Primitive Equation Model Research Articles

The Adiabatic Evolution of 3D Annular Vortices with a Double-Eyewall Structure

Tropical cyclones (TCs) can be characterized as a 3D annular structure of elevated potential vorticity (PV). However, strong mature TCs often develop a secondary eyewall, leading to a 3D annular vortex with a double-eyewall structure. Using 2D linear stability analysis, it is shown that three types of barotropic instability (BI) are present for annular vortices with a double-eyewall structure: Type-1 BI across the secondary eyewall, Type-2 BI across the moat of the vortex, and Type-3 BI across the primary eyewall. The overall stability of these vortices (and the type of BI that develops) depends principally upon five vortex parameters: the thickness of the primary eyewall, the thickness of the secondary eyewall, the moat width, the vorticity ratio between the eye and the primary eyewall, and the vorticity ratio between the primary and secondary eyewall. The adiabatic evolution of 3D annular vortices with a double-eyewall structure is examined using a primitive equation model in normalized isobaric coordinates. It is shown that Type-2 BI is the most common type of BI for 3D annular vortices whose vortex parameters mimic TCs with a double-eyewall structure. During the onset of Type-2 BI, eddy kinetic energy budget analysis indicates that barotropic energy conversion from the mean azimuthal flow is the dominant energy source of the eddies, which produces a radial velocity field with a quadrupole structure. Absolute angular momentum budget analysis indicates that Type-2 BI generates azimuthally averaged radial outflow across the moat, and the eddies transport absolute angular momentum radially outward towards the secondary eyewall. The combination of these processes leads to the dissipation of the primary eyewall and the maintenance of the secondary eyewall for the vortex.

An Overlooked Component of the Meridional Overturning Circulation

Abstract Upwelling along the western boundary of the major ocean basin subtropical gyres has been diagnosed in a wide range of ocean models and state estimates. This vertical transport is O(5 × 106) m3 s−1, which is on the same order of magnitude as the downward Ekman pumping across the subtropical gyres and zonally integrated meridional overturning circulation. Two approaches are used here to understand the reason for this upwelling and how it depends on oceanic parameters. First, a kinematic model that imposes a density gradient along the western boundary demonstrates that there must be upwelling with a maximum vertical transport at middepths in order to maintain geostrophic balance in the western boundary current. The second approach considers the vorticity budget near the western boundary in an idealized primitive equation model of the wind- and buoyancy-forced subtropical and subpolar gyres. It is shown that a pressure gradient along the western boundary results in bottom pressure torque that injects vorticity into the fluid. This is balanced on the boundary by lateral viscous fluxes that redistribute this vorticity across the boundary current. The viscous fluxes in the interior are balanced primarily by the vertical stretching of planetary vorticity, giving rise to upwelling within the boundary current. This process is found to be nearly adiabatic. Nonlinear terms and advection of planetary vorticity are also important locally but are not the ultimate drivers of the upwelling. Additional numerical model calculations demonstrate that the upwelling is a nonlocal consequence of buoyancy loss at high latitudes and thus represents an integral component of the meridional overturning circulation in depth space but not in density space. Significance Statement The purpose of this study is to better understand what is forcing water to upwell along the western boundary at midlatitudes of the major ocean basins. This is a potentially important process since upwelling can bring heat and nutrients closer to the surface, where they can be exchanged with the atmosphere. Also, since ocean currents vary with depth, pathways followed in the upper ocean are different from those found for the deeper ocean, so the amount and location of upwelling influence where these waters go. Idealized numerical models and theory are used to demonstrate that the upwelling is ultimately driven by density changes along the western boundary of the basin that result from heat loss at high latitudes.

