Energy transfers between multidecadal and turbulent variability

AbstractOne of the proposed mechanisms to explain the multidecadal variability observed in sea surface temperature of the North Atlantic consists of a large-scale low-frequency internal mode spontaneously developing because of the large-scale baroclinic instability of the time-mean circulation. Even though this mode has been extensively studied in terms of the buoyancy variance budget, its energetic properties remain poorly known. Here we perform the full mechanical energy budget including available potential energy (APE) and kinetic energy (KE) of this internal mode and decompose the budget into three frequency bands: mean, low frequency (LF) associated with the large-scale mode and high frequency (HF) associated with mesocale eddy turbulence. This decomposition allows us to diagnose the energy fluxes between the different reservoirs and to understand the sources and sinks. Due to the large-scale of the mode, most of its energy is contained in the APE. In our configuration, the only source of LF APE is the transfer from mean APE to LF APE that is attributed to the large-scale baroclinic instability. In return the sinks of LF APE are the parameterized diffusion, the flux toward HF APE and to a much lesser extent toward LF KE. The presence of an additional wind-stress component weakens multidecadal oscillations and modifies the energy fluxes between the different energy reservoirs. The KE transfer appears to only have a minor influence on the multidecadal mode compared to the other energy sources involving APE, in all experiments. These results highlight the utility of the full APE/ KE budget.

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Properties of Steady Geostrophic Turbulence with Isopycnal Outcropping

Journal of Physical Oceanography ◽

10.1175/jpo-d-11-09.1 ◽

2012 ◽

Vol 42 (1) ◽

pp. 18-38 ◽

Cited By ~ 25

Author(s):

G. Roullet ◽

J. C. McWilliams ◽

X. Capet ◽

M. J. Molemaker

Keyword(s):

High Resolution ◽

Large Scale ◽

Mechanical Energy ◽

Baroclinic Instability ◽

Three Dimensional ◽

Primary Source ◽

Eddy Diffusivity ◽

Energy Cycle ◽

Geostrophic Turbulence ◽

Pv Gradient

Abstract High-resolution simulations of β-channel, zonal-jet, baroclinic turbulence with a three-dimensional quasigeostrophic (QG) model including surface potential vorticity (PV) are analyzed with emphasis on the competing role of interior and surface PV (associated with isopycnal outcropping). Two distinct regimes are considered: a Phillips case, where the PV gradient changes sign twice in the interior, and a Charney case, where the PV gradient changes sign in the interior and at the surface. The Phillips case is typical of the simplified turbulence test beds that have been widely used to investigate the effect of ocean eddies on ocean tracer distribution and fluxes. The Charney case shares many similarities with recent high-resolution primitive equation simulations. The main difference between the two regimes is indeed an energization of submesoscale turbulence near the surface. The energy cycle is analyzed in the (k, z) plane, where k is the horizontal wavenumber. In the two regimes, the large-scale buoyancy forcing is the primary source of mechanical energy. It sustains an energy cycle in which baroclinic instability converts more available potential energy (APE) to kinetic energy (KE) than the APE directly injected by the forcing. This is due to a conversion of KE to APE at the scale of arrest. All the KE is dissipated at the bottom at large scales, in the limit of infinite resolution and despite the submesoscales energizing in the Charney case. The eddy PV flux is largest at the scale of arrest in both cases. The eddy diffusivity is very smooth but highly nonuniform. The eddy-induced circulation acts to flatten the mean isopycnals in both cases.

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Simulated Relationships between Regional Temperatures and Large-Scale Circulation: 125 kyr BP (Eemian) and the Preindustrial Period

Journal of Climate ◽

10.1175/jcli3469.1 ◽

2005 ◽

Vol 18 (19) ◽

pp. 4032-4045 ◽

Cited By ~ 21

Author(s):

Nikolaus Groll ◽

Martin Widmann ◽

Julie M. Jones ◽

Frank Kaspar ◽

Stephan J. Lorenz

Keyword(s):

