North American Monsoon Region Research Articles

CORDEX‐NA: factors inducing dry/wet years on the North American Monsoon region

ABSTRACTThe output of four regional climate models (RCMs) from the Coordinated Regional Climate Downscaling Experiment (CORDEX)‐North America (NA) region was analysed for the 1990–2008 period, with particular interest on the mechanisms associated with wet and dry years over the North American Monsoon (NAM) core region. All RCMs (RCA3.5, HadGEM3‐RA, REMO, and RegCM4) were forced by the ERA‐Interim reanalysis. Model precipitation was compared against several observational gridded data sets at different time scales. Most RCMs capture well the annual cycle of precipitation and outperform ERA‐Interim, which is drier than the observations. RCMs underestimate (overestimate) the precipitation over the coastal plains (mountains) and have some problems to reproduce the interannual variability of the monsoon. To further investigate this, two extreme summers that showed the largest consistency among observations and RCMs were chosen: one wet (1990) and one dry (2005). The impact of the passage of tropical cyclones, the size of the Western Hemisphere Warm Pool (WHWP), the Intertropical Convergence Zone (ITCZ) position, and the initial intensity of the land–sea thermal contrast (LSTC) were analysed. During the wet year, the LSTC was stronger than the 2005 dry monsoon season and there were a larger number of hurricanes near the Gulf of California, the WHWP was more extended, and the ITCZ was located in a more northerly position than in 2005. All these processes contributed to a wetter NAM season. During the dry year, the LSTC was weaker, with a later onset, probably due to a previous very wet winter. The inverse precipitation relationship between winter and summer in the monsoon region was well captured by most of the RCMs. RegCM4 showed the largest biases and HadGEM3‐RA the smallest ones.

Lightning in the North American Monsoon: An Exploratory Climatology

Abstract Temporal and spatial distributions of the North American monsoon have been studied previously with rainfall and satellite data. In the current study, the monsoon is examined with lightning data from Vaisala’s Global Lightning Dataset (GLD360). GLD360 has been operating for over three years and provides sufficient data to develop an exploratory climatology with minimal spatial variation in detection efficiency and location accuracy across the North American monsoon region. About 80% of strokes detected by GLD360 are cloud to ground. This paper focuses on seasonal, monthly, and diurnal features of lightning occurrence during the monsoon season from Mazatlán north-northwest to northern Arizona and New Mexico. The goal is to describe thunderstorm frequency with a dataset that provides uniform spatial coverage at a resolution of 2–5 km and uniform temporal coverage with individual lightning events resolved to the millisecond, compared with prior studies that used hourly point rainfall or satellite data with a resolution of several kilometers. The monthly lightning stroke density over northwestern Mexico increases between May and June, as thunderstorms begin over the high terrain east of the Gulf of California. The monthly lightning stroke density over the entire region increases dramatically to a maximum in July and August. The highest stroke densities observed in Mexico approach those observed by GLD360 in subtropical and tropical regions in Africa, Central and South America, and Southeast Asia. The diurnal cycle of lightning exhibits a maximum over the highest terrain near noon, associated with daytime solar heating, a maximum near midnight along the southern coast of the Gulf, and a gradual decay toward sunrise.

Quantifying the strength of land–atmosphere coupling in the 2004 North American monsoon

AbstractThis study examines the land–atmosphere coupling strength during the 2004 North American monsoon (NAM) using the ΔΩ index from ensemble model simulations. The strength of coupling varies geographically over different monsoon phases. The coupling strength over the entire monsoon period is primarily attributed to the monsoon mature phase with strong signal of coupling in the eastern NAM region. A decomposition of the land–atmosphere coupling suggests that land segment contributes to the strong coupling during the monsoon onset, mature phase and the overall monsoon season, while the atmospheric segment dominates the strong coupling during the decay phase.

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.

