scholarly journals A Climatology of Strong Large-Scale Ocean Evaporation Events. Part I: Identification, Global Distribution, and Associated Climate Conditions

2018 ◽  
Vol 31 (18) ◽  
pp. 7287-7312 ◽  
Author(s):  
Franziska Aemisegger ◽  
Lukas Papritz
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Past History ◽  
Spatiotemporal Variability ◽  
Western Boundary ◽  
Near Surface ◽  
Cold Air ◽  
The North ◽  
South Indian ◽  
The North Atlantic

This paper presents an object-based, global climatology (1979–2014) of strong large-scale ocean evaporation (SLOE) and its associated climatic properties. SLOE is diagnosed using an “atmospheric moisture uptake efficiency” criterion related to the ratio of surface evaporation and integrated water vapor content in the near-surface atmosphere. The chosen Eulerian identification procedure focuses on events that strongly contribute to the available near-surface atmospheric humidity. SLOE is particularly frequent along the warm ocean western boundary currents, downstream of large continental areas, and at the sea ice edge in polar regions with frequent cold-air outbreaks. Furthermore, wind-driven SLOE occurs in regions with topographically enforced winds. On a global annual average, SLOE occurs only 6% of the time but explains 22% of total ocean evaporation. An analysis of the past history and fate of air parcels involved in cold season SLOE in the North Atlantic and south Indian Oceans shows that cold-air advection is the main mechanism that induces these events. Extratropical cyclones thereby play an important role in setting the necessary equatorward synoptic flow. Consequently, the interannual variability of SLOE associated with the North Atlantic Oscillation and the southern annular mode reveals a very high sensitivity of SLOE with respect to the location of the storm tracks. This study highlights the strong link between transient synoptic events and the spatiotemporal variability in ocean evaporation patterns, which cannot be deduced from thermodynamic steady-state and climate mean state considerations alone.

scholarly journals Stochastic Forcing of Ocean Variability by the North Atlantic Oscillation

2009 ◽  
Vol 39 (1) ◽  
pp. 162-184 ◽  
Author(s):  
Kettyah C. Chhak ◽  
Andrew M. Moore ◽  
Ralph F. Milliff
Keyword(s):  
North Atlantic Oscillation ◽  
Ocean Circulation ◽  
Rossby Wave ◽  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
Stochastic Forcing ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract At middle and high latitudes, the magnitude of stochastic wind stress forcing of the ocean by atmospheric variability on synoptic time scales (i.e., “weather” related variability) is comparable to that of the seasonal cycle. Stochastic forcing may therefore have a significant influence on the ocean circulation, climate, and ocean predictability. Here, the influence of stochastic forcing associated with the North Atlantic Oscillation on the subtropical gyre circulation of the North Atlantic is explored in an eddy-permitting quasigeostrophic framework. For the North Atlantic winds used in this study, the root-mean-square of the annual average Ekman pumping velocity of the seasonal cycle between 35° and 52°N is 1.3 × 10−7 m s−1, while the wintertime standard deviation of the stochastic component of the North Atlantic Oscillation over the same latitude band is 2.2 × 10−7 m s−1. Significant stochastically induced variability in the ocean circulation occurs near the western boundary region and along the western margins of the abyssal plains associated with vortex stretching, energy release from the mean flow, and the generation of topographic Rossby waves. Variability arises from a combination of two effects, depending on the measure of variance used: growth of unstable modes of the underlying circulation and modal interference resulting from their nonnormal nature, which dominates during the first 10 days or so of perturbation growth. Near the surface, most of the variability is associated with large-scale changes in the barotropic circulation, although more than 20% of the energy and enstrophy variability is associated with small-scale baroclinic waves. In the deep ocean, much of the stochastically induced variability is apparently due to topographic Rossby wave activity along the continental rise and ocean ridges. Previous studies have demonstrated that rectification of topographic Rossby wave–induced circulations in the western North Atlantic may contribute to the western boundary current recirculation zones. The authors suggest that a source of topographic Rossby wave energy, significant enough to rectify the mean ocean circulation, may arise from stochastic forcing by large-scale atmospheric forcing, such as the North Atlantic Oscillation and other atmospheric teleconnection patterns.

