nasa-gesdisc/es-publications-researchareas huggingface dataset:
Modulation of Saharan dust export by the North African dipole (2015)

Abstract. We have studied the relationship between the long-term interannual variability in large-scale meteorology in western North Africa  the largest and most active dust source worldwide  and Saharan dust export in summer, when enhanced dust mobilization in the hyper-arid Sahara results in maximum dust impacts throughout the North Atlantic. We address this issue by analyzing 28 years (19872014) of summer averaged dust concentrations at the high-altitude Izana observatory (~ 2400 m a.s.l.) on Tenerife, and satellite and meteorological reanalysis data. The summer meteorological scenario in North Africa (aloft 850 hPa) is characterized by a high over the the subtropical Sahara and a low over the tropics linked to the monsoon. We measured the variability of this highlow dipole-like pattern in terms of the North African dipole intensity (NAFDI): the difference of geopotential height anomalies averaged over the subtropics (3032 N, Morocco) and the tropics (1013 N, Bamako region) close to the Atlantic coast (at 58 W). We focused on the 700 hPa standard level due to dust export off the coast of North Africa tending to occur between 1 and 5 km a.s.l. Variability in the NAFDI is associated with displacements of the North African anticyclone over the Sahara and this has implications for wind and dust export. The correlations we found between the 19872014 summer mean of NAFDI with dust at Izana, satellite dust observations and meteorological re-analysis data indicate that increases in the NAFDI (i) result in higher wind speeds at the north of the Inter-Tropical Convergence Zone that are associated with enhanced dust export over the subtropical North Atlantic, (ii) influence the long-term variability of the size distribution of exported dust particles (increasing the load of coarse dust) and (iii) are associated with enhanced rains in the tropical and northern shifts of the tropical rain band that may affect the southern Sahel. Interannual variability in NAFDI is also connected to spatial distribution of dust over the North Atlantic; high NAFDI summers are associated with major dust export (linked to winds) in the subtropics and minor dust loads in the tropics (linked to higher rainfall), and vice versa. The evolution of the summer NAFDI values since 1950 to the present day shows connections to climatic variability (through the Sahelian drought, ENSO (El NinoSouthern Oscillation) and winds) that have implications for dust export paths. Efforts to anticipate how dust export may evolve in future decades will require a better understanding of how the large-scale meteorological systems represented by the NAFD will evolve.

Research areas: Atmospheric/Ocean Indicators
MODIS-Aqua detects Noctiluca scintillans and hotspots in the central Arabian Sea (2015)

Northern Arabian Sea is considered as an ecologically sensitive area as it experiences a massive upwelling and long-lasting algal bloom, Noctiluca scintillans (green tide) during summer and spring-winter, respectively. Diatom bloom is also found to be co-located with N. scintillans and both have an impact on ecology of the basin. In-house technique of detecting species of these blooms from Moderate Resolution Imaging Spectroradiometer (MODIS)-Aqua data was used to generate a time-series of images revealing their spatial distribution. A study of spatial-temporal variability of these blooms using satellite data expressed a cyclic pattern of their spread over a period of 13 years. An average distribution of the blooms for JanuaryMarch period revealed a peak in 2015 and minimum in 2013. Subsequently, a time-series of phytoplankton species images were generated for these 2 years to study their inter-annual variability and the associated factors. Species images during active phase of the bloom (February) in 2015 indicated development of N. scintillans and diatom in the central Arabian Sea also, up to 12 N. This observation was substantiated with relevant oceanic parameters measured from the ship as well as satellite data and the same is highlight of the paper. While oxygen depletion and release of ammonia associated with N. scintillans are detrimental for waters on the western side; it is relatively less extreme and supports the entire food chain on the eastern side. In view of these contrasting eco-sensitive events, it is a matter of concern to identify biologically active persistent areas, hot spots, in order to study their ecology in detail. An ecological index, persistence of the bloom, was derived from the time-series of species images and it is another highlight of our study.

