North American Monsoon Precipitation Research Articles

California margin temperatures modulate regional circulation and extreme summer precipitation in the desert Southwest

In August 2022, Death Valley, the driest place in North America, experienced record flooding from summertime rainfall associated with the North American monsoon (NAM). Given the socioeconomic cost of these type of events, there is a dire need to understand their drivers and future statistics. Existing theory predicts that increases in the intensity of precipitation is a robust response to anthropogenic warming. Paleoclimatic evidence suggests that northeast Pacific (NEP) sea surface temperature (SST) variability could further intensify summertime NAM rainfall over the desert southwest. Drawing on this paleoclimatic evidence, we use historical observations and reanalyzes to test the hypothesis that warm SSTs on the southern California margin are linked to more frequent extreme precipitation events in the NAM domain. We find that summers with above-average coastal SSTs are more favorable to moist convection in the northern edge of the NAM domain (southern California, Arizona, New Mexico, and the southern Great Basin). This is because warmer SSTs drive circulation changes that increase moisture flux into the desert southwest, driving more frequent precipitation extremes and increases in seasonal rainfall totals. These results, which are robust across observational products, establish a linkage between marine and terrestrial extremes, since summers with anomalously warm SSTs on the California margin have been linked to seasonal or multi-year NEP marine heatwaves. However, current generation earth system models (ESMs) struggle to reproduce the observed relationship between coastal SSTs and NAM precipitation. Across models, there is a strong negative relationship between the magnitude of an ESM’s warm SST bias on the California margin and its skill at reproducing the correlation with desert southwest rainfall. Given persistent NEP SST biases in ESMs, our results suggest that efforts to improve representation of climatological SSTs are crucial for accurately predicting future changes in hydroclimate extremes in the desert southwest.

The North American Monsoon precipitation response to climate warming at convection-permitting scales

The North American Monsoon precipitation response to climate warming at convection-permitting scales

Paleohydrologic history of pluvial lake San Agustin, New Mexico: Tracking changing effective moisture in southwest North America through the last glacial transition

Paleoclimate records across the Intermountain West region of North America show significant regional variation in timing and magnitude of wet conditions that accompanied the last glacial-interglacial transition. To understand the climate controls on paleohydrologic change, well-dated records are needed across the region. The Plains of San Agustin (New Mexico, USA) is a closed-basin watershed of the American Southwest influenced by both winter westerly and summer North American Monsoon precipitation. The flat valley floors of the Plains contain lake and groundwater discharge deposits that record multiple periods of past wet climate. We present a record of hydroclimate for the past 26,000 years based on radiocarbon, U–Th series, and OSL dating of these deposits and stratigraphic correlation across the three sub-basins of the lake system. We find that two major lake oscillations occurred, coincident with the global Last Glacial Maximum (∼23–18 ka) and with Heinrich Stadial 1 (∼17–14 ka). The LGM lake cycle created a deep lake in the lowermost sub-basin, fed by marsh/lake overflow in the upper sub-basins. The Heinrich Stadial 1 wet interval attained the highest recorded lake level between ∼17.0 and 15.3 ka, merging the lower two sub-basins into one lake. Both lake cycles agree well in timing and magnitude with other lake-based records from the southwestern U.S., supporting previous interpretations that a south-shifted cool season storm track brought a dipole-like pattern of enhanced moisture to the southwest at the expense of drier conditions in the northwest during the LGM and deglaciation. A transition from lake to groundwater discharge conditions followed during the Bølling-Allerød (14.7–12.9 ka) across the study area. Wet meadows prevailed in the lower sub-basin during the Younger Dryas (12.9–11.7 ka), with marsh and open lake conditions in the upper and middle sub-basins, respectively. During the early Holocene, discrete wet intervals are recorded by ages for wet meadow deposits in all sub-basins with centennial-millennial frequency at 9.9, 8.8, 8.2 ka. These events agree well with other Intermountain West records showing wetter-than-present conditions prior to 8 ka. Two additional wet periods, centered at 6.8 and 5.4 ka, occurred during the driest known interval of the middle Holocene, and likely were partly supported in the Plains of San Agustin by summer moisture associated with the peak strength of the North American Monsoon. Our record highlights that both winter and summer moisture support water resources in New Mexico watersheds.