The Various Aspects of the Large- scale Atmospheric Circulation Response to the Northern Hemispheric Ocean Western Boundary Currents

2020 ◽  
Author(s):  
Nour-Eddine Omrani ◽  
Fumiaki Ogawa ◽  
Hisashi Nakamura ◽  
Noel Keenlyside ◽  
Sandro Lubis ◽  
...  
Keyword(s):  
Northern Hemisphere ◽  
North Atlantic ◽  
Western Boundary ◽  
Significant Implication ◽  
Western Boundary Currents ◽  
Near Surface ◽  
Boundary Currents ◽  
The North ◽  
The North Atlantic ◽  
The Impact

<p>Semi-idealized Atmospheric General Circulation-Model (AGCM) experiments are used, in order to study the different aspects of the hemisphere-scale wintertime troposphere/stratosphere-coupled circulation that are maintained by the North Atlantic and Pacific Ocean Western Boundary Currents (OWBCs). Here we show that the North Atlantic and Pacific OWBCs jointly maintain and shape the wintertime hemispheric circulation and its leading mode of variability Northern Annular Mode (NAM). The OWBCs energize baroclinic waves that reinforce quasi-annular hemispheric structure in the tropospheric eddy-driven jetstreams and NAM variability. Without the OWBCs, the wintertime NAM variability is much weaker and its impact on the continental and maritime surface climate is largely insignificant. Atmospheric energy redistribution caused by the OWBCs acts to damp the near-surface atmospheric baroclinicity and compensates the associated oceanic meridional energy transport in agreement with the Bjerknes compensation. Furthermore, the OWBCs substantially weaken the wintertime stratospheric polar vortex by enhancing the upward planetary wave propagation, and thereby affecting both stratospheric and tropospheric NAM-annularity. It is shown that the impact of OWBCs on northern hemisphere circulation has significant implication for stratosphere/troposphere dynamical coupling, time-scales on the NAM, frequency of Sudden stratospheric warming and potential formation of polar stratospheric clouds.</p><p> </p><p>Reference:</p><p>Omrani et al., 2019: Key Role of the ocean Western Boundary currents in shaping the Northern Hemisphere climate, Scientific Reports, https://doi.org/10.1038/s41598-019-39392-y</p><p> </p>

scholarly journals Generation of Internal Waves by Eddies Impinging on the Western Boundary of the North Atlantic

2016 ◽  
Vol 46 (4) ◽  
pp. 1067-1079 ◽  
Author(s):  
L. Clément ◽  
E. Frajka-Williams ◽  
K. L. Sheen ◽  
J. A. Brearley ◽  
A. C. Naveira Garabato
Keyword(s):  
Internal Waves ◽  
North Atlantic ◽  
High Frequency ◽  
Large Scale ◽  
Acoustic Doppler Current Profiler ◽  
Relative Vorticity ◽  
Western Boundary ◽  
The North ◽  
Current Profiler ◽  
The North Atlantic

AbstractDespite the major role played by mesoscale eddies in redistributing the energy of the large-scale circulation, our understanding of their dissipation is still incomplete. This study investigates the generation of internal waves by decaying eddies in the North Atlantic western boundary. The eddy presence and decay are measured from the altimetric surface relative vorticity associated with an array of full-depth current meters extending ~100 km offshore at 26.5°N. In addition, internal waves are analyzed over a topographic rise from 2-yr high-frequency measurements of an acoustic Doppler current profiler (ADCP), which is located 13 km offshore in 600-m deep water. Despite an apparent polarity independence of the eddy decay observed from altimetric data, the flow in the deepest 100 m is enhanced for anticyclones (25.2 cm s−1) compared with cyclones (−4.7 cm s−1). Accordingly, the internal wave field is sensitive to this polarity-dependent deep velocity. This is apparent from the eddy-modulated enhanced dissipation rate, which is obtained from a finescale parameterization and exceeds 10−9 W kg−1 for near-bottom flows greater than 8 cm s−1. The present study underlines the importance of oceanic western boundaries for removing the energy of low-mode westward-propagating eddies to higher-mode internal waves.

scholarly journals Detecting flow features in scarce trajectory data using networks derived from symbolic itineraries: an application to surface drifters in the North Atlantic