Inferring Tracer Diffusivity From Coherent Mesoscale Eddies

AbstractMixing along isopycnals plays an important role in the transport and uptake of oceanic tracers. Isopycnal mixing is commonly quantified by a tracer diffusivity. Previous studies have estimated the tracer diffusivity using the rate of dispersion of surface drifters, subsurface floats, or numerical particles advected by satellite‐derived velocity fields. This study shows that the diffusivity can be more efficiently estimated from the dispersion of coherent mesoscale eddies. Coherent eddies are identified and tracked as the persistent sea surface height extrema in both a two‐layer quasigeostrophic (QG) model and an idealized primitive equation (PE) model. The Lagrangian diffusivity is estimated using the tracks of these coherent eddies and compared to the diagnosed Eulerian diffusivity. It is found that the meridional coherent eddy diffusivity approaches a stable value within about 20–40 days in both models. In the QG model, the coherent eddy diffusivity is a good approximation to the upper‐layer tracer diffusivity in a broad range of flow regimes, except for small values of bottom friction or planetary vorticity gradient, where the motions of same‐sign eddies are correlated over long distances. In the PE model, the tracer diffusivity has a complicated vertical structure and the coherent eddy diffusivity is correlated with the tracer diffusivity at the e‐folding depth of the energy‐containing eddies where the intrinsic speed of the coherent eddies matches the rms eddy velocity. These results suggest that the oceanic tracer diffusivity at depth can be estimated from the movements of coherent mesoscale eddies, which are routinely tracked from satellite observations.

Eddy‐Mean Flow Interaction With a Multiple Scale Quasi Geostrophic Model

AbstractParameterization of mesoscale eddies in coarse resolution ocean models is necessary to include the effect of eddies on the large‐scale oceanic circulation. We propose to use a multiple‐scale Quasi‐Geostrophic (MSQG) model to capture the eddy dynamics that develop in response to a prescribed large‐scale flow. The MSQG model consists in extending the traditional quasi geostrophic (QG) dynamics to include the effects of a variable Coriolis parameter and variable background stratification. Solutions to this MSQG equation are computed numerically and compared to a full primitive equation model. The large‐scale flow field permits baroclinically unstable QG waves to grow. These instabilities saturate due to non‐linearities and a filtering method is applied to remove large‐scale structures that develop due to the upscale cascade. The resulting eddy field represents a dynamically consistent response to the prescribed background flow, and can be used to rectify the large‐scale dynamics. Comparisons between Gent‐McWilliams eddy parameterization and the present solutions show large regions of agreement, while also indicating areas where the eddies feed back onto the large scale in a manner that the Gent‐McWilliams parameterization cannot capture. Also of interest is the time variability of the eddy feedback which can be used to build stochastic eddy parameterizations.

Stable singularity formation for the inviscid primitive equations

The primitive equations (PEs) model large-scale dynamics of the oceans and the atmosphere. While it is by now well known that the three-dimensional viscous PEs are globally well posed in Sobolev spaces, and that there are solutions to the inviscid PEs (also called the hydrostatic Euler equations) that develop singularities in finite time, the qualitative description of the blowup still remains undiscovered. In this paper, we provide a full description of two blow-up mechanisms, for a reduced PDE that is satisfied by a class of particular solutions to the PEs. In the first one a shock forms, and pressure effects are subleading, but in a critical way: they localize the singularity closer and closer to the boundary near the blow-up time (with a logarithmic-in-time law). This first mechanism involves a smooth blow-up profile and is stable among smooth enough solutions. In the second one the pressure effects are fully negligible; this dynamics involves a two-parameter family of nonsmooth profiles, and is stable only by smoother perturbations.

Turbulence modeling to aid tidal energy resource characterization in the Western Passage, Maine, USA

Numerical models combined with field measurements are regularly used to characterize tidal energy resources at potential energetic sites. However, most existing works only focus on the tidal hydrodynamic characteristics, and turbulence parameters are often not reported because of the lack of high-quality turbulence measurements and the limitations of numerical models in resolving turbulent eddies. In this study, we used FVCOM – a hydrostatic primitive equation (HPE) model – to characterize the tidal energy resource in the Western Passage, Maine, USA, by taking care of the essential macro-scale turbulence properties. We observed an excellent model performance using the Mellor–Yamada Level 2.5 Turbulence Model; estimating the spatial and vertical distribution of the turbulent kinetic energy and intensity added a new perspective to the site ranking for tidal energy converter (TEC) deployments. In addition, we also examined the role of channel geometry and bathymetry, such as headlands and underwater sills, in enhancing turbulent eddies around potential TEC siting locations. Ultimately, the detailed analysis of the turbulent flow characteristics has changed the site-ranking results and demonstrated that the regional-scale HPE models could be used for the relative understanding of more or less turbulent sites for a refined resource assessment.

