North American Monsoon System Research Articles

Role of Antecedent Land Surface Conditions in Warm Season Precipitation over Northwestern Mexico

Abstract The role of antecedent land surface conditions including precipitation (P), surface skin temperature (Ts), soil moisture (Sm), and snow water equivalent (SWE) anomalies on the onset and intensity of the monsoon during the 1950–99 period in the core of the North American monsoon system (NAMS) region in northwestern Mexico (termed MSa here) is explored. A statistically significant positive relationship is found between monsoon onset date in MSa and previous winter precipitation in the southwestern United States (SW) and northwestern (NW) Mexico, and winter SWE in the southern Rocky Mountains. The linkages are strong during the 1960s–80s and weak otherwise, which is a much shorter period than had been found previously for an SW target area termed monsoon west (MW). In the MW study, the following land surface feedback hypothesis was proposed: more winter P and SWE lead to more spring Sm, hence lower spring and early summer Ts, which induce a weaker onset of the NAMS. This hypothesis broke down in MW due to the small contribution of land surface memory to surface thermal condition, and hence to monsoon strength. The same hypothesis is in this work for MSa by examining three links. First, it is found that in May not only the total column, but also the near-surface Sm, in both SW and NW Mexico have memory from the previous winter precipitation. The spring Sm anomalies correlate negatively with Ts anomalies over most of the continental United States and Mexico except for the desert region of SW and NW Mexico. The monsoon onset is negatively correlated with May Ts over an area roughly consisting of New Mexico and some adjacent areas, suggesting that antecedent land surface conditions may influence the premonsoon surface thermal condition, which then affects monsoon onset. The monsoon-driving force concept that states that the strength of the monsoon should be related to premonsoon land–sea surface temperature contrasts is also confirmed. The confirmation of this concept shows that late monsoon years are associated with colder land and warmer adjacent ocean than early monsoon years. In addition to the apparent land surface feedback, a strong positive relationship between May Ts anomalies and the large-scale midtropospheric circulation (Z500) anomalies is found, which suggests that large-scale circulation may play a strong (possibly more important than land feedback) role in modulating the monsoon onset.

Seasonal Shifts in the North American Monsoon

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.

Changes in Vegetation Condition and Surface Fluxes during NAME 2004

Abstract The vegetation in the core region of the North American monsoon (NAM) system changes dramatically after the onset of the summer rains so that large changes may be expected in the surface fluxes of radiation, heat, and moisture. Most of this region lies in the rugged terrain of western Mexico and very few measurements of these fluxes have been made in the past. Surface energy balance measurements were made at seven sites in Sonora, Mexico, and Arizona during the intensive observation period (IOP) of the North American Monsoon Experiment (NAME) in summer 2004 to better understand how land surface vegetation change alters energy flux partitioning. Satellite data were used to obtain time series for vegetation indices and land surface temperature for these sites. The results were analyzed to contrast conditions before the onset of the monsoon with those afterward. As expected, precipitation during the 2004 monsoon was highly variable from site to site, but it fell in greater quantities at the more southern sites. Likewise, large changes in the vegetation index were observed, especially for the subtropical sites in Sonora. However, the changes in the broadband albedo were very small, which was rather surprising. The surface net radiation was consistent with the previous observations, being largest for surfaces that are transpiring and cool, and smallest for surfaces that are dry and hot. The largest evaporation rates were observed for the subtropical forest and riparian vegetation sites. The evaporative fraction for the forest site was highly correlated with its vegetation index, except during the dry spell in August. This period was clearly detected in the land surface temperature data, which rose steadily in this period to a maximum at its end.

