Momentum Advection Research Articles

Diurnal Variations of Low-Level Winds and Precipitation Response to Large-Scale Circulations during a Heavy Rainfall Event

AbstractA succession of MCSs developed during the last week of October 2016 and produced extreme heavy rainfall in central China. The event underwent an evident shift from a mei-yu-like warm scenario to an autumn cold scenario. Diurnal cycles of rainfall and low-level winds may be modulated by the shifting of large-scale atmospheric conditions. We conducted observational analyses and numerical experiments to examine how large-scale circulations influenced rainfall systems through diurnally varying processes. The results show that, in the first half (warm) period of the event, intense rainfall mostly occurred in eastern-central China with an early morning peak. It was closely related to a nocturnal southwesterly low-level jet (NLLJ) on the flank of the western Pacific subtropical high. The NLLJ formed near midnight in southern China where ageostrophic wind rotated clockwise due to Blackadar’s inertial oscillation. The NLLJ extended downstream to central China during the predawn hours due to the horizontal advection of momentum. Both the formation and extension of the NLLJ were supported by an enhanced subtropical high that provided relatively warm conditions with surface heating for boundary layer inertial oscillation and strong background southwesterly winds for momentum transport. The NLLJ induced MCSs at its northern terminus where the low-level ascent, moisture flux convergence, and convective instability were enhanced during the predawn hours. In the second half period with an intrusion of cold air, the diurnal amplitude of low-level winds became small under relatively cold and cloudy conditions. Moderate rainfall tended to occur in western-central China with a peak after midnight, most likely due to frontogenetic processes, upslope lifting, and nighttime cloud-top cooling.

On the seasonal and inter-annual variability of the equatorial Indian Ocean surface winds

Variability of the Equatorial Indian Ocean (EIO) winds play crucial roles in driving the upper EIO dynamics and modifying the ocean–atmosphere interactions in the Indian Ocean (IO). This study, using a satellite-sensed high-resolution (0.25° × 0.25°) monthly winds, has revealed many salient features of the seasonal and inter-annual variability of winds at various dynamically significant regions of the EIO. Though annual mean wind pattern of the EIO shows westerlies east of 60°E, the different local areas in this ocean exhibit significant seasonal wind variations. Migrations and fluctuations of southerlies and westerlies primarily determine the EIO wind variability. In the western (eastern) EIO, SW and NE monsoon winds are stronger (weaker) and of longer (shorter) duration. Near to the equator (1°S–1°N), in the central EIO, weaker winds occur during SW and NE monsoons, whereas stronger winds during transition periods. Near the equator, meridional winds show a significant annual period, especially in the western EIO. But zonal winds exhibit semi-annual period east of 55°E with a peak in the central ocean associated with the active westerlies during spring and fall, and annual period west of 55°E in the western EIO due to monsoon reversals. Westerlies are stronger during the fall compared to spring. Zonal wind variability in the central EIO is the essential deciding factor for the zonal wind variability occurring in the whole EIO. Both Zonal Sea Level Pressure Gradient (ZSLPG) and Momentum Advection (MA) fields determine the dynamics of zonal winds in the EIO. Zonal winds are weak in the western EIO because of the semi-annual harmonics of both ZSLPG and MA, though with substantial amplitude, exhibiting opposite accelerations there. But stronger zonal winds occur in the central EIO (60°E–80°E) because of the same accelerations set by the relatively weaker ZSLPG and MA fields. The zonal winds show substantial inter-annual variability compared to the meridional winds. The inter-annual variability of zonal winds shows the predominance of two modes, with EOF1 50% and EOF2 20% of the total variance. Zonal wind pattern of EOF1 resembles the monsoon transition seasons, while that of EOF2 describes the SW monsoon. In the EIO, Indian Ocean Dipole (IOD) influences the patterns of winds and SST anomalies better than that of El Nino–Southern Oscillation (ENSO). Spatiotemporal variability of the EIO winds during ENSO and IOD indicates the prevalence of stronger zonal wind anomalies in the eastern and central EIO and weaker anomalies in the western EIO. Although spatial patterns of wind anomalies are similar during El Nino and Positive IOD (PIOD) periods, the monthly variations, spatial extensions, and intensities of anomalies are much different while comparing. Prospectus for understanding the EIO winds variations can help to understand the changes occurring in the wind-driven surface and subsurface zonal currents.