2020 ◽  
Vol 27 (4) ◽  
pp. 501-518 ◽  
Author(s):  
David Wichmann ◽  
Christian Kehl ◽  
Henk A. Dijkstra ◽  
Erik van Sebille
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
Boundary Current ◽  
Trajectory Data ◽  
Surface Transport ◽  
The North ◽  
The North Atlantic ◽  
Surface Drifters ◽  
Double Gyre

Abstract. The basin-wide surface transport of tracers such as heat, nutrients and plastic in the North Atlantic Ocean is organized into large-scale flow structures such as the Western Boundary Current and the Subtropical and Subpolar gyres. Being able to identify these features from drifter data is important for studying tracer dispersal but also for detecting changes in the large-scale surface flow due to climate change. We propose a new and conceptually simple method to detect groups of trajectories with similar dynamical behaviour from drifter data using network theory and normalized cut spectral clustering. Our network is constructed from conditional bin-drifter probability distributions and naturally handles drifter trajectories with data gaps and different lifetimes. The eigenvalue problem of the respective Laplacian can be replaced by a singular value decomposition of a related sparse data matrix. The construction of this matrix scales with O(NM+Nτ), where N is the number of particles, M the number of bins and τ the number of time steps. The concept behind our network construction is rooted in a particle's symbolic itinerary derived from its trajectory and a state space partition, which we incorporate in its most basic form by replacing a particle's itinerary by a probability distribution over symbols. We represent these distributions as the links of a bipartite graph, connecting particles and symbols. We apply our method to the periodically driven double-gyre flow and successfully identify well-known features. Exploiting the duality between particles and symbols defined by the bipartite graph, we demonstrate how a direct low-dimensional coarse definition of the clustering problem can still lead to relatively accurate results for the most dominant structures and resolve features down to scales much below the coarse graining scale. Our method also performs well in detecting structures with incomplete trajectory data, which we demonstrate for the double-gyre flow by randomly removing data points. We finally apply our method to a set of ocean drifter trajectories and present the first network-based clustering of the North Atlantic surface transport based on surface drifters, successfully detecting well-known regions such as the Subpolar and Subtropical gyres, the Western Boundary Current region and the Caribbean Sea.

Marine cold air outbreaks in Fram Strait – General increase in March and a special case in 2020

2021 ◽  
Author(s):  
Sandro Dahlke ◽  
Amelie Solbes ◽  
Marion Maturilli
Keyword(s):  
Sea Ice ◽  
North Atlantic ◽  
Potential Temperature ◽  
Fram Strait ◽  
Near Surface ◽  
Cold Air ◽  
The North ◽  
The North Atlantic ◽  
Ice Edge ◽  
Cold Air Outbreaks

<p>Marine Cold Air Outbreaks (MCAOs) are common features above the open water surfaces of the Nordic Seas. They are characterized by marked vertical temperature gradients, which typically persist over several days, and strongly shape air-sea heat exchanges, convection, weather and boundary layer characteristics in the affected region. Based on the novel ERA-5 reanalysis product, we are analyzing climatological and recent aspects of MCAOs in the Fram Strait region of the North Atlantic, which is a “hot spot” particularly during winter and early spring. MCAOs in Fram Strait occur preferably when persistent low pressure systems occupy Northern Scandinavia and the Barents/Kara Sea, which exerts strong zonal pressure gradients across Fram Strait. Based on the vertical gradients of potential temperature, occurrence frequencies of MCAOs of different strengths are investigated.  It is found that MCAOs of moderate strength occur at an average of 7-9 days per month between December and March, while especially strong MCAOs occur at an average of 1-3 days in that time. Regarding the former, March is the only month for which a significant trend of +1.7 days/month/decade was found over the 1979-2020 period. While regional MCAO expression is dependent on both the relative location of the ice edge and on the atmospheric circulation, MCAO increase in Fram Strait in March can be explained mainly with the latter and the associated zonal pressure gradient.</p><p>February and March 2020 serve as examples of particularly strong and persistent MCAOs in Fram Strait. The record-breaking strong polar vortex at that time, which had received global attention in the media and literature, had left its associated footprint in near surface and tropospheric circulation fields, hence providing anomalous northerly flow across the ice edge in Fram Strait. While this clearly shaped MCAOs in Fram Strait, associated anomalies were also observed in the North Atlantic Sea Ice edge, and were even detected in upper air profiles and sea ice conditions on Svalbard.</p><p>For the detailed study of such northerly advection events, atmospheric data gathered during the year-long MOSAiC expedition 2019/2020 in the central Arctic are expected to provide valuable information in the upstream direction of the anomalies in Fram Strait.</p>

scholarly journals Spatiotemporal variability of lightning activity in Europe and the relation to the North Atlantic Oscillation teleconnection pattern