A Multiplatform-Merged (MPM) SST Analysis

AbstractPrevious observational studies indicated that local sea surface temperatures (SSTs) near the west coast of the United States, in the Gulf of California, and in the Gulf of Mexico have strong impacts on the North American monsoon (NAM) system. Simulations of the NAM by numerical models are also found to be sensitive to the specification of SSTs. Accordingly, a reliable SST dataset is essential for improving the understanding, simulation, and prediction of the NAM system. In this study, a new fine-resolution SST analysis is constructed by merging in situ observations from ships and buoys with retrievals from National Oceanic and Atmospheric Administration (NOAA) satellites (NOAA-16 and NOAA-17), Geostationary Operational Environmental Satellites (GOES), the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI), and the Advanced Microwave Scanning Radiometer (AMSR). Called the multiplatform-merged (MPM) SST analysis, this new product of 3-hourly SST is defined on a 0.25° × 0.25° latitude–longitude grid over the Western Hemisphere (30°S–60°N, 180°–30°W). The analysis for the period of 15 May–30 September 2004 shows that the MPM is capable of capturing small-scale disturbances such as those associated with the tropical instability waves. It also depicts local sharp gradients around Baja California and the Gulf Stream with reasonable accuracy compared with the existing analyses. Experiments have been conducted to examine the impacts of the addition of satellite observations on the quality of the MPM analysis. Results showed that inclusion of observations from more satellites progressively improves the quantitative accuracy, especially for diurnal amplitude of the analysis, indicating the importance of accommodating observations from multiple platforms in depicting critical details in an SST analysis with high temporal and spatial resolutions.

Submesoscale Spatiotemporal Variability of North American Monsoon Rainfall over Complex Terrain

Abstract The authors analyze information from rain gauges, geostationary infrared satellites, and low earth orbiting radar in order to describe and characterize the submesoscale (<75 km) spatial pattern and temporal dynamics of rainfall in a 50 km × 75 km study area located in Sonora, Mexico, in the periphery of the North American monsoon system core region. The temporal domain spans from 1 July to 31 August 2004, corresponding to one monsoon season. Results reveal that rainfall in the study region is characterized by high spatial and temporal variability, strong diurnal cycles in both frequency and intensity with maxima in the evening hours, and multiscaling behavior in both temporal and spatial fields. The scaling parameters of the spatial rainfall fields exhibit dependence on the rainfall rate at the synoptic scale. The rainfall intensity exhibits a slightly stronger diurnal cycle compared to the rainfall frequency, and the maximum lag time between the two diurnal peaks is within 2.4 h, with earlier peaks observed for rainfall intensity. The time of maximum cold cloud occurrence does not vary with the infrared threshold temperature used (215–235 K), while the amplitude of the diurnal cycle varies in such a way that deep convective cells have stronger diurnal cycles. Furthermore, the results indicate that the diurnal cycle of cold cloud occurrence can be used as a surrogate for some basic features of the diurnal cycle of rainfall. The spatial pattern and temporal dynamics of rainfall are modulated by topographic features and large-scale features (circulation and moisture fields as related to geographical location). As compared to valley areas, mountainous areas are characterized by an earlier diurnal peak, an earlier date of maximum precipitation, closely clustered rainy hours, frequent yet small rainfall events, and less dependence of precipitation accumulation on elevation. As compared to the northern section of the study area, the southern section is characterized by strong convective systems that peak late diurnally. The results of this study are important for understanding the physical processes involved, improving the representation of submesoscale variability in models, downscaling rainfall data from coarse meteorological models to smaller hydrological scales, and interpreting and validating remote sensing rainfall estimates.

Synthesis of Results from the North American Monsoon Experiment (NAME) Process Study

Abstract An international team of scientists from the United States, Mexico, and Central America carried out a major field campaign during the summer of 2004 to develop an improved understanding of the North American monsoon system leading to improved precipitation forecasts. Results from this campaign, which is the centerpiece of the North American Monsoon Experiment (NAME) Process Study, are reported in this issue of the Journal of Climate. In addition to a synthesis of key findings, this brief overview article also raises some important unresolved issues that require further attention. More detailed background information on NAME, including motivating science questions, where NAME 2004 was conducted, when, and the experimental design, was published previously by Higgins et al.

Extreme weather change and the dynamics of oviposition behavior in the pipevine swallowtail, Battus philenor

Prospects of global increases in extreme weather change provide incentive to examine how such change influences animal behavior, for example, behavior associated with resource use. In this study, we examined how oviposition behavior in a southern Arizona population of pipevine swallowtails (Battus philenor L.) responded to changes in their Aristolochia host resource and vegetative background caused by the North American monsoon system. Summer monsoon rains resulted in a flush of non-host vegetation and a more than doubling in rate of landings by host-searching females on non-host vegetation. Rates of discovery of the host species A. watsoni Woot. Standl. decreased by 50% after monsoon rains. Rains did not alter host density appreciably, but resulted in significant increases in host plant size and new growth, two indicators of host suitability for B. philenor larvae. After the rains, mean clutch size on individual host plants increased by a factor of 2.5; the mean proportion of host plants encountered on which a female laid eggs also increased significantly. Females were discriminating about the host plants on which they laid eggs after alightment; plants accepted for oviposition were larger, bore more new growth, and bore fewer larvae than rejected plants. Contrary to predictions from foraging theory, degree of discrimination did not change seasonally. Finally, the rate at which eggs were laid increased seasonally, suggesting that oviposition rates were limited more before monsoon rains by the relatively low quality of hosts than they were after the rains by the relatively low rate at which hosts were found. This latter result suggests that, while butterflies possess behavioral flexibility to respond to extreme weather change, such flexibility may have limits. In particular, expected increases in the severity and frequency of droughts may result in reduced oviposition rates, reductions that could have adverse demographic consequences.

