Estimation of monsoon wind characteristics in India

A detailed statistical analysis of monthly average wind speed data of monsoon period (June-September) for the year 1921-90 for 57 stations spread all over India have been reported. Probability densities, average wind speeds, standard deviations, kurtosis and skewness of wind speed frequency distribution for each station have been worked out. Histograms depicting relative frequency distribution of average wind speeds have also been prepared. It is observed that the different histograms do not exhibit any similarity among themselves indicating thereby that no single distribution is uniformly applicable for all the stations. It is also seen that the average wind speeds during monsoon period over major part of India varies from 7 to 14 kmph. Further, at most of the stations average monsoon wind speed is generally higher than average annual wind speeds. It is also noted that most of the time the wind speed exceeds 10 kmph in coastal regions of Gujarat and southern parts of the peninsular India. The information generated is of multi fold application such as (i) Identification of sites suitable for installation of Wind Energy Conversion Systems (ii) Development of Driving Rain Index and (iii) Design of buildings for creating comfortable environment indoors.

Influence of variability to coastline dynamic in the small uninhabited reef islands of Kepulauan Seribu

Small reef islands provide habitable land for coastal communities in many parts of the world. However, the small, low lying reef islands are commonly considered among the most geomorphically sensitive landforms to changes in sea level, wave processes, sediment supply and anthropogenic impacts. Kepulauan Seribu in the Java Sea comprise of numerous reef islands. By 2019, the islands chain is host to more than 24 thousand people. Kepulauan Seribu is affected by monsoon wind cycle. The monsoon wind also known to interact with an interannual phenomenon such as Indian Ocean Dipole (IOD) which affecting regional and local wind circulation. This study aims to examine the reef shoreline response to seasonal and interannual climate variability using satellite data that encompasses yearly monsoon cycle and IOD event. Strengthens (weakens) of winds speed in the study area during the East (West) Monsoon, which in some year also coincides with a positive (negative) IOD event, are observed from 2009 to 2018 ERA - Interim by The European Centre for Medium - Range Weather Forecasts (ECMWF) data. This variability influences the shoreline shifting in the uninhabited reef islands of Kepulauan Seribu as identified based on satellite imagery analysis. More pronounce shifted of large sediment flux are perceptible on opposing monsoon which coincides with positive/negative IOD event. Small uninhabited reef islands have ecological and economical value. Hence, enhancing coastal resilience from erosion by using conservation-types approach should be taking into consideration. Ultimately, a good understanding of climate variability that controlled changes in beach systems of reef islands is important for adequate coastal management decisions.

Fast local warming is the main driver of recent deoxygenation in the northern Arabian Sea

The Arabian Sea (AS) hosts one of the most intense oxygen minimum zones (OMZs) in the world. Observations suggest a decline in O2 in the northern AS over the recent decades accompanied by an intensification of the suboxic conditions there. Over the same period, the local sea surface temperature has risen significantly, particularly over the Arabian Gulf (also known as Persian Gulf, hereafter the Gulf), while summer monsoon winds may have intensified. Here, we simulate the evolution of dissolved oxygen in the AS from 1982 through 2010 and explore its controlling factors, with a focus on changing atmospheric conditions. To this end, we use a set of eddy-resolving hindcast simulations forced with winds and heat and freshwater fluxes from an atmospheric reanalysis. We find a significant deoxygenation in the northern AS, with O2 inventories north of 20∘ N dropping by over 6 % per decade between 100 and 1000 m. These changes cause an expansion of the OMZ volume north of 20∘ N at a rate of 0.6 % per decade as well as an increase in the volume of suboxia and the rate of denitrification by 14 and 15 % per decade, respectively. We also show that strong interannual and decadal variability modulate dissolved oxygen in the northern AS, with most of the O2 decline taking place in the 1980s and 1990s. Using a set of sensitivity simulations we demonstrate that deoxygenation in the northern AS is essentially caused by reduced ventilation induced by the recent fast warming of the sea surface, including in the Gulf, with a contribution from concomitant summer monsoon wind intensification. This is because, on the one hand, surface warming enhances vertical stratification and increases Gulf water buoyancy, thus inhibiting vertical mixing and ventilation of the thermocline. On the other hand, summer monsoon wind intensification causes a rise in the thermocline depth in the northern AS that lowers O2 levels in the upper ocean. Our findings confirm that the AS OMZ is strongly sensitive to upper-ocean warming and concurrent changes in the Indian monsoon winds. Finally, our results also demonstrate that changes in the local climatic forcing play a key role in regional dissolved oxygen changes and hence need to be properly represented in global models to reduce uncertainties in future projections of deoxygenation.

Seasonal Variability of Waves Within the Indonesian Seas and Its Relation With the Monsoon Wind

The previous studies have simulated the variability of the wave within the Indonesian seas which showed that the variability of wave follows the seasonal pattern. However, their analysis only consider the influence of local wind forcings. The bias and error of their simulated wave were also unclear. In the present study, we investigate the variability of wave within the Indonesian seas and its relation with the surface wind speed using the combination of reanalysis and remote sensing data with high accuracies. We split the analysis into swell and wind wave to obtain the influence of local and remote wind forcings. We show that at the inner seas (i.e., the South China Sea, Java Sea, Flores Sea, Banda Sea and Arafura Sea), the variability of significant wave height (SWH) is majorly influenced by the variability of the speed of monsoon wind. The maximum SWH during Northwest monsoon (NWM) season is located at the South China Sea while during Southeast monsoon (SEM) season is at Arafura Sea. This indicates that the wind wave (sea) is dominant at the inner seas. At the open seas (i.e., Pacific Ocean and Indian Ocean) the variability of SWH less corresponds to the the speed of monsoon wind. The remote wind forcings control the wave variability in the open ocean area. This indicates that swell is dominant at the open seas. In general, the magnitude of SWHswell is also more than SWHsea within the Indonesian seas.

