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Mechanical forcing of the North American monsoon by orography



The core of the North American monsoon consists of a band of intense rainfall along the west coast of Mexico[1, 2] and is commonly thought to be caused by thermal forcing from both land and the elevated terrain of that region[3-5]. Here we use observations, a global climate model, and stationary wave solutions to show that this rainfall maximum is instead generated when Mexico's Sierra Madre mountains mechanically force an adiabatic stationary wave by diverting extratropical eastward winds toward the equator; eastward, upslope flow in that wave lifts warm and moist air to produce convective rainfall. Land surface heat fluxes do precondition the atmosphere for convection, particularly in summer afternoons, but even if amplified are insufficient for producing the observed rainfall maximum. These results, together with dynamical structures in observations and models, indicate that the core monsoon should be understood as convectively enhanced orographic rainfall in a mechanically forced stationary wave, not as a classic, thermally forced tropical monsoon. This has implications for the response of the North American monsoon to past and future global climate change, making trends in jet stream interactions with orography of central importance.
Mechanical forcing of the North American monsoon
by orography
William Boos ( )
University of California, Berkeley
Salvatore Pascale
University of Bologna
Physical Sciences - Article
Keywords: monsoon, rainfall, global climate change
License: This work is licensed under a Creative Commons Attribution 4.0 International License. 
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Mechanical forcing of the North American monsoon1
by orography2
William R. Boos1,2,* and Salvatore Pascale3
1Department of Earth and Planetary Science, University of California, Berkeley, USA4
2Climate and Ecosystem Sciences Division, Lawrence Berkeley National Laboratory, USA5
3Department of Physics and Astronomy, University of Bologna, Italy6
The core of the North American monsoon consists of a band of intense rainfall along the west coast of Mexico
and is
commonly thought to be caused by thermal forcing from both land and the elevated terrain of that region
. Here we use
observations, a global climate model, and stationary wave solutions to show that this rainfall maximum is instead generated
when Mexico’s Sierra Madre mountains mechanically force an adiabatic stationary wave by diverting extratropical eastward
winds toward the equator; eastward, upslope flow in that wave lifts warm and moist air to produce convective rainfall. Land
surface heat fluxes do precondition the atmosphere for convection, particularly in summer afternoons, but even if amplified are
insufficient for producing the observed rainfall maximum. These results, together with dynamical structures in observations and
models, indicate that the core monsoon should be understood as convectively enhanced orographic rainfall in a mechanically
forced stationary wave, not as a classic, thermally forced tropical monsoon. This has implications for the response of the North
American monsoon to past and future global climate change, making trends in jet stream interactions with orography of central
1 Introduction10
Tropical monsoons occur when a surface of low heat capacity transfers the energy of intense summer solar radiation to the
overlying atmosphere, creating thermally direct, precipitating flow. Such circulations supply water to billions of people and set
the climate of large swaths of Earth’s surface. The North American monsoon (NAM) is commonly viewed in this paradigm,
being a low-latitude summer circulation crucial for the hydrology of western Mexico and the southwestern US1,2,6,7.14
As in many tropical regions, North American orography alters the simple description of monsoon flow provided above. The
core NAM consists of a narrow tongue of high precipitation stretching over 1,000 km north-south along the western side of
the Sierra Madre Occidental (SMO) mountains (Fig. 1a). Drier conditions lie east of this precipitation maximum, in central
Mexico and Texas, and atmospheric subsidence occurs to the west-northwest due to baroclinic Rossby waves forced by the
latent heating810.19
The mechanisms that produce this strong organization of NAM precipitation around orography remain unclear. Early global
climate model (GCM) simulations showed that NAM rainfall decreases greatly when mountains are flattened globally
. Some
argue that this occurs because sensible heat fluxes from orography into elevated levels of the atmosphere cause NAM rainfall
drawing water vapor from the Gulf of California up SMO slopes to condense and precipitate
. The high-amplitude diurnal
cycle of precipitation in the NAM has also been taken to suggest the importance of orographic thermal forcing. Shallow
convection begins around noon over SMO peaks and deep convection follows a few hours later on the western slopes
Near-surface air flows upslope during the day and downslope at night
, as expected for a sea breeze or mountain-valley
breeze driven by solar heating. Despite the prominence of this diurnal cycle, horizontal moisture fluxes produced by transients
(e.g., diurnally reversing sea breeze circulations) are an order of magnitude smaller in the core NAM than those produced by
seasonal mean winds
. Thus, sea breeze circulations are observationally striking in the NAM, but their winds average to zero
over 24 hours; core NAM precipitation must be controlled by the forcings that produce seasonal mean flow.30
The mechanical, rather than thermal, effects of orography are known to drive the summer circulation east and northeast of