Atmospheric forcing of the eastern tropical Pacific: A review

The increase in marine, land surface, atmospheric and satellite data during recent decades has led to an improved understanding of the air–sea interaction processes in the eastern tropical Pacific. This is also thanks to extensive diagnoses from conceptual and coupled ocean–atmosphere numerical models. In this paper, mean fields of atmospheric variables, such as incoming solar radiation, sea level pressure, winds, wind stress curl, precipitation, evaporation, and surface energy fluxes, are derived from global atmospheric data sets in order to examine the dominant features of the low level atmospheric circulations of the region. The seasonal march of the atmospheric circulations is presented to depict the role of radiative forcing on atmospheric perturbations, especially those dominating the atmosphere at low levels. In the tropics, the trade winds constitute an important north–south energy and moisture exchange mechanism (as part of the low level branch of the Hadley circulation), that determines to a large extent the precipitation distribution in the region, i.e., that associated with the Inter-Tropical Convergence Zone (ITCZ). Monsoonal circulations also play an important role in determining the warm season precipitation distribution over the eastern tropical Pacific through a large variety of air–sea–land interaction mechanisms. Westward traveling waves, tropical cyclones, low latitude cold air intrusions, and other synoptic and mesoscale perturbations associated with the ITCZ are also important elements that modulate the annual rainfall cycle. The low-level jets of the Gulf of California, the Intra-Americas Sea (Gulf of Mexico and Caribbean Sea) and Chocó, Colombia are prominent features of the eastern tropical Pacific low-level circulations related to sub-regional and regional scale precipitation patterns. Observations show that the Intra-Americas Low-Level Jet intensity varies with El Niño/Southern Oscillation (ENSO) phases, however its origin and role in the westward propagation and development of disturbances that may hit the eastern tropical Pacific, such as easterly waves and tropical cyclones, are still unclear. Changes in the intensity of the trade winds in the Caribbean Sea and the Gulf of Mexico (associated with eastern tropical Pacific wind jets) exert an important control on precipitation by means of wind–topography interactions. Gaps in the mountains of southern Mexico and Central America allow strong wind jets to pass over the continent imprinting a unique signal in sea surface temperatures and ocean dynamics of the eastern tropical Pacific. The warm pools of the Americas constitute an important source of moisture for the North American Monsoon System. The northeastern tropical Pacific is a region of intense cyclogenetic activity, just west of the coast of Mesoamerica. Over the oceanic regions, large-scale properties of key variables such as precipitation, moisture, surface energy fluxes and wind stress curl are still uncertain, which inhibits a more comprehensive view of the region and stresses the importance of regional field experiments. Progress has been substantial in the understanding of the ocean and atmospheric dynamics of the eastern tropical Pacific, however, recent observational evidence such as that of a shallow meridional circulation cell in that region, in contrast to the classic concept of the Hadley-type deep meridional circulation, suggests that more in situ observations to validate theories are still necessary. This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific Ocean.

Hydroclimatology of the North American Monsoon region in northwest Mexico

The North American Monsoon (NAM) system controls the warm season climate over much of southwestern North America. In this semi-arid environment, understanding the regional behavior of the hydroclimatology and its associated modes of variability is critically important to effectively predicting and managing perpetually stressed regional water resources. Equally as important is understanding the relationships through which warm season precipitation is converted into streamflow. This work explores the hydroclimatology of northwestern Mexico, i.e. the core region of the NAM, by (a) presenting a thorough review of recent hydroclimatic investigations from the region and (b) developing a detailed hydroclimatology of 15, unregulated, headwater basins along the Sierra Madre Occidental mountains in western Mexico. The present work is distinct from previous studies as it focuses on the intra-seasonal evolution of rainfall-runoff relationships, and contrasts the sub-regional behavior of the rainfall-runoff response. It is found that there is substantial sub-regional coherence in the hydrological response to monsoon precipitation. Three physically plausible regions emerge from a rotated Principal Components Analysis of streamflow and basin-averaged precipitation. Month-to-month streamflow persistence, rainfall-runoff correlation scores and runoff coefficient values demonstrate regional coherence and are generally consistent with what is currently known about sub-regional aspects of NAM precipitation character.

