Upper‐tropospheric troughs and North American monsoon rainfall in a long‐term track dataset

AbstractSummer monsoon rainfall supplies over 55% of annual precipitation to global monsoon regions. As shown by more than 70% of models, including 30 models from CMIP5 and 30 models from CMIP6 under high-emission scenarios, North American (NAM) monsoon rainfall decreases in a warmer climate, in sharp contrast to the robust increase in Asian–African monsoon rainfall. A hierarchy of model experiments is analyzed to understand the mechanism for the reduced NAM monsoon rainfall in this study. Modeling evidence shows that the reduction of NAM monsoon rainfall is related to both direct radiative forcing of increased CO2 concentration and SST warming, manifested as fast and slow responses to abrupt CO2 quadrupling in coupled GCMs. A cyclone anomaly forms over the Eurasian–African continental area due to enhanced land–sea thermal contrast under increased CO2 concentration, and this leads to a subsidence anomaly on its western flank, suppressing the NAM monsoon rainfall. The SST warming acts to further reduce the rainfall over the NAM monsoon region, and the El Niño–like SST warming pattern with enhanced SST warming over the equatorial Pacific plays a key role in suppressing NAM rainfall, whereas relative cooling over the subtropical North Atlantic has no contribution. A positive feedback between monsoon precipitation and atmospheric circulation helps to amplify the responses of monsoon rainfall.

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Vegetation Dynamics within the North American Monsoon Region

Journal of Climate ◽

10.1175/2010jcli3847.1 ◽

2011 ◽

Vol 24 (6) ◽

pp. 1763-1783 ◽

Cited By ~ 39

Author(s):

Giovanni Forzieri ◽

Fabio Castelli ◽

Enrique R. Vivoni

Keyword(s):

North American ◽

Vegetation Dynamics ◽

Vegetation Index ◽

North American Monsoon ◽

Terrain Attributes ◽

The North ◽

Short Period ◽

Long Term Trends ◽

Northwestern Mexico

Abstract The North American monsoon (NAM) leads to a large increase in summer rainfall and a seasonal change in vegetation in the southwestern United States and northwestern Mexico. Understanding the interactions between NAM rainfall and vegetation dynamics is essential for improved climate and hydrologic prediction. In this work, the authors analyze long-term vegetation dynamics over the North American Monsoon Experiment (NAME) tier I domain (20°–35°N, 105°–115°W) using normalized difference vegetation index (NDVI) semimonthly composites at 8-km resolution from 1982 to 2006. The authors derive ecoregions with similar vegetation dynamics using principal component analysis and cluster identification. Based on ecoregion and pixel-scale analyses, this study quantifies the seasonal and interannual vegetation variations, their dependence on geographic position and terrain attributes, and the presence of long-term trends through a set of phenological vegetation metrics. Results reveal that seasonal biomass productivity, as captured by the time-integrated NDVI (TINDVI), is an excellent means to synthesize vegetation dynamics. High TINDVI occurs for ecosystems with a short period of intense greening tuned to the NAM or with a prolonged period of moderate greenness continuing after the NAM. These cases represent different plant strategies (deciduous versus evergreen) that can be adjusted along spatial gradients to cope with seasonal water availability. Long-term trends in TINDVI may also indicate changing conditions favoring ecosystems that intensively use NAM rainfall for rapid productivity, as opposed to delayed and moderate greening. A persistence of these trends could potentially result in the spatial reorganization of ecosystems in the NAM region.