A Simple Family of Tropical Cyclone Models

This review discusses a simple family of models capable of simulating tropical cyclone life cycles, including intensification, the formation of the axisymmetric version of boundary layer shocks, and the development of an eyewall. Four models are discussed, all of which are axisymmetric, f-plane, three-layer models. All four models have the same parameterizations of convective mass flux and air–sea interaction, but differ in their formulations of the radial and tangential equations of motion, i.e., they have different dry dynamical cores. The most complete model is the primitive equation (PE) model, which uses the unapproximated momentum equations for each of the three layers. The simplest is the gradient balanced (GB) model, which replaces the three radial momentum equations with gradient balance relations and replaces the boundary layer tangential wind equation with a diagnostic equation that is essentially a high Rossby number version of the local Ekman balance. Numerical integrations of the boundary layer equations confirm that the PE model can produce boundary layer shocks, while the GB model cannot. To better understand these differences in GB and PE dynamics, we also consider two hybrid balanced models (HB1 and HB2), which differ from GB only in their treatment of the boundary layer momentum equations. Because their boundary layer dynamics is more accurate than GB, both HB1 and HB2 can produce results more similar to the PE model, if they are solved in an appropriate manner.

Diagnosing Scale-Dependent Energy Cycles in a High-Resolution Isopycnal Ocean Model

Abstract Energy exchanges between large-scale ocean currents and mesoscale eddies play an important role in setting the large-scale ocean circulation but are not fully captured in models. To better understand and quantify the ocean energy cycle, we apply along-isopycnal spatial filtering to output from an isopycnal 1/32° primitive equation model with idealized Atlantic and Southern Ocean geometry and topography. We diagnose the energy cycle in two frameworks: 1) a non-thickness-weighted framework, resulting in a Lorenz-like energy cycle, and 2) a thickness-weighted framework, resulting in the Bleck energy cycle. This paper shows that framework 2 is more useful for studying energy pathways when an isopycnal average is used. Next, we investigate the Bleck cycle as a function of filter scale. Baroclinic conversion generates mesoscale eddy kinetic energy over a wide range of scales and peaks near the deformation scale at high latitudes but below the deformation scale at low latitudes. Away from topography, an inverse cascade transfers kinetic energy from the mesoscales to larger scales. The upscale energy transfer peaks near the energy-containing scale at high latitudes but below the deformation scale at low latitudes. Regions downstream of topography are characterized by a downscale kinetic energy transfer, in which mesoscale eddies are generated through barotropic instability. The scale- and flow-dependent energy pathways diagnosed in this paper provide a basis for evaluating and developing scale- and flow-aware mesoscale eddy parameterizations. Significance Statement Blowing winds provide a major energy source for the large-scale ocean circulation. A substantial fraction of this energy is converted to smaller-scale eddies, which swirl through the ocean as sea cyclones. Ocean turbulence causes these eddies to transfer part of their energy back to the large-scale ocean currents. This ocean energy cycle is not fully simulated in numerical models, but it plays an important role in transporting heat, carbon, and nutrients throughout the world’s oceans. The purpose of this study is to quantify the ocean energy cycle by using fine-scale idealized numerical simulations of the Atlantic and Southern Oceans. Our results provide a basis for how to include unrepresented energy exchanges in coarse global climate models.

A Lagrange–Laplace Integration Scheme for Weather Prediction and Climate Modelling

A time integration scheme based on semi-Lagrangian advection and Laplace transform adjustment has been implemented in a baroclinic primitive equation model. The semi-Lagrangian scheme makes it possible to use large time steps. However, errors arising from the semi-implicit scheme increase with the time step size. In contrast, the errors using the Laplace transform adjustment remain relatively small for typical time steps used with semi-Lagrangian advection. Numerical experiments confirm the superior performance of the Laplace transform scheme relative to the semi-implicit reference model. The algorithmic complexity of the scheme is comparable to the reference model, making it computationally competitive, and indicating its potential for integrating weather and climate prediction models.