Dynamics of ITCZ Width: Ekman Processes, Non-Ekman Processes, and Links to Sea Surface Temperature

Abstract The dynamical processes controlling the width of the intertropical convergence zone (ITCZ) are investigated using idealized and CMIP5 simulations. ITCZ width is defined in terms of boundary layer vertical velocity. The tropical boundary layer is approximately in Ekman balance, suggesting that wind stress places a strong constraint on ITCZ width. A scaling based on Ekman balance predicts that ITCZ width is proportional to the wind stress and inversely proportional to its meridional gradient. A toy model of an Ekman boundary layer illustrates the effects of wind stress perturbations on ITCZ width. A westerly wind perturbation widens the ITCZ whereas an easterly perturbation narrows the ITCZ. Multiplying the wind stress by a constant factor does not shift the ITCZ edge, but ITCZ width is sensitive to the latitude of maximum wind stress. Scalings based on Ekman balance cannot fully capture the behavior of ITCZ width across simulations, suggesting that non-Ekman dynamical processes need to be accounted for. An alternative scaling based on the full momentum budget explains variations in ITCZ width and highlights the importance of horizontal and vertical momentum advection. Scalings are also introduced linking ITCZ width to surface temperature. An extension to Lindzen–Nigam theory predicts that ITCZ width scales with the latitude where the Laplacian of SST is zero. The supercriticality theory of Emanuel is also invoked to show that ITCZ width is dynamically linked to boundary layer moist entropy gradients. The results establish a dynamical understanding of ITCZ width that can be applied to interpret persistent ITCZ biases in climate models and the response of tropical precipitation to climate change.

Acceleration and Overturning of the Antarctic Slope Current by Winds, Eddies, and Tides

AbstractAll exchanges between the open ocean and the Antarctic continental shelf must cross the Antarctic Slope Current (ASC). Previous studies indicate that these exchanges are strongly influenced by mesoscale and tidal variability, yet the mechanisms responsible for setting the ASC’s transport and structure have received relatively little attention. In this study the roles of winds, eddies, and tides in accelerating the ASC are investigated using a global ocean–sea ice simulation with very high resolution (1/48° grid spacing). It is found that the circulation along the continental slope is accelerated both by surface stresses, ultimately sourced from the easterly winds, and by mesoscale eddy vorticity fluxes. At the continental shelf break, the ASC exhibits a narrow (~30–50 km), swift (>0.2 m s−1) jet, consistent with in situ observations. In this jet the surface stress is substantially reduced, and may even vanish or be directed eastward, because the ocean surface speed matches or exceeds that of the sea ice. The shelfbreak jet is shown to be accelerated by tidal momentum advection, consistent with the phenomenon of tidal rectification. Consequently, the shoreward Ekman transport vanishes and thus the mean overturning circulation that steepens the Antarctic Slope Front (ASF) is primarily due to tidal acceleration. These findings imply that the circulation and mean overturning of the ASC are not only determined by near-Antarctic winds, but also depend crucially on sea ice cover, regionally-dependent mesoscale eddy activity over the continental slope, and the amplitude of tidal flows across the continental shelf break.

Enhanced estimation of boundary layer advective properties to improve space‐to‐time conversion processes for wind energy applications

AbstractRemote sensing instruments that scan have the ability to provide high‐resolution spatial measurements of atmospheric boundary layer winds across a region. However, the ability to use these spatially distributed measurements to extract temporal variations in the flow at time scales less than the measurement revisit period is historically limited. As part of this work, the framework for an enhanced space‐to‐time conversion technique is established, allowing for time histories of atmospheric boundary layer wind characteristics to be reliably extracted for locations within the measurement domain. This space‐to‐time conversion technique is made possible by quantifying momentum advection within the measurement domain, rather than simply assuming a uniform advection based on a singular mean wind speed and direction. The use of this technique enables the extraction of long lead‐time (ie, upwards of 60 seconds) forecasts of wind speed and direction at individual locations within the measurement domain, thereby expanding the application and potential benefits of scanning instruments. For example, these long lead‐time forecasts can be used to enhance proactive wind turbine control and more accurately define wind turbine wake statistics.