Research areas: Ecosystems
Modification of cumulus convection and planetary boundary layer schemes in the GRAPES global model (2015)

Cumulus convection is a key linkage between hydrological cycle and large-scale atmospheric circulation. Cumulus parameterization scheme is an important component in numerical weather and climate modeling studies. In the Global/Regional Assimilation and Prediction Enhanced System (GRAPES), turbulent mixing and diffusion approach is applied in its shallow convection scheme. This method overestimates the vertical transport of heat and moisture fluxes but underestimates cloud water mixing ratio over the region of stratocumulus clouds. As a result, the simulated low stratocumulus clouds are less than observations. To overcome this problem, a mass flux method is employed in the shallow convection scheme to replace the original one. Meanwhile, the deep convection scheme is adjusted correspondingly. This modification is similar to that in the US NCEP Global Forecast System (GFS), which uses the simplified Arakawa Schubert Scheme (SAS). The planetary boundary layer scheme (PBL) is also revised by considering the coupling between the PBL and stratocumulus clouds. With the modification of both the cumulus and PBL schemes, the GRAPES simulation of shallow convective heating rate becomes more reasonable; total amounts of stratocumulus clouds simulated over the eastern Pacific and their vertical structure are more consistent with observations; the underestimation of stratocumulus clouds simulated by original schemes is less severe with the revised schemes. Precipitation distribution in the tropics becomes more reasonable and spurious precipitation is effectively suppressed. The westward extension and northward movement of the western Pacific subtropical high simulated with the revised schemes are more consistent with Final Operational Global Analysis (FNL) than that simulated with the original schemes. The statistical scores for the global GRAPES forecast are generally improved with the revised schemes, especially for the simulation of geopotential height in the Northern Hemisphere and winds in the tropics. Root mean square errors (RMSEs) decrease in the lower and upper troposphere with the revised schemes. The above results indicate that with the revised cumulus and PBL schemes, model biases in the tropics decrease and the global GRAPES performance is greatly improved.

Research areas: Validation
Modeling the effects of dust-radiative forcing on the movement of Hurricane Helene (2006) (2015)

The influence of direct dust-radiative forcing on the movement and track of Hurricane Helene (2006) is examined numerically using the Weather Research and Forecasting dust model. Numerical simulations show that the model-generated dust plume modifies the thermal field, causing a clockwise turning of the vertical shear surrounding the plume, which changes the deep layer steering flow. The change in the steering flow modifies Helene's moving speed and direction as it transits the plume. As Helene exits the plume, it has a different trajectory than it would have had in the absence of dust-radiative forcing. Consequently, the difference in the tracks with and without dust-radiative forcing continues to grow with distance from the plume. The dust-induced changes in temperature and wind together cause Helene's modeled storm track to be in closer agreement with the observed track; the dust-radiative forcing reduces the error in the model's 7-day track forecasts by an average of 27% (205 km).

Research areas: Severe Storms
Modeling daytime sky access for a satellite quantum key distribution downlink (2015)

Daytime sky radiance can lead to quantum-bit-error rates (QBER) sufficiently high to preclude quantum key distribution over satellite-to-Earth downlinks. For circular orbits, sky radiance and propagation losses are both pointing-angle-dependent quantities. This dependence makes it possible to map the sky as a function of the QBER in certain protocols. Sky radiance distributions are generated by libRadtran software with aerosol values representative of summer and winter. Hemispherical maps of the QBER are created for orbit altitudes of 400 and 800 km. Sunrise-to-sunset simulations are presented for a system with commercial optical components and compared to a case employing an atomic-line spectral filter.