the NAM, in the central US. A GCM and stationary wave model were used to show that the eastern Sierra Madre deflect trade
winds northward to become the Great Plains low level jet
, which transports water into the central US from the Gulf of
Mexico but is not traditionally seen as a main NAM component. Some authors
have considered orographic elevated heating
and orographic blocking of zonal winds to both be plausible NAM causes, but models integrated at resolutions fine enough to
resolve the SMO and core NAM2022 have not been used to distinguish between these possibilities.36
Our goal is to determine the mechanisms that cause the intense rainfall maximum in the core NAM. Is it generated primarily
by a thermodynamic forcing (e.g. elevated heating) or a mechanical one (mechanical blocking)? Given the prior finding that
time-mean vertically integrated moisture flux convergence in the core NAM is produced by time-mean winds
, this task
amounts to determining the cause of the seasonal-mean eastward, upslope flow over the SMO.40
2 Net response to orography41
We integrate a high-resolution (0.25
-grid spacing) GCM twice: once with observed orography (Control) and again with
surface height set to zero over nearly all of Mexico (FlatMex). This GCM, which uses prescribed sea surface temperature
(SST), produces a realistic seasonal cycle and spatial pattern of NAM precipitation and wind in the Control run (Fig. 1a, b
and Supplementary Figs. S1-S3; the model has a positive precipitation bias but its climatology falls in the range of observed
interannual variability). The model resolves the SMO as a
3 km-high ridge along Mexico’s west coast, and reproduces
observed eastward low-level winds extending roughly 1000 km west of that ridge (Fig. 1a, b). This wind distribution is
suggestive of the midlatitude eastward jet being deflected toward the equator by the SMO; the broader North American
cordillera is known to deflect the jet in such a stationary wave
, but the equatorial part of that wave has not previously been
argued to play a role in the NAM, nor has it been adequately resolved in stationary wave models of the region.50
We obtain the net response to all dynamic and thermodynamic effects of Mexico’s orography by subtracting the FlatMex
state from the Control. Nearly all core NAM precipitation is caused by local orography, with the rainfall maximum on Mexico’s
west coast disappearing in the FlatMex state despite the continued land surface thermal forcing (Fig. 1c). Without the SMO,
westward trade winds span most of Mexico, separating two zones of eastward flow: one in the extratropics and another in
the oceanic intertropical convergence zone (ITCZ) south of Mexico (near 15
N). The region of high near-surface moist static
energy (MSE), which in observations and the Control is confined to the Gulf of California and the Gulf of Mexico, expands
inland to cover central Mexico when orography is flattened (Fig. 1d; surface air MSE is hereafter written
and expressed in
temperature units through normalization by the specific heat of air). In the FlatMex state, the distributions of
, precipitation,
and wind behave as expected for tropical monsoons, with peak rainfall and low-level eastward flow on the equatorial side of the
The wind and
response to the SMO suggests that core NAM precipitation is not forced primarily by orographic elevated
heating, which would work by driving the overlying atmospheric column toward a higher-energy state than it would realize
over the same surface at sea level
. The dynamical response to tropical heatings typically includes poleward flow through the
heated region, in Sverdrup balance, with a low-level cyclone to the west
. Instead, we see anomalous eastward flow over the
orographic forcing, with a low-level cyclone to the north and anticyclonic flow to the southwest (Fig. 1d). Orography decreases
over the SMO and continental Mexico, whereas orographic elevated heating would increase it
. However, since much
of this reasoning employs comparisons with previous idealized solutions that might be complicated by strong background flows,
we now systematically assess the response to separate mechanical and thermal forcings.68
3 Mechanically forced response69
We estimate the response to the mechanical influence of orography with a stationary wave model that has been used to study
the Great Plains low-level jet
and other orographically influenced circulations
, but integrated at finer resolution than
used previously (see Methods). We impose as a basic state the three-dimensional summer-mean flow from the FlatMex GCM,
then use this model to find the adiabatic response to Mexico’s orography (the forcing is the Control-FlatMex surface height
This mechanically forced response consists of a meridional dipole in low-level vorticity, with a cyclone over much of the
western US and an anticyclone southwest of Mexico (Fig. 2c). The dynamical structure strongly resembles the GCM response
(Control-FlatMex; Fig. 2a), even though the GCM also includes diabatic feedbacks and any orographic thermal forcing. The
stationary wave includes anomalous eastward flow upstream of and over the SMO, with a vertical structure and amplitude
similar to that of the net GCM response (Fig. 2b, d). This anomalous flow opposes the westward trade winds stretching across
Mexico in the basic state. Between the surface and
850 hPa, the total flow (basic state plus stationary wave anomaly) is
eastward upstream of and over the SMO western slopes (orange contours in Fig. 2b, d). The stationary wave thus produces the
time-mean upslope wind over the western SMO that is observationally associated with moisture convergence and precipitation
The stationary wave is nonlinear, with isentropes (constant potential temperature surfaces) intersecting orography instead
of bowing upward around it