Primary production responses to extreme changes in North American Monsoon precipitation vary by elevation and plant functional composition through time

Abstract Primary production in dryland ecosystems is limited by water availability and projected to be strongly affected by future shifts in seasonal precipitation. Warm‐season precipitation derived from the North American Monsoon contributes 40% of annual precipitation to dryland ecosystems in the southwestern United States and is projected to become more variable. However, there is large uncertainty on whether this variability will be expressed as either extreme wet or dry years and how primary production of different plant functional types will respond across widespread elevation gradients in this region. We experimentally imposed extreme drought and water addition treatments from 2016 to 2020, during which ambient warm‐season precipitation declined to reach historic lows, to understand production sensitivity of dominant plant functional types along a 1000 m elevation gradient. We found that the production responses of plant functional types to monsoon precipitation extremes were dependent on the number of treatment years that occurred across sites along the elevation gradient. C4 perennial grasses were most responsive to precipitation manipulation treatments, followed by C3 perennial grasses and annuals, while perennial forbs and shrubs had weak or no responses. C4 perennial grass reductions due to extreme drought were generally stronger or occurred earlier at low elevation sites, while multi‐year extreme drought extended negative effects to C3 perennial grasses at high elevation, and all sites showed delayed responses to multi‐year water addition. We found that the sensitivity of C3 perennial grass production differed for extreme drought and water addition compared to ambient precipitation at one site, but other sites and plant functional types had similar sensitivities to the different treatment types. Synthesis. The upward advance of primary production responsiveness from single‐ to multi‐year extreme changes in warm‐season precipitation suggests more immediate shifts in functional composition and carbon cycling at low elevation, while high elevation ecosystems may become less resistant as the effects of extreme precipitation compound through time.

Sub‐Seasonal Predictability of North American Monsoon Precipitation

AbstractNorth American Monsoon (NAM) rainfall is a vital water resource in the United States Southwest, providing 60–80% of the region's annual precipitation. However, NAM rainfall is highly variable and water managers lack skillful guidance on summer rainfall that could help inform their management decisions and operations. Here we show that NAM season (June–October) precipitation can be forecasted by the European Centre for Medium‐Range Weather Forecast's model months ahead at catchment scales. This is possible by identifying the frequency of days with synoptic‐scale moisture advection into the NAM region, which greatly improves predictability over directly utilizing modeled precipitation. Other forecasting systems fail to provide useful guidance due to deficiencies in their data assimilation systems and biases in representing key synoptic features of the NAM including its teleconnections.

Stable Isotopes in Precipitation and Meteoric Water: Sourcing and Tracing the North American Monsoon in Arizona, New Mexico, and Utah

AbstractThe North American Monsoon (NAM) is an important source of precipitation across the southwestern United States. The approximate northern boundary of this feature crosses the Navajo Nation (NN), in the Four Corners region, where NAM rains have long been important to the livelihoods of Native Americans. Relatively little is known about the characteristics and hydrological significance of the NAM in this region. Here, we report a new 4 yr record of stable H and O isotope ratios in monsoon‐season rainfall and water resources across the NN. Monthly precipitation samples collected at 39 sites document a characteristic pattern of 2H‐ and 18O‐enrichment associated with monsoonal precipitation. These changes are weakly correlated with local precipitation amount, however, and the correlation that does exist is dominated by sub‐cloud evaporation effects. In contrast to precipitation amount, monsoon‐season isotopic values exhibited little spatial variability across the region, and after correction for sub‐cloud evaporation NN values were similar to those from a site in southern Arizona. Airmass back trajectory analysis suggests that the uniformly high NAM precipitation water isotope ratios across the region may reflect (a) a region‐wide shift from mid‐latitude to low‐latitude moisture sources at the onset of the peak monsoon, and (b) substantial land‐surface recycling of NAM moisture in upwind regions. Comparison of precipitation water isotope data with surface and groundwater values implies that, despite its hydroclimatic significance, monsoon rainfall contributes little to subsurface water resources. This highlights the monsoon's importance for warm‐season land‐surface ecology and hydrology critical to residents in the Four Corners region.

