Editing IPCC:AR6/WGI/Chapter-10 (section)

==== 10.4.2.3 The South-western North America Drought ====

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Persistent hydroclimatic drought in south-western North America remains a much-studied event. Drought is a regular feature of the south-western North America’s climate regime, as can be seen in both the modern record, and through paleoclimate reconstructions ( [[#Cook--2010|Cook et al., 2010]] ; [[#Woodhouse--2010|Woodhouse et al., 2010]] ; [[#Williams--2020|Williams et al., 2020]] ), as well as in future climate model projections ( [[#Cook--2015a|Cook et al., 2015a]] ). Since the early 1980s, which were relatively wet in terms of precipitation and streamflow, the region has experienced major multi-year droughts such as the turn-of-the-century drought that lasted from 1999 to 2005, and the most recent and extreme 2012–2014 drought that in certain locations is perhaps unprecedented in the last millennium ( [[IPCC:Wg1:Chapter:Chapter-8#8.3.1.6|Section 8.3.1.6]] ; [[#Griffin--2014|Griffin and Anchukaitis, 2014]] ; [[#Robeson--2015|Robeson, 2015]] ). Shorter dry spells also happened between these multi-year droughts making 1980 to present a period with an exceptionally steep trend from wet to dry (Figure 10.13a), leading to strong declines in Rio Grande and Colorado river flows ( [[#Lehner--2017b|Lehner et al., 2017b]] ; [[#Udall--2017|Udall and Overpeck, 2017]] ). While robust attribution of this trend is complicated by the large natural variability in this region, the 20th century warming has been suggested to increase the chances for hydrological drought periods by lowering runoff efficiency ( [[#Woodhouse--2016|Woodhouse et al., 2016]] ; [[#Lehner--2017b|Lehner et al., 2017b]] ; [[#Woodhouse--2018|Woodhouse and Pederson, 2018]] ) and affecting evapotranspiration ( [[#Williams--2020|Williams et al., 2020]] ). There is some evidence suggesting that the Last Glacial Maximum, a period of low atmospheric CO <sub>2</sub> , about 21 ka ago, has a thermodynamically-driven zonal mean precipitation response similar to that of the current state with relatively high CO <sub>2</sub> levels when compared with the pre-industrial period. Pluvial conditions at that time and a reduction in precipitation from the Last Glacial Maximum to the pre-industrial period are consistent with drying trends for the region in models with GHG concentrations exceeding pre-industrial levels. However, the dominant large-scale drivers responsible for the precipitation changes observed during these two transitions are markedly different: mainly ice-sheet retreat and increasing insolation on one hand, increasing GHGs on the other hand. This suggests that the Last Glacial Maximum correspondence is fortuitous which strongly limits its use to capture future hydrological cycle changes ( [[IPCC:Wg1:Chapter:Chapter-8#8.3.2.4.4|Section 8.3.2.4.4]] ; [[#Morrill--2018|Morrill et al., 2018]] ; [[#Lowry--2019|Lowry and Morrill, 2019]] ). Furthermore, the conclusion of the Last Glacial Maximum drying versus wetting seems to strongly depend on the physical property of interest, hydrologic or vegetation indicators ( [[#Scheff--2017|Scheff et al., 2017]] ). Droughts are characterized by deficits in total soil moisture content that can be caused by a combination of decreasing precipitation and warming temperature, which promotes greater evapotranspiration. Regional-scale attribution of the prevalence of south-western North America drought since 1980 then mostly focuses on the attribution of change in these two variables.

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[[File:724be120d5dc061c3a1e9a1bf1604e0e IPCC_AR6_WGI_Figure_10_13.png]]

'''Figure 10.13''' '''|''' '''Attribution of the south-western North America precipitation decline during the 1983–2014 period. (a)''' Water year (October to September) precipitation spatial linear trend (in percent per decade) over North America from 1983 to 2014. Trends are estimated using ordinary least squares. Top row: observed trends from CRU TS, REGEN, GPCC, and the Global Precipitation Climatology Project (GPCP). Middle row: driest, mean and wettest trends (relative to the region enclosed in the black quadrilateral, bottom row) from the 100 members of the MPI-ESM coupled SMILE. Bottom row: driest, mean and wettest trends relative to the above region from the 100 members of the d4PDF atmosphere-only SMILE. '''(b)''' Time series of water year precipitation anomalies (%, baseline 1971–2000) over the above south-western North America region for CRU TS (grey bar charts). Black, brown and green lines show low-pass filtered time series for CRU TS, driest and wettest members of the d4PDF SMILE, respectively. The filter is the same as the one used in Figure 10.10. '''(c)''' Distribution of south-western region-averaged water-year precipitation 1983–2014 trends (in percent per decade) for observations (CRU TS, REGEN, GPCC and GPCP, black crosses), CMIP6 all-forcing historical simulations (red circles), the MIROC6, CSIRO-Mk3-6-0, MPI-ESM and d4PDF SMILEs (grey box-and-whisker plots). Grey squares refer to ensemble mean trends of their respective SMILE and the red circle refers to the CMIP6 multi-model mean. Box-and-whisker plots follow the methodology used in Figure 10.6. Further details on data sources and processing are available in the chapter data table (Table 10.SM.11).

