Search

Currents and pressure records from the DeepLev mooring station and drifter data in the eastern Levantine Basin were analyzed to identify the dominant tidal constituents and their seasonal and depth variability. Harmonic and spectral analysis of seasonal segments of currents and pressure reveal key attributes of the tidal regime: (1) dominant semidiurnal sea level variability; (2) seasonal variation of semidiurnal and diurnal tides in both currents and pressure datasets; and (3) significant diurnal currents with weak semidiurnal currents across all seasons. The most dominant tidal constituent from the pressure dataset is the M2 (12.4 h). Results from pressure datasets align with previous models and observations of semidiurnal tides. In contrast, the diurnal tides are larger than previously reported by 8--9 cm in the winter and 1--2 cm in the summer. The surface current tidal regime differs from prior reports in the eastern Levantine Basin, with M2 magnitudes weaker by 1 cm s-1, while the diurnal tides (K1, O1) are 1--2 cms-1 larger. Seasonal segments showed seasonal differences in the local tidal regime's amplitudes. The most pronounced seasonal differences were with the K1 and S2 tides, with differences between winter and fall of 7 cmfor the K1 and 4 cm between summer and fall for the S2. We utilized the DeepLev datasets to compare a moored device with surface drifters near DeepLev by analyzing the M2 and S2 tides. Additionally, we examined data at different dataset lengths, considering the time constraints needed to resolve the tides adequately. Longer datasets improved the resolution of the tidal analysis and reduced amplitude leakages from nearby frequencies, resulting in a more realistic and accurate analysis of the tidal currents. Conversely, longer datasets resulted in fewer drifters remaining our study region for the allotted dataset length. From 32 available segments in 15 day dataset to 7 with a 30 day dataset, reducing the effectiveness of drifters as a local tidal research tool. [ABSTRACT FROM AUTHOR]

The Mediterranean Sea releases approximately 1 Sv of water into the North Atlantic through the Gibraltar Straits, forming the saline Mediterranean Outflow Water (MOW). Its impact on large‐scale flow and specifically its northbound Lagrangian pathways are widely debated, yet a comprehensive overview of MOW pathways over recent decades is lacking. We calculate and analyze synthetic Lagrangian trajectories in 1980–2020 reanalysis velocity data. Sixteen percent of the MOW follow a direct northbound path to the sub‐polar gyre, reaching a 1,000 m depth crossing window at the southern tip of Rockall Ridge in about 10 years. Surprisingly, time‐dependent chaotic advection, not steady currents, drives over half of the northbound transport. Our results suggest a potential 15–20 years predictability in the direct northbound transport. Additionally, monthly variability appears more significant than inter‐annual variability in Lagrangian mixing and spreading the MOW. Plain Language Summary: The Mediterranean Sea and the North Atlantic Ocean sustain a constant exchange of water through the shallow, narrow Straits of Gibraltar. Due to intense evaporation, the Mediterranean water is saltier and denser than its Atlantic counterpart and flows out of the Mediterranean Sea into the middepth North Atlantic Ocean to create the Mediterranean Outflow Water (MOW). This salty input reaches 1,000 m depth and spreads and mixes into the North Atlantic Ocean, and is thought to have a non‐negligible impact on its flow regime, specifically through its northbound transport and direct contribution to the salinity of the northern water of the North Atlantic. However, a comprehensive survey of the various pathways taken by the MOW is lacking. In this work, we track the 3D pathways of virtual trajectories starting at the Gibraltar Straits at various depths for 20 years. We find that 16% of the particles take a direct northbound path. Surprisingly, over half of this transport is not due to steady currents that lead the MOW to the north. Rather, it is a result of time‐dependent chaotic advection, a phenomenon that allows pathways in a time‐dependent flow to circumvent stationary barriers of transport. Key Points: Synthetic Lagrangian trajectories initialized at the Gibraltar Straits, based on SODA3.4.2 reanalysis velocity data from 1980 to 2020, are used to study the pathways, Lagrangian mixing, and spreading of the Mediterranean Outflow Water (MOW)16% of the MOW particles take a direct northbound routeOver half of the direct northbound transport results from time‐dependent chaotic advection and not from a steady northbound current [ABSTRACT FROM AUTHOR]

Processed and analyzed sea surface wave characteristics derived from an up-looking acoustic Doppler current profiler (ADCP) for the period 2016–2022 are presented as a dataset available from the public open-access repository of SEA scieNtific Open data Edition (SEANOE) at 10.17882/96904. The collected data include full two-dimensional wave fields, along with computed bulk parameters, such as wave heights, periods, and directions of propagation. The ADCP was mounted on the submerged Deep Levantine (DeepLev) mooring station located 50 km off the Israeli coast to the west of Haifa (bottom depth ∼1470 m). It meets the need for accurate and reliable in situ measurements in the eastern Mediterranean Sea as the area significantly lacks wave data compared to other Mediterranean sub-basins. The developed long-term time series of wave parameters contribute to the monitoring and analysis of the region's wave climate and the quality of wind–wave forecasting models. [ABSTRACT FROM AUTHOR]

