scholarly journals Multiscale Variability of the Flow during the North American Monsoon Experiment

2007 ◽  
Vol 20 (9) ◽  
pp. 1628-1648 ◽  
Author(s):  
Richard H. Johnson ◽  
Paul E. Ciesielski ◽  
Brian D. McNoldy ◽  
Peter J. Rogers ◽  
Richard K. Taft
Keyword(s):  
United States ◽  
North American ◽  
Gulf Of California ◽  
Large Scale ◽  
Regional Scale ◽  
North American Monsoon ◽  
Return Flow ◽  
Southwestern United States ◽  
The North ◽  
Upper Level

Abstract The 2004 North American Monsoon Experiment (NAME) provided an unprecedented observing network for studying the structure and evolution of the North American monsoon. This paper focuses on multiscale characteristics of the flow during NAME from the large scale to the mesoscale using atmospheric sounding data from the enhanced observing network. The onset of the 2004 summer monsoon over the NAME region accompanied the typical northward shift of the upper-level anticyclone or monsoon high over northern Mexico into the southwestern United States, but in 2004 this shift occurred slightly later than normal and the monsoon high did not extend as far north as usual. Consequently, precipitation over the southwestern United States was slightly below normal, although increased troughiness over the Great Plains contributed to increased rainfall over eastern New Mexico and western Texas. The first major pulse of moisture into the Southwest occurred around 13 July in association with a strong Gulf of California surge. This surge was linked to the westward passages of Tropical Storm Blas to the south and an upper-level inverted trough over northern Texas. The development of Blas appeared to be favored as an easterly wave moved into the eastern Pacific during the active phase of a Madden–Julian oscillation. On the regional scale, sounding data reveal a prominent sea breeze along the east shore of the Gulf of California, with a deep return flow as a consequence of the elevated Sierra Madre Occidental (SMO) immediately to the east. Subsidence produced a dry layer over the gulf, whereas a deep moist layer existed over the west slopes of the SMO. A prominent nocturnal low-level jet was present on most days over the northern gulf. The diurnal cycle of heating and moistening (Q1 and Q2) over the SMO was characterized by deep convective profiles in the mid- to upper troposphere at 1800 LT, followed by stratiform-like profiles at midnight, consistent with the observed diurnal evolution of precipitation over this coastal mountainous region. The analyses in the core NAME domain are based on a gridded dataset derived from atmospheric soundings only and, therefore, should prove useful in validating reanalyses and regional models.

scholarly journals The Impact of Horizontal Resolution on North American Monsoon Gulf of California Moisture Surges in a Suite of Coupled Global Climate Models

2016 ◽  
Vol 29 (21) ◽  
pp. 7911-7936 ◽  
Author(s):  
Salvatore Pascale ◽  
Simona Bordoni ◽  
Sarah B. Kapnick ◽  
Gabriel A. Vecchi ◽  
Liwei Jia ◽  
...  
Keyword(s):  
United States ◽  
North American ◽  
Gulf Of California ◽  
Large Scale ◽  
Horizontal Resolution ◽  
North American Monsoon ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Southwestern United States ◽  
Near Surface ◽  
The Impact

Abstract The impact of atmosphere and ocean horizontal resolution on the climatology of North American monsoon Gulf of California (GoC) moisture surges is examined in a suite of global circulation models (CM2.1, FLOR, CM2.5, CM2.6, and HiFLOR) developed at the Geophysical Fluid Dynamics Laboratory (GFDL). These models feature essentially the same physical parameterizations but differ in horizontal resolution in either the atmosphere (≃200, 50, and 25 km) or the ocean (≃1°, 0.25°, and 0.1°). Increasing horizontal atmospheric resolution from 200 to 50 km results in a drastic improvement in the model’s capability of accurately simulating surge events. The climatological near-surface flow and moisture and precipitation anomalies associated with GoC surges are overall satisfactorily simulated in all higher-resolution models. The number of surge events agrees well with reanalyses, but models tend to underestimate July–August surge-related precipitation and overestimate September surge-related rainfall in the southwestern United States. Large-scale controls supporting the development of GoC surges, such as tropical easterly waves (TEWs), tropical cyclones (TCs), and trans-Pacific Rossby wave trains (RWTs), are also well captured, although models tend to underestimate the TEW and TC magnitude and number. Near-surface GoC surge features and their large-scale forcings (TEWs, TCs, and RWTs) do not appear to be substantially affected by a finer representation of the GoC at higher ocean resolution. However, the substantial reduction of the eastern Pacific warm sea surface temperature bias through flux adjustment in the Forecast-Oriented Low Ocean Resolution (FLOR) model leads to an overall improvement of tropical–extratropical controls on GoC moisture surges and the seasonal cycle of precipitation in the southwestern United States.