Observed Influence of Soil Moisture on the North American Monsoon: An Assessment Using the Stepwise Generalized Equilibrium Feedback Assessment Method

AbstractThe evidence shows that soil moisture has an important influence on North American monsoon (NAM) precipitation. This study evaluates the local and nonlocal feedbacks of soil moisture on summer (June–September) precipitation in the NAM region using observational data. We applied a multivariate statistical method known as the Stepwise Generalized Equilibrium Feedback Assessment (SGEFA) to control for internal atmospheric variability and sea surface temperature (SST) forcings so that we could isolate the impact of soil moisture feedbacks on NAM precipitation. Our results identify feedback pathways between soil moisture and precipitation in the NAM region and in the southern Rocky Mountains (SRM) region. Wet soils in the SRM result in lower-than-normal local surface temperature, weaker water vapor transport from the eastern Pacific and the Gulf of California (GOC), and less monsoon precipitation. Precipitation over the U.S. Great Plains also significantly increases when there are wet soils in the SRM. This occurs due to an enhanced water vapor influx into this region. On the other hand, anomalously wet soils in the NAM region increase NAM precipitation by enhancing local moist static energy and increasing the strength of the monsoonal circulation. Our observational results using SGEFA agree well with previous numerical modeling studies. This study highlights the critical role of land–atmosphere interactions for understanding NAM variability.

The Impact of Assimilating GPS Precipitable Water Vapor in Convective-Permitting WRF-ARW on North American Monsoon Precipitation Forecasts over Northwest Mexico

AbstractWe assess the impact of GPS precipitable water vapor (GPS-PWV) data assimilation (DA) on short-range North American monsoon (NAM) precipitation forecasts, across 38 days with weak synoptic forcing, during the NAM GPS Hydrometeorological Network field campaign in 2017 over northwest Mexico. Utilizing an ensemble-based data assimilation technique, the GPS-PWV data retrieved from 18 observation sites are assimilated every hour for 12 hours into a 30-member ensemble convective-permitting (2.5 km) Advanced Research version of the Weather Research and Forecasting (WRF-ARW) model. As the assimilation of the GPS-PWV improves the initial condition of WRF by reducing the root mean square error and bias of PWV across 1200-1800 UTC, this also leads to an improvement in capturing nocturnal convection of mesoscale convective systems (MCSs; after 0300 UTC) and to an increase by 0.1 mm h-1 in subsequent precipitation during the 0300-0600 UTC period relative to no assimilation of the GPS-PWV (NODA) over the area with relatively more observation sites. This response is consistent with observed precipitation from the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement Final Precipitation product. Moreover, compared to the NODA, we find that the GPS-PWV DA decreases cloud top temperature, increases most unstable convective available energy and surface dewpoint temperature, and thus creates a more favorable condition for convective organization in the region.

A Review of Climate Change Impacts on the USA-Mexico Transboundary Santa Cruz River Basin

In the parched Upper Santa Cruz River Basin (USCRB), a binational USA–Mexico basin, the water resources depend on rainfall-triggered infrequent flow events in ephemeral channels to recharge its storage-limited aquifers. In-situ data from the basin highlight a year-round warming trend since the 1980s and a concerning decline in average precipitation (streamflow) from 1955–2000 to 2001–2020 by 50% (87.6%) and 17% (63%) during the winter and summer, respectively. Binational sustainable management of the basins water resources requires a careful consideration of prospective climatic changes. In this article we review relevant studies with climate projections for the mid-21st century of four weather systems that affect the region’s precipitation. First, the North American Monsoon (NAM) weather system accounts for ~60% of the region’s annual rainfall. The total NAM precipitation is projected to decline while heavy rainfall events are expected to intensify. Second, the frequency of the pacific cold fronts, the region’s prevalent source of winter precipitation, is projected to decline. Third, the frequency and intensity of future atmospheric rivers, a weather system that brings winter rainfall to the region, are projected to increase. Fourth, the frequency and intensity of large eastern pacific tropical cyclones (TC) are expected to increase. On rare occasions, remnants of TC make their way to the USCRB to cause storms with considerable impact on the region’s water resources. In contrast to the high confidence projections for the warming trend to persist throughout the mid-21st century, the precipitation projections of these four weather systems affecting the region encompass large uncertainties and studies have often reported contradicting trends. An added source of uncertainty is that the USCRB is located at the periphery of the four rain-bearing weather systems and small mesoscale changes in these weather systems may have accentuated impacts on their edges. Despite the high uncertainty in the projections of future precipitation, the early 21st century drying trend and the projected mid-21st century decline in precipitation events serve as a pressing call for planning and actions to attain sustainable water resources management that reliably satisfies future demands.