Accurately quantifying air–sea heat and gas exchange is crucial for comprehending thermoregulation processes and modeling ocean dynamics; these models incorporate bulk formulae for air–sea exchange derived in unstable atmospheric conditions. Therefore, their applicability in stable atmospheric conditions, such as desert-enclosed basins in the Gulf of Eilat/Aqaba (coral refugium), Red Sea, and Persian Gulf, is unclear. We present 2-year eddy covariance results from the Gulf of Eilat, a natural laboratory for studying air–sea interactions in stable atmospheric conditions, which are directly related to ocean dynamics. The measured mean evaporation, 3.22 myr-1 , approximately double that previously estimated by bulk formulae, exceeds the heat flux provided by radiation. Notably, in arid environments, the wind speed seasonal trend drives maximum evaporation in summer, with a minimum winter rate. The higher evaporation rate appears when elevated wind, particularly in the afternoon, coincides with an increase in vapor pressure difference. The inability of the bulk formulae approach to capture the seasonal (opposite from our measurements) and annual trend of evaporation is linked to errors in quantifying the atmospheric boundary layer stability parameter. Most of the year, there is a net cooling effect of surface water (- 79 Wm-2), primarily through evaporation. The substantial heat deficit is compensated by the advection of heat via northbound currents from the Red Sea, which we indirectly quantify from energy balance considerations. Cold and dry synoptic-scale winds induce extreme heat loss through air–sea fluxes and are correlated with the destabilization of the water column during winter and initiation of vertical water-column mixing. [ABSTRACT FROM AUTHOR]

The modification of the dynamics on the f-plane due to the inclusion of the latitudinal variation of the Coriolis frequency, quantified by β, is examined by comparing steady and time-dependent solutions for different values of β, including 0. The comparisons are made for analytic expressions and numerical simulations of the solutions of two physical problems: geostrophic adjustment and Ekman transport. By varying the extent of the meridional domain, L, and the value of β, while keeping the same value of the deformation radius, Rd, we find that the relative change of the Coriolis frequency across L is not a uniformly valid predictor of the effect of β on the dynamics. Instead, the magnitude of this effect varies with the physical problem, the type of dynamics, i.e., steady or time dependent, and with the particular values of β and L. In contrast to the effect of β, the effect of curvature on the zonally invariant f-plane dynamics is negligible in all cases for L ≤ 1200 km and R d = 30 km employed here. [ABSTRACT FROM AUTHOR]

The isotopic ratio 18O/16O of dissolved O2 in aquatic systems is affected by the preferential biological uptake of 16O (ε). Studies over the past six decades reveal that during incubation experiments, the isotopic effect of microorganism respiration (εorganism) varies in the range of −18‰ to −22‰. In contrast, natural variations in the deep‐ocean O2 concentration and δ18O levels show a considerably weaker effect (~ −10‰). The differences between these observations have been explained to result from either O2 uptake by sediments or organic particles that, due to diffusion‐limited respiration, are expected to weakly fractionate oxygen isotopes, by mixing processes or by weak fractionation at low temperatures. To gain better insight, we studied oxygen demand and δ18O in the deep, cold hypolimnion of Lake Stechlin between 2018 and 2021 as well as in various laboratory incubations. Our incubation results demonstrate an εorganism of about −24‰. Simple model calculations demonstrate a sediment O2 demand isotope effect (εSOD) of about −8.4‰, and a variated water‐column O2 demand isotope effect (εWOD) which is lower than εorganism, ranging from −13.9‰ in 2019 to −23‰ in 2021. Accompanying experiments indicate that the lower magnitude of εWOD may be related to respiration at organic particles lending to a weaker fractionation effect. Thus, variations in εWOD may reflect a changing partitioning of hypolimnion oxygen uptake between suspended particles and their containing microorganisms. Based on own incubation experiments with Daphnia carcasses, we discuss how possible changes of particle rigidity might influence εWOD. [ABSTRACT FROM AUTHOR]