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SST data improve modeling of North American monsoon rainfall

Eos ◽

10.1029/2003eo430001 ◽

2003 ◽

Vol 84 (43) ◽

pp. 457 ◽

Cited By ~ 6

Author(s):

X. Gao ◽

S. Sorooshian ◽

J. Li ◽

J. Xu

Keyword(s):

North American ◽

Monsoon Rainfall ◽

North American Monsoon

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Seasonal Shifts in the North American Monsoon

Journal of Climate ◽

10.1175/jcli4091.1 ◽

2007 ◽

Vol 20 (9) ◽

pp. 1923-1935 ◽

Cited By ~ 58

Author(s):

Katrina Grantz ◽

Balaji Rajagopalan ◽

Martyn Clark ◽

Edith Zagona

Keyword(s):

United States ◽

North American ◽

Gulf Of California ◽

Monsoon Rainfall ◽

Monsoon Season ◽

North American Monsoon ◽

Southwestern United States ◽

Monsoon Period ◽

The North ◽

Ocean Conditions

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

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Flash Flooding in Arid/Semiarid Regions: Dissecting the Hydrometeorology and Hydrology of the 19 August 2014 Storm and Flood Hydroclimatology in Arizona

Journal of Hydrometeorology ◽

10.1175/jhm-d-17-0089.1 ◽

2017 ◽

Vol 18 (12) ◽

pp. 3103-3123 ◽

Cited By ~ 15

Author(s):

Long Yang ◽

James Smith ◽

Mary Lynn Baeck ◽

Efrat Morin ◽

David C. Goodrich

Keyword(s):

North American ◽

Hydrological Modeling ◽

Rainfall Variability ◽

Flash Floods ◽

Flood Frequency ◽

North American Monsoon ◽

The North ◽

Flood Response ◽

Central Arizona

The hydroclimatology, hydrometeorology, and hydrology of flash floods in the arid/semiarid southwestern United States are examined through empirical analyses of long-term, high-resolution rainfall and stream gauging observations, together with hydrological modeling analyses of the 19 August 2014 storm based on the Kinematic Runoff and Erosion Model (KINEROS2). The analyses presented here are centered on identifying the structure and evolution of flood-producing storms, as well as the interactions of space–time rainfall variability and basin characteristics in determining the upper-tail properties of rainfall and flood magnitudes over this region. This study focuses on four watersheds in Maricopa County, Arizona, with contrasting geomorphological properties. Flash floods over central Arizona are concentrated in both time and space, reflecting controls of the North American monsoon and complex terrain. Thunderstorm systems during the North American monsoon, as represented by the 19 August 2014 storm, are the dominant flood agents that determine the upper tail of flood frequency over central Arizona and that also shape the envelope curve of floods for watersheds smaller than 250 km2. Flood response for the 19 August 2014 storm is associated with storm elements of comparable spatial extent to the drainage area and slow movement for the three compact, headwater watersheds. Flood response for the elongated and relatively flat Skunk Creek highlights the importance of the spatial distribution of rainfall for transmission losses in arid/semiarid watersheds.

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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

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-16-0358.1 ◽

2017 ◽

Vol 56 (9) ◽

pp. 2509-2529 ◽

Cited By ~ 28

Author(s):

Thang M. Luong ◽

Christopher L. Castro ◽

Hsin-I Chang ◽

Timothy Lahmers ◽

David K. Adams ◽

...

Keyword(s):

United States ◽

North American ◽

Extreme Event ◽

Severe Weather ◽

Climate Simulation ◽

North American Monsoon ◽

Monsoon Precipitation ◽

Southwestern United States ◽

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.

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Understanding Future Change of Global Monsoons Projected by CMIP6 Models

Journal of Climate ◽

10.1175/jcli-d-19-0993.1 ◽

2020 ◽

Vol 33 (15) ◽

pp. 6471-6489 ◽

Cited By ~ 9

Author(s):

Bin Wang ◽

Chunhan Jin ◽

Jian Liu

Keyword(s):