Role of Antecedent Land Surface Conditions on North American Monsoon Rainfall Variability*

Abstract Possible links between North American Monsoon System (NAMS) seasonal [June–July–August–September (JJAS)] precipitation and premonsoon seasonal land surface conditions including precipitation (P), surface air temperature (Ts), soil moisture (Sm), and snow water equivalent (SWE) anomalies are explored during the 1950–2000 period. A statistically significant inverse relationship is found between monsoon precipitation in an area defined as the Monsoon West (Arizona and western New Mexico) and antecedent winter precipitation in the southwestern (SW) United States and the mountainous region in Utah and Colorado (the predictor area). This linkage is strong during 1965–90 and weak otherwise, as has been suggested by previous studies. A land surface feedback hypothesis is proposed to explain this relationship: more winter P leads to more winter and early spring SWE in the predictor area, hence more spring and early summer Sm, and lower spring and early summer Ts, which induces a weaker onset (and less precipitation) of the NAMS and vice versa. All three links in this hypothesis were tested and the existence of a land memory associated with winter precipitation and snow, which can persist until June, was confirmed. However, the results show that this land memory contributes little to the magnitude of NAM precipitation. Winter snow is negatively correlated to late spring Ts in the SW mountainous region, but not in extreme years. In fact, the premonsoon (June) Ts over the U.S. southwest is inversely related to monsoon precipitation, which is the reverse of what is expected based on the hypothesis. The lack of a significant Sm–Ts–P relationship in most of the SW suggests, based on the constructed Sm dataset, that local premonsoon soil wetness conditions play a minor role in the strength of the monsoon. A strong positive relationship between June Ts anomalies and the large-scale midtropospheric circulation before the onset of the monsoon was found, suggesting that the controlling factor for the premonsoon Ts anomalies may not be local (i.e., not from the land surface). The results suggest that further research is needed to elucidate the nature of land–sea–atmosphere interactions as related to the onset of the monsoon.

The Effects of Satellite-Derived Vegetation Cover Variability on Simulated Land–Atmosphere Interactions in the NAMS

Abstract Substantial evolution of Normalized Difference Vegetation Index (NVDI)-derived vegetation cover (Fg) exists in the southwestern United States and Mexico. The intraseasonal and wet-/dry-year fluctuations of Fg are linked to observed precipitation in the North American monsoon system (NAMS). The manner in which the spatial and temporal variability of Fg influences the land–atmosphere energy and moisture fluxes, and associated likelihood of moist convection in the NAMS regions, is examined. For this, the regional climate model (RCM) is employed, with three different Fg boundary conditions to examine the influence of intraseasonal and wet-/dry-year vegetation variability. Results show that a strong link exists between evaporative fraction (EF), surface temperature, and relative humidity in the boundary layer (BL), which is consistent with a positive soil moisture feedback. However, contrary to expectations, higher Fg does not consistently enhance EF across the NAMS region. This is because the low soil moisture values simulated by the land surface model (LSM) yield high canopy resistance values throughout the monsoon season. As a result, the experiment with the lowest Fg yields the greatest EF and precipitation in the NAMS region, and also modulates regional atmospheric circulation that steers the track of tropical cyclones. In conclusion, the simulated influence of vegetation on land–atmosphere exchanges depends strongly on the canopy stress index parameterized in the LSM. Therefore, a reliable dataset, at appropriate scales, is needed to calibrate transpiration schemes and to assess simulated and realistic vegetation–atmosphere interactions in the NAMS region.