North Atlantic ◽

Large Scale ◽

Temperature Sensitive ◽

Proxy Data ◽

Large Scale Circulation ◽

The North ◽

Temperature Signal ◽

Northern Winter ◽

The Mean ◽

Mean Circulation

Abstract To investigate relationships between large-scale circulation and regional-scale temperatures during the last (Eemian) interglacial, a simulation with a general circulation model (GCM) under orbital forcing conditions of 125 kyr BP is compared with a simulation forced with the Late Holocene preindustrial conditions. Consistent with previous GCM simulations for the Eemian, higher northern summer 2-m temperatures are found, which are directly related to the different insolation. Differences in the mean circulation are evident such as, for instance, stronger northern winter westerlies toward Europe, which are associated with warmer temperatures in central and northeastern Europe in the Eemian simulation, while the circulation variability, analyzed by means of a principal component analysis of the sea level pressure (SLP) field, is very similar in both periods. As a consequence of the differences in the mean circulation the simulated Arctic Oscillation (AO) temperature signal in the northern winter, on interannual-to-multidecadal time scales, is weaker during the Eemian than today over large parts of the Northern Hemisphere. Correlations between the AO index and the central European temperature (CET) decrease by about 0.2. The winter and spring SLP anomalies over the North Atlantic/European domain that are most strongly linearly linked to the CET cover a smaller area and are shifted westward over the North Atlantic during the Eemian. However, the strength of the connection between CET and these SLP anomalies is similar in both simulations. The simulated differences in the AO temperature signal and in the SLP anomaly, which is linearly linked to the CET, suggest that during the Eemian the link between the large-scale circulation and temperature-sensitive proxy data from Europe may differ from present-day conditions and that this difference should be taken into account when inferring large-scale climate from temperature-sensitive proxy data.

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Internal tide energy flux over a ridge measured by a co-located ocean glider and moored ADCP

10.5194/os-2019-15 ◽

2019 ◽

Author(s):

Rob Hall ◽

Barbara Berx ◽

Gillian Damerell

Keyword(s):

Energy Flux ◽

Large Scale ◽

Low Frequency ◽

Internal Tide ◽

Small Scale ◽

Potential Density ◽

The North ◽

Model Study ◽

Continental Shelves ◽

Alternative Approach

Abstract. Internal tide energy flux is an important diagnostic for the study of energy pathways in the ocean, from large-scale input by the surface tide, to small-scale dissipation by turbulent mixing. Accurate calculation of energy flux requires repeated full-depth measurements of both potential density (ρ) and horizontal current velocity (u) over at least a tidal cycle and over several weeks to resolve the internal spring-neap cycle. Typically, these observations are made using full-depth oceanographic moorings that are vulnerable to being fished-out by commercial trawlers when deployed on continental shelves and slopes. Here we test an alternative approach to minimise these risks, with u measured by a low-frequency ADCP moored near the seabed and ρ measured by an autonomous ocean glider holding station by the ADCP. The method is used to measure the M2 internal tide radiating from the Wyville Thompson Ridge in the North Atlantic. The observed energy flux (4.2 ± 0.2 kW m−1) compares favourably with historic observations and a previous numerical model study. Error in the energy flux calculation due to imperfect co-location of the glider and ADCP is estimated by sub-sampling potential density in an idealised internal tide field along pseudorandomly distributed glider paths. The error is considered acceptable (

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Energetics of the Western Hemisphere Circulation Pattern

Journal of Climate ◽

10.1175/jcli-d-19-0211.1 ◽

2019 ◽

Vol 32 (22) ◽

pp. 7857-7870 ◽

Cited By ~ 1

Author(s):

Xin Tan ◽

Ming Bao ◽

Xuejuan Ren

Keyword(s):

North Atlantic ◽

Large Scale ◽

Low Frequency ◽

Lower Troposphere ◽

Circulation Pattern ◽

Western Hemisphere ◽

Self Organizing Maps ◽

Atlantic Sector ◽

The North ◽

Energy Maintenance

Abstract The Western Hemisphere (WH) circulation pattern, identified by self-organizing maps cluster analysis, is a low-frequency atmospheric regime that influences the fluctuations of large-scale circulation over the North Pacific–North American–North Atlantic areas. The reanalysis datasets from ECMWF are used to estimate the energetics of the WH pattern in this study. The composite results based on monthly WH events reveal that the kinetic energy (KE) associated with the WH pattern is maintained through the barotropic conversion from the climatological-mean westerlies, mainly in the Atlantic jet exit regions. The KE could also be gained through the barotropic feedback forcing from transient eddies. The corresponding baroclinic conversion of available potential energy (APE) from the climatological-mean state, which contributes most efficiently to the energy maintenance of the WH pattern, is obvious in the middle and lower troposphere, owing to the thermal contrast of the colder continent and warmer ocean over the North America–North Atlantic sector. The baroclinic conversion associated with the heat flux on the climatological temperature gradient is consistent with the southwestward-tilting height anomalies from 850 to 500 hPa. The baroclinic feedback from transient eddies contributes negatively to the energy conversion and destroys the maintenance of the WH pattern.

