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Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze
Get PDFEvery year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo-Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one- and four-dimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (±10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii) are used to develop a composite indirect effect scheme. The Indo-Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (–20 ± 4 W M–2) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate
Flexible elemental stoichiometry in Trichodesmium spp. and its ecological implications
Get PDFWe conducted laboratory experiments to assess the bioelemental plasticity of cultures of Trichodesmium IMS101 under phosphorus (P)-replete, P-restricted, and light-limited conditions. The results reveal a high degree of stoichiometric flexibility. Specifically, Trichodesmium IMS101 is capable of growth with carbon (C) : nitrogen (N) : P ratios of C₅₈₅±₅₆ : N₉₀±₁₀ : P₁, approximately six times higher than would be predicted by the Redfield reference ratio (C₁₀₆ : N₁₆ : P₁), thus signifying low cellular P quotas relative to C and N. Luxury consumption of P occurs rapidly after periods of prolonged P restriction, under both light and dark conditions, resulting in substantial increases in P quotas and reductions of C : N : P ratios (C₉₆±₈ : N₁₆±₁ : P₁). Comparisons of laboratory culture data to our field observations from the Northwest Atlantic and the North Pacific indicate that, while natural populations of Trichodesmium exhibit persistently low P content relative to C and N (C₂₉₀±₁₅ : N ₅₃±₃ : P₁), the highest and lowest C : P and N : P ratios recorded in the laboratory are rarely observed in nature. We have also performed laboratory experiments intended to simulate the energetic and nutritional extremes that would occur as naturally migrating populations of Trichodesmium sink out of the euphotic zone into P-rich regions of the upper disphotic zone. The duration of dark survival for this isolate is on the order of 3–6 d, after which time cells are unable to recover from light deprivation. This finding provides a constraint on the temporal scale of vertical migration.Individual authors are also free to post PDF copies of their work published in ASLO journals on their individual or institutional websites. It is not necessary to request permission from ASLO so long as such postings are not used for commercial purposes
Lipid/bitumen maturation by hydrothermal activity in sediments of Middle Valley, leg 139
Get PDFThe accelerated diagenesis, maturation, and catagenesis of organic matter to hydrothermal petroleum was studied in sediments from Ocean Drilling Program Leg 139 in Middle Valley, northern Juan de Fuca Ridge. Sediments at Sites 855 and 856 have experienced high heat flow resulting in accelerated diagenesis of the immature organic matter without product migration. Ruid migration through these sediments has resulted in an influx of mature organic components, yielding diverse bitumen mixtures. At Site 857 the alteration has occurred in situ to the catagenetic stage and at Site 858, the hottest holes, the hydrothermal petroleums have migrated after generation. Maturation for the rc-alkanes proceeds from high CPI values to 250 dalton)
Geological and geochemical contraints on the isotopic composition of interstitial waters from the Hydrate Ridge region, Cascadia Continental margin
Get PDFThe isotopic compositions of interstitial waters collected from Hydrate
Ridge during Ocean Drilling Program Leg 204 were measured to
evaluate the fluid evolution of this accretionary prism. At shallow
depths, the dissolved Cl- concentrations and δD and δ¹⁸O values of the
interstitial water reflect changes in the salinity and the isotopic compositions
of seawater from the Last Glacial Maximum to the present. The
presence of disseminated gas hydrates, which is well identified by discrete
low Cl- anomalies within the gas hydrate stability zone, is accompanied
by high δD and δ¹⁸O values of the freshened fluids. This is
consistent with incorporation of heavy isotopes into the gas hydrate
lattice, which is also apparent in the signals observed at the ridge summit.