Model Study of Evolution and Diurnal Variations of Rainfall in the North American Monsoon during June and July 2002

Abstract Rainfall evolution and diurnal variation are important components in the North American monsoon system (NAMS). In this study these components are numerically studied using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with high resolution (12-km grids) in contrast to most previous model studies that used relatively coarse spatial resolutions (>25 km grids). The model was initialized at the start of each month and allowed to run for 31 days. The study shows that, in general, the model results broadly matched the patterns of satellite-retrieved rainfall data for monthly rainfall accumulation. The rainfall timing evolution in the monsoon core region predicted by the model generally matched the gauge observations. However, the differences among the three precipitation estimates (model, satellite, and gauge) are obvious, especially in July. The rainfall diurnal cycle pattern was reproduced in the monsoon core region of west...

An Index of Interannual Precipitation Variability in the Core of the North American Monsoon Region

Abstract Seasonal precipitation anomalies associated with the continental North American monsoon system are characterized using a land-based dataset derived from in situ observations across the southwestern United States and northwestern Mexico. Coherent regions of interannual continental precipitation variability are derived from principal component analysis, after defining separate “early” and “late” summer monsoon seasons. The gravest mode of late-season interannual variability captures precipitation anomalies in the core of the continental monsoon domain. A simple spatial average is developed as an index of this core variability. The seasonal separation allows examination of persistence of precipitation anomalies as an indicator of practical late-season predictability. Possible influences of large-scale oceanic interannual fluctuations [ENSO and Pacific decadal oscillation (PDO)] on core index precipitation anomalies are also considered. The core precipitation index exhibits considerably more early-to-late-season persistence than an ocean-centered precipitation index. Implications of these results for monthly/seasonal predictability of warm-season precipitation are discussed.

Model Climatology of the North American Monsoon Onset Period during 1980–2001

Abstract In this study, the seasonal development of the North American monsoon system (NAMS), as simulated by a mesoscale model during a 22-yr simulation from 1980 through 2001, is assessed. Comparison between model simulations and observations shows that the model simulation reproduces the precipitation, skin temperature, and wind field patterns in the seasonal development (May–July) of the NAMS reasonably well and that the mesoscale features and spatial heterogeneity of the NAMS are described correctly. The onset of the monsoon in the central and southern Sierra Madre Occidental (SMO) in Mexico occurs on 20 June, about 2 weeks earlier than the onset in Sonora, Mexico (6 July), the Sonoran Desert, and central Arizona and New Mexico (8 July). The temperature in Mexico is highest after the onset of the monsoon and then decreases with the increasing monsoon rainfall. However, the temperature in the Sonoran Desert and central Arizona and New Mexico is highest just prior to the onset of the monsoon, and high ...

Soil moisture–precipitation feedback on the North American monsoon system in the MM5‐OSU model

AbstractIn this study, the Pennsylvania State University/National Center for Atmospheric Research fifth generation Mesoscale Model (MM5) linked to the Oregon State University (OSU) land‐surface scheme, is used to assess the strength of soil moisture–precipitation feedback in the region of influence of the North American monsoon (NAM). Two control simulations are made with external forcing taken from the National Centers for Environmental Prediction re‐analysis, and with a nested horizontal resolution of 30 km, for the period 1 June to 30 September in wetter than average (1999) and drier than average (2000) monsoon seasons. These two model runs are then repeated with a prescribed precipitation rate anomaly in July over the entire NAM region, and comparisons made between atmospheric and land‐surface states in the two control runs and the two runs with anomalous precipitation. The results show that size and importance of soil moisture–precipitation feedbacks in the NAM region have substantial interannual variability, and that the resulting behaviour has a strong dependency on the intensity of the prescribed precipitation anomaly. It is also shown that a marked precipitation anomaly in the NAM region results in modified soil moisture, rainfall, and surface temperature, which persist for about one month, and that a precipitation anomaly within the NAM region not only has an impact on soil moisture locally, but also causes a remote, downwind soil moisture anomaly one month later. Analysis of the modelled response to the soil moisture anomaly indicates that not only land–atmosphere interactions, but also the large‐scale atmospheric circulation act together to determine the modified precipitation and soil moisture fields in the NAM system. Copyright © 2004 Royal Meteorological Society

Simulations of the North American Monsoon System. Part I: Model Analysis of the 1993 Monsoon Season

Abstract The North American monsoon system is known to produce significant summertime precipitation on the west coast of Mexico and the southwestern United States, with some areas receiving greater than 50% of their yearly rainfall between the months of July and September. The onset of the monsoon is attributed to a shift in the large-scale upper-level anticyclonic flow over the central United States, and the associated increases in moisture flux and resulting precipitation are tied to the low-level jets from the Gulf of California and the Gulf of Mexico. Individual monsoon surge events vary in intensity, as does the magnitude of the diurnal cycle of the low-level jets and precipitation. Numerical modeling and forecasting of these interacting large-and mesoscale monsoon features is often difficult in terms of accurately recreating the varying flow regimes aloft and near the surface and over both the flat and steep terrain that are encompassed within the monsoon region of influence. The Regional Atmospheri...