MODIS Consistent Vegetation Parameter Specifications and Their Impacts on Regional Climate Simulations

Abstract A consistent set of Moderate Resolution Imaging Spectroradiometer (MODIS) vegetation parameters, including leaf and stem area index (LAI and SAI, respectively), land-cover category (LCC), fractional vegetation cover (FVC), and albedo parameterization are developed, and their impacts on North American regional climate are evaluated based on 10-yr Climate–Weather and Research Forecasting Model (CWRF) simulations. As compared with the previous Advanced Very High Resolution Radiometer (AVHRR) set, MODIS LCC increases grassland and cropland fractions in the central Great Plains and Midwest, respectively. Evergreen needleleaf forest converts to mixed forest in the Southeast, and mixed forest converts to evergreen needleleaf in Canada. FVC decreases by 0.05–0.3 over the central Great Plains but increases by 0.1–0.35 over the northern Rocky Mountains, Canada, and the U.S. Southeast. MODIS LAI is less than AVHRR by 2–6, except in the central Great Plains, eastern Rocky Mountains, and central Mexico. LCC and FVC changes over the central Great Plains reduce CWRF warm biases by 0.71°C and wet biases by 0.36 mm day−1. Large LAI reductions cause latent and sensible heat fluxes to decrease by 0.78–5.81 and 0.91–6.54 W m−2, respectively. They also lessen cold biases over the Gulf States and Southeast and wet biases over the North American monsoon region and Canada during summer. In densely vegetated regions including eastern Canada, the Ohio Valley, and the mid-Atlantic region, spring and summer precipitation decreases and temperature increases result from LAI reductions that cause positive evapotranspiration–precipitation–soil moisture feedbacks. Conversely, precipitation and temperature decreases in sparely vegetated regions, such as the Great Plains, result from FVC reductions that cause negative albedo–evapotranspiration–precipitation–soil moisture feedbacks.

Spatial and temporal variations in ecosystem response to monsoon precipitation variability in southwestern North America

AbstractDue to its marked vegetation phenology and precipitation gradients, the North American Monsoon Region (NAMR) is a useful domain for studying ecosystem responses to climate variability and change. To this end, we analyze long‐term dynamics (1982–2004) in monsoon precipitation (Pr), time‐integrated Normalized Difference Vegetation Index (TINDVI) used as proxy of net primary productivity, and rain‐use efficiency (RUE). The analysis focuses on six ecoregions, spanning from desert environments to tropical dry forests, to investigate (1) how net primary productivity and rain‐use efficiency vary along a precipitation gradient, (2) if interannual variability in net primary productivity is linked to the interannual variability in precipitation, and (3) if there is evidence of a long‐term signal imposed on the interannual variability in rain‐use efficiency. Variations in TINDVI and RUE with Pr along the NAMR precipitation gradient differ among ecoregions exhibiting intensive or extensive water use strategies. We explain the nonlinear behaviors along the precipitation gradient as resulting from different physiological responses to climatological means and the impact of topographic effects. Statistical analysis indicates that the interannual variability in vegetation response is significantly related to the interannual variability in Pr, but their correlation declines with time. A long‐term positive signal in RUE imposed on its interannual variability is identified and results from a constant TINDVI under negative long‐term trends of Pr. This important finding suggests the combined long‐term effects of ecosystem acclimation to reduced water availability and increasing CO2 concentration across the varied ecosystems of the North American Monsoon Region.

Land–Sea Thermal Contrast and Intensity of the North American Monsoon under Climate Change Conditions

Abstract The hypothesis that global warming during the twenty-first century will increase the land–sea thermal contrast (LSTC) and therefore the intensity of early season precipitation of the North American monsoon (NAM) is examined. To test this hypothesis, future changes (2075–99 minus 1979–2004 means) in LSTC, moisture flux convergence (MFC), vertical velocity, and precipitation in the region are analyzed using six global climate models (GCMs) from phase 5 of the Coupled Model Intercomparison Project (CMIP5) under the representative concentration pathway 8.5 (RCP8.5) emission scenario. A surface LSTC index shows that the continent becomes warmer than the ocean in May in the North American Regional Reanalysis (NARR) and ECMWF Interim Re-Analysis (ERA-Interim) and in June in the mean ensemble of the GCMs (ens_GCMs), and the magnitude of the positive LSTC is greater in the reanalyses than in the ens_GCMs during the historic period. However, the reanalyses underestimate July–August precipitation in the NAM region, while the ens_GCMs reproduces the peak season surprisingly well but overestimates it the rest of the year. The future ens_GCMs projects a doubling of the magnitude of the positive surface LSTC and an earlier start of the continental summer warming in mid-May. Contrary to the stated hypothesis, however, the mean projection suggests a slight decrease of monsoon coastal precipitation during June–August (JJA), which is attributed to increased midtropospheric subsidence, a reduced midtropospheric LSTC, and reduced MFC in the NAM coastal region. In contrast, the future ens_GCMs produces increased MFC and precipitation over the adjacent mountains during JJA and significantly more rainfall over the entire NAM region during September–October, weakening the monsoon retreat.