(Fig. 2b, d; Supplementary Fig. S4 shows the linear response). Nevertheless, this response is
straightforward to understand. When orography is high enough to block zonal winds, adiabatic flow, which in the time mean
follows isentropes, must deviate northward or southward depending on where isentropes intersect the ground. In contrast with
the basic state used in prior studies of flow perturbed by narrow orography
, peak temperatures lie near 38
N, so isentropes over
Mexico tilt downward to the north, intersecting the ground over the southwestern US (Fig. 2a, c, and Supplementary Fig. S5).89
Adiabatic zonal flow must thus ascend and turn southward as it encounters the SMO, because northward flow is blocked as it
follows isentropes into the ground. Lower-resolution stationary wave solutions have a weaker anticyclone south of Mexico
and give greater prominence to the Great Plains low-level jet (Supplementary Fig. S6), perhaps explaining why orographic
mechanical forcing has previously been more closely associated with that circulation17.93
4 Seasonal and diurnal thermodynamic maxima94
How do we reconcile observations of a strong diurnal cycle of precipitation in the core NAM
with evidence that upslope
flow there is produced by a stationary wave? Moist convection requires both a reservoir of convective available potential
energy (CAPE) and, typically, some lifting to overcome convective inhibition or release conditional instability. CAPE generally
increases with
, and high time-mean
lies over the warm Gulf of California and Gulf of Mexico (Fig. 1d). However,
is achieved in late afternoon over western SMO slopes, at least in one observational estimate (Fig. 3a). The strong
diurnal cycle of
, particularly prominent over elevated terrain (Fig. 3b), is caused by solar heating of land, which increases
through surface enthalpy fluxes (there is observational uncertainty in the magnitude of the
maximum over the western SMO,
but all estimates show high
there with a large diurnal cycle). Thus, a warm and moist air layer from the Gulf of California
flows eastward at low levels in the mechanically forced stationary wave, and its MSE is increased further by daytime surface
heat fluxes while its temperature drops adiabatically due to upslope flow. In a convectively stable atmosphere this scenario
would produce large-scale, stratiform condensation, but nonzero CAPE allows convection to occur. Prior work19 showed that105
the observed CAPE distribution does not explain why NAM precipitation favors the west coast of Mexico versus the east coast;
release of CAPE through upslope flow in the stationary wave resolves this issue.107
These effects can be synthesized by examining the seasonal cycle of
and near-surface zonal wind averaged in and
upstream of the core NAM region, respectively. Upslope flow peaks in spring, before the observed rainy season, but
is low
then so ample convective precipitation is not produced (Fig. 3c). Peak precipitation occurs a few months later when upslope flow
is still strong and
has increased to its summer peak. Flattening Mexico’s orography produces a slight increase in summer
presumably because orography blocks the inland penetration of warm and moist oceanic air, yet NAM precipitation decreases
greatly as upslope flow is reduced (Fig. 1c). The seasonality of NAM precipitation thus seems to arise from the seasonal cycle
(and CAPE) but, consistent with CAPE being a necessary but insufficient condition for convection, mechanically forced
ascent in the stationary wave is needed to turn that thermodynamic seasonal cycle into rainfall.115
distribution (Fig. 3a) also illustrates the deviation of the spatial structure of the NAM from that of a classic tropical
monsoon. In the latter we expect peak rainfall and peak low-level eastward wind on the equatorial side of the
Instead, we observe peak NAM rainfall slightly east of (or even directly over) the peak
, and low-level eastward winds west
of the peak
. This suggests that the thermally forced tropical monsoon in the North American region consists of the oceanic
precipitation maximum just south of Mexico, which would exist without Mexico’s orography (Fig. 1c); southward deflection of
prevailing extratropical winds by the North American cordillera (including the SMO) superimposes on that tropical monsoon
the intense band of rainfall along Mexico’s west coast.122
5 Response to a pure thermal forcing123
We now show that if the core NAM were driven primarily by a thermal forcing, it would have a dynamical structure distinct
from that in observations. We conduct a third GCM integration with the albedo of the surface that was flattened (most of
Mexico) reduced to 0.05 (FlatMexLowAlb). This provides a strong thermal forcing, with land albedo in much of the NAM
region reduced below that of open ocean, yielding a local increase of about 20 W m
in the net energy input to the atmosphere
(NEI; the sum of radiative and surface turbulent fluxes into each atmospheric column; Fig. 4a). In response, the high
expands poleward and the oceanic precipitation maximum follows, expanding inland (compare Figs. 4b and 1c, d). Anomalous
low-level poleward flow over the region in which the albedo forcing was applied is consistent with the Sverdrup balance
achieved in the linear response to tropical thermal forcings
. As expected for a thermally forced tropical monsoon
, peak
rainfall lies on the equatorial side of the high-
region, and precipitation increases by about 2 mm day
over the broad region
of the albedo forcing (Fig. 1c). In FlatMexLowAlb, there is no precipitation maximum along Mexico’s west coast and no
eastward flow extending 1000 km west of the SMO.134
These GCM simulations also suggest that core NAM rainfall is not produced by rectification of the diurnal cycle of
