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Changes in biological productivity associated with Ningaloo Niño / Niña events in the southern subtropical Indian Ocean in recent decades
Get PDFUsing observations and long term simulations of an ocean-biogeochemical coupled model, we investigate the biological response in the southern subtropical Indian Ocean (SIO) associated with Ningaloo Niño and Niña events. Ningaloo events have large impact on sea surface temperature (SST) with positive SST anomalies (SSTA) seen off the west coast of Australia in southern SIO during Ningaloo Niño and negative anomalies during Niña events. Our results indicate that during the developing period of Ningaloo Niño, low chlorophyll anomaly appears near the southwest Australian coast concurrently with high SSTA and vice-versa during Niña, which alter the seasonal cycle of biological productivity. The difference in the spatiotemporal response of chlorophyll is due to the southward advection of Leeuwin current during these events. Increased frequency of Ningaloo Niño events associated with cold phase of Pacific Decadal Oscillation (PDO) resulted in anomalous decrease in productivity during Austral summer in the SIO in the recent decades
Indian summer monsoon rainfall simulation and prediction skill in the CFSv2 coupled model: Impact of atmospheric horizontal resolution
No full textThis study compares the simulation and prediction skill of the Indian summer monsoon at two different horizontal resolutions viz., T126 (~100 km) and T382 (~38 km) using 28 years of hindcast runs of the National Centers for Environmental Prediction Climate Forecast System version 2 (CFSv2) model. It is found that the simulation of the mean state of the South Asian summer monsoon, its variance, and prediction skill of the all India summer monsoon rainfall (AISMR) are better represented in the high-resolution configuration (T382) of the CFSv2 compared to the low-resolution (T126) configuration. In the high-resolution run, the systematic bias in the teleconnection between the AISMR and Indian Ocean Dipole (IOD) has considerably reduced and the teleconnections between the AISMR and El Niño–Southern Oscillation (ENSO) remained same. We hypothesize that the better simulation of mean climate and IOD-AISMR teleconnection in high-resolution configuration (T382) of CFSv2 are responsible for the improved prediction skill of AISMR in T382 configuration. Although the T382 configuration of CFSv2 has shown a significant improvement in the simulation and prediction of Indian summer monsoon as compared to the T126 configuration, several parallel efforts are still essential to understand the processes controlling some of the systematic biases of CFSv2 and those efforts are underway as part of the Monsoon Mission project
Precipitation extremes during Indian summer monsoon: role of cyclonic disturbances
No full textThe Indian summer (JJAS) shows high variability in both space and timescales. Changes in precipitation extremes play an important role on the regional scale due to their serious socio-economic consequences. This study, therefore, is mainly focused on understanding the variation of precipitation extremes during summer monsoon season in the presence of cyclonic disturbances forming over the Bay of Bengal (BOB), Arabian Sea, Land Area (LA) and Total. For this, several indices of observed precipitation extremes, in terms of frequencies, intensities and spell duration have been computed for the period 1951–2007 using daily APHRODITE data of 0.5° latitude × 0.5° longitude resolution. Correlation analysis reveals that a large part of the country exhibits positive relationship between the indices of precipitation extremes and frequency of cyclonic disturbances. Correlations with the indices of frequencies defined as seasonal count of days when rainfall exceeds 30, 20 and 10 mm show that spatial extent and strength of the positive relationship decreases with increase in threshold values. Disturbances forming over BOB play dominant role in precipitation during Indian summer monsoon
Potential predictability of Indian summer monsoon rainfall in NCEP CFSv2
No full textThe potential predictability of the Indian summer monsoon rainfall (ISMR), soil moisture, and sea surface temperature (SST) is explored in the latest version of the NCEP Climate Forecast System (CFSv2) retrospective forecast at five different lead times. The focus of this study is to find out the sensitivity of the potential predictability of the ISMR to the initial condition through analysis of variance technique (ANOVA), information-based measure, including relative entropy (RE), mutual information (MI), and classical perfect model correlation. In general, the all methods show an increase in potential predictability with a decrease in lead time. Predictability is large over the Pacific Ocean basin as compared to that of the Indian Ocean basin. However, over the Indian land region the potential predictability increases from lead-4 to lead-2 and then decreases at lead-1 followed by again increase at lead-0. While the actual ISMR prediction skill is highest at lead-3 forecast (second highest at lead-1), the potential predictability is highest at lead-2. It is found that highest and second highest actual prediction skill of the ISMR in CFSv2 is due to the combined effects of initial Eurasian snow and SST over Indian, west Pacific and eastern equatorial Pacific Ocean region. While the teleconnection between the ISMR and El Niño-Southern Oscillation is too strong, the ISMR and Indian Ocean dipole have completely out of phase relation in the model as compared to the observation. Furthermore, the actual prediction skill of the ISMR is now very close to the potential predictability limit. Therefore, in order to improve the ISMR prediction skill further, development of model physics as well as improvements in the initial conditions is required
Influence of meteorological parameters and atmospheric pollutants on lightning, rainfall and normalized difference vegetation index in the Indo-Gangetic Plain