A Modeling Study of the Responses of Mesosphere and Lower Thermosphere Winds to Geomagnetic Storms at Middle Latitudes

AbstractThermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIMEGCM) simulations are diagnostically analyzed to investigate the causes of mesosphere and lower thermosphere (MLT) wind changes at middle latitudes during the 17 April 2002 storm. In the early phase of the storm, middle‐latitude upper thermospheric wind changes are greater and occur earlier than MLT wind changes. The horizontal wind changes cause downward vertical wind changes, which are transmitted to the MLT region. Adiabatic heating and heat advection associated with downward vertical winds cause MLT temperature increases. The pressure gradient produced by these temperature changes and the Coriolis force then drive strong equatorward meridional wind changes at night, which expand toward lower latitudes. Momentum advection is minor. As the storm evolves, the enhanced MLT temperatures produce upward vertical winds. These upward winds then lead to a decreased temperature, which alters the MLT horizontal wind pattern and causes poleward wind disturbances at higher latitudes.

The Role of Downflows in Establishing Solar Near-surface Shear

Abstract The dynamical origins of the Sun’s tachocline and near-surface shear layer (NSSL) are still not well understood. We have attempted to self-consistently reproduce an NSSL in numerical simulations of a solar-like convection zone by increasing the density contrast across rotating 3D spherical shells. We explore the hypothesis that high density contrast leads to near-surface shear by creating a rotationally unconstrained layer of fast flows near the outer surface. Although our high-contrast models do have near-surface shear, it is confined primarily to low latitudes (between ±15°). Two distinct types of flow structures maintain the shear dynamically: rotationally constrained Busse columns aligned with the rotation axis and fast, rotationally unconstrained downflow plumes that deplete angular momentum from the outer fluid layers. The plumes form at all latitudes and, in fact, are more efficient at transporting angular momentum inward at high latitudes. The presence of Busse columns at low latitudes thus appears essential to creating near-surface shear in our models. We conclude that a solar-like NSSL is unobtainable from a rotationally unconstrained outer fluid layer alone. In numerical models, the shear is eliminated through the advection of angular momentum by the meridional circulation. Therefore, a detailed understanding of how the solar meridional circulation is dynamically achieved will be necessary to elucidate the origin of the Sun’s NSSL.

Lateral Circulation in a Partially Stratified Tidal Inlet

Using a three-dimensional, hydrostatic, primitive-equation ocean model, this study investigates the dynamics of lateral circulation in a partially stratified tidal inlet, the Barataria Pass in the Gulf of Mexico, over a 25.6 h diurnal tidal cycle. Model performance is assessed against observational data. During flood tide, the lateral circulation exhibits the characteristics similar to those induced by differential advection, i.e., lateral flow consists of two counter-rotating cells and is convergent at the surface. The analysis of momentum balance indicates that, in addition to the pressure gradient and vertical stress divergence, nonlinear advection and horizontal stress divergence are also important contributors. During ebb phase, the lateral circulation is mostly toward the right shoal (when looking into the estuary) for the whole water column and persisting for almost the whole period. The surface divergence suggested by the differential advection mechanism lasts for a very short period, if it ever exists. The main momentum balance across most of the transect during ebb is between the along-channel advection of cross-channel momentum and pressure gradient. The sectional averaged lateral velocity magnitude during ebb is comparable to that during flood, which is different from the idealized numerical experiment result.

A 2D Tide-Averaged Model for the Long-Term Evolution of an Idealized Tidal Basin-Inlet-Delta System

We present a model for the morphodynamics of tidal basin-inlet-delta systems at the centennial time scales. Tidal flow is calculated through a friction dominated model, with a semi-empirical correction to account for the advection of momentum. Transport of non-cohesive sediment (sand) is simulated through tidal dispersion, i.e., without explicitly resolving sediment advection. Sediment is also transported downslope through a bed elevation diffusion process. The model is compared to a high-resolution tide-resolving model (Delft3D) with good agreement for different hydrodynamic and sedimentary settings. The model has low sensitivity with respect to temporal and spatial discretization. For the same spatial resolution, the model is about five orders of magnitude faster than tide-resolving models (e.g., Delft3D), and about three orders of magnitude faster than tide-resolving models that use a morphological acceleration factor. This numerical efficiency makes the model suitable to assess long-term changes of large coastal areas. The model’s simplicity makes it suitable for coupling with other physical, ecological, and socio-economic dynamics.