Analysis of watershed topography effects on summer precipitation variability in the southwestern United States

With climate change, precipitation variability is projected to increase. The present study investigates the potential interactions between watershed characteristics and precipitation variability. The watershed is considered as a functional unit that may impact seasonal precipitation. The study uses historical precipitation data from 370 meteorological stations over the last five decades, and digital elevation data from regional watersheds in the southwestern United States. This domain is part of the North American Monsoon region, and the summer period (June–July–August, JJA) was considered. Based on an initial analysis for 1895–2011, the JJA precipitation accounts, on average, for 22–43% of the total annual precipitation, with higher percentages in the arid part of the region. The unique contribution of this research is that entropy theory is used to address precipitation variability in time and space. An entropy-based disorder index was computed for each station’s precipitation record. The JJA total precipitation and number of precipitation events were considered in the analysis. The precipitation variability potentially induced by watershed topography was investigated using spatial regionalization combining principal component and cluster analysis. It was found that the disorder in precipitation total and number of events tended to be higher in arid regions. The spatial pattern showed that the entropy-based variability in precipitation amount and number of events gradually increased from east to west in the southwestern United States. Regarding the watershed topography influence on summer precipitation patterns, hilly relief has a stabilizing effect on seasonal precipitation variability in time and space. The results show the necessity to include watershed topography in global and regional climate model parameterizations.

Observed Local and Remote Influences of Vegetation on the Atmosphere across North America Using a Model-Validated Statistical Technique That First Excludes Oceanic Forcings*

Abstract The observed local and nonlocal influences of vegetation on the atmosphere across North America are quantified after first removing the oceanic impact. The interaction between vegetation and the atmosphere is dominated by forcing from the atmosphere, making it difficult to extract the forcing from vegetation. Furthermore, the atmosphere is not only influenced by vegetation but also the oceans, so in order to extract the vegetation impact, the oceanic forcing must first be excluded. This study identified significant vegetation impact in two climatically and ecologically unique regions: the North American monsoon region (NAMR) and the North American boreal forest (NABF). A multivariate statistical method, a generalized equilibrium feedback assessment, is applied to extract vegetation influence on the atmosphere. The statistical method is validated using a dynamical experiment for the NAMR in a fully coupled climate model, the Community Climate System Model, version 3.5 (CCSM3.5). The observed influence of NAMR vegetation on the atmosphere peaks in June–August and is primarily attributed to both roughness and hydrological feedbacks. Elevated vegetation amount increases evapotranspiration and surface roughness, which leads to a local decline in sea level pressure and generates an atmospheric teleconnection response. This atmospheric response leads to moister and cooler (drier and warmer) conditions over the western and central United States (Gulf states). The observed influence of the NABF on the atmosphere peaks in March–May, related to a thermal feedback. Enhanced vegetation greenness increases the air temperature locally. The atmosphere tends to form a positive Pacific–North American (PNA)-like pattern, and this anomalous atmospheric circulation and associated moisture advection lead to moister (drier) conditions in the western (eastern) United States.

The response of the North American Monsoon to increased greenhouse gas forcing

We analyze the response of the North American Monsoon (NAM) to increased greenhouse gas (GHG) forcing (emissions scenario RCP 8.5) using new simulations available through the Coupled Model Intercomparison Project version 5 (CMIP5). Changes in total monsoon season rainfall with GHG warming are small and insignificant. The models do, however, show significant declines in early monsoon season precipitation (June‐July) and increases in late monsoon season (September‐October) precipitation, indicating a shift in seasonality toward delayed onset and withdrawal of the monsoon. Early in the monsoon season, tropospheric warming increases vertical stability, reinforced by reductions in available surface moisture, inhibiting precipitation and delaying the onset of the monsoon. By the end of the monsoon season, moisture convergence is sufficient to overcome the warming induced stability increases, and precipitation is enhanced. Even with no change in total NAM rainfall, shifts in the seasonal distribution of precipitation within the NAM region are still likely to have significant societal and ecological consequences, reinforcing the need to not only understand the magnitude, but also the timing, of future precipitation changes.