precipitation to produce seasonal-mean heating, a hypothesis proposed to explain the enhancement of time-mean precipitation
over small islands
although not explicitly raised for the NAM. If such a mechanism operated in the NAM, it would need to
have an effect stronger than that achieved by the increase in
produced in the FlatMexLowAlb model (Fig. 4) and be confined
to the western slopes of the SMO; it would not explain how the SMO produces eastward low-level winds stretching 1000 km
west of Mexico.140
6 Discussion and conclusions141
The NAM is commonly categorized as a thermally forced tropical monsoon. Although one early study stated that it was difficult
to determine whether mechanical or thermal effects of orography dominated in forcing the NAM
, most previous work has
described the NAM as either (i) similar to though smaller in scale than the South Asian monsoon
, with a central role played
by elevated plateau heating3, or (ii) caused by land-ocean thermal contrast4,8,33.145
We found that a mechanically forced stationary wave produces the seasonal-mean upslope flow associated in prior work
with the water vapor convergence needed to sustain core NAM rainfall. Such stationary waves dominate North American
climate in winter
, but in summer they have been identified primarily with the Great Plains low level jet
. Stationary waves
are also forced by the Rockies and other parts of the North American Cordillera, and Baja peninsula orography seems to steer
eastward winds toward the equator (Fig. 1c). However, our results suggest that core NAM precipitation requires the SMO,
which produce eastward, upslope flow in the region of high hs.151
Mechanically forced stationary waves will be modified in the real atmosphere by moist convective heating, but the
resemblance between horizontal winds in the adiabatic stationary wave solutions and in the moist GCM suggests this has only a
modest effect on horizontal flow (Fig. 2). Upward motions will be amplified by moist convection; such amplification has been
represented using various forms of a reduced effective static stability for convectively coupled equatorial waves
, transient
off-equatorial vortices
, and extratropical precipitation extremes
, but a similar theory for moist convective amplification of
orographic upslope flow has not been developed.157
These findings have implications for past and future NAM variability, placing new emphasis on the jet stream and trade
winds, and their interaction with orography. Changes in the jet stream and trade winds may have been of central importance for
NAM changes in paleoclimates. Accurate dynamical forecasts of NAM rainfall will require models with an unbiased jet stream,
in addition to resolutions fine enough to represent the SMO. Thermodynamic controls on convection, long thought to dominate
NAM rainfall, are important, but their representation in models should be evaluated in terms of how they affect convection in
upslope flow. In contrast, surface conditions and convective stability over central Mexico may primarily affect the low amounts
(1-2 mm day
) of local summer rainfall received there. Finally, global climate change may alter the NAM through changes
in the extratropical jet stream and through changes in convective stability in regions of upslope flow, rather than through its
influence on more general land-ocean thermodynamic contrasts.166
7 Methods167
7.1 Observations168
We obtain estimates of Earth’s atmospheric state from ERA5, the fifth-generation atmospheric reanalysis from the European
Centre for Medium-Range Weather Forecasts
. For years 1979-2019, we use ERA5 surface air temperature, surface air
dewpoint (which we convert to specific humidity to calculate
), surface height, and 100-meter zonal wind. We also obtain
surface air temperature, surface air dewpoint, and surface height from the Modern-Era Retrospective analysis for Research and
Applications, Version 2 (MERRA2)
. Precipitation estimates are drawn from the Global Precipitation Measurement Mission
(GPM) Integrated Multi-satellitE Retrievals for GPM (IMERG), Final Precipitation L3 Daily 0.1 degree
0.1 degree V06
product (GPM_3IMERGDF)42. We averaged years 2001-2020 to obtain the precipitation climatology shown in Fig. 1a. Plots175
of surface height use estimates from the ETOPO1 global relief model
at 1 arc-minute resolution; surface height used in
calculating reanalyzed hsis taken from ERA5 and MERRA2. All quantities are averaged July-September.177
Surface air MSE is also computed for stations along a transect near 28
N using observations of temperature, specific
humidity, and height from the North American Monsoon GPS Transect Experiment 2013
(measurements collected June-
September 2013), and the 2017 North American Monsoon GPS Hydrometeorological Network
(hereafter referred to as GPS
Hydromet 2017, measurements collected June-September 2017), which uses some of the permanent observation sites of the
Trans-boundary, Land and Atmosphere Long-term Observational and Collaborative Network (TLALOCNet)
. Data from the
GPS Transect Experiment 2013 are available every minute while GPS Hydromet 2017 are at 5-minute intervals. We compute183
for all minutes within the 01 UTC and 13 UTC hours, corresponding to late afternoon and early morning in local time,
respectively. We average for all days from July through September for both datasets, and retain only those stations for which
there are less then ten days of missing data. Data for stations within 0.5latitude of 28N were used for the transect.186
7.2 Models187
7.2.1 Global climate model188
Simulations were performed using the Community Atmospheric Model, version 5.1 (CAM5)
coupled to the Community
Land Model, version 4
, within the software infrastructure of the Community Earth System Model (CESM) version 2.1.3.