A high-resolution Navier–Stokes solver for direct numerical simulation of free shear flow

A high-resolution, Navier–Stokes solver is developed for direct numerical simulation (DNS) of free shear flow. All terms in Navier–Stokes equations are discretized using higher order methods. Diffusion term is discretized using fourth order central difference scheme while second order Adams–Bashforth is used for time derivative. Advecting velocity is approximated using fourth order Lagrangian interpolation. For the approximation of advected velocity, a blended fifth order-upwind scheme is proposed. Developed high resolution solver is used for DNS of round jet in transitional and turbulent regimes. A novel open outlet boundary condition (OOBC) is proposed which has the ability to dynamically adjust according to prevailing local condition at the outlet thereby minimizing reflections from outlet. Ability of blended fifth order upwind scheme and fifth order WENO is assessed in terms of algorithmic efficiency as well as fidelity of simulations. It is demonstrated that the proposed blended fifth order upwind scheme outperforms the WENO scheme in terms of algorithmic efficiency. Assessment of fidelity of simulations reveals that WENO displays a tendency to over-predict momentum advection in transitional as well as fully turbulent regime of the round jet. In contrast, the proposed advection scheme is not faced with such limitation.

The Role of Polygonal Eyewalls in Rapid Intensification of Typhoon Megi (2010)

Abstract High-resolution numerical experiments for Typhoon Megi (2010) in the western North Pacific are conducted using the Advanced Research version of the Weather Research and Forecasting (WRF) Model to understand the mechanisms of rapid intensification (RI). With a dynamically initialized vortex, sensitivity experiments are carried out focusing on the planetary boundary layer (PBL) and the following microphysics schemes: WRF single-moment 6-class (WSM6) and WRF double-moment 6-class (WDM6) microphysics with Yonsei University, Mellor–Yamada–Janjić (MYJ), and Mellor–Yamada–Nakanishi–Niino (MYNN) 2.5-level (MN2.5) and 3.0-level (MN3) PBL schemes. The largest differences are found between WSM6-MN3 and WDM6-MN3, and we therefore examine RI mechanisms based on the results of these experiments. Prior to RI, WDM6-MN3 shows a drier environment and stronger downdrafts in the lower troposphere than in WSM6-MN3. As a result, during the RI period, WSM6-MN3 (WDM6-MN3) significantly intensifies with the minimum sea level pressure decreasing by 51 (29) hPa and the maximum surface wind increasing by 28 (12) m s−1 in 24 h. In both experiments, the maximum values of surface heat fluxes, potential vorticity (PV), radial absolute angular momentum advection, inertial stability, supergradient wind, and convective bursts inside the radius of maximum winds are frequently observed at each vertex of polygonal eyewalls in the lower troposphere. In particular, WSM6-MN3 exhibits more convective cells inside the inner-core region, a more persistent and thicker polygonal eyewall in the lower troposphere, and a more robust vertical structure of hydrometeors and vertical velocity than WDM6-MN3. This study suggests that within the inner-core region, polygonal eyewalls like WSM6-MN3 provide favorable conditions for RI.