Impact of land surface conditions on 2004 North American monsoon in GCM experiments

In this study, two sets of six‐member ensemble simulations were performed for the boreal summer of 2004 using the Finite Volume General Circulation model to investigate the sensitivity of the North American monsoon (NAM) system to land surface conditions and further to identify the mechanisms by which land surface processes control the NAM precipitation. The control simulation uses a fully interactive land surface model, whereas the sensitivity experiment uses prescribed land surface fields from the Global Land Data Assimilation System.The response of the monsoon precipitation to land surface changes varies over different regions modulated by two different soil moisture–precipitation feedbacks. The vast northern NAM region, including most of Arizona and New Mexico, as well as the northwestern Mexico shows that soil moisture has a positive feedback with precipitation primarily due to local recycling mechanisms. The reduction of soil moisture decreases latent heat flux and increases sensible heat flux and consequently increases the Bowen ratio and surface temperature, leading to a deep (warm and dry) boundary layer, which suppresses convection and hence reduces precipitation. Over the west coast of Mexico near Sinaloa, a negative soil moisture–precipitation relationship is noted to be associated with a large‐scale mechanism. The reduced soil moisture changes surface fluxes and hence boundary layer instability and ultimately low‐level circulation. As a result, the changes in surface pressure and large scale wind field increase moisture flux convergence and consequently moisture content, leading to increased atmospheric instability and in turn enhancing convection and accordingly precipitation. These results further reinforce the important role of land surface conditions on surface process, boundary structure, atmospheric circulation, and rainfall during the NAM development.

Can a Regional Climate Model Improve the Ability to Forecast the North American Monsoon?

Abstract Global climate models are challenged to represent the North American monsoon, in terms of its climatology and interannual variability. To investigate whether a regional atmospheric model can improve warm season forecasts in North America, a retrospective Climate Forecast System (CFS) model reforecast (1982–2000) and the corresponding NCEP–NCAR reanalysis are dynamically downscaled with the Weather Research and Forecasting model (WRF), with similar parameterization options as used for high-resolution numerical weather prediction and a new spectral nudging capability. The regional model improves the climatological representation of monsoon precipitation because of its more realistic representation of the diurnal cycle of convection. However, it is challenged to capture organized, propagating convection at a distance from terrain, regardless of the boundary forcing data used. Dynamical downscaling of CFS generally yields modest improvement in surface temperature and precipitation anomaly correlations in those regions where it is already positive in the global model. For the North American monsoon region, WRF adds value to the seasonally forecast temperature only in early summer and does not add value to the seasonally forecast precipitation. CFS has a greater ability to represent the large-scale atmospheric circulation in early summer because of the influence of Pacific SST forcing. The temperature and precipitation anomaly correlations in both the global and regional model are thus relatively higher in early summer than late summer. As the dominant modes of early warm season precipitation are better represented in the regional model, given reasonable large-scale atmospheric forcing, dynamical downscaling will add value to warm season seasonal forecasts. CFS performance appears to be inconsistent in this regard.

Response of the North American monsoon to regional changes in ocean surface temperature

The North American monsoon (NAM), an onshore wind shift occurring between July and September, has evolved in character during the Holocene largely due to changes in Northern Hemisphere insolation. Published paleoproxy and modeling studies suggest that prior to ∼8000 cal years BP, the NAM affected a broader region than today, extending westward into the Mojave Desert of California. Holocene proxy SST records from the Gulf of California (GoC) and the adjacent Pacific provide constraints for this changing NAM climatology. Prior to ∼8000 cal years BP, lower GoC SSTs would not have fueled northward surges of tropical moisture up the GoC, which presently contribute most of the monsoon precipitation to the western NAM region. During the early Holocene, the North Pacific High was further north and SSTs in the California Current off Baja California were warmer, allowing monsoonal moisture flow from the subtropical Pacific to take a more direct, northwesterly trajectory into an expanded area of the southwestern U.S. west of 114°W. A new upwelling record off southwest Baja California reveals that enhanced upwelling in the California Current beginning at ∼7500 cal year BP may have triggered a change in NAM climatology, focusing the geographic expression of NAM in the southwest USA into its modern core region east of ∼114°W, in Arizona and New Mexico. Holocene proxy precipitation records from the southwestern U.S. and northwestern Mexico, including lakes, vegetation/pollen, and caves are reviewed and found to be largely supportive of this hypothesis of changing Holocene NAM climatology.

