Cut-off low systems over Iraq: Contribution to annual precipitation and synoptic analysis of extreme events

We combine daily in situ precipitation data with meteorological reanalysis data in order to explore the contribution of cut-off low systems to the seasonal and interannual rainfall variations over Baghdad from 2005 to 2016. During these 12 years (average rainfall of 131 ± 67 mm/year), 38 rainy cut-off lows brought 43% of the total precipitation, with extreme inter-annual variations. Indeed, precipitation associated with autumn cut-off lows was the principal factor that turned an arid into a wet year: during the three most arid years cut-off lows contributed about 25% of the average rainfall (10 out of 40 mm/year) while during the three wettest years they contributed near 67% (171 out of 254 mm/year). The extreme-rain cut-off low systems displayed analogous synoptic characteristics: upper-atmosphere divergence, upwards vertical motions in the middle atmosphere, and lower-atmosphere winds into central Iraq at times when the surface Red Sea and Persian Gulf waters were warmer than the surface air. During those days previous to an extreme event, the surface waters cooled substantially and the amount of precipitable water increased largely, suggesting high latent heat transfer. In order to characterize those conditions that favour rainfall, we focus on the November 18–20, 2013 cut-off low system, which led to the largest flooding and wettest year in Baghdad between 2005 and 2016. The distribution of properties in the middle (500 hPa) and upper (250 hPa) troposphere shows that the region was affected by intense horizontal divergence and upwards motions, coinciding with a surface low over the Arabian Peninsula that caused intense northwards winds over the Persian Gulf and brought substantial moisture to central Iraq. The analysis of several stability indexes indicates that convective instability played a secondary role during the episode.Peer ReviewedPostprint (author's final draft

Diapycnal Nutrient Fluxes in the Cape Ghir upwelling region

European Geosciences Union General Assembly 22-27 April 2012, Vienna, Austria.-- 1 pageAn oceanographic survey was carried out from 18 to 29 October 2010 in the Canary Basin (PROMECA project). Near Cape Ghir, in the Northwest Africa coastal upwelling, 17 CTD casts were made to obtain continuous records of conductivity and temperature with depth, and to collect waters samples for nutrients analyses. Additionally, free-fall turbulence profiles were obtained at each station. 14 Expandable bathythermographs (XBTs) were deployed between stations to increase the grid resolution of the temperature field. Velocity data were acquired with a vessel-mounted Acoustic Doppler Current Profiler (ADCP) with a vertical bin size of 8 m. Water samples for nutrients: nitrate + nitrite (N+N), phosphates and silicates, were collected from 12 depths (down to 2000 m or the maximum depth), with 12-l Niskin bottles mounted on the rosettes sampler. The first results show low to moderate concentrations of nutrients offshore, with average values in the upper 150 m of 2.45 ± 1.98, 0.37 ± 0.18 and 1.47 ± 0.94 μM for N+N, phosphate and silicate, respectively. However, for stations near the Cape Ghir upwelling filament or influenced by this feature, a significant increase in the concentration of nutrients (up to 10 µM of N+N at 200 m) was observed. We have estimated and compared the diapycnal nutrient flux in the region by using two different approaches. First, we used the dissipation rates of turbulent kinetic energy and thermal variance estimated from microstructure data acquired from turbulence profilers, and applied a model based on the dissipation ratio. This way we obtain net turbulence diffusivities in regions where there is an interaction of processes of double diffusion and turbulence induced by vertical shear of the flow. The second approach is based on obtaining diapycnal diffusivities with parameterizations of the gradient Richardson number and density ratioPeer Reviewe

Turbulence as a driver for vertical plankton distribution in the subsurface upper ocean