Flow and passive transport in planar multipolar flows

We study the flow and transport of heat or mass, modelled as passive scalars, within a basic geometrical unit of a three-dimensional multipolar flow – a triangular prism – characterised by a side length$L$, a normalised thickness$0.01\leqslant \unicode[STIX]{x1D700}\leqslant 0.1$and an apex angle$0<\unicode[STIX]{x1D6FC}<\unicode[STIX]{x03C0}$, and connected to inlet and outlet pipes of equal normalised radius$0.01\leqslant \unicode[STIX]{x1D6FF}\leqslant 0.1$perpendicularly to the plane of the flow. The flow and scalar fields are investigated over the range$0.1\leqslant Re_{p}\leqslant 10$and$0.1\leqslant Pe_{p}\leqslant 1000$, where$Re_{p}$and$Pe_{p}$are respectively the Reynolds and Péclet numbers imposed at the inlet pipe when either a Dirichlet ($\text{D}$) or a Neumann ($\text{N}$) scalar boundary condition is imposed at the wall unattached to the inlets and outlets. A scalar no-flux boundary condition is imposed at all the other walls. An axisymmetric model is applied to understand the flow and scalar transport in the inlet and outlet regions, which consist of a turning region close to the pipe centreline and a channel region away from it. A separate two-dimensional model is then developed for the channel region by solving the integral form of the momentum and scalar advection–diffusion equations. Analytical relations between geometrical, flow and scalar transport parameters based on similarity and integral methods are generated and agree closely with numerical solutions. Finally, three-dimensional numerical calculations are undertaken to test the validity of the axisymmetric and depth-averaged analyses. Dominant flow and scalar transport features vary dramatically across the flow domain. In the turning region, the flow is a largely irrotational straining flow when$\unicode[STIX]{x1D700}\geqslant \unicode[STIX]{x1D6FF}$and a dominantly viscous straining flow when$\unicode[STIX]{x1D700}\ll \unicode[STIX]{x1D6FF}$. The thickness of the scalar boundary layer scales to the local Péclet number to the power$1/3$. The diffusive flux$j_{d}$and the scalar$C_{s}$at the wall where ($\text{D}$) or ($\text{N}$) is imposed, respectively, are constant. In the channel region, the flow is parabolic and dominated by a source flow near the inlet and an irrotational straining flow away from it. When$(\text{D})$is imposed the scalar decreases exponentially with distance from the inlet and the normalised scalar transfer coefficient converges to$\unicode[STIX]{x1D6EC}_{\infty }=2.5694$. When$(\text{N})$is imposed,$C_{s}$varies proportionally to surface area. Transport in the straining region downstream of the inlet is diffusion-limited, and$j_{d}$and$C_{s}$are functions of the geometrical parameters$L$,$\unicode[STIX]{x1D700}$,$\unicode[STIX]{x1D6FC}$and$\unicode[STIX]{x1D6FF}$. In addition to describing the fundamental properties of the flow and passive transport in multipolar configurations, the present work demonstrates how geometrical and flow parameters should be set to control transfers in the different regions of the flow domain.

Discretization of generalized Coriolis and friction terms on the deformed hexagonal C‐grid

This article discusses the generalized Coriolis and friction terms on the hexagonal C‐grid from two perspectives: (a) within the linearized discretized momentum equations on an equilateral grid, and (b) as nonlinear terms on a distorted mesh.The discrete linearized momentum equations are formulated using a trivariate coordinate system. The tendencies of the different forcing terms for each wind component must be linearly dependent. This constraint determines unique discretizations for each term. The linearized vorticity flux term around a zonal mean current requires only the four rhombus potential vorticities (PVs) next to an edge. The vector Laplacian must be formulated with the vorticity on vertices defined as the average of three rhombus vorticities.A modified generalized Coriolis term is defined on the deformed mesh. The baroclinic wave test on the sphere does not reveal any sign of nonlinear Hollingsworth instability, even though it is demonstrated that the vector‐invariant form and the advective form of momentum advection are not equivalent.Physical constraints determine the shape of the stress tensor. These are invariance to the addition of solid‐body rotation and a resulting positive‐definite dissipation rate. An appropriate stress‐tensor formulation does not deliver Laplacian momentum diffusion in the linear case. On the deformed mesh, parts of this stress tensor are obtained by a least‐squares reconstruction of wind gradients. This approach avoids spurious deformations diagnosed for constant flow in the vicinity of pentagon cells.

Quantification of Preferential Contribution of Reynolds Shear Stresses and Flux of Mean Kinetic Energy Via Conditional Sampling in a Wind Turbine Array

Conditional statistics are employed in analyzing wake recovery and Reynolds shear stress (RSS) and flux directional out of plane component preference. Examination of vertical kinetic energy entrainment through describing and quantifying the aforementioned quantities has implications on wind farm spacing, design, and power production, and also on detecting loading variation due to turbulence. Stereographic particle image velocimetry measurements of incoming and wake flow fields are taken for a 3 × 4 model wind turbine array in a scaled wind tunnel experiment. Reynolds shear stress component is influenced by ⟨uv⟩ component, whereas ⟨vw⟩ is more influenced by streamwise advection of the flow; u, v, and w being streamwise, vertical, and spanwise velocity fluctuations, respectively. Relative comparison between sweep and ejection events, ΔS⟨uiuj⟩, shows the role of streamwise advection of momentum on RSS values and direction. It also shows their tendency to an overall balanced distribution. ⟨uw⟩ intensities are associated with ejection elevated regions in the inflow, yet in the wake, ⟨uw⟩ is linked with sweep dominance regions. Downward momentum flux occupies the region between hub height and top tip. Sweep events contribution to downward momentum flux is marginally greater than ejection events'. When integrated over the swept area, sweeps contribute 55% of the net downward kinetic energy flux and 45% is the ejection events contribution. Sweep dominance is related to momentum deficit as its value in near wake elevates 30% compared to inflow. Understanding these quantities can lead to improved closure models.