Here, massive gas hydrate formation in the upper 20 meters below
seafloor leads the formation of brines with dissolved Cl- concentrations
as high as 1400 mM. The interstitial waters sampled near massive gas
hydrates at the ridge summit are extremely depleted in D and ¹⁸O. Clay
mineral dehydration within the deep prism results in a progressive decrease
in Cl¯ and δD with depth. Dehydration temperature estimates
based on those data likely suggest a progressive increase in the temperature
of isotopic fractionation between clay and water with distance
from the prism toe. The oxygen isotope data probably reflect the combined
effects of clay dehydration, carbonate precipitation, and alteration of oceanic basement;
however, there are not enough data to
constrain the relative contribution of these processes to the observed
signals
Bottom boundary layer flow and salt injection from the continental shelf to slope
Get PDFAustral winter oceanographic measurements from the northwest Australian continental shelf reveal salty water forming evaporatively inshore, moving across the wide shelf near the bottom and into the adjacent open ocean when the shelf edge alongshore flow is equatorward. The salt tongue is absent during more normal conditions, when the poleward Leeuwin Current is present. We hypothesize that the flow reversal enables shelf-wide bottom boundary layer (Ekman) transport and thus creates the shelf-edge convergence that accounts for the observed salt tongue. This flow is absent under sustained normal conditions because of buoyancy arrest in the bottom boundary layer
Acoustic radiation from the head of echolocating harbor porpoises
Get PDFAn experiment was conducted to investigate the sound pressure patterns on the melon of odontocetes by using four broadband hydrophones embedded in suction cups to measure echolocation signals on the surface of the forehead of two harbor porpoises (Phocoena phocoena). It has long been hypothesized that the special lipids found in the melon of odontocetes, and not in any other mammals, focus sounds produced in the nasal region that then propagate through the melon, producing a beam that is directional in both the horizontal and vertical planes. The results of our measurements supported the melon-focusing hypothesis, with the maximum click amplitude, representing the axis of the echolocation beam, located approximately 5.6–6.1 cm from the edge of the animal’s upper lip along the midline of the melon. The focusing is not sharp but is sufficient to produce a transmission beam of about 16°. Click amplitude dropped off rapidly at locations away from the location of site of maximum amplitude. Based on comparisons of forehead anatomy from similar sized porpoises, the beam axis coincided with a pathway extending from the phonic lips through the axis of the low-density/low sound velocity lipid core of the melon. The significant interaction between click number and hydrophone position suggests that the echolocation signals can take slightly different pathways through the melon, probably as a result of how the signals are launched by the production mechanism and the position of the acoustically reflective air sacs.Keywords: melon,
contact hydrophone,
sound pressure level,
echolocation signals,
harbor porpoise,
Phocoena phocoena,
lipi
Internal Waves in the Upper Ocean During MILE
No full textWe describe the spectral analysis of temperature and velocity measurements made in the northeast Pacific as part of the Mixed Layer Experiment (MILE) and attempt to relate the observed fluctuations to internal-wave models of the upper ocean. From the inertial frequency to 1 cph there is good agreement between these upper-ocean data and typical deep-ocean observations as described by the WKB-scaled Garrett-Munk model. The largest deviations from the Garrett-Munk model occur in the vertical-displacement field at high frequency, 1–5 cph, where there is a spectral peak or shoulder and high vertical coherence. These high-frequency features in vertical displacement are successfully modeled using a few standing modes and un-correlated noise, though the velocity spectra are poorly modeled—probably because of contamination by mooring motion. There are significant temporal fluctuations of the high-frequency energy that are not correlated with the local winds but are perhaps associated with the advection of an eddy-like feature
Turbulence and Internal Waves at the Equator. Part I: Statistics from Towed Thermistors and a Microstructure Profiler
No full textHigh correlations between turbulent dissipation rates and high-wavenumber internal waves and the high values of turbulent dissipation associated with internal wave activity suggest that internal waves are the main direct source of mixing in the thermocline above the core of the Equatorial Undercurrent. An extensive dataset obtained using a microstructure profiler and thermistor chain towed along the equator was analyzed to examine the correspondence between turbulent mixing and high-wavenumber internal waves. In the low Richardson number (Ri) thermocline below the mixed layer but above the core of the Equatorial Undercurrent, and when winds were moderate and steadily westward, it was found that:
• the spectrum of vertical isotherm displacement was dominated by a narrow wavenumber band (corresponding to 150–250-m zonal wavelength) of internal waves;
• both turbulence and internal waves varied diurnally—hourly averaged values of turbulent dissipation rate and wave potential energy were greater by a factor of 100 at night; and
• correlations between turbulent dissipation rate and several measures of internal wave activity (wave isotherm displacement, wave slope, and wave potential energy) were high.