QBO Wind Influence on MJO-Induced Temperature Anomalies in the Upper Troposphere and Lower Stratosphere in an Idealized Model

Abstract The dynamical mechanism by which the quasi-biennial oscillation (QBO) might influence the temperature anomaly, associated with the Madden–Julian oscillation (MJO), in the equatorial upper troposphere and lower stratosphere (UTLS) is examined by conducting a series of initial-value experiments using a dry primitive equation model. The observed temperature response to the MJO convection becomes colder and more in phase with the convection during easterly QBO (EQBO) than westerly QBO (WQBO) phases. This QBO-dependent MJO temperature anomaly in the UTLS is qualitatively reproduced by model experiments in which EQBO or WQBO background state is artificially imposed above 250 hPa while leaving the troposphere unaltered. As in the observations, the localized cold anomaly in the UTLS becomes strengthened and steepened with EQBO-like background state than WQBO-like one. It turns out that the QBO zonal wind, instead of temperature, plays a major role in determining the localized UTLS temperature anomaly by modulating wave energy dispersion.

NeverWorld2: an idealized model hierarchy to investigate ocean mesoscale eddies across resolutions

Abstract. We describe an idealized primitive-equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable. The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography. The model is perfectly adiabatic and spans the Equator and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally include diabatic processes and buoyancy forcing. We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence.

The Mid‐ to High‐Latitude Migrating Semidiurnal Tide: Results From a Mechanistic Tide Model and SuperDARN Observations

AbstractSimulations of the solar thermal migrating semidiurnal (SW2) tide in the mesosphere‐lower‐thermosphere (MLT) are compared against meteor wind observations from a longitudinal chain of high‐latitude Super Dual Auroral Radar Network radars. The simulations span the year 2015 and are performed using a mechanistic primitive equation model. The model employs a whole‐atmosphere tide forcing based on temperature tendency fields from the Specified Dynamics Whole Atmosphere Community Climate Model with Thermosphere and Ionosphere Extension, and a background atmospheric specification based on zonal wind and temperature data from the Navy Global Environmental Model‐High Altitude meteorological analysis system. Results show that the model accurately reproduces the observed seasonal variability of the SW2 tide in both the amplitude and phase. Numerical experiments are performed to investigate how the tidal forcing, dissipation terms, and seasonal variations in the background atmosphere shape the seasonal variations of the simulated SW2 tide. Notable results are that the background atmosphere most strongly impacts the SW2 tide forced in the troposphere, and that the specification of a narrow surface friction profile enhances the net SW2 amplitude in the MLT between April and October. Eddy diffusion is found to damp the simulated tide predominantly around summer solstice and in December.

Comparing numerical accuracy and stability for different horizontal discretizations in MPAS-Ocean

This manuscript investigates the effectiveness of two possible horizontal discretizations for the global ocean model MPAS-Ocean, both applied to Spherical Centroidal Voronoi Tessellations (SCVTs). The first discretization is TRiSK, a C-grid, finite-volume method, that possesses many desirable mimetic properties, but has a low order accuracy. The second discretization was introduced for the first time by Peixoto (2016), and consists of modifications to the TRiSK scheme designed to achieve at least first-order accuracy in the L∞ norm, with the loss of some of the mimetic properties. Tests on shallow-water and primitive-equation models show that the scheme due to Peixoto is indeed more accurate, but presents stability issues with respect to TRiSK. TRiSK is indeed found to be often more stable in time and more robust with respect to errors in the geometric properties of the grid.

Genesis of Easterly Waves over the Tropical Eastern Pacific and the Intra-Americas Sea

AbstractThis paper explores a new mechanism for in situ genesis of easterly waves (EWs) over the tropical eastern Pacific Ocean (EPAC). Using an idealized primitive equation model, it is shown that EWs can be triggered by finite-amplitude transient heating close to the midlevel jet at about 15°N over the EPAC and intra-Americas sea region. The atmospheric response to heating initiates EWs downstream, showing an EW structure within 4 days, with a wavelength and propagation speed of about 2000 km and 4.6 m s−1, respectively, resembling EWs described in the literature. The most sensitive location for EW initiation from finite-amplitude transient heating is located over the northern part of South America and extends to the EPAC. The closer the heating is to the jet, the bigger the response is. A stratiform heating profile is the most efficient at triggering EPAC EWs. Comparisons of simulated EWs over the EPAC and West Africa reveal similar structures but with a shorter wavelength and much weaker amplitudes over the EPAC. EPAC EWs are dominated by horizontal tilts against the shear on the equatorial side of the jet, consistent with barotropic growth, with weaker low-level amplitudes relative to those seen over West Africa. These differences arise from differences in the mean state EPAC having a shorter and weaker midlevel jet with less baroclinicity.