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LOFAR observations of the XMM-LSS field

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833906 ◽

2019 ◽

Vol 622 ◽

pp. A4 ◽

Cited By ~ 8

Author(s):

C. L. Hale ◽

W. Williams ◽

M. J. Jarvis ◽

M. J. Hardcastle ◽

L. K. Morabito ◽

...

Keyword(s):

Large Scale ◽

Angular Resolution ◽

Low Frequency ◽

Primary Beam ◽

Electromagnetic Spectrum ◽

Radio Sources ◽

Spectral Indices ◽

The North ◽

True Source ◽

Scale Calibration

We present observations of the XMM Large-Scale Structure (XMM-LSS) field observed with the LOw Frequency ARray (LOFAR) at 120–168 MHz. Centred at a J2000 declination of −4.5°, this is a challenging field to observe with LOFAR because of its low elevation with respect to the array. The low elevation of this field reduces the effective collecting area of the telescope, thereby reducing sensitivity. This low elevation also causes the primary beam to be elongated in the north-south direction, which can introduce side lobes in the synthesised beam in this direction. However the XMM-LSS field is a key field to study because of the wealth of ancillary information, encompassing most of the electromagnetic spectrum. The field was observed for a total of 12 h from three four-hour LOFAR tracks using the Dutch array. The final image presented encompasses ∼27 deg2, which is the region of the observations with a >50% primary beam response. Once combined, the observations reach a central rms of 280μJy beam−1at 144 MHz and have an angular resolution of 7.5 × 8.5″. We present our catalogue of detected sources and investigate how our observations compare to previous radio observations. This includes investigating the flux scale calibration of these observations compared to previous measurements, the implied spectral indices of the sources, the observed source counts and corrections to obtain the true source counts, and finally the clustering of the observed radio sources.

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Revisiting the humid Roman hypothesis: novel analyses depict oscillating patterns

Climate of the Past Discussions ◽

10.5194/cpd-7-2355-2011 ◽

2011 ◽

Vol 7 (4) ◽

pp. 2355-2389 ◽

Cited By ~ 4

Author(s):

B. J. Dermody ◽

H. J. de Boer ◽

M. F. P. Bierkens ◽

S. L. Weber ◽

M. J. Wassen ◽

...

Keyword(s):

North Atlantic ◽

Large Scale ◽

Mediterranean Climate ◽

Eastern Mediterranean ◽

Low Frequency ◽

Interdisciplinary Approach ◽

Archaeological Site ◽

Roman Period ◽

Spatial And Temporal Patterns ◽

The North

Abstract. Previous studies have proposed that potential vegetation in the Mediterranean maintained a wetter climate during the Roman Period until the initiation of large scale deforestation. The reduction in evapotranspirative fluxes associated with deforestation is suggested to have caused climatic aridification leading to the establishment of the present-day Mediterranean climate. There is also evidence to indicate that during the Roman Period Mediterranean climate was influenced by low frequency fluctuations in sea level pressure over the North Atlantic, termed here: the Centennial North Atlantic Oscillation (CNAO). In order to understand the importance of each of these mechanisms and disentangle their respective signals in the proxy record, we have employed an interdisciplinary approach that exploits a range of tools and data sources. An analysis of archaeological site distribution and historical texts demonstrate that climate did not increase in aridity since the Roman Period. Using an Earth system model of intermediate complexity prescribed with a reconstruction of ancient deforestation, we find that Mediterranean climate was insensitive to deforestation in the Late Holocene. A novel analysis of a composite of proxy indicators of climatic humidity depicts spatial and temporal patterns consistent with the CNAO. The link between the CNAO during the Roman Period and climatic humidity signals manifest in our composite analysis are demonstrated using a modelling approach. Finally, we present evidence indicating that fluctuations in the CNAO contributed to triggering a societal tipping point in the Eastern Mediterranean at the end of the Roman Period.