Vertical distributions of turbulent energy dissipation rates and fluorescence were measured simultaneously with a high-resolution micro-profiler in four different oceanographic regions, from temperate to polar and from coastal to open waters settings. High fluorescence values, forming a deep chlorophyll maximum (DCM), were often located in weakly stratified portions of the upper water column, just below layers with maximum levels of turbulent energy dissipation rate. In the vicinity of the DCM, a significant negative relationship between fluorescence and turbulent energy dissipation rate was found. We discuss the mechanisms that may explain the observed patterns of planktonic biomass distribution within the ocean mixed layer, including a vertically variable diffusion coefficient and the alteration of the cells’ sinking velocity by turbulent motion. These findings provide further insight into the processes controlling the vertical distribution of the pelagic community and position of the DCM

Carbon dynamics within cyclonic eddies: insights from a biomarker study

It is generally assumed that episodic nutrient pulses by cyclonic eddies into surface waters support a significant fraction of the primary production in subtropical low-nutrient environments in the northern hemisphere. However, contradictory results related to the influence of eddies on particulate organic carbon (POC) export have been reported. As a step toward understanding the complex mechanisms that control export of material within eddies, we present here results from a sediment trap mooring deployed within the path of cyclonic eddies generated near the Canary Islands over a 1.5-year period. We find that, during summer and autumn (when surface stratification is stronger, eddies are more intense, and a relative enrichment in CaCO3 forming organisms occurs), POC export to the deep ocean was 2-4 times higher than observed for the rest of the year. On the contrary, during winter and spring (when mixing is strongest and the seasonal phytoplankton bloom occurs), no significant enhancement of POC export associated with eddies was observed. Our biomarker results suggest that a large fraction of the material exported from surface waters during the late-winter bloom is either recycled in the mesopelagic zone or bypassed by migrant zooplankton to the deep scattering layer, where it would disaggregate to smaller particles or be excreted as dissolved organic carbon. Cyclonic eddies, however, would enhance carbon export below 1000 m depth during the summer stratification period, when eddies are more intense and frequent, highlighting the important role of eddies and their different biological communities on the regional carbon cycle

Life history of an anticyclonic eddy

19 pages, 11 figures, 1 table, supporting information https://doi.org/10.1029/2004JC002526We use the trajectory of three buoys dragged below the surface mixed layer, together with sea surface temperature imagery, to examine the evolution of an anticyclonic warm-core eddy since its generation by the Canary Islands. Two buoys remain within the eddy during some 100 days, and the third one remains almost 200 days, while drifting southwestward up to 500 km with the mean Canary Current. The eddy merges with several younger anticyclonic and cyclonic eddies, in each occasion, suffering substantial changes. The eddy core, defined as a region with near-solid-body-type rotation and radial convergence, initially occupies the whole eddy. After interacting with another vortex the inner core markedly slows down, although it continues displaying radial convergence and relatively small radial oscillations, and an uncoupled outer ring is formed or enhanced, which revolves even more slowly and displays large radial fluctuations. The vortex extensive life is consistent with its inertially stable character and observations of radial convergence. A very simple model of vortex merging, where cylinders fuse conserving mass and angular momentum, gives fair results. The observations suggest that the eddy changes, as the result of its own slow evolution and sporadic mixing events, from a young stage, where the core retains its vorticity and occupies most of the eddy, through a mature stage, where the eddy has a reduced inner core and a slowly revolving outer ring, to a decay stage, where the vorticity maximum is substantially reducedThis work has been supported by the Spanish government through projects FRENTES (AMB95-0731), COCA (REN2000-1471-C02-02MAR), and BREDDIES (REN2001-2650/ANT) and the special action MAR1999-1489-E and by the European Union through projects CANIGO (MAS3-CT96-0060) and OASIS (EVK3-CT-2002-00073-OASIS)Peer Reviewe

Experiments on layer formation in stratified shear flow

We discuss the experimental methodology and present the results of several experiments on the onset and growth of instabilities in stratified shear flow. Our results include the assessment of the effective diffusivity prior to the development of the Kelvin-Helmholtz (K-H) type instability and the measurement of the velocity field. This allows us to determine that the onset of the instability takes place when the Richardson number is close to 0.25, this value slightly decreasing with increasing initial interface thickness. The formation of a fine layered structure at the interface is suggested by density profiles taken immediately after the generation of K-H billows, but the size of the full interface as well as the layers themselves decrease in the following few seconds. We hypothesize that the time scale of the K-H billows in our laboratory experiments is so short that there is no time for added mixing to further distribute and smoothen the initial layer structureThis work has been supported by the Spanish government throught CICYT´s grant number MAR96-1893Peer reviewe