Using Observational and Reanalysis Data to Explore the Southern Gulf of California Boundary Layer During the North American Monsoon Onset

AbstractThis paper uses rawinsondes and pilot balloon data from the 2004 North American Monsoon (NAM) Experiment, as well as satellite‐based products and reanalysis datasets that span 1982–2018, to analyze the mixing mechanisms responsible for the temporal and spatial variations of the Gulf of California (GoC) boundary layer during the NAM onset in the core monsoon region. We show that the regional diurnal cycle is strongly affected by low‐level convergence and divergence associated with local land‐sea breezes and by the presence of a thermal inversion over the gulf. Earlier starting monsoons have less moisture available for precipitation than those starting later in the calendar year. Therefore, early onset monsoons have less rainfall during their first month, which is a result that is in contrast with previous studies that have analyzed the timing of the NAM but only reported seasonal precipitation totals. The GoC boundary layer height at the time of monsoon onset, found to be controlled by the gulf’s surface temperature, has a significant impact on the precipitation over Sonora, Sinaloa, and southern Arizona. After the erosion of the thermal inversion over the GoC that coincides with the NAM onset, wind shear produced by the region’s unique geographic and topographic features is the largest source of turbulence for the mixing of the boundary layer. Our results suggest that a numerical model used to forecast or analyze NAM precipitation must have enough spatial resolution to adequately reproduce the effects that the GoC’s features have on its complex diurnal circulation systems.

Potential Influences of Volcanic Eruptions on Future Global Land Monsoon Precipitation Changes

AbstractThe global monsoon system is of exceptional socioeconomic importance owing to its impacts on two‐thirds of the globe’s population. Major volcanic eruptions strongly influence global land monsoon (GLM) precipitation change. By using 60 plausible eruption scenarios sampled from reconstructed volcanic proxies over the past 2,500 years, 21st century volcanic influences on GLM precipitation projections are examined with an Earth system model under a moderate emission scenario. The decadal‐scale ensemble spread with realistic eruptions (VOLC) increases by 17.5% and 20.1% compared to no‐volcanic (NO‐VOLC) and constant background‐volcanic (VOLC‐CONST) scenarios, respectively. Compared with NO‐VOLC, the centennial mean VOLC GLM precipitation shows a 10% overall reduction and regionally, Asia is the most impacted. Changes in atmospheric circulation in the aftermath of large volcanic eruptions match the global warming response patterns well with opposite sign, with the North American monsoon precipitation enhanced following large volcanic eruptions, which is in sharp contrast to the robust decrease in Asian monsoon rainfall. Volcanic activity could delay the time of emergence of anthropogenic influence by five years on average over about 60% of the GLM area. Our results demonstrate the importance of statistical representation of potential volcanism for the projections of future monsoon variability. Quantifying volcanic impacts on regional climate projections and their socioeconomic influences on infrastructure planning, food security, and disaster management should be a priority of future work.