2017 ◽  
Vol 17 (8) ◽  
pp. 1319-1336 ◽  
Author(s):  
David Piper ◽  
Michael Kunz
Keyword(s):  
North Atlantic Oscillation ◽  
North Atlantic ◽  
Large Scale ◽  
Lightning Activity ◽  
Spatiotemporal Variability ◽  
Convective Activity ◽  
Diurnal Cycles ◽  
The North ◽  
The North Atlantic ◽  
The North Atlantic Oscillation

Abstract. Comprehensive lightning statistics are presented for a large, contiguous domain covering several European countries such as France, Germany, Austria, and Switzerland. Spatiotemporal variability of convective activity is investigated based on a 14-year time series (2001–2014) of lightning data. Based on the binary variable thunderstorm day, the mean spatial patterns of lightning activity and regional peculiarities regarding seasonality are discussed. Diurnal cycles are compared among several regions and evaluated with respect to major seasonal changes. Further analyses are performed regarding interannual variability and the impact of teleconnection patterns on convection. Mean convective activity across central Europe is characterized by a strong northwest-to-southeast gradient with pronounced secondary features superimposed. The zone of maximum values of thunderstorm days propagates southwestward along the southern Alpine range from April to July. Diurnal cycles vary substantially between both different months and regions, particularly regarding the incidence of nighttime lightning. The North Atlantic Oscillation (NAO) is shown to have a significant impact on convective activity in several regions, which is primarily caused by variations of the large-scale lifting pattern in both NAO phases. This dynamical effect is partly compensated for by thermodynamical modifications of the pre-convective environment. The results point to a crucial role of large-scale flow in steering the spatiotemporal patterns of convective activity.

scholarly journals Detecting flow features in scarce trajectory data using networks derived from symbolic itineraries: an application to surface drifters in the North Atlantic

2020 ◽  
Author(s):  
David Wichmann ◽  
Christian Kehl ◽  
Henk A. Dijkstra ◽  
Erik van Sebille
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Western Boundary ◽  
Boundary Current ◽  
Trajectory Data ◽  
Surface Transport ◽  
The North ◽  
The North Atlantic ◽  
Surface Drifters ◽  
Double Gyre

Abstract. The basinwide surface transport of tracers such as heat, nutrients and plastic in the North Atlantic Ocean is organized into large scale flow structures such as the Western Boundary Current and the Subtropical and Subpolar Gyres. Being able to identify these features from drifter data is important for studying tracer dispersal, but also to detect changes in the large scale surface flow due to climate change. We propose a new and conceptually simple method to detect groups of trajectories with similar dynamical behaviour from drifter data using network theory and normalized cut spectral clustering. Our network is constructed from conditional bin-drifter probability distributions and naturally handles drifter trajectories with data gaps and different lifetimes. The eigenvalue problem of the respective Laplacian can be replaced by a singular value decomposition of a related sparse data matrix. The construction of this matrix scales with O(NM + Nτ), where N is the number of particles, M the number of bins and τ the number of time steps. The concept behind our network construction is rooted in a particle's symbolic itinerary derived from its trajectory and a state space partition, which we incorporate in its most basic form by replacing a particle's itinerary by a probability distribution over symbols. We represent these distributions as the links of a bipartite graph, connecting particles and symbols. We apply our method to the periodically driven double-gyre flow and successfully identify well-known features. Exploiting the duality between particles and symbols defined by the bipartite graph, we demonstrate how a direct low-dimensional coarse definition of the clustering problem can still lead to relatively accurate results for the most dominant structures, and resolve features down to scales much below the coarse graining scale. Our method also performs well in detecting structures with incomplete trajectory data, which we demonstrate for the double-gyre flow by randomly removing data points. We finally apply our method to a set of ocean drifter trajectories and present the first network-based clustering of the North Atlantic surface transport based on surface drifters, successfully detecting well-known regions such as the Subpolar and Subtropical Gyres, the Western Boundary Current region and the Carribean Sea.