Decadal Variation of Rainfall Seasonality in the North American Monsoon Region and Its Potential Causes

Abstract This study shows that the North American monsoon system’s (NAMS) strength, onset, and retreat over northwestern Mexico exhibit multidecadal variations during the period 1948–2009. Two dry regimes, associated with late onsets, early retreats, and weaker rainfall rates, occurred in 1948–70 and 1991–2005, whereas a strong regime, associated with early onsets, late retreats, and stronger rainfall rates, occurred in 1971–90. A recovery of the monsoon strength was observed after 2005. This multidecadal variation is linked to the sea surface temperature anomalies’ (SSTAs) variability, which is a combination of the Atlantic multidecadal oscillation (AMO) and the warming SST trends. These SST modes appear to cause an anomalous cyclonic circulation and enhanced rainfall over the southeastern United States and the Gulf of Mexico, which in turn increases the atmospheric stability over the monsoon region. However, these SST modes cannot fully explain the circulation and rainfall anomalies observed during the early-retreat monsoons. An expansion of the North Atlantic surface high (NASH) in recent decades also contributes to the anomalous circulation associated with the early retreats of the NAMS. A northwestward expansion of the NASH further enhances the anomalous cyclonic circulation and rainfall over the southeastern United States and the Gulf of Mexico. Its associated northwestward shift of the subtropical jets over the western United States enhances subsidence over the NAMS region. The combined effects of the AMO, the warming trends, and the NASH expansion on atmospheric circulation contribute to a stronger and more persistent earlier retreat during the recent dry regime (1991–2005), while the earlier dry regime (1948–70) appears to be only influenced by the positive phase of the AMO.

Flow, Moisture, and Thermodynamic Variability Associated with Gulf of California Surges within the North American Monsoon

Abstract This study uses an improved surge identification method to examine composites of 29 yr of surface observations and reanalysis data alongside 10 yr of satellite precipitation data to reveal connections between flow, thermodynamic parameters, and precipitation, both within and outside of the North American monsoon (NAM) region, associated with Gulf of California (GoC) moisture surges. The North American Regional Reanalysis (NARR), examined using composites of flow during all detected moisture surges at Yuma, Arizona, and so-called wet and dry surges (those producing anomalously high and low precipitation, respectively, over Arizona and New Mexico), show markedly different flow and moisture patterns that ultimately lead to the differing observed precipitation distributions in the region. Wet surges tend to be associated with moister precursor air masses over the southwestern United States, have a larger contribution of enhanced easterly cross–Sierra Madre Occidental (SMO) moisture transport, and tend to result from a transient cyclonic disturbance tracking across northern Mexico. Dry surges tend to be associated with a more southerly tracking disturbance, are associated with less convection over the SMO, and tend to be associated with a drier presurge air mass over Arizona and New Mexico.

Mechanisms for Precipitation Enhancement in a North American Monsoon Upper-Tropospheric Trough

Abstract Tropical upper-tropospheric troughs (TUTTs), transient summertime disturbances over the Pacific and Atlantic Oceans, are also observed to frequently occur in the North American monsoon (NAM) region. However, unlike TUTTs over the Pacific and Atlantic, which feature a predominance of precipitation on the eastern flank of the disturbances, TUTTs in the NAM region have been shown to enhance precipitation on their western flank as they pass over the mountains of northern Mexico. To investigate this phenomenon, convection-permitting simulations are performed over the core NAM region for the 12–14 July 2004 TUTT event that occurred during the North American Monsoon Experiment (NAME). The effects of the TUTT are isolated using an approach that removes the vorticity anomaly associated with it. Six simulations, three with the TUTT and three without it, are then executed. It is found that the mean of the TUTT simulations has increased surface–500-hPa and 700–400-hPa shear and that there is incrementally more convective available potential energy (CAPE) along the Sierra Madre Occidental (SMO). These differences lead to convective changes, with the TUTT simulations having more convection, larger maximum updraft velocities, and more precipitation in lower elevations. Overall, the TUTT simulation mean has about 15% more precipitation during the primary period of TUTT interaction with the northern SMO. There are also slight simulated microphysical differences that agree with nearby polarimetric radar observations. Finally, TUTT removal impacts the gulf surge event on 13 July via the convective modifications.