We use the finite-volume dynamical core, which is typically configured with a horizontal resolution of 0.9
(latitude) by 1.3
(longitude); to better resolve the topography of the NAM region, we use a global horizontal resolution of 0.23
approximately 25 km at the equator) with 30 vertical levels. We use the Sea ICE model (CICE) version 5 with prescribed ice
cover and prescribed cyclic sea surface temperature (SST) from the year 2000. This model configuration is largely the same
as that used in projections of the future behavior of tropical cyclones
, and prior work has shown that the finer horizontal
resolution used here improves the representation of the NAM in CAM522.196
As discussed in previous work
, climate models with relatively coarse horizontal resolution fail to resolve features
like the Gulf of California and the Sierra Madres, thereby misrepresenting key NAM processes such as Gulf of California
moisture surges
, land-sea contrast
, and mechanical flow-blocking by orography
. Furthermore, SST biases in coupled
GCMs can have a detrimental impact on simulation of the NAM, biasing its seasonal evolution to produce a late withdrawal
and thus an overly wet late summer and autumn
. Therefore, using a high resolution configuration with climatological SST
reduces the model’s bias and brings the regional circulation closer to observations (Supplementary Figs. S1-S3).202
To assess the influence of elevated terrain on the core NAM, we integrate the model with standard orography (Control) and
again with flattened orography over most of Mexico (FlatMex). In the integration with flattened orography over Mexico, we
set both the surface height and the subgrid-scale standard deviation of orography to zero within a quadrilateral having these
vertices: (33
N, 245
E), (29
N, 265
E), (15
N, 257
E), and (15
N, 265
E). Orography on the Baja Peninsula is unaltered (it
lies outside this quadrilateral). To avoid creating a high vertical wall of orography at the northern edge of this quadrilateral,
where Mexico’s orography joins the greater North American cordillera, the surface height is set to decrease linearly to zero over
of latitude immediately south of the northern edge of the quadrilateral; the same procedure is used for the subgrid-scale
standard deviation of orography. To help distinguish between the thermal and mechanical influence of orography, we conduct a
third integration in which the surface albedo of the flattened land is set to 0.05 (FlatMexLowAlb); this is done for both the
direct and diffuse albedo by altering the land model (CLM4). To be clear, this third integration has both flattened orography
over Mexico and reduced surface albedo, in an attempt to impose an enhanced thermal forcing without the mechanical effects
of orography. All three of these model configurations are run for 11 years of simulated time, with the last 10 years analyzed.214
To understand how orography deflects the midlatitude westerlies toward the equator and then forces convection through
upslope flow (Fig. 1), we analyze the time-mean zonal wind on a terrain-following level located within a typical subcloud layer
(the atmospheric layer that lies below cloud base). For ERA5 we choose the level 100 m above Earth’s surface, while for the
GCM we use the horizontal wind on the third model level above the surface (level 957.5).218
7.2.2 Stationary wave model219
To isolate the mechanical influence of Mexico’s orography on the atmospheric circulation we use a fully nonlinear stationary
wave model. The model was introduced by Ting and Yu (1998)
, and solves the primitive equations in terms of vorticity,
divergence, temperature, and the logarithm of surface pressure, using spherical harmonics
. Important distinctions with
the GCM are that the stationary wave model (i) solves these equations for anomalies relative to a specific three-dimensional
basic state and (ii) is adiabatic aside from a 15-day Newtonian relaxation of temperature toward the basic state, as used in
prior work
. Transients, such as midlatitude baroclinic instabilities, are suppressed using drag and scale-selective diffusion.
Specifically, interior Rayleigh drag on the anomalies is imposed with a 15-day time scale, with surface drag represented by
gradually reducing this time scale to 0.3 days over the lowest 4 levels. Biharmonic diffusion with a coefficient of
acts on vorticity, divergence, and temperature. The original version of this stationary wave model
was created with a
rhomboidal truncation at wavenumber 15 (R15 spectral resolution) and 12 vertical levels. Later work integrated the model at
R30 resolution with 14 vertical levels28 and R30 resolution with 24 vertical levels29. We enhanced the resolution to R63 with230
24 levels, based on code supplied by Isla Simpson.231
The model was forced by imposing Mexico’s orography on a basic state obtained by time-averaging the summer atmospheric
state from the GCM without that orography. Specifically, we obtain the basic state by taking the 10-year July-September
average atmospheric state from the FlatMex GCM run, and use the surface height difference between the Control and FlatMex
GCM runs as the forcing. The stationary wave model nears a steady state after about 20 days, and is run for 90 days of simulated
time with the last 20 used for analysis.236
Data Availability237
The ERA5 monthly averaged data by hour of day were downloaded from the Copernicus Climate Change Service Climate Data
Store (identifiers cited in Methods). MERRA-2 and GPM data were downloaded from the NASA Goddard Earth Sciences
Data and Information Services Center (GES DISC; identifiers cited in Methods). ETOPO1 data were downloaded from the
National Centers for Environmental Information at NOAA, identifiers cited in Methods). David K. Adams provided access to
GPS Hydromet 2017, TLALOCNet, and GPS Transect Experiment 2013 data.242
Code Availability243
The CESM model, which is supported primarily by the National Science Foundation, was obtained from
. Isla Simpson provided code for the stationary wave model, the original version of which was written by
Mingfang Ting and Linhai Yu.246
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This material is based on work supported by the U.S. Department of Energy, Office of Science, Office of Biological and
Environmental Research, Climate and Environmental Sciences Division, Regional and Global Model Analysis Program, under
Award DE-SC0019367. It used resources of the National Energy Research Scientific Computing Center (NERSC), which is a
DOE Office of Science User Facility. This paper benefited from discussions with Quentin Nicolas, David Adams, Inez Fung,
and John C. H. Chiang.377
Author contributions378
WRB conceived the study, devised and performed the GCM and stationary wave model integrations, and analyzed model output.
