Mean flow due to momentum flux convergence from internal tides upstream of a submarine ridge near Merir Island, Palau

Abstract For many climate forcings the dominant response of the extratropical circulation is a latitudinal shift of the tropospheric midlatitude jets. The magnitude of this response appears to depend on climatological jet latitude in general circulation models (GCMs): lower-latitude jets exhibit a larger shift. The reason for this latitude dependence is investigated for a particular forcing, heating of the equatorial stratosphere, which shifts the jet poleward. Spinup ensembles with a simplified GCM are used to examine the evolution of the response for five different jet structures. These differ in the latitude of the eddy-driven jet but have similar subtropical zonal winds. It is found that lower-latitude jets exhibit a larger response due to stronger tropospheric eddy–mean flow feedbacks. A dominant feedback responsible for enhancing the poleward shift is an enhanced equatorward refraction of the eddies, resulting in an increased momentum flux, poleward of the low-latitude critical line. The sensitivity of feedback strength to jet structure is associated with differences in the coherence of this behavior across the spectrum of eddy phase speeds. In the configurations used, the higher-latitude jets have a wider range of critical latitude locations. This reduces the coherence of the momentum flux anomalies associated with different phase speeds, with low phase speeds opposing the effect of high phase speeds. This suggests that, for a given subtropical zonal wind strength, the latitude of the eddy-driven jet affects the feedback through its influence on the width of the region of westerly winds and the range of critical latitudes on the equatorward flank of the jet.

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INTRA-SEASONAL VARIABILITY OF NEAR-BOTTOM CURRENT IN THE HALMAHERA SEA

Jurnal Ilmu dan Teknologi Kelautan Tropis ◽

10.29244/jitkt.v6i2.9003 ◽

2015 ◽

Vol 6 (2) ◽

Author(s):

Marlin C Wattimena ◽

Agus S Atmadipoera ◽

Mulia Purba ◽

Ariane Koch-Larrouy

Keyword(s):

Pacific Ocean ◽

Seasonal Variability ◽

Sea Surface Height ◽

Equatorial Pacific ◽

Internal Tides ◽

Sea Surface ◽

Bottom Current ◽

Equatorial Pacific Ocean ◽

Mean Flow ◽

Zonal Winds

The secondary entry portal of the Indonesian Throughflow (ITF) from the Pacific to Indian Oceans is considered to be via the Halmahera Sea (HS). However, few ITF studies have been done within the passage. This motivated the Internal Tides and Mixing in the Indonesian Througflow (INDOMIX) program to conduct direct measurements of currents and its variability across the eastern path of the ITF. This study focused on the intra-seasonal variability of near-bottom current in HS (129°E, 0°S), its origin and correlation with surface zonal winds and sea surface height over the equatorial Pacific Ocean. The result showed a strong northwestward mean flow with velocity exceeding 40 cm/s, which represented the current-following topography with the northwest orientation. Meridional current component was much stronger than the zonal component. The energy of power spectral density (PSD) of the current peaked on 14-days and 27-days periods. The first period was presumably related to the tidal oscillation, but the latter may be associated with surface winds perturbation. Furthermore, cross-PSD revealed a significant coherency between the observed currents and the surface zonal winds in the central equatorial Pacific zonal winds (180°E-160°W), which corroborates westward propagation of intra-seasonal sea surface height signals along the 5°S with its mean phase speeds of 50 cm/s, depicting the low-latitude westward Rossby waves on intra-seasonal band. Keywords: current, equatorial Pacific Ocean, zonal winds, sea surface height, Halmahera Sea

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Forcing of oceanic mean flows by dissipating internal tides

Journal of Fluid Mechanics ◽

10.1017/jfm.2012.303 ◽

2012 ◽

Vol 708 ◽

pp. 250-278 ◽

Cited By ~ 19

Author(s):

Nicolas Grisouard ◽

Oliver Bühler

Keyword(s):

Numerical Study ◽

Three Dimensional ◽

Wave Drag ◽

Perturbation Series ◽

Internal Tides ◽

Time Averaging ◽

Mean Flow ◽

Wave Source ◽

Lee Waves ◽

Lee Wave

AbstractWe present a theoretical and numerical study of the effective mean force exerted on an oceanic mean flow due to the presence of small-amplitude internal waves that are forced by the oscillatory flow of a barotropic tide over undulating topography and are also subject to dissipation. This extends the classic lee-wave drag problem of atmospheric wave–mean interaction theory to a more complicated oceanographic setting, because now the steady lee waves are replaced by oscillatory internal tides and, most importantly, because now the three-dimensional oceanic mean flow is defined by time averaging over the fast tidal cycles rather than by the zonal averaging familiar from atmospheric theory. Although the details of our computation are quite different, we recover the main action-at-a-distance result from the atmospheric setting, namely that the effective mean force that is felt by the mean flow is located in regions of wave dissipation, and not necessarily near the topographic wave source. Specifically, we derive an explicit expression for the effective mean force at leading order using a perturbation series in small wave amplitude within the framework of generalized Lagrangian-mean theory, discuss in detail the range of situations in which a strong, secularly growing mean-flow response can be expected, and then compute the effective mean force numerically in a number of idealized examples with simple topographies.