Research areas: Validation
Microbial community composition and nitrogen availability influence DOC remineralization in the South Pacific Gyre (2015)

Many environmental factors are thought to control the bioavailability of marine dissolved organic matter (DOM) for marine microbes including its composition, the microbial community structure, and nutrient availability, yet which factors dominate at the ocean basin scale remains uncertain. Understanding the controls on DOM lability is an important goal given the role of DOM in the marine carbon cycle. We performed DOM lability experiments at two contrasting stations, one oligotrophic and one mesotrophic, in the eastern tropical South Pacific (ETSP) to investigate the controls on microbial remineralization of surface ocean DOM. Surface layer dissolved organic carbon (DOC) and nitrogen (DON) were recalcitrant to remineralization over 9 to 14days when exposed to the microbial communities from the surface mixed layer, however exposure to microbial communities from the upper mesopelagic (twilight zone) allowed consumption of DOC but not DON. The DOC remineralization response differed between the mesotrophic site (~21M consumed), likely experiencing allochthonous inputs of DOM from the adjacent eastern boundary upwelling system, versus the oligotrophic station (~3M consumed) further offshore in the South Pacific gyre. DNA fingerprinting of the microbial communities across the ETSP with terminal restriction fragment length polymorphism (T-RFLP) analyses revealed greater differences between microbial communities in surface vs. subsurface (e.g., 100m) waters at the same station than between surface water microbial communities separated by 1000s of kilometers. The subsurface microbial community at the mesotrophic station responsible for the greatest observed DOC remineralization, with a concomitant consumption of nitrate, consumed DOC to concentrations below that observed in situ (at 100m), suggesting a potential role for co-metabolism of relatively labile with more recalcitrant DOC or relief from micronutrient limitation, in driving the additional DOC consumption. DOC remineralization by the mesopelagic (200m) microbial community was much less at the oligotrophic station and similar to previously published results from the Sargasso Sea. Both microbial community composition and nutrient availability contribute to DOM persistence over weekly timescales in the surface mixed layer with varying degrees of DOC lability in the subsurface waters of the ETSP.

Research areas: Ecosystems, Water Quality
Meteorological, atmospheric and climatic perturbations during major dust storms over Indo-Gangetic Basin (2015)

During the pre-monsoon season (AprilJune), the Indo-Gangetic Basin (IGB) suffers from frequent and intense dust storms originated from the arid and desert regions of southwest Asia (Iran, Afghanistan), Arabia and Thar desert blanketing IGB and Himalayan foothills. The present study examines the columnar and vertical aerosol characteristics and estimates the shortwave (0.254.0m) aerosol radiative forcing (ARF) and atmospheric heating rates over Kanpur, central IGB, during three intense dust-storm events in the pre-monsoon season of 2010. MODIS images, meteorological and AERONET observations clearly show that all the dust storms either originated from the Thar desert or transported over, under favorable meteorological conditions (low pressure and strong surface winds) affecting nearly the whole IGB and modifying the aerosol loading and characteristics (Angstrom exponent, single scattering albedo, size distribution and refractive index). CALIPSO observations reveal the presence of high-altitude (up to 35km) dust plumes that strongly modify the vertical aerosol profile and are transported over Himalayan foothills with serious climate implications (atmospheric warming, enhanced melting of glaciers). Shortwave ARF calculations over Kanpur using SBDART model show large negative forcing values at the surface (93.27, 101.60 and 66.71Wm2) during the intense dusty days, associated with planetary (top of atmosphere) cooling (18.16, 40.95, 29.58Wm2) and significant atmospheric heating (75.11, 60.65, 37.13Wm2), which is translated to average heating rates of 1.57, 1.41 and 0.78Kday1, respectively in the lower atmosphere (below 3.5km). The ARF estimates are in satisfactory agreement with the AERONET ARF retrievals over Kanpur.