Pacific sea surface temperature related influences on North American monsoon precipitation within North American Regional Climate Change Assessment Program models

Climate inter‐annual variability over the North American monsoon (NAM) region is associated with El Niño‐Southern Oscillation (ENSO) and Pacific decadal variability (PDV), which drive a warm season atmospheric teleconnection response. Using the North American Regional Climate Change Assessment Program (NARCCAP) simulations, previous studies have found that regional models forced with an atmospheric reanalysis (NARCCAP Phase I) represent the NAM reasonably well as a climatological feature. However, when these same regional models are forced with global climate model projections (NARCCAP Phase II), their ability to represent the NAM as a salient feature substantially degrades. The present study evaluates NAM inter‐annual climate variability through the continental‐scale patterns of summer precipitation within the NARCCAP simulations (Phases I and II), in relation to ENSO–PDV, and the presence of the driving atmospheric teleconnection response. Multivariate statistical analyses are applied to sea surface temperature and precipitation data sets to determine dominant variability at continental scale, with focus on the southwest. The analysis reveals that NARCCAP Phase I simulations are able to portray the spatial pattern of precipitation associated with ENSO–PDV in a similar way to observations. However, all NARCCAP Phase II simulations, with the exception of the HRM(Hadcm3) regional–global model pair, fail to reproduce this climate variability. Although including all possible NARCCAP model simulations to generate a multi‐model ensemble mean would increase the statistical degree of confidence in climate projections, this type of result would not increase confidence in the physical climatology of model representations of warm season climate variability. More physically based, process‐oriented metrics are needed to evaluate model quality in assessing the uncertainty of future climate change in multi‐model ensemble products used for climate change impacts assessments.

Improvement in the Modeled Representation of North American Monsoon Precipitation Using a Modified Kain–Fritsch Convective Parameterization Scheme

A commonly noted problem in the simulation of warm season convection in the North American monsoon region has been the inability of atmospheric models at the meso-β scales (10 s to 100 s of kilometers) to simulate organized convection, principally mesoscale convective systems. With the use of convective parameterization, high precipitation biases in model simulations are typically observed over the peaks of mountain ranges. To address this issue, the Kain–Fritsch (KF) cumulus parameterization scheme has been modified with new diagnostic equations to compute the updraft velocity, the convective available potential energy closure assumption, and the convective trigger function. The scheme has been adapted for use in the Weather Research and Forecasting (WRF). A numerical weather prediction-type simulation is conducted for the North American Monsoon Experiment Intensive Observing Period 2 and a regional climate simulation is performed, by dynamically downscaling. In both of these applications, there are notable improvements in the WRF model-simulated precipitation due to the better representation of organized, propagating convection. The use of the modified KF scheme for atmospheric model simulations may provide a more computationally economical alternative to improve the representation of organized convection, as compared to convective-permitting simulations at the kilometer scale or a super-parameterization approach.

The More Extreme Nature of North American Monsoon Precipitation in the Southwestern United States as Revealed by a Historical Climatology of Simulated Severe Weather Events

AbstractLong-term changes in North American monsoon (NAM) precipitation intensity in the southwestern United States are evaluated through the use of convective-permitting model simulations of objectively identified severe weather events during “historical past” (1950–70) and “present day” (1991–2010) periods. Severe weather events are the days on which the highest atmospheric instability and moisture occur within a long-term regional climate simulation. Simulations of severe weather event days are performed with convective-permitting (2.5 km) grid spacing, and these simulations are compared with available observed precipitation data to evaluate the model performance and to verify any statistically significant model-simulated trends in precipitation. Statistical evaluation of precipitation extremes is performed using a peaks-over-threshold approach with a generalized Pareto distribution. A statistically significant long-term increase in atmospheric moisture and instability is associated with an increase in extreme monsoon precipitation in observations and simulations of severe weather events, corresponding to similar behavior in station-based precipitation observations in the Southwest. Precipitation is becoming more intense within the context of the diurnal cycle of convection. The largest modeled increases in extreme-event precipitation occur in central and southwestern Arizona, where mesoscale convective systems account for a majority of monsoon precipitation and where relatively large modeled increases in precipitable water occur. Therefore, it is concluded that a more favorable thermodynamic environment in the southwestern United States is facilitating stronger organized monsoon convection during at least the last 20 years.