No full textVariation of lightning flash distribution, rain, and normalized difference vegetation index (NDVI) and their dependency on meteorological parameters were studied along the Indo-Gangetic Plain from 21°N to 35°N, dividing it into four regions of equal area (R1, R2, R3 and R4). Time series and correlation analysis were used to study the variations and relations among the parameters. Maximum lightning flashes were observed in region R1 where rain, surface temperature, convective available potential energy (CAPE), cloud cover, water vapour, and NDVI were found to be minimum. Lightning flashes and rain are positively correlated with surface temperature, CAPE, and aerosol optical depth (AOD), on both an annual and seasonal basis, over the whole region. Seasonal variation in NDVI showed a double-peak distribution, which can be understood by considering its dependence on water vapour, soil moisture, accumulated rain and surface temperature, and their variation from season to season and region to region. The effects of the atmospheric pollutants (AOD, carbon dioxide (CO2), nitrogen dioxide (NO2), and ozone (O3)) on NDVI showed a mixed response. A small variation in atmospheric CO2 showed no statistically significant impact in any season or region. AOD had a negative effect in general except that a small positive tendency was observed in R3 and R4 during summer months. NO2 showed a negative impact in all regions and seasons. O3 had a negative effect in R3 and R4 during the monsoon months and in R2–R4 during the summer months, and a positive effect in the other regions and seasons. When all four regions are merged, correlation analysis on an annual basis showed that the effects in localized regions are lost and the impact from the larger area predominates. To understand the regional localized effect, analysis of the smaller regions seems to be important
Evidence of atmospheric nanoparticle formation from emissions of marine microorganisms
No full textEarth, as a whole, can be considered as a living organism emitting gases and particles into its atmosphere, in order to regulate its own temperature. In particular, oceans may respond to climate change by emitting particles that ultimately will influence cloud coverage. At the global scale, a large fraction of the aerosol number concentration is formed by nucleation of gas-phase species, but this process has never been directly observed above oceans. Here we present, using semicontrolled seawater-air enclosures, evidence that nucleation may occur from marine biological emissions in the atmosphere of the open ocean. We identify iodine-containing species as major precursors for new particle clusters' formation, while questioning the role of the commonly accepted dimethyl sulfide oxidation products, in forming new particle clusters in the region investigated and within a time scale on the order of an hour. We further show that amines would sustain the new particle formation process by growing the new clusters to larger sizes. Our results suggest that iodine-containing species and amines are correlated to different biological tracers. These observations, if generalized, would call for a substantial change of modeling approaches of the sea-to-air interactions
Study of spatial-temporal variations in the green Noctiluca scintillans and diatom blooms in the Arabian Sea using MODIS data
Get PDFPhytoplankton blooms of green Noctiluca scintillans (a dinoflagellate) and diatom, which appear in the Northern and Central Arabian Sea during winter are far-reaching and persistent. Generation of phytoplankton species images revealed a massive winter bloom with huge spatial extent in 2015. In contrast to this, the classified species images for 2013 indicated relatively weaker bloom with respect to its spread. A plot of total number of pixels classified as diatom and Noctiluca scintillans for different years revealed a cyclic pattern of the spread. The report deals with an approach to forecast the bloom / productivity of the oceanic waters in the Northern-Central Arabian Sea in a qualitative way making use of the systematic pattern of its distribution across the year
Heat content of the Arabian Sea Mini Warm Pool is increasing
No full textSea surface temperature in the Arabian Sea Mini Warm Pool has been suggested to be one of the factors that affects the Indian summer monsoon. In this paper, we analyze the annual ocean heat content (OHC) of this region during 1993-2010, using in situ data, satellite observations, and a model simulation. We find that OHC increases significantly in the region during this period relative to the north Indian Ocean, and propose that this increase could have caused the decrease in Indian Summer Monsoon Rainfall that occurred at the same time
Classification of case-II waters using hyperspectral (HICO) data over North Indian Ocean
No full textState of the art Ocean color algorithms are proven for retrieving the ocean constituents (chlorophyll-a, CDOM and Suspended Sediments) in case-I waters. However, these algorithms could not perform well at case-II waters because of the optical complexity. Hyperspectral data is found to be promising to classify the case-II waters. The aim of this study is to propose the spectral bands for future Ocean color sensors to classify the case-II waters. Study has been performed with Rrs’s of HICO at estuaries of the river Indus and GBM of North Indian Ocean. Appropriate field samples are not available to validate and propose empirical models to retrieve concentrations. The sensor HICO is not currently operational to plan validation exercise. Aqua MODIS data at case-I and Case-II waters are used as complementary to in- situ. Analysis of Spectral reflectance curves suggests the band ratios of Rrs 484 nm and Rrs 581 nm, Rrs 490 nm and Rrs 426 nm to classify the Chlorophyll –a and CDOM respectively. Rrs 610 nm gives the best scope for suspended sediment retrieval. The work suggests the need for ocean color sensors with central wavelength’s of 426, 484, 490, 581 and 610 nm to estimate the concentrations of Chl-a, Suspended Sediments and CDOM in case-II waters
Penetrative radiative flux in the Bay of Bengal
Get PDFThe Bay of Bengal (BoB), a semi-enclosed basin in the northern Indian Ocean, is a complex region with large freshwater inputs and strong vertical stratification that result in a shallow, spatially variable mixed layer. With the exception of shortwave insolation, the air-sea heat exchange occurs at the sea surface and is vertically redistributed by mixing and advection. Strongly stratified, shallow mixed layers inhibit vertical mixing, and the penetration of solar radiation through the base of the mixed layer can lead to redistribution of upper-ocean heat. This paper compiles observations of hyperspectral downwelling irradiance (Ed) from 67 profiles collected during six research cruises in the BoB that span a broad range of regions and seasons between 2009 and 2014. We report attenuation length scales computed using double and single exponential models and quantify the penetration of radiative flux below the mixed layer depth (Qpen. We then evaluate estimates of Qpen obtained from published chlorophyll-based models and compare them to our observations. We find that the largest penetrative heat flux (up to 40 of the incident Ed) occurs near 16°N where the mixed layers are shallow and the water is optically clear