ALBERT

Description: [1]  Structural evolution of monsoon clouds in the core monsoon region of India has been examined using multi-sensor data. Invigoration of warm clouds above 4.5 km (occurring in only 15.4% days of the last 11 monsoon seasons) is associated with a transition from negative to positive normalized rainfall anomaly. Cloud top pressure reduces with an increase in aerosol optical depth at a higher rate of invigoration in drier condition (characterized by large fraction of absorbing aerosols) than wet condition. Cloud effective radius for warm clouds does not show any significant change with an increase in aerosol concentration in presence of high liquid water path, probably due to strong buffering role of meteorology. The structural evolution of monsoon clouds is influenced by both dynamic and microphysical processes that prolong the cloud lifetime, resulting in infrequent rainfall. Our results call for improved representation of aerosol and cloud vertical structures in the climate models to resolve this issue.

Description: A thin layer of fresh water from summer monsoon rain and river runoff in the Bay of Bengal (BoB) has profound influence on air-sea interaction across the south Asian region, but the mechanisms that sustain the low-salinity layer are as yet unknown. Using the first long time series of high-frequency observations from a mooring in the north BoB and satellite salinity data, we show that fresh water from major rivers is transported by large-scale flow and eddies, and shallow salinity stratification persists from summer through winter. The moored observations show frequent 0.2-1.2 psu salinity jumps with time scales of 10 minutes to days, due to O(1-10) km sub-mesoscale salinity fronts moving past the mooring. In winter, satellite sea surface temperature shows 10 km-wide filaments of cool water, in line with moored data. Rapid salinity and temperature changes at the mooring are highly coherent, suggesting slumping of salinity-dominated fronts. Based on these observations, we propose that sub-mesoscale fronts may be one of the important drivers for the persistent fresh layer in the north BoB.

Description: Journal of Physical Oceanography, Ahead of Print. 〈br/〉

Description: The effect of upwelling events, storm water discharge, and local circulation on phytoplankton blooms in the central California and Southern California Bight (SCB) coastal zones was analyzed using 10+ years (1997–2007) of remotely sensed surface chlorophyll concentration (CHL, derived from SeaWiFS ocean color), sea surface temperature, and modeled freshwater discharge. Analysis of variability and factors associated with phytoplankton blooms was conducted using the offshore extension of zones of CHL 〉 5 mg m−3; this method excludes terrestrial interference that complicates the use of ocean color to investigate phytoplankton blooms in coastal waters. In the SCB, blooms were most frequent in spring and associated with the spring transition to an upwelling regime. Along the Central Coast, blooms persisted from spring to autumn during seasonal intensification of upwelling. Offshore CHL extensions showed a significant positive trend during 1997–2007, with maxima in 2000–2001 and 2005–2006 that coincided with higher than normal frequency of upwelling events. Upwelling was found to be a major factor driving phytoplankton blooms, although the standard upwelling index derived from large-scale atmospheric circulation was decoupled from the frequencies of both upwelling events and phytoplankton blooms. Areas of longer residence time associated with natural boundaries between coastal ocean regions had more extensive and persistent blooms. The influence of storm water discharge on offshore CHL extension appeared to be limited to areas in close proximity to major river mouths. These “hot spots” were also co-located with ocean outfalls of Publicly Owned Treatment Works (POTW) discharge and, in some cases, longer residence time of coastal waters.

Description: Abstract Retarding potential analyzers are frequently flown on small satellites as in situ ion probes, from which can be derived a number of ion plasma parameters from a current‐voltage relationship (I‐V curve). The traditional method of analyzing retarding potential analyzer data produces inaccuracies in derived estimates when there is significant noise present in the instrument measurements. In this study we investigate the dependencies between parameters that produce uncertainties in noisy I‐V curves. It is found that multiple combinations of ion velocity and spacecraft floating potential can produce I‐V curves that lie within the noise envelope, which renders it difficult for a traditional curve fitting technique to objectively and accurately estimate parameters from a noisy I‐V curve. In this paper we propose BATFORD—a bootstrap resampling‐based technique to improve the accuracies of parameter estimates. It is particularly useful when signal‐to‐noise ratios are low. The algorithm is tested against a traditional curve fitting method for a simulated data set comprising I‐V curves for the middle‐ and low‐latitude ionosphere at low Earth orbit altitudes around 450 km, where O+ is the predominant species. BATFORD is found to provide more robust and reliable estimates assuming generalized noise distribution characteristics. As further validation, the algorithm is applied to satellite data from an orbit with deep plasma bubbles and hence low signal levels.

Description: Swimming organisms can enhance mixing in their natural environments by creating eddies in their wake and by dragging water along. However, these mixing mechanisms are inefficient for microorganisms, because swimming-induced variations in velocity, temperature and dissolved substances are evened out before they can be advected. In bioconvection, however, microorganisms induce water movement not by propulsion directly, but by locally changing the fluid density, which drives convection. Observations of bioconvection have so far mainly been limited to laboratory settings. We report the first observation and quantification of bioconvection within a stratified natural water body. Using in-situ measurements, laboratory experiments and simulations, we demonstrate that the bacterium Chromatium okenii is capable of mixing 0.3- to 1.2-m-thick water layers at around 12-m-water depth in the Alpine Lake Cadagno (Switzerland). As many species are capable of driving bioconvection, this phenomenon potentially plays a role in species distributions and influences large-scale phenomena like algal blooms.