River computations: artificial backwater from the momentum advection scheme

ABSTRACTThe established method for determining dike heights and dimensioning river training structures is to assess the resulting backwater by numerical modelling. The common consensus is that bottom friction determines the backwater and that momentum advection only has a local effect. We demonstrate that the numerical/artificial backwater contribution from the momentum advection approximation can be of the same order of magnitude as the bottom friction contribution, depending on the advection scheme. This is realized using a one-dimensional analysis and verified using a set of one- and two-dimensional test problems including a wavy bed case, flow over emerged and submerged groynes and finally an actual river. We compare first- and second-order accurate advection schemes and compute their artificial contribution to the backwater, for a range of practically-feasible grid resolutions. The tests demonstrate that the conservation/constancy properties of the scheme determine the size of this contribution, rather than the order of the scheme.

Subrainband Structure and Dynamic Characteristics in the Principal Rainband of Typhoon Hagupit (2008)

Abstract The principal rainband in tropical cyclones is currently depicted as a solitary and continuous precipitation region. However, the airborne radar observations of the principal rainband in Typhoon Hagupit (2008) reveal multiple subrainband structures. These subbands possess many characteristics of the squall lines with trailing stratiform in the midlatitudes and are different from those documented in previous principal rainband studies. The updraft and reflectivity cores are upright and elevated. The updraft is fed by a low-level radial outflow from the inner side. The tangential wind speed shows a clear midlevel jet on the inner side of the reflectivity core. Except for the structural similarities, the dynamics of the subbands is also similar to the squall lines. The local environment near the subbands shows little convective inhibition, modest instability, and vertical wind shear. The temperature retrieval shows a cold pool structure in the stratiform precipitation region. The estimated vertical wind shear induced by the cold pool is close to that of the local environment. The structural and dynamic similarities to the squall lines imply that the variation of principal rainbands is subjected to convective-scale dynamics related to the local environment in addition to storm-scale dynamics. The subbands show positive impacts to the vortex intensity in terms of potential vorticity redistribution and absolute angular momentum advection. The positive impacts are closely related to specific structural characteristics of the subbands, which suggests the importance of understanding the convective-scale structure and dynamics of the principal rainband.

Impacts of the IOD-associated temperature and salinity anomalies on the intermittent equatorial undercurrent anomalies

The study of Equatorial Undercurrent (EUC) has attracted a broad attention in recent years due to its strong response and feedback to the Indian Ocean Dipole. In this paper, we first produce a high-quality simulation of three-dimensional temperature, salinity and zonal current simulation from 1982 to 2014, using a high-resolution ocean general circulation model. On this basis, with two sensitivity experiments, we investigate the role of temperature and salinity anomalies in driving and enhancing the EUC during the positive IOD events by examining the variation of the EUC seasonal cycle and diagnosing the zonal momentum budget along the equatorial Indian Ocean. Our results show that during January–March, the EUC can appear along the entire equatorial Indian Ocean in all years, but during August–November, the EUC can appear and reach the eastern Indian Ocean only during the positive IOD events. The zonal momentum budget analysis indicates that the pressure gradient force contributes most to the variation of the eastward acceleration of zonal currents in the subsurface. During the positive IOD events, strong negative subsurface temperature anomalies exist in the eastern Indian Ocean, with negative surface salinity anomalies in the central and eastern Indian Ocean, resulting in a large pressure gradient force to drive EUC during the August–November. Further, the results of two sensitivity experiments indicate that the temperature anomalies significantly impact the pressure gradient force, playing a leading role in driving the EUC, while the surface salinity anomalies can secondarily help to intensify the eastward EUC through increasing the zonal density gradient in the eastern Indian Ocean and impacting the vertical momentum advection in the subsurface.