Australian tidal currents – assessment of a barotropic model (COMPAS v1.3.0 rev6631) with an unstructured grid

Abstract. While the variations of tidal range are large and fairly well known across Australia (less than 1 m near Perth but more than 14 m in King Sound), the properties of the tidal currents are not. We describe a new regional model of Australian tides and assess it against a validation dataset comprising tidal height and velocity constituents at 615 tide gauge sites and 95 current meter sites. The model is a barotropic implementation of COMPAS, an unstructured-grid primitive-equation model that is forced at the open boundaries by TPXO9v1. The mean absolute error (MAE) of the modelled M2 height amplitude is 8.8 cm, or 12 % of the 73 cm mean observed amplitude. The MAE of phase (10∘), however, is significant, so the M2 mean magnitude of vector error (MMVE, 18.2 cm) is significantly greater. The root sum square over the eight major constituents is 26 % of the observed amplitude. We conclude that while the model has skill at height in all regions, there is definitely room for improvement (especially at some specific locations). For the M2 major axis velocity amplitude, the MAE across the 95 current meter sites, where the observed amplitude ranges from 0.1 to 156 cm s−1, is 6.9 cm s−1, or 22 % of the 31.7 cm s−1 observed mean. This nationwide average result is encouraging, but it conceals a very large regional variation. Relative errors of the tidal current amplitudes on the narrow shelves of New South Wales (NSW) and Western Australia exceed 100 %, but tidal currents are weak and negligible there compared to non-tidal currents, so the tidal errors are of little practical significance. Looking nationwide, we show that the model has predictive value for much of the 79 % of Australia's shelf seas where tides are a major component of the total velocity variability. In descending order this includes the Bass Strait, the Kimberley to Arnhem Land, and southern Great Barrier Reef regions. There is limited observational evidence to confirm that the model is also valuable for currents in other regions across northern Australia. We plan to commence publishing “unofficial” tidal current predictions for chosen regions in the near future based on both our COMPAS model and the validation dataset we have assembled.

The shelf sources of estuarine inflow

AbstractThe inflow to an estuary originates on the shelf. It flushes the estuary and can bring in nutrients, heat, salt, and hypoxic water, having consequences for estuarine ecosystems and fjordic glacial melt. However, the source of estuarine inflow has only been explored in simple models that do not resolve interactions between inflow and outflow outside of the estuarine channel. This study addressed the estuary inflow problem using variations on a three-dimensional primitive equation model of an idealized estuarine channel next to a sloping, unstratified shelf with mixing provided by a single frequency, 12-hour tide. Inflow was identified using particle tracking, momentum budgets, and Total Exchange Flow. Inflow sources were found in shelf water downstream of the estuary, river plume water, and shelf water upstream of the estuary. Downstream is defined here with respect to the direction of coastal trapped wave propagation, which is to the right for an observer looking seaward from the estuary mouth in the northern hemisphere. Downstream of the estuary and offshore of the plume, the dynamics were quasi-geostrophic, consistent with previous simple models. The effect of this inflowing current on the geometry of the river plume front was found to be small. Novel sources of inflow were identified which originated from within the plume and upstream of the estuary.

The Role of Inertial Instability in Cross-Hemispheric Coupling

AbstractRecent studies suggest linkages between anomalously warm temperatures in the winter stratosphere, and the high-latitude summer mesopause. The summer temperature anomaly is manifested in the decline of polar mesospheric clouds. The 2-day wave is a strong-amplitude and transient summer feature that interacts with the background state so as to warm the high-latitude summer mesopause. This wave has been linked to a low-latitude phenomenon called inertial instability, which is organized by breaking planetary waves in the winter stratosphere. Hence, inertial instability has been identified as a possible nexus between the disturbed winter stratosphere, and summer mesopause warming. We investigate a sustained occurrence of inertial instability during 19 July–8 August 2014. During this period, stratospheric winter temperatures warmed by about 10 K, while a steep decline in polar mesospheric clouds was reported between 26 July and 6 August. We present, for the first time, wave driving associated with observed inertial instability. The effect of inertial instability is to export eastward momentum from the winter hemisphere across the equator into the summer hemisphere. Using a primitive equation model, we demonstrate that the wave stresses destabilize the stratopause summer easterly jet. The reconfigured wind profile excites the wavenumber-4 component of the 2-day wave, leading to enhanced warming of the summer mesopause. This work supports previous numerical investigations that identified planetary wave–driven inertial instability as a source of the 2-day wave.