Seasonal Shifts in the North American Monsoon

2007 ◽  
Vol 20 (9) ◽  
pp. 1923-1935 ◽  
Author(s):  
Katrina Grantz ◽  
Balaji Rajagopalan ◽  
Martyn Clark ◽  
Edith Zagona
Keyword(s):  
United States ◽  
North American ◽  
Gulf Of California ◽  
Monsoon Rainfall ◽  
Monsoon Season ◽  
North American Monsoon ◽  
Southwestern United States ◽  
Monsoon Period ◽  
The North ◽  
Ocean Conditions

Abstract Analysis is performed on the spatiotemporal attributes of North American monsoon system (NAMS) rainfall in the southwestern United States. Trends in the timing and amount of monsoon rainfall for the period 1948–2004 are examined. The timing of the monsoon cycle is tracked by identifying the Julian day when the 10th, 25th, 50th, 75th, and 90th percentiles of the seasonal rainfall total have accumulated. Trends are assessed using the robust Spearman rank correlation analysis and the Kendall–Theil slope estimator. Principal component analysis is used to extract the dominant spatial patterns and these are correlated with antecedent land–ocean–atmosphere variables. Results show a significant delay in the beginning, peak, and closing stages of the monsoon in recent decades. The results also show a decrease in rainfall during July and a corresponding increase in rainfall during August and September. Relating these attributes of the summer rainfall to antecedent winter–spring land and ocean conditions leads to the proposal of the following hypothesis: warmer tropical Pacific sea surface temperatures (SSTs) and cooler northern Pacific SSTs in the antecedent winter–spring leads to wetter than normal conditions over the desert Southwest (and drier than normal conditions over the Pacific Northwest). This enhanced antecedent wetness delays the seasonal heating of the North American continent that is necessary to establish the monsoonal land–ocean temperature gradient. The delay in seasonal warming in turn delays the monsoon initiation, thus reducing rainfall during the typical early monsoon period (July) and increasing rainfall during the later months of the monsoon season (August and September). While the rainfall during the early monsoon appears to be most modulated by antecedent winter–spring Pacific SST patterns, the rainfall in the later part of the monsoon seems to be driven largely by the near-term SST conditions surrounding the monsoon region along the coast of California and the Gulf of California. The role of antecedent land and ocean conditions in modulating the following summer monsoon appears to be quite significant. This enhances the prospects for long-lead forecasts of monsoon rainfall over the southwestern United States, which could have significant implications for water resources planning and management in this water-scarce region.

scholarly journals Flash Flooding in Arid/Semiarid Regions: Climatological Analyses of Flood-Producing Storms in Central Arizona during the North American Monsoon

2019 ◽  
Vol 20 (7) ◽  
pp. 1449-1471 ◽  
Author(s):  
Long Yang ◽  
James Smith ◽  
Mary Lynn Baeck ◽  
Efrat Morin
Keyword(s):  
United States ◽  
North American ◽  
Large Scale ◽  
North American Monsoon ◽  
Time Variability ◽  
Southwestern United States ◽  
Flash Flooding ◽  
The North ◽  
Synoptic Conditions ◽  
Central Arizona