Description: We develop an experimental daily surface heat flux dataset based on satellite observations to study subseasonal variability (periods shorter than 90 days) in the tropical Indian Ocean. We use incoming shortwave and longwave radiation from the International Satellite Cloud Climatology Project, and sea surface temperature (SST) from microwave sensors, to estimate net radiative flux. Latent and sensible heat fluxes are estimated from scatterometer winds and near-surface air temperature and specific humidity from Atmospheric Infrared Sounder (AIRS) observations calibrated to buoy data. Seasonal biases in net heat flux are generally within 10 Wm −2 of estimates from moorings, and the phases and amplitudes of subseasonal variability of heat fluxes are realistic. We find that the contribution of subseasonal changes in air-sea humidity gradients to latent heat flux equals or exceeds the contribution of subseasonal changes in wind speed in all seasons. SST responds coherently to subseasonal oscillations of net heat flux associated with active and suppressed phases of atmospheric convection in the summer hemisphere. Thus subseasonal SST changes are mainly forced by heat flux in the northeast Indian Ocean in northern summer, and in the 15 o S-5 o N latitude belt in southern summer. In the winter hemisphere, subseasonal SST changes are not a one-dimensional response to heat flux, implying that they are mainly due to oceanic advection, entrainment or vertical mixing. The coherent evolution of subseasonal SST variability and surface heat flux suggests active coupling between SST and large-scale, organized tropical convection in the summer season. This article is protected by copyright. All rights reserved.

Description: We present here, for the first time, comprehensive depth specific (near surface to 1800m) oxygen isotope (δ 18 O) dataset of 175 water samples collected from 22 different locations in the Bay of Bengal (BoB) during July-August, 2009 and December-January, 2012-13. The study is aimed to constrain the water mass parameters using oxygen isotope and salinity. Four water masses are identified on the basis of temperature-salinity relationship- (a) Bay of Bengal water (BoBW;0-50m), (b) Mixed zone (MZ; 60-120m) (c) Indonesian throughflow (ITF; 200-500m) and (d) Indian Ocean Deep water (IDW; 600-1800m). Our study shows- (1) δ 18 O-salinity correlation (slope = 0.15 ± 0.01; intercept = -4.58 ± 0.24; r 2 = 0.85; n = 54) for the BoBW is found to be consistent with surface BoB water reported earlier, (2) relatively lower correlation (r 2 = 0.24; n = 47) in underlying mixed zone suggesting mixing of various water masses (3) significant linear correlation between δ 18 O and salinity in IDW (r 2 = 0.70) indicating depleted less saline water from southern hemisphere spread upto 18 º N, and (4) vertical profiles at various sampling locations showing a continuous enrichment of δ 18 O (0.6-0.8‰) within top 60-100 meters irrespective of their proximity to coastline. In conjunction with a sharp decrease in Δ 14 C data below the same depth zone (60-100 m) as reported earlier, we suggest the δ 18 O value reaches to its acme at a zone of mixing between younger BoBW and relatively older ITF. This preliminary study indicates oxygen isotope and salinity together can be used to identify different water masses in the BoB.

Description: [1]  We present evidence that the springtime western boundary current (WBC) in the Bay of Bengal is a continuous northward-flowing current from about 12°N to 17°N, which then separates from the coast at around 18°N. We first revisit a hydrographic dataset collected in 1987 ( Babu et al ., 2003), from a potential vorticity perspective, and then analyze absolute dynamic height maps from satellite altimeters during the period 2000–2010. The altimetric maps suggest that the mean configuration of the WBC is that of an intense current with two anticyclonic eddies on the offshore side, which are part of the basin-wide anticyclonic circulation. The WBC consistently separates from the coast at around 18°N in all years between 2000 and 2010. The path of the eastward-flowing mean stream after separation appears to be consistent with isolines of f / H and with Ertel's potential vorticity, based on an analysis of the hydrographic data from 1987.

Description: Evolving over millions of years, hair-like natural flow sensors called cilia, which are found in fish, crickets, spiders, and inner ear cochlea, have achieved high resolution and sensitivity in flow sensing. In the pursuit of achieving such exceptional flow sensing performance in artificial sensors, researchers in the past have attempted to mimic the material, morphological, and functional properties of biological cilia sensors, to develop MEMS-based artificial cilia flow sensors. However, the fabrication of bio-inspired artificial cilia sensors involves complex and cumbersome micromachining techniques that lay constraints on the choice of materials, and prolongs the time taken to research, design, and fabricate new and novel designs, subsequently increasing the time-to-market. In this work, we establish a novel process flow for fabricating inexpensive, yet highly sensitive, cilia-inspired flow sensors. The artificial cilia flow sensor presented here, features a cilia-inspired high-aspect-ratio titanium pillar on an electrospun carbon nanofiber (CNF) sensing membrane. Tip displacement response calibration experiments conducted on the artificial cilia flow sensor demonstrated a lower detection threshold of 50 µm. Furthermore, flow calibration experiments conducted on the sensor revealed a steady-state airflow sensitivity of 6.16 mV/(m s−1) and an oscillatory flow sensitivity of 26 mV/(m s−1), with a lower detection threshold limit of 12.1 mm/s in the case of oscillatory flows. The flow sensing calibration experiments establish the feasibility of the proposed method for developing inexpensive, yet sensitive, flow sensors; which will be useful for applications involving precise flow monitoring in microfluidic devices, precise air/oxygen intake monitoring for hypoxic patients, and other biomedical devices tailored for intravenous drip/urine flow monitoring. In addition, this work also establishes the applicability of CNFs as novel sensing elements in MEMS devices and flexible sensors.