The Effect of Anthropogenic Aerosols on the Aleutian Low

AbstractPast studies have suggested that regional trends in anthropogenic aerosols can influence the Pacific decadal oscillation (PDO) through modulation of the Aleutian low. However, the robustness of this connection is debated. This study analyzes changes to the Aleutian low in an ensemble of climate models forced with large, idealized global and regional black carbon (BC) and sulfate aerosol perturbations. To isolate the role of ocean feedbacks, the experiments are performed with an interactive ocean and with prescribed sea surface temperatures. The results show a robust weakening of the Aleutian low forced by a global tenfold increase in BC in both experiment configurations. A linearized steady-state primitive equation model is forced with diabatic heating anomalies to investigate the mechanisms through which heating from BC emissions influences the Aleutian low. The heating from BC absorption over India and East Asia generates Rossby wave trains that propagate into the North Pacific sector, forming an upper-tropospheric ridge. Sources of BC outside of East Asia enhance the weakening of the Aleutian low. The responses to a global fivefold and regional tenfold increase in sulfate aerosols over Asia show poor consistency across climate models, with a multimodel mean response that does not project strongly onto the Aleutian low. These findings for a large, idealized step increase in regional sulfate aerosol differ from previous studies that suggest the transient increase in sulfate aerosols over Asia during the early twenty-first century weakened the Aleutian low and induced a transition to a negative PDO phase.

Submesoscale Dynamics in the Gulf of Aden and the Gulf of Oman

We have investigated the surface and subsurface submesoscale dynamics in the Gulf of Aden and the Gulf of Oman. Our results are based on the analyses of regional numerical simulations performed with a primitive equation model (HYCOM) at submesoscale permitting horizontal resolution. A model zoom for each gulf was embedded in a regional mesoscale-resolving simulation. In the Gulf of Aden and the Gulf of Oman, the interactions of mesoscale structures and fronts instabilities form submesoscale eddies and filaments. Rotational energy spectra show that the Gulf of Aden has a higher ratio of submesoscale to mesocale energy than the Gulf of Oman. Fast waves (internal gravity waves, tidal waves, Kelvin waves) and slow waves (Rossby waves) were characterized via energy spectra of the divergent velocity. Local upwelling systems which shed cold filaments, coastal current instabilities at the surface, and baroclinic instability at capes in subsurface were identified as generators of submesocale structures. In particular, the Ras al Hamra and Ras al Hadd capes in the Gulf of Oman, and the Cape of Berbera in the Gulf of Aden, are loci of submesoscale eddy generation. To determine the instability mechanisms involved in these generations, we diagnosed the Ertel potential vorticity and the energy conversion terms: the horizontal and vertical Reynolds stresses and the vertical buoyancy flux. Finally, the impacts of the subsurface submesoscale eddy production at capes on the diffusion and fate of the Red Sea Water (in the Gulf of Aden) and the Persian Gulf Water (in the Gulf of Oman) are highlighted.

The Role of Extratropical Background Flow in Modulating the MJO Extratropical Response

AbstractThe Madden–Julian oscillation (MJO) is the dominant mode of tropical intraseasonal variability. Many studies have found that the MJO, which acts as a tropical heating source, can excite Rossby waves that propagate into the midlatitude and modulate midlatitude circulation. The extratropical mean flow can modulate the MJO extratropical response. Rossby waves can grow or decay in different extratropical background flows, and the propagation of the Rossby waves also varies as the background flow acts as a waveguide. In this study, how extratropical mean flow modulates the MJO extratropical response is explored by using a nonlinear baroclinic primitive equation model. MJO-associated heating, as an external forcing of the model, is imposed into scenarios with different extratropical background flows. Different background flow modulates the generation and advection of the vorticity anomalies induced by the MJO, which determines the initial location and strength of the Rossby waves. The midlatitude waveguides can be different as the background flow changes. As the propagation of Rossby waves follows the waveguides, the background flow determines whether the Rossby waves are trapped in the Pacific Ocean region or can propagate to the north and to the east into North America. The experiments also show that the anomalies associated with the Rossby waves can extract energy from the midlatitude jet over the jet exit region and the southern flank of the jet. This further modulates the strength, location, and duration of the MJO extratropical response.