North American ◽

Rainy Season ◽

Monsoon Rainfall ◽

North American Monsoon ◽

Convective Heating ◽

Monsoon Circulation ◽

Future Change ◽

Global Monsoon ◽

African Monsoon ◽

The Future

AbstractProjecting future change of monsoon rainfall is essential for water resource management, food security, disaster mitigation, and infrastructure planning. Here we assess the future change and explore the causes of the changes using 15 models that participated in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The multimodel ensemble projects that, under the shared socioeconomic pathway (SSP) 2–4.5, the total land monsoon rainfall will likely increase in the Northern Hemisphere (NH) by about 2.8% per one degree Celsius of global warming (2.8% °C−1) in contrast to little change in the Southern Hemisphere (SH; −0.3% °C−1). In addition, in the future the Asian–northern African monsoon likely becomes wetter while the North American monsoon becomes drier. Since the humidity increase is nearly uniform in all summer monsoon regions, the dynamic processes must play a fundamental role in shaping the spatial patterns of the global monsoon changes. Greenhouse gas (GHG) radiative forcing induces a “NH-warmer-than-SH” pattern, which favors increasing the NH monsoon rainfall and prolonging the NH monsoon rainy season while reducing the SH monsoon rainfall and shortening the SH monsoon rainy season. The GHG forcing induces a “land-warmer-than-ocean” pattern, which enhances Asian monsoon low pressure and increases Asian and northern African monsoon rainfall, and an El Niño–like warming, which reduces North American monsoon rainfall. The uncertainties in the projected monsoon precipitation changes are significantly related to the models’ projected hemispheric and land–ocean thermal contrasts as well as to the eastern Pacific Ocean warming. The CMIP6 models’ common biases and the processes by which convective heating drives monsoon circulation are also discussed.

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Influence of sea surface temperature anomalies in the Gulf of California on North American monsoon rainfall

Journal of Geophysical Research Atmospheres ◽

10.1029/2002jd002403 ◽

2003 ◽

Vol 108 (D3) ◽

pp. n/a-n/a ◽

Cited By ~ 17

Author(s):

Kingtse C. Mo ◽

H. M. Henry Juang

Keyword(s):

Surface Temperature ◽

Sea Surface Temperature ◽

North American ◽

Gulf Of California ◽

Monsoon Rainfall ◽

Sea Surface ◽

North American Monsoon ◽

Temperature Anomalies ◽

Sea Surface Temperature Anomalies

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The Last 1200 Years of Rainfall/Runoff Variability along the Central Mexico Pacific Coast Associated with the North American Monsoon

Oceans ◽

10.3390/oceans2030030 ◽

2021 ◽

Vol 2 (3) ◽

pp. 530-545

Author(s):

Steve Lund ◽

Emily Mortazavi ◽

Ellen Platzman ◽

Caitlin Tems ◽

William Berelson ◽

...

Keyword(s):

Magnetic Material ◽

North American ◽

Pacific Coast ◽

Ice Age ◽

Magnetic Data ◽

North American Monsoon ◽

Cycle Duration ◽

Rainfall Runoff ◽

Runoff Variability

This study presents new evidence for long-term variability in the late Holocene North American Monsoon (NAM), Pacific coast of Mexico. We have carried out a rock magnetic study on two deep-sea sediment cores from the Pacific coast Pescadero Basin. The magnetic intensities estimate total magnetic material and are a proxy for total clastic sediment. Ratios of magnetic intensities estimate the grain size of magnetic material. The rock magnetic data show a decimeter scale, multi-decadal oscillation with fourteen cycles (A-N) over the last 1200 years. These oscillations reflect alternating intervals of stronger/coarser magnetic/clastic flux to the coastal ocean and intervals of weaker/finer magnetic flux. We think these variations are caused by variations in long-term dominance of the NAM; summer (wet) monsoons produce rainy conditions (with runoff) while winter (dry) monsoons produce significant offshore winds, increased upwelling/biological productivity. We can correlate our variability to two other published studies southeast of Pescadero Basin, coastal lake sediments in Laguna de Juanacatlan and a Juxtlahuaca Cave stalagmite. Both of these studies estimate local rainfall. We see evidence of the same pattern of multi-decadal rainfall-runoff variability in these records as we see in Pescadero Basin, which is synchronous to within ±25 years over the last 1200 years. The multi-dacadal pattern of hydrologic variability in all three records varies in cycle duration from ~90-years wet/dry cycles in the Little Ice Age (1400–1850 AD) to ~60-years cycles in the Medieval Climate Optimum (1100–1400 AD). This variability in cycle duration suggests some chaotic nature to the regional NAM climate pattern or some long-term non-linear forcing (PDO?).

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