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A Diagnosis of Thickness Fluxes in an Eddy-Resolving Model

Journal of Physical Oceanography ◽

10.1175/jpo2987.1 ◽

2007 ◽

Vol 37 (3) ◽

pp. 727-742 ◽

Cited By ~ 59

Author(s):

Carsten Eden ◽

Richard J. Greatbatch ◽

Jürgen Willebrand

Keyword(s):

Large Scale ◽

North Atlantic Ocean ◽

A Priori ◽

Baroclinic Instability ◽

Optimization Technique ◽

Subtropical Gyre ◽

Equatorial Atlantic ◽

The North ◽

Eddy Fluxes ◽

The Mean

Abstract Output from an eddy-resolving model of the North Atlantic Ocean is used to estimate values for the thickness diffusivity κ appropriate to the Gent and McWilliams parameterization. The effect of different choices of rotational eddy fluxes on the estimated κ is discussed. Using the raw fluxes (no rotational flux removed), large negative values (exceeding −5000 m2 s−1) of κ are diagnosed locally, particularly in the Gulf Stream region and in the equatorial Atlantic. Removing a rotational flux based either on the suggestion of Marshall and Shutts or the more general theory of Medvedev and Greatbatch leads to a reduction of the negative values, but they are still present. The regions where κ < 0 correspond to regions where eddies are acting to increase, rather than decrease (as in baroclinic instability) the mean available potential energy. In the subtropical gyre, κ ranges between 500 and 2000 m2 s−1, rapidly decreasing to zero below the thermocline in all cases. Rotational fluxes and κ are also estimated using an optimization technique. In this case, |κ| can be reduced or increased by construction, but the regions where κ < 0 are still present and the optimized rotational fluxes also remain similar to a priori values given by the theoretical considerations. A previously neglected component (ν) of the bolus velocity is associated with the horizontal flux of buoyancy along, rather than across, the mean buoyancy contours. The ν component of the bolus velocity is interpreted as a streamfunction for eddy-induced advection, rather than diffusion, of mean isopycnal layer thickness, showing up when the lateral eddy fluxes cannot be described by isotropic diffusion only. All estimates show a similar large-scale pattern for ν, implying westward advection of isopycnal thickness over much of the subtropical gyre. Comparing ν with a mean streamfunction shows that it is about 10% of the mean in midlatitudes and even larger than the mean in the Tropics.

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Low-Frequency Variability in the Midlatitude Baroclinic Atmosphere Induced by an Oceanic Thermal Front

Journal of the Atmospheric Sciences ◽

10.1175/jas3780.1 ◽

2007 ◽

Vol 64 (1) ◽

pp. 97-116 ◽

Cited By ~ 40

Author(s):

Yizhak Feliks ◽

Michael Ghil ◽

Eric Simonnet

Keyword(s):

Energy State ◽

Baroclinic Instability ◽

Low Frequency ◽

High Energy ◽

Horizontal Resolution ◽

Free Atmosphere ◽

Thermal Front ◽

Atmospheric Flow ◽

The North ◽

Barotropic Atmosphere

Abstract This study examines the flow induced by an east–west-oriented oceanic thermal front in a highly idealized baroclinic model. Previous work showed that thermal fronts could produce energetic midlatitude jets in an equivalent-barotropic atmosphere and that barotropic instabilities of this jet had dominant periods of 25–30 and 65–75 days. The present study extends this work to a two-mode baroclinic free atmosphere. The baroclinic jet produced in this case is subject to both barotropic and baroclinic instabilities. A barotropic symmetric instability propagates westward with periods of roughly 30 days and is similar to those found in the equivalent-barotropic model. A baroclinic instability results in standing-dipole anomalies and oscillates with a period of 6–8 months. A mixed barotropic–baroclinic instability results in anomalies that propagate northward, perpendicular to the jet, with a period of 2–3 months. The later anomalies are reminiscent of the 70-day oscillation found over the North Atlantic in observed fields. The atmospheric flow has two distinct states: the flow in the high-energy state exhibits two large gyres and a strong eastward jet; its antisymmetric component is dominant. The low-energy flow is characterized by small gyres and a weak jet. The model’s dynamics depends on the layer-depth ratio. When the model is nearly equivalent-barotropic, symmetric oscillatory modes dominate. As the two layers become nearly equal, antisymmetric oscillatory modes become significant and the mean energy of the flow increases. When the oceanic thermal front’s strength T* is weak (T* ≤ 1.5°C), the flow is steady. For intermediate values of the strength (1.5°C < T* < 3°C), several oscillatory instabilities set in. As the frontal strength increases further (T* ≥ 3°C), the flow becomes more turbulent. These results all depend on the atmospheric model’s horizontal resolution being sufficiently high.