Processed and analyzed sea surface wave characteristics derived from an up-looking Acoustic Doppler Current Profiler (ADCP) for the period 2016-2022 are presented as a dataset. The collected data include full two-dimensional wave fields along with computed bulk parameters, such as wave heights, periods, and directions of propagation. The ADCP was mounted on the submerged Deep Levantine mooring station located 50 km off the Israeli coast to the west of Haifa (bottom depth ~1470m). It meets the need for accurate and reliable in situ measurements in the Eastern Mediterranean Sea, as the area significantly lacks wave data compare to other Mediterranean sub-basins. The developed long term timeseries of wave parameters allow for monitoring and analysis of the region's wave climate, as well as a deeper understanding of wind-wave generation mechanisms. [ABSTRACT FROM AUTHOR]

The Cretaceous 'greenhouse' period (~145 to ~66 million years ago, Ma) in Earth's history is relatively well documented by multiple paleoproxy records, which indicate that the meridional sea surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO2 concentration, solar constant, and paleogeography are the primary drivers of variations in the spatiotemporal distribution of SST. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to compare the relative effects of atmospheric CO2 variability and paleogeographic changes on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the Northern Hemisphere paleogeography weaken the circulation in subtropical ocean gyres, leading to an increase in extratropical SSTs. Simulations using a coupled atmosphere-ocean model show that paleogeography-driven reduction in the intensity of surface ocean circulation explains much of the increase in the mid-latitudinal sea surface temperature gradient during the Cretaceous. [ABSTRACT FROM AUTHOR]

Analytical and numerical solutions of the Linearized Rotating Shallow Water Equations are combined to study the geostrophic adjustment on the mid-latitude β -plane. The adjustment is examined in zonal periodic channels of width Lᵧ = 4 Rd ('narrow' channel, where Rd is the radius of deformation) and Lᵧ =60 Rd ('wide' channel) for the particular initial conditions of a resting fluid with a step-like height distribution, η ₀. In the one-dimensional case, where η ₀ = η ₀(y) we find that: (i) β affects the geostrophic state (determined from the conservation of the meridional vorticity gradient) only when b = cot (Φ ₀) Rd / R ≥ 0.5 (where Φ ₀ is the channel's central latitude and R is Earth's radius); (ii) The energy conversion ratio varies by less than 10 % when b increases from 0 to 1; (iii) In 'wide' channels, β affects the waves significantly even for small b (e.g. b = 0.005). (iv) For b = 0.005, harmonic waves approximate the waves in 'narrow' channels, and trapped waves approximate the waves in 'wide' channels. In the two-dimensional case, where η ₀ = η ₀(x) we find that: (i) At short times the spatial structure of the steady solution is similar to that on the f -plane, while at long times the steady state drifts westward at the speed of Rossby waves – harmonic Rossby waves in 'narrow' channels and trapped Rossby waves in 'wide' channels; (ii) In 'wide' channels, trapped wave dispersion causes the equatorward segment of the wavefront to move faster than the northern segment; (iii) The energy of Rossby waves on the β -plane approaches that of the steady-state on the f -plane; (iv) The results outlined in (iii) and (iv) of the one-dimensional case also hold in the two-dimensional case. [ABSTRACT FROM AUTHOR]

Anthropogenic‐induced variations of the Atlantic Meridional Overturning Circulation (AMOC) and the associated Deep‐Water Formation (DWF) are a major concern. Using measurements of triple oxygen isotopes in the deep North Atlantic, we present novel evidence for a dramatic decadal to centennial shift in ocean conditions at the source region of DWF. These measurements suggest a recent decrease in the percentage of photosynthetic O2 derived from the source regions of AMOC in the Nordic Seas compared to the Little Ice Age. 1‐D model simulations suggest that a reduction in photosynthetic O2 production can explain the observed decrease. Alternatively, it may indicate a substantial decrease in sea‐ice cover and thus increased air‐sea gas exchange, bringing the isotopic composition of O2 closer to equilibrium with the atmosphere. Our novel data can serve as a benchmark for climate models. Plain Language Summary: In this study, we use the isotopic signature of dissolved oxygen gas in the deep North Atlantic Ocean to demonstrate a recent dramatic change in the Nordic Seas. Water in the deep Atlantic at present was at the surface of the Nordic Seas several hundred years ago, during the Little Ice Age (LIA). The isotopic signature shows that the percentage of oxygen sourced from photosynthesis was greater during the LIA. This may indicate a greater rate of photosynthesis than at present. However, we know that when sea ice covers the ocean surface, it prevents the influx of atmospheric oxygen, while sunlight penetration through the ice keeps a high rate of photosynthesis and production of biological oxygen. Therefore, it is also possible that during the LIA, sea‐ice cover was much more extensive than today. Key Points: Oxygen isotopic evidence for a recent dramatic shift in the North Atlantic source region of deep water‐massesThe isotopic measurements suggest high extent of ice cover and/or higher primary production rates during the Little Ice AgeThese new observations can serve as a benchmark for climate models [ABSTRACT FROM AUTHOR]