Research areas: Atmospheric/Ocean Indicators
Meteorological regimes modulating dust outbreaks in southwest Asia: The role of pressure anomaly and Inter-Tropical Convergence Zone on the 13 July 2014 case (2015)

The present work examines the characteristics (source regions, pathways, vertical profiles) as well as the meteorological conditions and the role of the Inter-Tropical Convergence Zone (ITCZ) during a dust event on 13 July 2014 over southwest (SW) Asia. NCEP/NCAR reanalysis, meteorological and satellite remote sensing (MODIS, Meteosat, CALIPSO) observations are utilized. The major dust storm (AOD550 up to 1.52.0) was generated in the southern Karakum desert in Turkmenistan as a result of intense north winds (Levar), due to enhanced pressure gradient between the Caspian Sea and Hindu Kush Mountains. The dust storm was vertically extended up to 5km over the arid terrain of SW Asia, as a consequence of the deep mixing layer and increased convection, while over northern Arabian Sea it was mixed with another dust plume coming from Arabia and marine aerosols due to strong monsoon winds within the boundary layer. The regional topography, variations in pressure, monsoon circulation and the position of the ITCZ play a decisive role on modulation of the wind field, dust-storm pathways and vertical distribution of dust. Enhanced knowledge of atmospheric circulation and processes responsible for dust export over SW Asia and Arabian Sea in linkage to the Levar wind and Indian summer monsoon is essential for improving dust forecasts and simulations over the region.

Research areas: Atmospheric/Ocean Indicators
Mathematical modelling of the seasonal variability of plankton and forage fish in the Gulf of Kachchh (2015)

Seasonal variability of plankton and small fish in the coastal water of west coast of India is studied with a non-linear mathematical model (NPZF model) having four compartments viz., nutrient, phytoplankton, zooplankton and forage fish. The inclusion of an additional compartment of forage fish increases the complexity of the earlier proposed NPZ model as it introduces additional seven unknown parameters, which need to be specified for performing numerical simulations. The key problem is therefore to estimate the range of values of these parameters in the dynamical system. With the help of local stability analysis, sensitivity analysis and numerical simulations, these parameters have been estimated. As a case study for the evaluation of the model, simulations are performed to explain the aquatic ecology of Gulf of Kachchh (2220 N2340 N, 6820 E7040 E), an inlet on west coast of India in the Arabian Sea. An important conclusion of this study is that by using the prescribed parameters as estimated from the sensitivity analysis, the NPZF model reproduces some salient features of the Gulf of Kachchh, especially the bimodal oscillations observed in the plankton and forage fish. The 4-compartment model could therefore serve as an ecological model of the Gulf of Kuchchh.

Research areas: Ecosystems
Marine climate influences on interannual variability of tropical cyclones in the eastern Caribbean: 19792008 (2015)

Interannual variability of tropical cyclones (TCs) in the eastern Caribbean is studied using MIT-Hurdat fields during the JulyOctober season from 1979 to 2008. TC intensity shows local climate sensitivity particularly for upper ocean currents, salinity and mixed-layer depth, and 200850 mb wind shear. Remote influences from the Southern Oscillation, Saharan dust, and the South American monsoon are also identified as important. Ocean currents diminish along the coast of South America, so interbasin transfer between the North Brazil and Caribbean Currents declines in seasons of frequent and intense TCs. This is related to a dipole pattern in the sea surface height formed mainly by reduced trade wind upwelling northeast of Venezuela. A low-salinity plume from the Orinoco River spreads across the eastern Caribbean. It is the weaker currents and shallower mixed layer that conspire with surplus heat to build thermodynamic energy available for TC intensification.

Research areas: Severe Storms
Manifestations of the rim current, coccolithophore blooms, and continental runoff in the long-term monthly mean distributions of satellite reflectance coefficients of the Black Sea (2015)

Based on the archived data of the ocean color scanner MODIS-Aqua for 20032011, we constructed the long-term monthly mean distributions of the reflectance Rrs of the Black Sea for April, May, June, and September in order to visualize the contributions of seasonal factors to the long-term variability of the basins images. In April, the Rim Currents branch west of 34 E is visualized by higher Rrs in different regions of the visible spectrum due to the transport of suspended matter caused by intensification of the Rim Current during the winter-spring period. During the June coccolithophore blooms, the long-term Rrs level is 23 times higher when compared to the previous and subsequent months. This excess is particularly considerable outside the shelf and coastal areas. The open sea Rrs distribution in June features horizontal inhomogeneity. The seasonal trend of the Rrs spectra on the Black Sea NW shelf is explicitly related to the annual cycle of the continental runoff effects.