Abstract Flash flooding in the arid/semiarid southwestern United States is frequently associated with convective rainfall during the North American monsoon. In this study, we examine flood-producing storms in central Arizona based on analyses of dense rain gauge observations and stream gauging records as well as North American Regional Reanalysis fields. Our storm catalog consists of 102 storm events during the period of 1988–2014. Synoptic conditions for flood-producing storms are characterized based on principal component analyses. Four dominant synoptic modes are identified, with the first two modes explaining approximately 50% of the variance of the 500-hPa geopotential height. The transitional synoptic pattern from the North American monsoon regime to midlatitude systems is a critical large-scale feature for extreme rainfall and flooding in central Arizona. Contrasting spatial rainfall organizations and storm environment under the four synoptic modes highlights the role of interactions among synoptic conditions, mesoscale processes, and complex terrains in determining space–time variability of convective activities and flash flood hazards in central Arizona. We characterize structure and evolution properties of flood-producing storms based on storm tracking algorithms and 3D radar reflectivity. Fast-moving storm elements can be important ingredients for flash floods in the arid/semiarid southwestern United States. Contrasting storm properties for cloudburst storms highlight the wide spectrum of convective intensities for extreme rain rates in the arid/semiarid southwestern United States and exhibit comparable vertical structures to their counterparts in the eastern United States.

scholarly journals Simulation of a North American Monsoon Gulf Surge Event and Comparison to Observations

2012 ◽  
Vol 140 (8) ◽  
pp. 2534-2554 ◽  
Author(s):  
Andrew J. Newman ◽  
Richard H. Johnson
Keyword(s):  
United States ◽  
North American ◽  
Gulf Of California ◽  
Moisture Flux ◽  
North American Monsoon ◽  
Southwest United States ◽  
Northwest Mexico ◽  
The North ◽  
The Pacific ◽  
Gulf Surges

Abstract Gulf surges are transient disturbances that propagate along the Gulf of California (GoC) from south to north, transporting cool moist air toward the deserts of northwest Mexico and the southwest United States during the North American monsoon. They have been shown to modulate precipitation and have been linked to severe weather and flooding in northern Mexico and the southwest United States. The general features and progression of surge events are well studied, but their detailed evolution is still unclear. To address this, several convection-permitting simulations are performed over the core monsoon region for the 12–14 July 2004 gulf surge event. This surge event occurred during the North American Monsoon Experiment, which allows for extensive comparison to field observations. A 60-h reference simulation is able to reproduce the surge event, capturing its main characteristics: speed and direction of motion, thermodynamic changes during its passage, and strong northward moisture flux. While the timing of the simulated surge is accurate to within 1–3 h, it is weaker and shallower than observed. This deficiency is likely due to a combination of weaker convection and lack of stratiform precipitation along the western slopes of the Sierra Madre Occidental than observed, hence, weaker precipitation evaporation to aid the surge. Sensitivity simulations show that convective outflow does modulate the intensity of the simulated surge, in agreement with past studies. The removal of gap flows from the Pacific Ocean across the Baja Peninsula into the GoC shows they also impact surge intensity.

Dynamics of a Simulated North American Monsoon Gulf Surge Event

2013 ◽  
Vol 141 (9) ◽  
pp. 3238-3253 ◽  
Author(s):  
Andrew J. Newman ◽  
Richard H. Johnson
Keyword(s):  
United States ◽  
North American ◽  
Gulf Of California ◽  
Dynamical Evolution ◽  
North American Monsoon ◽  
The Core ◽  
Southwest United States ◽  
Northwest Mexico ◽  
The North ◽  
Internal Bores

Abstract Gulf surges are transient disturbances that propagate along the Gulf of California (GoC) from south to north, transporting cool moist air toward the deserts of northwest Mexico and the southwest United States during the North American monsoon. They have been shown to modulate precipitation and have been linked to severe weather and flooding in northern Mexico and the southwest United States. The general features and progression of surge events are well documented but their detailed dynamical evolution is still unclear. In this study, a convection-permitting simulation is performed over the core monsoon region for the 12–14 July 2004 gulf surge event and the dynamics of the simulated surge are examined. Initially, convection associated with the tropical easterly wave precursor to Tropical Cyclone Blas creates a disturbance in the southern GoC on early 12 July. This disturbance is a precursor to the gulf surge on 13 July and is a Kelvin shock (internal bore under the influence of rotation) that dissipates in the central GoC. The surge initiates from inflow from the mouth of the GoC along with convective outflow impinging on the southern GoC. Continued convective outflow along the GoC generates multiple gravity currents and internal bores while intensifying the simulated surge as it propagates up the GoC. As the core of the surge reaches the northern GoC, a Kelvin shock is again the best dynamical fit to the phenomenon. Substantial low-level cooling and moistening are associated with the modeled surge along the northern GoC as is observed.