A Method for Measuring Sub-Annual Ring Widths ofPinus Edulisfor Seasonal Climate Reconstructions

Pinus edulis is one of the most ubiquitous tree species in the US Southwest. It accounts for over a fifth of the total number of trees in New Mexico alone. Its prevalence and relatively long-lived nature makes it an ideal candidate for dendroclimatological studies of the North American Monsoon. The problem occurs with delineating the boundary of the earlywood and latewood for sub-annual reconstructions. In this study, we present a novel method (“the resin duct method”) for delineating the latewood boundary using resin ducts of P. edulis from three sites in New Mexico. The climate sensitivity of partial ring widths of P. edulis is then explored and compared to co-occurring Pinus ponderosa, which has a clear latewood boundary. The method of using resin ducts for delineating latewood in P. edulis resulted in a statistically significant relationship when compared to the latewood widths of co-occurring P. ponderosa. Although we found a similar climate response of P. edulis when compared to P. ponderosa, P. edulis latewood was a poor predictor of North American Monsoon precipitation unlike P. ponderosa. However, P. edulis earlywood has a statistically significant correlation with cool-season precipitation, making it useful for cool-season climate reconstructions in the Southwest.

Bias correction of the CCSM4 for improved regional climate modeling of the North American monsoon

This study investigates how a form of bias correction using linear regression improves the limitations of the community climate system model (CCSM) version 4 when it is dynamically downscaled with the Weather Research and Forecasting (WRF) model for the North American monsoon (NAM). Long-term biases in the CCSM dataset were removed using the climate forecast system reanalysis (CFSR) dataset as a baseline, from which a physically consistent set of bias-corrected variables were created. To quantitatively identify the effects of CCSM data on the NAM simulations, three 32-year climatologies were generated with WRF driven by (1) CFSR, (2) original CCSM, and (3) bias-corrected CCSM data. The WRF-CFSR simulations serve as a baseline for comparison. With the bias correction, onset dates simulated by WRF bias-corrected CCSM data were generally within a week of the WRF-CFSR climatology, while WRF using the original CCSM data occur up to 3–4 weeks too early over the core of the NAM. Additionally, bias-correction led to improvements in the mature phase of the NAM, reducing August root-mean-square-error values by 26 % over the core of the NAM and 36 % over the northern periphery. Comparison of the CFSR and the bias-corrected CCSM climatologies showed marked consistency in the general evolution of the NAM system. Dry biases in the NAM precipitation existed in each climatology with the original CCSM performing the poorest when compared to observations. The poor performance of the original CCSM simulations stem from biases in the thermodynamic profile supplied to the model through lateral boundary conditions. Bias-correction improved the excessive capping inversions, and mid-level mixing ratio dry biases (2–3 g kg−1) present in the CCSM simulations. Improvements in the bias-corrected CCSM data resulted in greater convective activity and a more representative seasonal distribution of precipitation.

Causes of interannual to decadal variability of Gila River streamflow over the past century

Abstract Study region The Gila River, New Mexico, is characterized by two peaks in streamflow: one in the winter–spring (December–May), and summer (August–September). The region is influenced both by Pacific SST variability as well as the North American Monsoon. Study focus The mechanisms responsible for the variability of the winter–spring and summer streamflow peaks are investigated by correlation of streamflow with precipitation and sea surface temperature for 1928–2012. Decadal variability in the flow record is examined for a longer term perspective on Gila River streamflow using tree ring-based reconstructions of the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Index (SPI). New hydrological insights for the region Results indicate a strong influence of winter–spring precipitation and Pacific SST anomalies on the winter–spring streamflow, with El Nino conditions in the Pacific causing increased precipitation and streamflow. Decadal Pacific variability helps explain the transition from high winter flow in the late 20th century to lower flows in the most recent decade. The summer streamflow has a somewhat weaker correlation with precipitation and Pacific SST than the winter–spring streamflow. Its variability is more likely influenced by local North American Monsoon precipitation variability. PDSI and SPI reconstructions indicate much more severe and extended periods of droughts and pluvials in past centuries as well as periods of concurrent winter and summer drought.

