on 33 years of daily low-level wind observations and 24 years of satellite observations of outgoing long wave radiation, the predictability of the Asian summer monsoon region is co...

Active and break phases of the Indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region. Using genesis data of monsoon low pressure systems (LPS) and circulation data for the period 1954 to 1993, it is shown that the frequency of occurrence of LPS is nearly 3.5 times higher in the active ph...

How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956-97) daily circulation data from National Centers for Environmental Prediction-National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoi...

A method of separating the contributions from slowly varying boundary forcing and internal dynamics (e.g. intraseasonal oscillations) that determine the predictability of the monthly mean tropical climate is presented. Based on 33 years of daily low level wind observations and 24 years of satellite observations of outgoing long wave radiation, we s...

How and to what extent the intraseasonal oscillations (ISOs) influence the seasonal mean and its interannual variability of the Indian summer monsoon is investigated using 42-yr (1956–97) daily circulation data from National Centers for Environmental Prediction–National Center for Atmospheric Research 40-Year Reanalysis and satellite-derived outgoi...

How and to what extent the intra-seasonal oscillations (ISOs) of the Indian summer monsoon influence the seasonal mean and its inter-annual variability is investigated using long records of daily circulation data (1956–1997) and outgoing long wave radiation (OLR) data (1974–1997). The underlying spatial structure of a typical ISO cycle that is inva...

1] Active and break phases of the Indian summer monsoon are characterized by enhancement and decrease of precipitation over the monsoon trough region. Using genesis data of monsoon low pressure systems (LPS) and circulation data for the period 1954 to 1993, it is shown that the frequency of occurrence of LPS is nearly 3.5 times higher in the active...