Regional Surges of Monsoonal Moisture into the Southwestern United States

2013 ◽  
Vol 141 (1) ◽  
pp. 182-191 ◽  
Author(s):  
James E. Favors ◽  
John T. Abatzoglou
Keyword(s):  
United States ◽  
Pacific Northwest ◽  
North American ◽  
Gulf Of California ◽  
Precipitable Water ◽  
Western United States ◽  
Southwestern United States ◽  
Vapor Flux ◽  
The North ◽  
Easterly Wave

Abstract Episodic surges of moisture into the southwestern United States are an important attribute of the North American monsoon. Building upon prior studies that identified mesoscale gulf surges using station-based diagnostics, regional surges in monsoonal moisture are identified using precipitable water and integrated water vapor flux from the North American Regional Reanalysis. These regional surge diagnostics exhibit increased skill over gulf surge diagnostics in capturing widespread significant multiday precipitation over the state of Arizona and are associated with the northward intrusion of moisture and precipitation into the southwestern United States. Both tropical and midlatitude circulation patterns are associated with identified regional surge events. In the tropics, the passage of a tropical easterly wave across the Sierra Madre and through the Gulf of California facilitates a northeastward flux of moisture toward the southwestern United States. In midlatitudes, the breakdown and eastward shift of an upper-level ridge over the western United States ahead of an eastward-propagating trough off the Pacific Northwest coast helps destabilize the middle troposphere ahead of the easterly wave and provides a conduit for subtropical moisture advection into the interior western United States.

Distinguishing Pronounced Droughts in the Southwestern United States: Seasonality and Effects of Warmer Temperatures

2009 ◽  
Vol 22 (22) ◽  
pp. 5918-5932 ◽  
Author(s):  
Jeremy L. Weiss ◽  
Christopher L. Castro ◽  
Jonathan T. Overpeck
Keyword(s):  
United States ◽  
North American ◽  
North American Monsoon ◽  
Southwestern United States ◽  
Atmospheric Humidity ◽  
The Core ◽  
Monsoon Area ◽  
The 1950S ◽  
Time Of Year ◽  
Moisture Conditions

Abstract Higher temperatures increase the moisture-holding capacity of the atmosphere and can lead to greater atmospheric demand for evapotranspiration, especially during warmer seasons of the year. Increases in precipitation or atmospheric humidity ameliorate this enhanced demand, whereas decreases exacerbate it. In the southwestern United States (Southwest), this means the greatest changes in evapotranspirational demand resulting from higher temperatures could occur during the hot–dry foresummer and hot–wet monsoon. Here seasonal differences in surface climate observations are examined to determine how temperature and moisture conditions affected evapotranspirational demand during the pronounced Southwest droughts of the 1950s and 2000s, the latter likely influenced by warmer temperatures now attributed mostly to the buildup of greenhouse gases. In the hot–dry foresummer during the 2000s drought, much of the Southwest experienced significantly warmer temperatures that largely drove greater evapotranspirational demand. Lower atmospheric humidity at this time of year over parts of the region also allowed evapotranspirational demand to increase. Significantly warmer temperatures in the hot–wet monsoon during the more recent drought also primarily drove greater evapotranspirational demand, but only for parts of the region outside of the core North American monsoon area. Had atmospheric humidity during the more recent drought been as low as during the 1950s drought in the core North American monsoon area at this time of year, greater evapotranspirational demand during the 2000s drought could have been more spatially extensive. With projections of future climate indicating continued warming in the region, evapotranspirational demand during the hot–dry and hot–wet seasons possibly will be more severe in future droughts and result in more extreme conditions in the Southwest, a disproportionate amount negatively impacting society.