Evaluation of Oceanic and Terrestrial Sources of Moisture for the North American Monsoon Using Numerical Models and Precipitation Stable Isotopes

Abstract This work evaluates the oceanic and terrestrial moisture sources that contribute to North American monsoon (NAM) precipitation over a 30-yr period using the modified analytical dynamic recycling model. This computationally efficient modeling framework reveals previously overlooked moisture source regions such as Central America and the Caribbean Sea in addition to the well-known Gulf of California and Gulf of Mexico source regions. The results show that terrestrial evapotranspiration is as important as oceanic evaporation for NAM precipitation, and terrestrial sources contribute to approximately 40% of monsoonal moisture. There is a northward progression of terrestrial moisture sources, beginning with Central America during the early season and transitioning north into northern Mexico and the NAM region itself during the peak of the monsoon season. The most intense precipitation occurs toward the end of the season and tends to originate in the Gulf of California and the tropical Pacific, associated with tropical cyclones and gulf surges. Heavy stable isotopes of hydrogen and oxygen in precipitation (δD and δ18O) collected for every precipitation event measured in Tucson, Arizona, for the period 1981–2008 complement the numerical results. The analysis shows that precipitation events linked to sources from the Gulf of Mexico and Caribbean Sea are more isotopically enriched than sources from the Gulf of California and tropical Pacific. It is also seen that terrestrial regions that derive their precipitation from the Gulf of Mexico are also more isotopically enriched than moisture sources from the Pacific.

A possible link between wildfire aerosol and North American Monsoon precipitation in Arizona–New Mexico

ABSTRACTPrevious research highlights the dominant role of Pacific sea surface temperatures (SSTs) and their associated large‐scale teleconnections in modulating the North American monsoon (NAM). At the regional scale, feedbacks associated with land‐surface boundary conditions have been shown to provide ‘memory’ in the system. Here, a previously unexplored second‐order linkage between aerosol generated by late‐spring wildfires and subsequent summer precipitation delivered by the NAM in the Arizona‐New Mexico (AZNM) region is proposed. Correlations between June/July organic carbon and elemental carbon (OC/EC) in Inter‐agency Monitoring of Protected Visual Environments (IMPROVE) aerosol data (Gila Wilderness and Bandelier) and NAM precipitation in southern AZNM over the period 1994–2012 are shown to be negative (r = –0.4), suggesting that active antecedent wildfire seasons tend to be immediately followed by a weak late summer monsoon. This result is consistent with a previous study linking precipitation and area burned in wildfires in AZNM. A survey of extant literature suggests a sound basis for potential mechanisms related to convective processes and cloud microphysics, and furthermore suggests that this forcing could be of similar magnitude to well‐documented land‐based second‐order NAM forcings (antecedent snowpack and soil moisture, vegetation, and mineral dust). Based on these results, we believe that the role of aerosols in modulating summer precipitation deserves further investigation both observationally and in modeling studies. If indeed wildfire smoke does contribute to the modulation of NAM intensity, by virtue of its close temporal association with NAM season, it may well represent a factor that could contribute effectively to improved seasonal prediction of summer precipitation in the NAM region.

The dominant synoptic‐scale modes of North American monsoon precipitation

ABSTRACTIn this study, we explore the mechanisms of synoptic rainfall variability using observations from the Tropical Rainfall Measuring Mission satellite. Although previously shown to have an important impact on North American monsoon rainfall, tropical cyclones are excluded from this analysis, in order to focus on more frequent synoptic disturbances within the region. A rotated empirical orthogonal function (EOF) analysis of North American monsoon rainfall during June to September 2002–2009 suggests that low‐level tropical disturbances contribute to the leading two modes of precipitation variability within this region. These disturbances result in gulf surges or low‐level surges of moisture up the Gulf of California, and provide a key moisture source to facilitate the development of organized convection. In the first mode, the low‐level trough brings precipitation to lower elevations along the western slopes of the Sierra Madre Occidental south of Hermosillo, Mexico and over the southern Baja Peninsula. In the second mode, the low‐level trough interacts with an upper‐level inverted trough enhancing precipitation into the southwestern United States and northwest Mexico. In particular, the upper‐level trough contributes to the easterly to northeasterly shear across the region, favouring mesoscale convective organization and enhanced deep convection over the Sierra Madre Occidental and higher elevations in southeast Arizona. The EOF methodology offers an objective approach for determining the dominant modes of precipitation for the monsoon region useful for identifying past and monitoring future low‐frequency impacts on these modes.