SP assessed the GCM bias. Both authors analyzed observations and contributed to writing the manuscript.380
Additional information381
Supplementary information is available in the online version of the paper. Reprints and permissions information is available
online at Correspondence and requests for materials should be addressed to WRB.383
Competing financial interests384
The authors declare no competing financial interests.385
Figure 1. Influence of orography on rain and low-level wind. Summer precipitation (shading, mm day1) and
near-surface eastward wind (orange contours, interval 1 m s
, with the zero contour bold and negative values omitted) for (a)
observations, (b) the control GCM, and (c) the GCM with flattened orography over Mexico. Panel (d) shows the anomalous 700
hPa horizontal wind (vectors) produced by Mexico’s orography in the GCM, and the extent of the region with high surface air
moist static energy (defined as a 2-meter value larger than 345 kJ kg1) in the control model (black stippling) and the model
with flattened orography over Mexico (red shading). Surface height of 1.5 km is contoured in magenta in all panels.
Figure 2. Generation of eastward flow across western Mexico by the mechanically forced stationary wave.
Left panels
show streamfunction of the anomalous 700 hPa horizontal wind (shading, in meters; air flows clockwise around maxima) for
(a) the Control-FlatMex GCM integrations and (c) the stationary wave model forced by the Control-FlatMex surface height (1.5
km surface height is contoured in green). The thick orange line is the zero contour of the basic state zonal wind, which near
35N divides westward trade winds from prevailing eastward extratropical flow. Thin blue lines show 700 hPa potential
temperature (in K). Right panels show anomalous zonal wind at 26N (shading, in m s1) for (b) the Control-FlatMex GCM
and (d) the stationary wave model, with isentropes plotted in blue (5 K contour interval) and orography masked in white; the
total zonal wind (basic state plus response to orography) is contoured in orange, with a contour interval of 2 m s1, negative
contours omitted, and the zero contour in bold. Note the total wind is eastward at low levels west of the SMO. Streamfunction
has been normalized by the gravitational acceleration and the value of the Coriolis parameter at 45N, giving it the units of
geopotential height.
Figure 3. Diurnal and seasonal cycles in the North American monsoon. (a) Observed surface air moist static energy
(MSE; shading, from ERA5) at the time of day when MSE peaks (6 pm local time in western Mexico) and orography (1.5 km
surface height contoured in magenta). Blue line marks the location of the zonal section of surface air MSE and surface height
shown in (b), which illustrates the migration of peak MSE from the Gulf of California at 6 am (blue) toward the western Sierra
Madre at 6 pm (orange) local time. In (b), MSE is from the ERA5 (solid lines) and MERRA2 (dashed lines) reanalyses
, and
from station data (stars), indicating robustness in the amplitude of the diurnal cycle and the rough location of maxima despite
observational uncertainty. (c) Seasonal cycle of surface air MSE averaged over the NAM region (red lines) and near-surface
zonal wind averaged over and upstream (i.e., west) of that region (black lines; see Supplementary Fig. S7 for averaging regions).
Note the large reduction in eastward flow and small increase in MSE during monsoon season when orography is flattened.
Figure 4. Response to a pure thermal forcing.
(a) Anomalies, produced by imposing a reduced surface albedo in the GCM
with flattened orography over Mexico, in 700 hPa horizontal wind (vectors) and net energy input through the top and bottom
boundaries of the atmosphere (shading, W m2). (b) Total precipitation (mm day1) and the extent of the region with high
surface air moist static energy (defined as a 2-meter value larger than 345 kJ kg1; black stippling) in the same GCM with
flattened orography and reduced albedo. Compare stippling in (b) with red shading in Fig. 1d to infer the MSE response to the
thermal forcing, and compare shading in (b) with shading in Fig. 1c to infer the precipitation response.