Little or no high wavenumber internal wave activity was observed when winds were light or eastward: Superposing plane waves with the observed characteristics on the observed background field suggests that they are inherently unstable to both adjective and shear instability above the core of the Equatorial Undercurrent. These waves are due either to locally generated internal gravity waves or to Kelvin-Helmholz-type instabilities generated in the shear flow; from our measurements these two phenomena could not be distinguished
Large-Scale Circulation with Locally Enhanced Vertical Mixing
Get PDFThe influence of localized regions of intensified vertical mixing on the stratification and circulation in a large-scale ocean model is investigated with idealized numerical experiments. Numerical solutions are obtained of a closed-basin, single-hemisphere ocean model based on the planetary geostrophic equations. Mesoscale eddy effects are minimized, and vertical mixing at the turbulent microscale is represented by a vertical diffusivity κ[subscript]υ. Solutions with uniform κ[subscript]υ are contrasted with a “localized mixing” solution, in which κ[subscript]υ increases by two orders of magnitude from its interior value (0.2 × 10⁻⁴ m² s⁻¹) in a region 500 km wide adjacent to the vertical eastern boundary. When κ[subscript]υ is uniform, the stratification beneath the ventilated thermocline is characterized by a single vertical scale. In contrast, the localized vertical mixing supports a deep diffusive thermocline with two distinct vertical scales: an internal boundary layer centered at the base of the ventilated thermocline (roughly 1000-m depth) and an abyssal thermocline whose vertical scale is set in the region of large κ[subscript]υ. This stratification is qualitatively similar to observed deep ocean stratification. In contrast to the Stommel–Arons meridional abyssal flow that arises in the model when κ[subscript]υ is uniform and small, the localized mixing solution has primarily zonal flow in the abyssal interior, with meridional motion confined to boundary layers. An advective–diffusive balance is established in the region of enhanced mixing. The near-surface circulation is dominated by westward zonal flow in the southern half of the interior, northward flow along the western boundary, and eastward flow in the northern half of the interior, while the pattern of flow in the abyssal interior is essentially the reverse. The circulation is closed by upwelling in the mixing region and downwelling along the northern boundary. Meridional motion in the mixing region is consistent with the Sverdrup vorticity balance, with northward flow at depth and southward flow near the surface. The source water for the deep circulation is confined to a narrow range of the coldest temperature classes in the basin, while the middepth subtropical thermocline is filled with warmer deep water that enters the gyre as cold deep water and then is modified in the eastern mixing region
On the Coupling of Wind Stress and Sea Surface Temperature
Get PDFA simple quasi-equilibrium analytical model is used to explore hypotheses related to observed spatial correlations between sea surface temperatures and wind stress on horizontal scales of 50–500 km. It is argued that a plausible contributor to the observed correlations is the approximate linear relationship between the surface wind stress and stress boundary layer depth under conditions in which the stress boundary layer has come into approximate equilibrium with steady free-atmospheric forcing. Warmer sea surface temperature is associated with deeper boundary layers and stronger wind stress, while colder temperature is associated with shallower boundary layers and weaker wind stress. Two interpretations of a previous hypothesis involving the downward mixing of horizontal momentum are discussed, and it is argued that neither is appropriate for the warm-to-cold transition or quasi-equilibrium conditions, while one may be appropriate for the cold-to-warm transition. Solutions of a turbulent large-eddy simulation numerical model illustrate some of the processes represented in the analytical model. A dimensionless ratio γτA is introduced to measure the relative influence of lateral momentum advection and local surface stress on the boundary layer wind profile. It is argued that when γ[subscript]τA < 1, and under conditions in which the thermodynamically induced lateral pressure gradients are small, the boundary layer depth effect will dominate lateral advection and control the surface stress