scholarly journals A new method to assess mesoscale contributions to meridional heat transport in the North Atlantic Ocean

Ocean Science ◽  
2020 ◽  
Vol 16 (4) ◽  
pp. 979-995
Author(s):  
Andrew Delman ◽  
Tong Lee
Keyword(s):  
Heat Transport ◽  
North Atlantic ◽  
Large Scale ◽  
Spatial Scales ◽  
Western Boundary ◽  
Meridional Heat Transport ◽  
The North ◽  
Basin Scale ◽  
Wide Range ◽  
The North Atlantic

Abstract. The meridional heat transport (MHT) in the North Atlantic is critically important to climate variability and the global overturning circulation. A wide range of ocean processes contribute to North Atlantic MHT, ranging from basin-scale overturning and gyre motions to mesoscale instabilities (such as eddies). However, previous analyses of “eddy” MHT in the region have mostly focused on the contributions of time-variable velocity and temperature, rather than considering the association of MHT with distinct spatial scales within the basin. In this study, a zonal spatial-scale decomposition separates large-scale from mesoscale velocity and temperature contributions to MHT, in order to characterize the physical processes driving MHT. Using this approach, we found that the mesoscale contributions to the time-mean and interannual/decadal (ID) variability of MHT in the latitude range 39–45∘ N are larger than large-scale horizontal contributions, though smaller than the overturning contributions. Considering the 40∘ N transect as a case study, large-scale ID variability is mostly generated close to the western boundary. In contrast, most ID MHT variability associated with mesoscales originates in two distinct regions: a western boundary region (70–60∘ W) associated with 1- to 4-year interannual variations and an interior region (50–35∘ W) associated with decadal variations. Surface eddy kinetic energy is not a reliable indicator of high MHT episodes, but the large-scale meridional temperature gradient is an important factor, by influencing the local temperature variance as well as the local correlation of velocity and temperature. Most of the mesoscale contribution to MHT at 40∘ N is associated with transient and propagating processes, but stationary mesoscale structures explain most of the mesoscale MHT south of the Gulf Stream separation, highlighting the differences between the temporal and spatial decomposition of meridional temperature fluxes.

scholarly journals Challenges for implementing the Marine Strategy Framework Directive in a climate of macroecological change

2012 ◽  
Vol 370 (1980) ◽  
pp. 5636-5655 ◽  
Author(s):  
Abigail McQuatters-Gollop
Keyword(s):  
North Atlantic ◽  
Large Scale ◽  
Marine Ecosystem ◽  
Ecological Change ◽  
Marine Strategy Framework Directive ◽  
Near Surface ◽  
The North ◽  
Basin Scale ◽  
The North Atlantic ◽  
European Seas

Unprecedented basin-scale ecological changes are occurring in our seas. As temperature and carbon dioxide concentrations increase, the extent of sea ice is decreasing, stratification and nutrient regimes are changing and pH is decreasing. These unparalleled changes present new challenges for managing our seas, as we are only just beginning to understand the ecological manifestations of these climate alterations. The Marine Strategy Framework Directive requires all European Member States to achieve good environmental status (GES) in their seas by 2020; this means management towards GES will take place against a background of climate-driven macroecological change. Each Member State must set environmental targets to achieve GES; however, in order to do so, an understanding of large-scale ecological change in the marine ecosystem is necessary. Much of our knowledge of macroecological change in the North Atlantic is a result of research using data gathered by the Continuous Plankton Recorder (CPR) survey, a near-surface plankton monitoring programme that has been sampling in the North Atlantic since 1931. CPR data indicate that North Atlantic and North Sea plankton dynamics are responding to both climate and human-induced changes, presenting challenges to the development of pelagic targets for achievement of GES in European Seas. Thus, the continuation of long-term ecological time series such as the CPR survey is crucial for informing and supporting the sustainable management of European seas through policy mechanisms.

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