Impact of the “Symmetric Instability of the Computational Kind” at mesoscale- and submesoscale-permitting resolutions

The energy- and enstrophy-conserving momentum advection scheme (EEN) used over the last 10 years in NEMO is subject to a spurious numerical instability. This instability, referred to as the Symmetric Instability of the Computational Kind (SICK), arises from a discrete imbalance between the two components of the vector-invariant form of momentum advection. The properties and the method for removing this instability have been documented by Hollingsworth et al. (1983), but the extent to which the SICK may interfere with processes of interest at mesoscale- and submesoscale-permitting resolutions is still unkown. In this paper, the impact of the SICK in realistic ocean model simulations is assessed by comparing model integrations with different versions of the EEN momentum advection scheme. Investigations are undertaken with a global mesoscale-permitting resolution (1/4 °) configuration and with a regional North Atlantic Ocean submesoscale-permitting resolution (1/60 °) configuration. At both resolutions, the instability is found to alter primarily the most energetic current systems, such as equatorial jets, western boundary currents and coherent vortices. The impact of the SICK is found to increase with model resolution with a noticeable impact at mesoscale-permitting resolution and a dramatic impact at submesoscale-permitting resolution. The SICK is shown to distort the normal functioning of current systems, by redirecting the slow energy transfer between balanced motions to a spurious energy transfer to internal inertia-gravity waves and to dissipation. Our results indicate that the SICK is likely to have significantly corrupted NEMO solutions (when run with the EEN scheme) at mesocale-permitting and finer resolutions over the last 10 years.

The role of boundary layer momentum advection in the mean location of the ITCZ

The inter-tropical convergence zones (ITCZ) form closer to the equator during equinoxes while they form well away from the equator during the boreal summer. A simple three-way balance between the pressure gradients, Coriolis force and effective Rayleigh friction has been classically used to diagnose the location of maximum boundary layer convergence in the near equatorial ITCZ. If such a balance can capture the dynamics of off-equatorial convergence was not known. We used idealized aqua planet simulations with fixed, zonally symmetric sea surface temperature boundary conditions to simulate the near equatorial and off-equatorial ITCZ. As opposed to the convergence of inter-hemispheric flows in the near equatorial convergence, the off-equatorial convergence forms due to the deceleration of cross-equatorial meridional flow. The detailed momentum budget of the off-equatorial convergence zone reveals that the simple balance is not sufficient to capture the relevant dynamics. The deceleration of the meridional flow is strongly modulated by the inertial effects due to the meridional advection of zonal momentum in addition to the terms in the simple balance. The simple balance predicts a spurious near equatorial convergence and a consistent off-equatorial convergence of the meridional flow. The spurious convergence disappears when inertial effects are included in the balance. As cross equatorial meridional flow decelerates to form convergence, the inertial effects cancel the pressure gradient effects near the equator while they add away from the equator. The contribution to the off-equatorial convergence induced by the pressure gradients is significantly larger than the contribution due to the inertial effects and hence pressure gradients appear to be the primary factor in anchoring the strength and location of the off-equatorial convergence.

Respective Roles of the Guinea Current and Local Winds on the Coastal Upwelling in the Northern Gulf of Guinea

AbstractThe northern Gulf of Guinea is a part of the eastern tropical Atlantic where oceanic conditions due to the presence of coastal upwelling may influence the regional climate and fisheries. The dynamics of this coastal upwelling is still poorly understood. A sensitivity experiment based on the Regional Oceanic Modeling System (ROMS) is carried out to assess the role of the detachment of the Guinea Current as a potential mechanism for coastal upwelling. This idealized experiment is performed by canceling the inertia terms responsible for the advection of momentum in the equations and comparing with a realistic experiment. The results exhibit two major differences. First, the Guinea Current is found to be highly sensitive to inertia, as it is no longer detached from the coast in the idealized experiment. The Guinea Current adjusts on an inertial boundary layer, the inertial terms defining its lateral extension. Second, the upwelling east of Cape Palmas disappears in absence of the Guinea Current detachment. This is in contrast with the upwelling east of Cape Three Points, which is still present. The results suggest that two different generation processes of the coastal upwelling need to be considered: the upwelling east of Cape Palmas (which is due to inertia, topographic variations, and advective terms effects resulting in important vertical pumping) and the upwelling east of Cape Three Points (which is principally induced by local winds). In addition to recent work ruling out the role of eddies, this study clarifies the processes responsible for this coastal upwelling.