Intraseasonal Teleconnection between North American and Western North Pacific Monsoons with 20-Day Time Scale

2008 ◽  
Vol 21 (11) ◽  
pp. 2664-2679 ◽  
Author(s):  
Xianan Jiang ◽  
Ngar-Cheung Lau
Keyword(s):  
United States ◽  
North American ◽  
North Pacific ◽  
Time Scale ◽  
Western North Pacific ◽  
Wave Train ◽  
Eastern Pacific ◽  
Southwestern United States ◽  
Low Level ◽  
The North

Abstract Based on a recently released, high-resolution reanalysis dataset for the North American region, the intraseasonal variability (ISV; with a time scale of about 20 days) of the North American monsoon (NAM) is examined. The rainfall signals associated with this phenomenon first emerge near the Gulf of Mexico and eastern Pacific at about 20°N. They subsequently migrate to the southwestern United States along the slope of the Sierra Madre Occidental. The rainfall quickly dissipates upon arrival at the desert region of Arizona and New Mexico (AZNM). The enhanced rainfall over AZNM is accompanied by strong southeasterly low-level flow along the Gulf of California. This pattern bears strong resemblance to the circulation related to “gulf surge” events, as documented by many studies. The southeasterly flow is associated with an anomalous low vortex over the subtropical eastern Pacific Ocean off California, and a midlatitude anticyclone over the central United States in the lower troposphere. This flow pattern is in broad agreement with that favoring the “wet surges” over the southwestern United States. It is further demonstrated that the aforementioned low-level circulations associated with ISV of the NAM are part of a prominent trans-Pacific wave train extending from the western North Pacific (WNP) to the Eastern Pacific/North America along a “great circle” path. The circulation anomalies along the axis of this wave train exhibit a barotropic vertical structure over most regions outside of the WNP, and a baroclinic structure over the WNP, thus suggesting the important role of convective activities over the WNP in sustaining this wave train. This inference is further substantiated by an analysis of the pattern of wave-activity–flux vectors. Variations in the WNP convection are correlated with the ISV of the monsoons in both North American and East Asian (EA)/WNP sectors. These relationships lead to notable teleconnections between NAM and the EA/WNP monsoon on 20-day time scales.

Observational Analysis of an Upper-Level Inverted Trough during the 2004 North American Monsoon Experiment

2010 ◽  
Vol 138 (9) ◽  
pp. 3540-3555 ◽  
Author(s):  
Zachary O. Finch ◽  
Richard H. Johnson
Keyword(s):  
North American ◽  
Cyclonic Circulation ◽  
Primary Data ◽  
North American Monsoon ◽  
The West ◽  
Colorado State University ◽  
Downward Motion ◽  
The North ◽  
Northwestern Mexico ◽  
Upper Level

Abstract Upper-level inverted troughs (IVs) associated with midlatitude breaking Rossby waves or tropical upper-troposphere troughs (TUTTs) have been identified as important contributors to the variability of rainfall in the North American monsoon (NAM) region. However, little attention has been given to the dynamics of these systems owing to the sparse observational network over the NAM region. High temporal and spatial observations taken during the 2004 North American Monsoon Experiment (NAME) are utilized to analyze a significant IV that passed over northwestern Mexico from 10 to 13 July 2004. The Colorado State University gridded dataset, which is independent of model analysis over land, is the primary data source used in this study. Results show that the 10–13 July IV disturbance was characterized by a warm anomaly around 100 hPa and a cold anomaly that extended from 200 to 700 hPa. The strongest cyclonic circulation was in the upper levels around 200 hPa. Quasigeostrophic (QG) diagnostics indicate that the upper-level low forced weak subsidence (weak rising motion) to the west (east) of its center. Net downward motion to the west was a result of the Laplacian of thermal advection (forcing subsidence) outweighing differential vorticity advection (forcing weak upward motion). Despite the QG forcing of downward motion west of the upper-level IV, enhanced convection occurred west of the IV center along the western slopes of the Sierra Madre Occidental (SMO). This seemingly contradictory behavior can be explained by noting that the upper-level IV induced a midlevel cyclonic circulation, with northeasterly (southeasterly) midlevel flow to the west (east) of its center. Increased mesoscale organization of convection along the SMO foothills was found to be collocated with IV-enhanced northeasterly midlevel flow and anomalous northeasterly shear on the western (leading) flank of the system. It is proposed that the upper-level IV increased the SMO-perpendicular midlevel flow as well as the wind shear, thereby creating an environment favorable for convective storms to grow upscale as they moved off the high terrain.

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