Level of proficiency and professional development needs in peripheral online teaching roles

Teaching in virtual environments demands mastery of several teaching competencies. Although the most accepted ones are pedagogical, in order to successfully teach online it becomes necessary to acquire and develop some other competencies, sometimes referred to as peripheral roles (Denis et al., 2004). The aim of this study is to analyse perceptions on the level of proficiency that online teachers have regarding these peripheral roles (social, evaluator, manager, technologist, advisor/counsellor, personal, and researcher), and their professional development needs required to improve their online teaching competencies. A questionnaire was specifically created and validated by experts, and data was gathered from 166 university teachers. The findings show that teachers highlight the importance of the peripheral roles for quality teaching, and thus, professional development programmes should be based on a balance between central and peripheral roles to better train online teachers and increase the quality of their teaching

Sensitivity of an idealized subtropical gyre to the eastern boundary conditions

The flow pattern of the North Atlantic Subtropical Gyre (NASG) is simulated using a highly idealised one-layer quasi-geostrophic wind-driven model. The novel feature of the model is the specification of the eastern boundary conditions. This is an upwelling favourable region with a quasi-permanent southward flowing coastal jet, which is fed by the eastern branch of the Canary Current. The corresponding boundary conditions are non-zero normal flux and constant potential vorticity, the latter being consistent with the generation of anticyclonic vorticity by the coastal jet. We examine the sensitivity of the model to the eastern boundary conditions and compare the results with recent observations for the regionThis work was supported by the European Union through project CANIGO (MAS3-CT96-1893) and the Spanish government trhough projects FRENTES (AMB95-0731) and TALUD (MAR96-1893)Peer reviewe

Rethinking the gradient Richardson number

IV Congress of Marine Sciences, Encuentro de la Oceanografía Física Española (EOF 2014), 11-13 June 2014, Las Palmas de Gran Canaria.-- 1 pageThe gradient Richardson number, Ri, is the classical index for dynamic vertical instability. It is generally viewed as a ratio between the stabilizing effects of buoyancy versus the destabilizing role of the (squared) vertical shear. The gradient Richardson number is small (subcritical when its value is less than one) when stratification is small enough and/or vertical shear is sufficiently large; under these circumstances the flow becomes unstable, prone to mixing. In this communication we carefully explore an alternative perspective, which arises in isopycnic coordinates: the gradient Richardson number is now a ratio between the inverse of vertical stratification and the (squared) shear in density coordinates (named the diapycnal shear). From this point of view the flow becomes unstable in well stratified conditions as long as the diapycnal shear remains moderately large (Pelegrí and Csanady, 1994; Pelegrí and Sangrà, 1998; Pelegrí et al., 1998). One important limitation of Ri, as an indicator of mixing, is that it cannot differentiate between mixing in stratified regions versus flow instability in already well-mixed waters. The isopycnic approach suggests that diapycnal shear is a most relevant variable for flow stability, yet it alone cannot assess the existence of unstable conditions. Therefore, we rewrite the instability condition as a reduced squared diapycnal shear, which is a function of both Ri and the stratification, and decreases monotonically with stratification. The above concepts are illustrated using data from three distinct regions: the shelf break south of Gran Canaria, the Gulf Stream and the Mediterranean outflow. It turns out that very often for Gran Canaria and the Mediterranean outflow, and only very rarely for the Gulf Stream, the conditions are subcritical. The variables are non-dimensionalized by means of the background stratification. The vertical shear, diapycnal shear and the reduced squared diapycnal shear are then plotted, as cloud points, as a function of stratification. The results confirm a dependence of the squared reduced diapycnal on stratification, which is characteristic for each particular flow dynamicsPeer Reviewe