Research areas: Water Quality, Ecosystems
Life in the sea of plenty: Seasonal and regional comparison of physiological performance of Euphausia hanseni in the northern Benguela upwelling system (2015)

Variability in upwelling events may lead to periods of constrained food availability in the northern Benguela upwelling system (NBUS), thereby affecting the physiological state and metabolic activity of euphausiids. Most attention has so far been paid to seasonal effects but little is known about regional variability. Metabolic activity (expressed by respiration and excretion rates) and physiological state (expressed by reproductive effort and moult activity) in Euphausia hanseni were examined at different stations during austral summer (minimum upwelling) and austral winter (maximum upwelling). Overall, regional differences in physiological state, influencing metabolic activity, were greater than seasonal ones, indicating favourable conditions for growth and reproduction year-round. Higher respiration rates were found for females in more advanced stages of sexual development. Moult stage did not affect oxygen consumption rates, however. The physiological state of E. hanseni at the time of capture may serve as a meaningful indicator of the associated hydrographic conditions in the NBUS, to be further used in eco-system analysis on seasonal or long-term time scales. A latitudinal comparison of species highlights the extraordinary physiological plasticity of euphausiids.

Research areas: Ecosystems, Atmospheric/Ocean Indicators
Latitudinal and temporal distributions of diatom populations in the pelagic waters of the Subantarctic and Polar Frontal zones of the Southern Ocean and their role in the biological pump (2015)

Abstract. The Subantarctic and Polar Frontal zones (SAZ and PFZ) represent a large portion of the total area of the Southern Ocean and serve as a strong sink for atmospheric CO2. These regions are central to hypotheses linking particle fluxes and climate change, yet multi-year records of modern flux and the organisms that control it are, for obvious reasons, rare. In this study, we examine two sediment trap records of the flux of diatoms and bulk components collected by two bottom-tethered sediment traps deployed at mesopelagic depths (~ 1 km) in the SAZ (2-year record; July 1999October 2001) and in the PFZ (6-year record; September 1997February 1998, July 1999August 2000, November 2002October 2004 and December 2005October 2007) along the 140 E meridian. These traps provide a direct measure of transfer below winter mixed layer depths, i.e. at depths where effective sequestration from the atmosphere occurs, in contrast to study of processes in the surface ocean. Total mass fluxes were about twofold higher in the PFZ (24  13 g m2 yr1) than in the SAZ (14  2 g m2 yr1). Bulk chemical composition of the particle fluxes mirrored the composition of the distinct plankton communities of the surface layer, being dominated by carbonate in the SAZ and by biogenic silica in the PFZ. Particulate organic carbon (POC) export was similar for the annual average at both sites (1.0  0.1 and 0.8  0.4 g m2 yr1 for the PFZ and SAZ, respectively), indicating that the particles in the SAZ were relatively POC rich. Seasonality in the particle export was more pronounced in the PFZ. Peak fluxes occurred during summer in the PFZ and during spring in the SAZ. The strong summer pulses in the PFZ are responsible for a large fraction of the variability in carbon sequestration from the atmosphere in this region. The latitudinal variation of the total diatom flux was found to be in line with the biogenic silica export with an annual flux of 31  5.5  108 valves m2 yr1 at the PFZ compared to 0.5  0.4  108 m2 yr1 at the SAZ. Fragilariopsis kerguelensis dominated the annual diatom export at both sites (43 % at the SAZ and 59 % in the PFZ). POC fluxes displayed a strong positive correlation with the relative contribution of a group of weakly silicified and bloom-forming species in the PFZ. Several lines of evidence suggests that the development of these species during the growth season facilitates the formation of aggregates and carbon export. Our results confirm previous work suggesting that F. kerguelensis plays a major role in the decoupling of the carbon and silicon cycles in the high-nutrient low-chlorophyll waters of the Southern Ocean.

Research areas: Ecosystems