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Seasonal variation of equatorial wave momentum fluxes at Gadanki (13.5° N, 79.2° E)

Annales Geophysicae ◽

10.5194/angeo-19-985-2001 ◽

2001 ◽

Vol 19 (8) ◽

pp. 985-990 ◽

Cited By ~ 10

Author(s):

M. N. Sasi ◽

V. Deepa

Keyword(s):

Momentum Flux ◽

Tropical Meteorology ◽

Radiosonde Data ◽

Mean Flow ◽

Flow Acceleration ◽

Four Seasons ◽

Equatorial Wave ◽

Autumnal Equinox ◽

Waves And Tides ◽

The Waves

Abstract. The vertical flux of the horizontal momentum associated with the equatorial Kelvin and Rossby-gravity waves are estimated from the winds measured by the Indian MST radar located at Gadanki (13.5° N, 79.2° E) during September 1995 to August 1996 in the tropospheric and lower stratospheric regions for all four seasons. The present study shows that momentum flux values are greater during equinox seasons than solstices, with values near the tropopause level being 16 × 10-3, 7.4 × 10-3, 27 × 10-3 and 5.5 × 10-3 m2 s-2 for Kelvin waves and 5.5 × 10-3, 3.5 × 10-3, 6.7 × 10-3 and 2.1 × 10-3 m2 s-2 for RG waves during autumnal equinox, winter, vernal equinox and summer seasons, respectively. Using these momentum flux values near the tropopause level, acceleration of the mean flow in the stratosphere up to a 29 km height were computed following Plumb (1984), by considering the wave-meanflow interaction and the deposition of the momentum through the radiative dissipation of the waves. A comparison of the estimated mean-flow acceleration in the stratosphere compares well, except at a few height levels, with the observed mean-flow accelerations in the stratosphere derived from the radiosonde data from a nearby station.Key words. Meteorology and atmosphenic dynamics (tropical meteorology; waves and tides)

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The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part I: Observations

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0297.1 ◽

2019 ◽

Vol 76 (4) ◽

pp. 1125-1144 ◽

Cited By ~ 2

Author(s):

Pablo Zurita-Gotor

Keyword(s):

Momentum Flux ◽

Large Scale ◽

Zonal Flow ◽

Stationary Wave ◽

Hadley Cell ◽

Momentum Transport ◽

Mean Flow ◽

Meridional Transport ◽

The Tropics

Abstract This work investigates the role played by the divergent circulation for meridional eddy momentum transport in the tropical atmosphere. It is shown that the eddy momentum flux in the deep tropics arises primarily from correlations between the divergent eddy meridional velocity and the rotational eddy zonal velocity. Consistent with previous studies, this transport is dominated by the stationary wave component, associated with correlations between the zonal structure of the Hadley cell (zonal anomalies in the meridional overturning) and the climatological-mean Rossby gyres. This eddy momentum flux decomposition implies a different mechanism of eddy momentum convergence from the extratropics, associated with upper-level mass convergence (divergence) over sectors with anomalous westerlies (easterlies). By itself, this meridional transport would only increase (decrease) isentropic thickness over regions with anomalous westerly (easterly) zonal flow. The actual momentum mixing is due to vertical (cross isentropic) advection, pointing to the key role of diabatic processes for eddy–mean flow interaction in the tropics.

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Equatorial deep jets and their influence on the equatorial mean circulation in an idealised model forced by intraseasonal momentum flux convergence

10.5194/egusphere-egu2020-3039 ◽

2020 ◽

Author(s):

Swantje Bastin ◽

Martin Claus ◽

Peter Brandt ◽

Richard J. Greatbatch

Keyword(s):

