Time filter

Source Type

Dall'Olmo G.,Plymouth Marine Laboratory | Dall'Olmo G.,National Center for Earth Observation | Mork K.A.,Norwegian Institute of Marine Research | Mork K.A.,Center for Climate Dynamics
Geophysical Research Letters | Year: 2014

Despite its fundamental role in controlling the Earth's climate, present estimates of global organic carbon export to the deep sea are affected by relatively large uncertainties. These uncertainties are due to lack of observations as well as disagreement among methods and assumptions used to estimate carbon export. Complementary observations are thus needed to reduce these uncertainties. Here we show that optical backscattering measured by Bio-Argo floats can detect a seasonal carbon export flux in the Norwegian Sea. This export was most likely due to small particles (i.e., 0.2-20 μm), was comparable to published export values, and contributed to long-term carbon sequestration. Our findings highlight the importance of small particles and of physical mixing in the biological carbon pump and support the use of autonomous platforms as tools to improve our mechanistic understanding of the ocean carbon cycle. Key Points Bio-Argo floats can track small particles in the mesopelagic region Export fluxes from small particles were significant Small particles contributed to long-term carbon sequestration ©2014. The Authors. Source

Esau I.,Gc Rieber Climate Institute | Esau I.,Center for Climate Dynamics | Esau I.,Nizhny Novgorod State Technical University | Luhunga P.,University of Pretoria | And 5 more authors.
Meteorology and Atmospheric Physics | Year: 2012

Links between spatial and temporal variability of Planetary Boundary Layer meteorological quantities and existing land-use patterns are still poorly understood due to the non-linearity of air-land interaction processes. This study describes the results of a statistical analysis of meteorological observations collected by a network of ten Automatic Weather Stations. The stations were in operation in the highveld priority area of the Republic of South Africa during 2008-2010. The analysis revealed localization, enhancement and homogenization in the inter-station variability of observed meteorological quantities (temperature, relative humidity and wind speed) over diurnal and seasonal cycles. Enhancement of the meteorological spatial variability was found on a broad range of scales from 20 to 50 km during morning hours and in the dry winter season. These spatial scales are comparable to scales of observed land-use heterogeneity, which suggests links between atmospheric variability and land-use patterns through excitation of horizontal meso-scale circulations. Convective motions homogenized and synchronized meteorological variability during afternoon hours in the winter seasons, and during large parts of the day during the moist summer season. The analysis also revealed that turbulent convection overwhelms horizontal meso-scale circulations in the study area during extensive parts of the annual cycle. © 2012 Springer-Verlag Wien. Source

Esau I.,G C Rieber Climate Institute | Esau I.,Center for Climate Dynamics
Geofizika | Year: 2012

The Ekman boundary layer (EBL) is a non-stratified turbulent layer of fluid in a rotated frame of reference. The EBL comprises two sub-layers, namely, the surface sub-layer, where small scale well-developed turbulence dominates, and the core sub-layer, where large scale self-organized turbulence dominates. This study reports self-organization of large scale turbulence in the EBL as simulated with the large-eddy simulation (LES) model LESNIC. The simulations were conducted in a large domain (144 km in the cross-flow direction, which is an equivalent to about 50 EBL depths) to resolve statistically significant number of the largest self-organized eddies. Analysis revealed that the latitude of the LES domain and, unexpectedly, the direction of the geostrophic wind forcing control the self-organization, turbulence scales, evolution and the quasi steady-state averaged vertical profiles in the EBL. The LES demonstrated destabilization of the EBL turbulence and its mean structure by the horizontal component of the Coriolis force. Visualisations of the EBL disclosed existence of quasi-regular large scale turbulent structures composed of counter-rotating vortices when the geostrophic flow was set from East to West. The corresponding structures are absent in the EBL when the geostrophic flow was set in the opposite (i.e. West to East) direction. These results finally resolve the long-standing controversy between the Leibovich-Lele and the Lilly-Brown instability mechanisms acting in the EBL. The LES demonstrated that the Lilly-Brown mechanism, which involves the vertical component of the Coriolis force, is working in the polar EBL where its impact is nevertheless rather small. The Leibovich-Lele mechanism, which involves the horizontal component of the Coriolis force, acts in low latitudes where it completely alters the turbulent structure of the EBL. Source

Esau I.,Gc Rieber Climate Institute | Esau I.,Center for Climate Dynamics | Repina I.,Russian Academy of Sciences
Advances in Meteorology | Year: 2012

This paper presents analysis of wind climate of the Kongsfjorden-Kongsvegen valley, Svalbard. The Kongsfjorden-Kongsvegen valley is relatively densely covered with meteorological observations, which facilitate joint statistical analysis of the turbulent surface layer structure and the structure of the higher atmospheric layers. Wind direction diagrams reveal strong wind channeled in the surface layer up to 300 m to 500 m. The probability analysis links strong wind channeling and cold temperature anomalies in the surface layer. To explain these links, previous studies suggested the katabatic wind flow mechanism as the leading driver responsible for the observed wind climatology. In this paper, idealized turbulence-resolving simulations are used to distinct between different wind driving mechanisms. The simulations were performed with the real surface topography at resolution of about 60 m. These simulations resolve the obstacle-induced turbulence and the turbulence in the non-stratified boundary layer core. The simulations suggest the leading roles of the thermal land-sea breeze circulation and the mechanical wind channeling in the modulation of the valley winds. The characteristic signatures of the developed down-slope gravity-accelerated flow, that is, the katabatic wind, were found to be of lesser significance under typical meteorological conditions in the valley. © 2012 Igor Esau and Irina Repina. Source

Repina I.,Russian State Hydrometeorological University | Artamonov A.,Russian Academy of Sciences | Chukharev A.,Ukrainian Academy of Sciences | Esau I.,G C Rieber Climate Institute | And 6 more authors.
Estonian Journal of Engineering | Year: 2012

This paper reports the results of field experiments performed at an offshore oceanographic platform in the Black Sea during spring and fall seasons 2005-2011. Observations of the air-sea interaction were made using direct and remote sensing methods in the coastal zone where the interaction is complex and still poorly understood. A specialized research platform, managed by the Marine Hydrophysical Institute (MHI), is placed on the shelf slope approximately 600 m offshore the Crimea coast, Ukraine. The water depth at the site is about 30 m. The experiment program included conventional turbulence measurements with the eddy-covariance method as well as remote radio-polarimetric measurements with a newly developed instrument. The study was concentrated on the air-sea interaction during episodes of weak wind in the atmosphere and upwelling events in the ocean. Analysis of the collected data confirmed significant dependence of the surface drag coefficient on the air-sea temperature difference under weak wind conditions. However, this analysis also demonstrated a new air-sea interaction regime, which is characterized by large quasi-periodic (periods about 3.5 h) turbulence oscillations developing initially in the atmosphere and later (after about 10-12 h) in the sub-surface water layer. The analysis of radiopolarimetric measurements provided the characteristics of the gravity-capillary wave field during these events. Source

Discover hidden collaborations