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The dependence of arid and semiarid ecosystems on seasonal rainfall is not well understood when sites have access to groundwater. Gradients in terrain conditions in northwest México can help explore this dependence as different ecosystems experience rainfall during the North American monsoon (NAM), but can have variations in groundwater access as well as in soil and microclimatic conditions that depend on elevation. In this study, we analyze water‐energy‐carbon fluxes from eddy covariance (EC) systems deployed at three sites: a subtropical scrubland, a riparian mesquite woodland, and a mountain oak savanna to identify the relative roles of soil and microclimatic conditions and groundwater access. We place datasets during the NAM season of 2017 into a wider context using previous EC measurements, nearby rainfall data, and remotely‐sensed products. We then characterize differences in soil, vegetation, and meteorological variables; latent and sensible heat fluxes; and carbon budget components. We find that lower elevation ecosystems exhibited an intense and short greening period leading to a net carbon release, while the high elevation ecosystem showed an extensive water use strategy with delayed greening of longer duration leading to net carbon uptake during the NAM. Access to groundwater appears to reduce the dependence of deep‐rooted riparian trees at low elevation and mountain trees on seasonal rainfall, allowing for a lower water use efficiency as compared to subtropical scrublands sustained by water in shallow soils. Thus, a transition from intensive to extensive water use strategies can be expected where there is reliable access to groundwater.
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Precipitation, geopotential height, and wind fields from 21 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are examined to determine how well this generation of general circulation models represents the North American monsoon system (NAMS). Results show no improvement since CMIP3 in the magnitude (root-mean-square error and bias) of the mean annual cycle of monthly precipitation over a core monsoon domain, but improvement in the phasing of the seasonal cycle in precipitation is notable. Monsoon onset is early for most models but is clearly visible in daily climatological precipitation, whereas monsoon retreat is highly variable and unclear in daily climatological precipitation. Models that best capture large-scale circulation patterns at a low level usually have realistic representations of the NAMS, but even the best models poorly represent monsoon retreat. Difficulty in reproducing monsoon retreat results from an in- accurate representation of gradients in low-level geopotential height across the larger region, which causes an unrealistic flux of low-level moisture from the tropics into the NAMS region that extends well into the post- monsoon season. Composites of the models with the best and worst representations of the NAMS indicate that adequate representation of the monsoon during the early to midseason can be achieved even with a large-scale circulation pattern bias, as long as the bias is spatially consistent over the larger region influencing monsoon development; in other words, as with monsoon retreat, it is the inaccuracy of the spatial gradients in geopotential height across the larger region that prevents some models from realistic representation of the early and mid- season monsoon system.
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This paper examines similarities and differences among major features of the North and South American monsoon systems. Over both North and South America the summertime circulation shows upper-level anticyclone/low-level heat low structures. These develop at different distances from the equator. It is argued that ascent to the east where convective and subtropical convergence zones develop, and subsidence over the cool waters of the eastern Pacific where stratocumulus decks provide a radiative heat sink to the tropical atmosphere are integral and unifying aspects of both monsoon systems. The intraseasonal and interannual variability of the systems are contrasted. The reported links between anomalies in soil conditions and sea surface temperatures are marginal, and consistently long-range predictability is low. Ropelewski et al. (2004) and Grimm et al. (2004) focus on each of the American monsoon systems in companion papers.
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The summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans. In winter, the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography. Here, the mechanics of the summer and winter lower-tropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations.By prescribing in the model the heatings associated with several of the world's monsoons, it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the Kelvin wave response to the monsoon heating over the continent to the west. A poleward-flowing low-level jet into a monsoon (such as the Great Plains jet) is required for Sverdrup vorticity balance. This jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region. The low-level jet into North America induced by its monsoon heating is augmented by a remote response to the Asian monsoon heating.The Rossby wave response to the west of subtropical monsoon heating, interacting with the midlatitude westerlies, produces a region of adiabatic descent. It is demonstrated here that a local `diabatic enhancement' can lead to a strengthening of the descent. Longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region. Below the descent, Sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through Ekman transport. Feedbacks, involving, for example, sea surface temperatures, may further enhance the descent in these regions. The conclusion is that the Mediterranean-type climates of regions such as California and Chile may be induced remotely by the monsoon to the east.Hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems, each involving monsoon rains, a low-level poleward jet, a subtropical anticyclone to the east, and descent and equatorward flow to the west.In winter, it is demonstrated how the nonlinear interaction between the strong zonal-mean circulation, associated with the winter `Hadley cell,' and the mountains can define many of the large-scale features of the subtropical circulation. The blocking effect of the longitudinal mountain chains is shown to be very important. Subsequent diabatic effects, such as a local diabatic enhancement, would appear to be essential for producing the observed amplitude of these features.
Using the percent of climatological stations reporting rain as a measure of the raininess of a particular day in Arizona, a large increase in rainfall within a few days is found to occur about July 1 in most Arizona summers. By means of flow charts, upper air sequences, mean soundings, and diurnal temperature ranges, this increase is shown to be the result of a rather sharp transition from one dominant air mass to another over the state. The occurrence appears to be related to index, and a hemispherical singularity also appears to be related to the phenomenon.