Fast local warming of sea-surface is the main factor of recent deoxygenation in the Arabian Sea

The Arabian Sea (AS) hosts one of the most intense oxygen minimum zones (OMZs) in the world. Observations show a decline of O2 in the northern AS over the recent decades accompanied by an intensification of the suboxic conditions there. Over the same period, the local sea-surface temperature has risen significantly, particularly over the Arabian Gulf (also known as Persian Gulf, hereafter the Gulf), while summer monsoon winds have intensified. Here, we reconstruct the evolution of dissolved oxygen in the AS from 1982 through 2010 and explore its controlling factors, with a focus on changing atmospheric conditions. To this end, we use a set of eddy-resolving hindcast simulations forced with observation-based winds and heat and freshwater fluxes. We find a significant deoxygenation in the northern AS with O2 inventories north of 20° N dropping by over 2 % decade-1 and 7 % decade-1 in the top 200 m and the 200–1000 m layer, respectively. These changes cause an increase in the volume of suboxia and the rate of denitrification by 10 % decade-1 and 13 % decade-1, respectively. Using a set of sensitivity simulations we demonstrate that deoxygenation in the northern AS is essentially caused by a reduced ventilation induced by the recent fast warming of the sea surface, in particular in the Gulf. Concomitant summer monsoon wind intensification contributes to deoxygenation at depth and in the upper ocean north of 20° N but enhances oxygenation of the upper ocean elsewhere. This is because surface warming enhances vertical stratification, thus limiting ventilation of the intermediate ocean, while summer monsoon wind intensification causes the thermocline depth to rise in the northern AS and deepen elsewhere, thus contributing to lowering O2 levels in the upper 200 m in the northern AS and increasing it in the rest of the AS. Our findings confirm that the AS OMZ is strongly sensitive to upper-ocean warming and concurrent changes in the Indian monsoon winds. Finally, our results also demonstrate that changes in the local climatic forcing play a key role in regional dissolved oxygen changes and hence need to be properly represented in global models to reduce uncertainties in future projections of deoxygenation.

Interannual monsoon wind variability over the South tropical Indian ocean drives East African small pelagic fisheries

<p>Small pelagic fisheries play a critical role in food security and economic stability for East African coastal communities ― a region of least developed countries. Using satellite and field observations together with modelling, we show the links between the small pelagic fisheries along the East African coast and the changes in Western Indian Ocean currents due to the interannual variability of the monsoonal wind field. The annual variations in phytoplankton biomass and fisheries yield are strongly associated. During the Northeast monsoon, the enhanced phytoplankton biomass is triggered by local wind-driven upwelling. During the Southeast monsoon, however, the enhanced phytoplankton biomass is due to two current induced mechanisms: coastal “dynamic uplift” upwelling; and westward advection of waters with higher nutrient concentrations. This biological response to the Southeast monsoon is greater than that to the Northeast monsoon. Interannually, an extreme increase (decrease) in chlorophyll concentrations is induced by strengthened (weakened) surface currents, which occur during anomalously “strong” (“weak”) Southeast monsoon years. For years where the effects of El Niño / La Niña are weak, the Southeast monsoon wind strength over the south tropical Indian Ocean is the main driver of year-to-year variability. Such changes have important implications for the predictability of fisheries yield, its response to climate change, policy and resource management.  </p>

Benthic Foraminifera as indicative of Austral Summer monsoon precipitation and winter monsoon wind-driven upwelling

<p>During IODP Expedition 363, a hemipelagic sediment succession was retrieved for the first time off NW Australia (Site U1483: 13°5.24ʹS, 121°48.25ʹE, water depth: 1733 m, sedimentation rate: ~10 cm/kyr). This carbonate- and clay-rich sequence provides an ideal archive to monitor intensity and variability of the Australian Monsoon (AM) and to better constrain monsoon sensitivity to changes in radiative forcing. Due to the location at the southern edge of the largest amplitude seasonal swing of the Intertropical Convergence Zone (ITCZ) within the large-scale Asian-Australian monsoon system, the AM subsystem is sensitive to tropical hydroclimate variability. However, this sensitivity to changing climate boundary conditions such as ice volume and greenhouse gas concentrations remain poorly understood across the Calabrian, in the Pleistocene Epoch (add ages?). Here we report on benthic foraminiferal assemblages as a tracer for terrigenous runoff (Austral Summer monsoon precipitation). We find shallow, fresh water-tolerant to transitional environments species Bolivina striatula, Buliminella elegantissima, Dentalina spp., Oolina sp., and paleo productivity indicator Melonis spp (winter monsoon wind-driven upwelling) from 1.34 Ma. through 1.61Ma. Principl component analysis (PCA) indicates that Melonis is present in 4 out of 5 PCA axes. It prefers organic matter in a more altered form, and migrates in the sediment depending on the quality of the organic matter supply and remineralization, which indicates surface upwelling during this time. The genera Stilostomella spp. is present in 3 out of 5 axes, and it is indicative of intermediate water temperature. These records will be compared to C org wt (%), TN wt (%) and a benthic foraminiferal stable isotope record to related faunal patterns to carbon cycling and global climate.</p><p> </p><p> </p>