Momentum Flux ◽

Intraseasonal Variability ◽

Zonal Flow ◽

Wind Forcing ◽

Vertical Resolution ◽

Mean Flow ◽

Geophysical Research ◽

Zonal Jets ◽

Total Magnitude ◽

Idealised Model

Equatorial deep jets (EDJ) are vertically stacked, downward propagating zonal jets that alternate in direction with depth. In the tropical Atlantic, they have been shown to influence both surface conditions and tracer variability. Despite their importance, the EDJ are absent in most ocean models, most likely due to a lack of vertical resolution. However, when the vertical resolution is sufficiently high, EDJ oscillating at a period of about 4.5 years are present in idealised model configurations driven by steady wind forcing. We have used such a model for a basin-wide reconstruction of the intraseasonal eddy momentum flux convergence, which has recently been shown to be a large contributor to the EDJ maintenance (Greatbatch et al., 2018). When we apply the diagnosed momentum flux convergence that is oscillating at the EDJ period as forcing in a model without wind forcing, EDJ develop, allowing us to verify the mechanism proposed in Greatbatch et al. (2018). Additionally, we can isolate the nonlinear effect that the EDJ have on the time mean zonal flow at intermediate depths. We can show that the EDJ drive time mean zonal flow that is similar in structure to the mean flow measured by Argo floats at 1000 m depth, contributing (in our idealised setup) about one fifth of the total magnitude of the observed flow, the rest likely resulting from direct forcing by downward propagating intraseasonal waves as shown before in other studies.&#160;References:Greatbatch, R. J., M. Claus, P. Brandt, J.-D. Matthie&#223;en, F. P. Tuchen, F. Ascani, M. Dengler, J. Toole, C. Roth, J. T. Farrar, 2018: Evidence for the Maintenance of Slowly Varying Equatorial Currents by Intraseasonal Variability. Geophysical Research Letters, 45, 1923-1929.

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Application of the IDEMIX Concept for Internal Gravity Waves in the Atmosphere

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0107.1 ◽

2020 ◽

Vol 77 (10) ◽

pp. 3601-3618

Author(s):

B. Quinn ◽

C. Eden ◽

D. Olbers

Keyword(s):

Energy Balance ◽

Wave Field ◽

Gravity Wave ◽

Gravity Waves ◽

Momentum Flux ◽

Middle Atmosphere ◽

Internal Gravity Waves ◽

Wave Drag ◽

Mean Flow ◽

The Mean

AbstractThe model Internal Wave Dissipation, Energy and Mixing (IDEMIX) presents a novel way of parameterizing internal gravity waves in the atmosphere. IDEMIX is based on the spectral energy balance of the wave field and has previously been successfully developed as a model for diapycnal diffusivity, induced by internal gravity wave breaking in oceans. Applied here for the first time to atmospheric gravity waves, integration of the energy balance equation for a continuous wave field of a given spectrum, results in prognostic equations for the energy density of eastward and westward gravity waves. It includes their interaction with the mean flow, allowing for an evolving and local description of momentum flux and gravity wave drag. A saturation mechanism maintains the wave field within convective stability limits, and a closure for critical-layer effects controls how much wave flux propagates from the troposphere into the middle atmosphere. Offline comparisons to a traditional parameterization reveal increases in the wave momentum flux in the middle atmosphere due to the mean-flow interaction, resulting in a greater gravity wave drag at lower altitudes. Preliminary validation against observational data show good agreement with momentum fluxes.

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Meridional Rossby Wave Generation and Propagation in the Maintenance of the Wintertime Tropospheric Double Jet

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-15-0197.1 ◽

2016 ◽

Vol 73 (5) ◽

pp. 2179-2201 ◽

Cited By ~ 6

Author(s):

Amanda K. O’Rourke ◽

Geoffrey K. Vallis

Keyword(s):

Zonal Wind ◽

Momentum Flux ◽

Baroclinic Instability ◽

Atmospheric Model ◽

Mean Flow ◽

Wind Maximum ◽

Short Waves ◽

Instability Theory ◽

Distinct Features ◽

Two Jets

Abstract The eddy-driven and subtropical jets are two dynamically distinct features of the midlatitude upper-troposphere circulation that are often merged into a single zonal wind maximum. Nonetheless, the potential for a distinct double-jet state in the atmosphere exists, particularly in the winter hemisphere, and presents a unique zonal-mean flow with two waveguides and an interjet region with a weakened potential vorticity gradient upon which Rossby waves may be generated, propagate, reflect, and break. The authors investigate the interaction of two groups of atmospheric waves—those with wavelengths longer and shorter than the deformation radius—within a double-jet mean flow in an idealized atmospheric model. Patterns of eddy momentum flux convergence for long and short waves differ greatly. Short waves behave following classic baroclinic instability theory such that their eddy momentum flux convergence is centered at the eddy-driven jet core. Long waves, on the other hand, reveal strong eddy momentum flux convergence along the poleward flank of the eddy-driven jet and within the interjet region. This pattern is enhanced when two jets are present in the zonal-mean zonal wind.

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