SST data improve modeling of North American monsoon rainfall

The North American Monsoon System (NAMS) is the large‐scale atmospheric circulation that results from complex ocean‐atmosphere‐land interactions. It strongly modulates the summer weather/climate over Mexico and the southwestern United States. From June to September, NAMS precipitation provides much‐needed water for the semi‐arid region, and is also the cause of frequent severe weather involving damaging winds, hail, lightning, and hydrological hazards, such as flash floods. In recent decades, the southwestern United States has experienced significant economic and population growth. Regional communities and agencies place strong demands on weather, climate, and hydrologic forecasters in an effort to better manage their water resources.

Numerical simulation of the North American monsoon system

¶This paper deals with the systematic evolution of the North American monsoon system by analyzing the evolution of tropospheric circulation, precipitation, moisture fluxes and the adjoining sea surface temperatures from observations and the Florida State University Nested Regional Spectral Model embedded within a coupled atmosphere–ocean coupled model. The atmospheric part is a global spectral model with triangular truncation T63 and a nested regional spectral model. The global atmospheric model is coupled with the Hamburg’s Global Ocean Model. This model was integrated for six-months with 50 km resolution in the North American monsoon domain. The NCEP/NCAR reanalysis were used as the observed state for comparison. The analyzed model results shows that the above model has the capability of capturing some of the features of the North American monsoon system. Besides the evolution of the monsoon system, the out-of-phase relationship between the Mexican monsoon with south-central United States precipitation and the largest moisture supply from the Gulf of Mexico are the principal findings of this study. It is also noted that during the mature phase of the monsoon, the Gulf of California and the Pacific Ocean dominates the Gulf of Mexico in order of moisture supply but immediately after the mature phase, the Gulf of Mexico dominates.

Links between Snow Cover, Surface Skin Temperature, and Rainfall Variability in the North American Monsoon System

The influence of land‐atmosphere interactions on the variability of the North American monsoon system (NAMS) is investigated using the Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (TOVS) Pathfinder, the Climate Prediction Center (CPC) gauge precipitation, and observed snow water equivalent (SWE). Three hypotheses are tested regarding the connection between land surface variables and precipitation in the NAMS region. First, there is a weak negative correlation between 1 April SWE and subsequent surface temperature in the southern Rocky Mountains (SRM) region. However, this connection persists only until June and, therefore, cannot directly influence monsoon rainfall in July and August. Second, there is a negative correlation between SRM surface temperature and NAMS precipitation during the monsoon season, rather than the positive correlation previously proposed. Third, there is a highly significant negative correlation between rainfall and surface temperature within the NAMS region. On the monthly timescale, surface temperature decreases by ;4 K per 1 mm day21 increase in rainfall, consistent with a positive soil moisture‐rainfall feedback. The substantial variability of SRM skin temperature (;10 K) may modulate the temperature gradient between land and ocean. However, these skin temperature anomalies persist only for ;1 month, so their effects are variable throughout the monsoon season.

Multiscale Diagnosis of the North American Monsoon System Using a Variable-Resolution GCM

Abstract The onset and evolution of the North American monsoon system during the summer of 1993 were examined from regional to large scales using the National Aeronautics and Space Administration (NASA) Goddard Earth Observing System (GEOS) stretched-grid GCM. The model's grid spacing for the dynamical core ranges from 0.4° × 0.5° in latitude–longitude over the United States to about 2.5° × 3.5° at the antipode, and the physical package is solved on an intermediate 1° × 1° uniform grid. A diagnostic analysis of the monsoon's onset reveals the development of a positive potential temperature (θ) anomaly at the surface that favors a lower-level cyclonic circulation, while a negative potential vorticity (PV) anomaly below the tropopause induces an upper-level anticyclonic circulation. Ignoring diabatic effects, this pattern is consistent with the superimposition of idealized PV and θ anomalies as previously discussed in the literature. The inclusion of the smaller-scale features of the core monsoon in the mod...