Distributed Hydrologic Modeling in Northwest Mexico Reveals the Links between Runoff Mechanisms and Evapotranspiration

Abstract A distributed hydrologic model is used to evaluate how runoff mechanisms—including infiltration excess (RI), saturation excess (RS), and groundwater exfiltration (RG)—influence the generation of streamflow and evapotranspiration (ET) in a mountainous region under the influence of the North American monsoon (NAM). The study site, the upper Sonora River basin (~9350 km2) in Mexico, is characterized by a wide range of terrain, soil, and ecosystem conditions obtained from best available data sources. Three meteorological scenarios are compared to explore the impact of spatial and temporal variations of meteorological characteristics on land surface processes and to identify the value of North American Land Data Assimilation System (NLDAS) forcing products in the NAM region. The following scenarios are considered for a 1-yr period: 1) a sparse network of ground-based stations, 2) raw forcing products from NLDAS, and 3) NLDAS products adjusted using available station data. These scenarios are discussed in light of spatial distributions of precipitation, streamflow, and runoff mechanisms during annual, seasonal, and monthly periods. This study identified that the mode of runoff generation impacts seasonal relations between ET and soil moisture in the water-limited region. In addition, ET rates at annual and seasonal scales were related to the runoff mechanism proportions, with an increase in ET when RS was dominant and a decrease in ET when RI was more important. The partitioning of runoff mechanisms also helps explain the monthly progression of runoff ratios in these seasonally wet hydrologic systems. Understanding the complex interplay between seasonal responses of runoff mechanisms and evapotranspiration can yield information that is of interest to hydrologists and water managers.

Downscaled Climate Change Scenarios for Baja California and the North American Monsoon during the Twenty-First Century

Abstract Regional climate change scenarios for Baja California/Southern California (BCC) and the North American monsoon (NAM) were produced as part of the Baja California State Climate Change Action Program (PEACC-BC). Bias-corrected and spatially downscaled scenarios (BCSD) from six general circulation models (GCMs) with a total of 12 realizations were analyzed for two scenarios of the Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES): B1 (low emissions) and A2 (high emissions) during the twenty-first century. A validation of the original GCM realizations and the BCSD scenarios with observed data during 1961–90 show that the ensemble GCM produces too much precipitation during autumn and winter, which could be the cause of the observed delay of the summer monsoon rains; the ensemble BCSD considerably improves the mean annual cycles and spatial distributions of precipitation and temperature in the region. However, both ensembles greatly underestimate the observed interannual variability of precipitation. BCSD scenarios of temperature and precipitation during the twenty-first century were evaluated on the basis of the multimodel median change relative to 1961–90. The scenarios of precipitation change show large interannual variations and larger uncertainties than the scenarios of temperature change. The A2 scenarios show the largest reductions of precipitation in the last 20 yr of the twenty-first century; a decrease of 30% is projected for BCC mainly in winter and spring, while precipitation in the NAM region could be weakened by 20% during winter, spring, and summer. After 2050, a significant reduction of precipitation is expected in northwestern Mexico and the southwestern United States south of 35°N, and temperature changes larger than 2°C warming.

Predictability of Evapotranspiration Patterns Using Remotely Sensed Vegetation Dynamics during the North American Monsoon

Abstract The links between vegetation, evapotranspiration (ET), and soil moisture (SM) are prominent in western Mexico—a region characterized by an abrupt increase in rainfall and ecosystem greenup during the North American monsoon (NAM). Most regional-scale land surface models use climatological vegetation and are therefore unable to capture fully the spatiotemporal changes in these linkages. Interannually varying and climatological leaf area index (LAI) were prescribed, both inferred from the space-borne Moderate Resolution Imaging Spectroradiometer (MODIS), as the source of vegetation parameter inputs to the Variable Infiltration Capacity (VIC) model applied over the NAM region for 2001–08. Results at two eddy covariance tower sites for three summer periods were compared and evaluated. Results show that both vegetation greening onset and dormancy dates vary substantially from year to year with a range of more than half a month. The model using climatological LAI tends to predict lower (higher) ET than the model using observed LAI when vegetation greening occurs earlier (later) than the mean greening date. These discrepancies were especially large during approximately two weeks at the beginning of the monsoon. The effect of LAI on ET estimates was about 10% in the Sierra Madre Occidental and 30% in the continental interior. VIC-estimated ET based on interannually varying LAI had high interannual variability at the greening onset and dormancy periods corresponding to the vegetation dynamics. The greening onset date was highly related to ET early in the monsoon season, indicating the potential usefulness of LAI anomalies for predicting early season ET.