The regional circulations that contribute moisture to the large precipitation over northwestern Mexico, the core region of the North American monsoon, are investigated using three summer seasons (July-September 1995-97) of Eta Model mesoscale analyses and forecasts. Analyses are produced by the Eta Model's own four-dimensional data assimilation system that includes a diverse mix of observations. Comparison of the forecast precipitation with satellite estimates and previous observational studies shows similarity in location, shape, and scale of the patterns over northwestern Mexico; the magnitude of the precipitation over the slopes of the Sierra Madre Occidental is also similar to that from climatologies based on rain gauge observations. Examination of the morning and evening forecast precipitation also reveals agreement with equivalent estimates from high resolution satellites. Excessive model forecast precipitation is found over the Isthmus of Tehuantepec in eastern Mexico, which seems related, at least in part, to deficiencies in the convective parameterization scheme. Special attention is given to the diurnal cycle that is needed to resolve the interactions between circulation and precipitation. The Gulf of California exhibits evaporation through the entire diurnal cycle. In contrast, moisture flux divergence has a marked diurnal cycle with the largest magnitude over the gulf during the afternoon; this divergence is associated with the afternoon sea and valley breezes that favor a net transport of moisture toward the western slopes of the Sierra Madre Occidental. At the same time. large convergence of moisture flux develops over the slopes of the Sierra Madre Occidental, and is followed by intense afternoon-evening precipitation. The reverse circulation during nighttime and early morning results in moisture flux convergence near the coastline and over water, where early morning precipitation develops. Large divergence of moisture flux is found over the northern sector of the Gulf of California at all times, and it results almost equally from transients and the time mean flow. The time mean flow is characterized by a nighttime and predawn low-level jet whose intensity is weaker than the Great Plains counterpart, but still appears to transport a significant amount of moisture into the southwestern United States. Northward transport of moisture is also accomplished by the transient fluxes that include, but are not limited to, the episodic northward moist surges frequently discussed in the literature.
North American monsoon is accompanied by large-scale changes in circulation and precipitation over much of Mexico and the United States during summer. Here, the influence of the North American monsoon is analyzed in terms of midlevel changes to the thermodynamic energy equation, circulation, and precipitation associated with the monsoon onset in northwest Mexico, for the 1948-2004 period. In addition to the well-known strong increase in rising motion over the core region of the monsoon during the onset, there is also a decrease in upward motion over the northern Baja California Peninsula and into the southwest United States, directly in the path of monsoon development. This area of decreased vertical motion is linked to cold advection caused by the onset itself, as the Gill-Matsuno response to the monsoon precipitation thermodynamically interacts with the mean circulation. It is in this sense that we propose that the monsoon is self-limiting.
The origins and transport of water vapor into the semi-arid Sonoran Desert region of southwestern North America are examined for the July-August wet season. Vertically integrated fluxes and flux divergences of water vapor are computed for the 8 summers 1985-1992 from ECMWF mandatory level analyses possessing a spectral resolution of triangular 106 (T106).The ECMWF analyses indicate that transports of water vapor by the time-mean flow dominate the transports by the transient eddies. Most of the moisture at upper levels (above 700 mb) over the Sonoran Desert arrives from over the Gulf of Mexico, while most moisture at low levels (below 700 mb) comes from the northern Gulf of California. There is no indication of moisture entering the Sonoran Desert at low levels directly from the southern Gulf of California or the tropical East Pacific. Water vapor from the tropical East Pacific can enter the region at upper levels after upward transport from low levels along the western slopes of the Sierra Madre Occidental of Mexico and subsequent horizontal transport aloft.The T106 ECMWF analyses, when only the mandatory level analyses are used, do not possess sufficient resolution to yield accurate estimates of highly differentiated quantities such as the divergence of the vertically integrated flux of water vapor. Even at a T1O6 resolution, the northern Gulf of California and the terrain of the Baja California peninsula are not adequately resolved.
The Mexican monsoon is a significant feature in the climate of the southwestern United States and Mexico during the summer months. Rainfall in northwestern Mexico during the months of July through September accounts for 60% to 80% of the total annual rainfall, while rainfall in Arizona for these same months accounts for over 40% of the total annual rainfall. Deep convection during the monsoon season produces frequent damaging surface winds, flash flooding, and hail and is a difficult forecast problem. Past numerical simulations frequently have been unable to reproduce the widespread, heavy rains over Mexico and the southwestern United States associated with the monsoon.The Pennsylvania State University/National Center for Atmospheric Research mesoscale model is used to simulate 32 successive 24-h periods during the monsoon season. Mean fields produced by the model simulations are compared against observations to validate the ability of the model to reproduce many of the observed features, including the large-scale midtropospheric wind field, southerly low-level winds over the Gulf of California, and the heavy rains over western Mexico. Preliminary analysis of the mean model fields also suggest that the Gulf of California is the dominant moisture source for deep convection over Mexico and the southwestern United States, with upslope flow along the Sierra Madre Occidental advecting low-level gulf moisture into western Mexico during the daytime and southerly flow at the northern end of the gulf advecting gulf moisture into Arizona on most days. These results illustrate the usefulness of four-dimensional data assimilation techniques to create proxy datasets containing realistic mesoscale features that can be used for detailed diagnostic studies.