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The North Atlantic Oscillation Response to Large-Scale Atmospheric Anomalies in the Northeastern Pacific

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0351.1 ◽

2013 ◽

Vol 70 (9) ◽

pp. 2854-2874 ◽

Cited By ~ 32

Author(s):

Marie Drouard ◽

Gwendal Rivière ◽

Philippe Arbogast

Keyword(s):

North Atlantic Oscillation ◽

North Atlantic ◽

Wave Breaking ◽

Large Scale ◽

Wave Train ◽

Low Frequency ◽

The North ◽

Northeastern Pacific ◽

The North Atlantic ◽

The North Atlantic Oscillation

Abstract Ingredients in the North Pacific flow influencing Rossby wave breakings in the North Atlantic and the intraseasonal variations of the North Atlantic Oscillation (NAO) are investigated using both reanalysis data and a three-level quasigeostrophic model on the sphere. First, a long-term run is shown to reproduce the observed relationship between the nature of the synoptic wave breaking and the phase of the NAO. Furthermore, a large-scale, low-frequency ridge anomaly is identified in the northeastern Pacific in the days prior to the maximum of the positive NAO phase both in the reanalysis and in the model. A large-scale northeastern Pacific trough anomaly is observed during the negative NAO phase but does not systematically precede it. Then, short-term linear and nonlinear simulations are performed to understand how the large-scale ridge anomaly can act as a precursor of the positive NAO phase. The numerical setup allows for analysis of the propagation of synoptic waves in the eastern Pacific in the presence of a large-scale ridge or trough anomaly and their downstream impact onto the Atlantic jet when they break. The ridge acts in two ways. First, it tends to prevent the downstream propagation of small waves compared to long waves. Second, it deflects the propagation of the wave trains in such a way that they mainly propagate equatorward in the Atlantic. The two modes of action favor the anticyclonic wave breaking and, therefore, the positive NAO phase. With the trough, the wave train propagation is more zonal, disturbances are more meridionally elongated, and cyclonic wave breaking is more frequent in the Atlantic than in the ridge case.

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The Antecedent Large-Scale Conditions of the “Perfect Storms” of Late October and Early November 1991

Monthly Weather Review ◽

10.1175/2010mwr3280.1 ◽

2010 ◽

Vol 138 (7) ◽

pp. 2546-2569 ◽

Cited By ~ 27

Author(s):

Jason M. Cordeira ◽

Lance F. Bosart

Keyword(s):

North America ◽

Kinetic Energy ◽

North Atlantic ◽

North Pacific ◽

Large Scale ◽

Baroclinic Instability ◽

Subsequent Development ◽

Eddy Kinetic Energy ◽

The North ◽

The North Atlantic

Abstract The “Perfect Storms” (PSs) were a series of three high-impact extratropical cyclones (ECs) that impacted North America and the North Atlantic in late October and early November 1991. The PSs included the Perfect Storm in the northwest Atlantic, a second EC over the North Atlantic that developed from the interaction of the PS with Hurricane Grace, and a third EC over North America commonly known as the “1991 Halloween Blizzard.” The PSs greatly impacted the North Atlantic and North America with large waves, coastal flooding, heavy snow, and accumulating ice, and they also provided an opportunity to investigate the physical processes that contributed to a downstream baroclinic development (DBD) episode across North America that culminated in the ECs. Downstream baroclinic development resulted from an amplification of the large-scale flow over the North Pacific that was influenced by anomalous tropical convection, the recurvature and extratropical transition of western North Pacific Tropical Cyclones Orchid, Pat, and Ruth, and the subsequent evolution of the extratropical flow. The progression of DBD occurred following the development of a negative PNA regime and the generation of baroclinic instability over North America associated with equatorward-displaced potential vorticity anomalies and poleward-displaced corridors of high moisture content. An analysis of the eddy kinetic energy tendency equation demonstrated that the resulting baroclinic conversion of eddy available potential energy into eddy kinetic energy during the cyclogenesis process facilitated the progression of DBD across North America and the subsequent development of the ECs.

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