The observed south-western North America drying fits the narrative of what might happen in response to increasing GHG concentrations due to a poleward expansion of the subtropics, that is conducive to drying trends over subtropical to mid-latitude regions ( [[#Hu--2013b|Hu et al., 2013b]] ; [[#Birner--2014|Birner et al., 2014]] ; [[#Lucas--2014|Lucas et al., 2014]] ). However, several studies based on modern reanalyses and CMIP5 models have recently shown that the current contribution of GHGs to Northern Hemisphere tropical expansion is much smaller than in the Southern Hemisphere and will remain difficult to detect due to large internal variability, even by the end of the 21st century ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.1|Section 3.3.3.1]] ; [[#Garfinkel--2015|Garfinkel et al., 2015]] ; [[#Allen--2017|Allen and Kovilakam, 2017]] ; [[#Grise--2018|Grise et al., 2018]] , 2019). In addition, the widening of the Northern Hemisphere tropical belt exhibits strong seasonality and zonal asymmetry, particularly in autumn and the North Atlantic ( [[#Amaya--2018|Amaya et al., 2018]] ; [[#Grise--2018|Grise et al., 2018]] ). Therefore, it seems that the recent Northern Hemisphere tropical expansion results from the interplay of internal and forced modes of tropical width variations and that the forced response has not robustly emerged from internal variability (Sections 3.3.3.1 and 10.4.3).

A second possible causal factor is the role for ocean-forced or internal atmospheric circulation change. Analysis of observed and CMIP5-simulated precipitation indicates that the drought prevalence since 1980 is linked to natural, internal variability in the climate system ( [[#Knutson--2018|Knutson and Zeng, 2018]] ). Based on observations and ensembles of SST-driven atmospheric simulations, [[#Seager--2014|Seager and Hoerling (2014)]] suggested that robust tropical Pacific and tropical North Atlantic forcing drove an important fraction of annual mean precipitation and soil moisture changes and that early 21st century multi-year droughts could be attributed to natural decadal swings in tropical Pacific and North Atlantic SSTs. A cold state of the tropical Pacific would lead by well-established atmospheric teleconnections to anomalous high pressure across the North Pacific and southern North America, favouring a weaker jet stream and a diversion of the Pacific storm track away from the south-west ( [[#Delworth--2015|Delworth et al., 2015]] ; [[#Seager--2017|Seager and Ting, 2017]] ). The multi-year drought of 2012–2016 has been linked to the multi-year persistence of anomalously high atmospheric pressure over the north-eastern Pacific Ocean, which deflected the Pacific storm track northward and suppressed regional precipitation during California’s rainy season ( [[#Swain--2017|Swain et al., 2017]] ). Going into more detail, [[#Prein--2016a|Prein et al. (2016a)]] used an assessment of changing occurrence of weather regimes to judge that changes in the frequency of certain regimes during 1979–2014 have led to a decline in precipitation by about 25%, chiefly related to the prevalence of anticyclonic circulation patterns in the north-east Pacific. Finally, the moderate model performance in representing Pacific SST decadal variability and its remote influence ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.6|Section 3.7.6]] ) as well as its change under warming may affect attribution results of observed and future precipitation changes ( [[#Seager--2019|Seager et al., 2019]] ).

It has also been suggested that the ocean-controlled influence is limited and internal atmospheric variability has to be invoked to fully explain the observed history of drought on decadal time scales ( [[#Seager--2014|Seager and Hoerling, 2014]] ; [[#Seager--2017|Seager and Ting, 2017]] ). From roughly 1980 to the present, the regional climate signals show an interesting mix between forced and internal variability. [[#Lehner--2018|Lehner et al. (2018)]] used a dynamical adjustment method and large ensembles of coupled and SST-forced atmospheric experiments to suggest that the observed south-western North America rainfall decline mainly results from the effects of atmospheric internal variability, which is in part driven by a PDV-related phase shift in Pacific SST around 2000 (Figure 10.13b,c). Based upon four SMILEs (three using a GCM and another one an AGCM constrained by observed SSTs) and a CMIP6 multi-model suite constrained by observed external forcings, Figure 10.13 shows, in agreement with [[#Lehner--2018|Lehner et al. (2018)]] , that observed SSTs with their associated atmospheric response are the main drivers of the south-western North America precipitation decrease during the 1983–2014 period. Once aspects of the internal variability are removed by dynamical adjustment, the observed precipitation change signal and simulated anthropogenically-forced components look more similar ( [[#Lehner--2018|Lehner et al., 2018]] ).

Importantly, as the AR6 assessment views the PDV as being mostly driven by internal variability ( [[IPCC:Wg1:Chapter:Chapter-3#3.7.6|Section 3.7.6]] ), the lines of evidence cited above suggest that the contribution of natural and anthropogenic forcings to the precipitation decline has a small amplitude. Unlike the precipitation deficit, the accompanying south-western North America warming is driven primarily by anthropogenic forcing from GHGs rather than atmospheric circulation variability and may help to enhance the drought through increased evapotranspiration ( [[#Knutson--2013|Knutson et al., 2013]] ; [[#Diffenbaugh--2015|Diffenbaugh et al., 2015]] ; [[#Williams--2015|Williams et al., 2015]] , [[#Williams--2020|Williams et al., 2020]] ; [[#Lehner--2018|Lehner et al., 2018]] , [[#Lehner--2020|2020]] ).

To conclude, there is ''high confidence'' ( ''robust evidence'' and ''medium agreement'' ) that most (&gt;50%) of the anomalous atmospheric circulation that caused the south-western North America negative precipitation trend can be attributed to teleconnections arising from tropical Pacific SST variations related to PDV. There is ''high confidence'' ( ''robust evidence'' and ''medium agreement'' ) that anthropogenic forcing has made a substantial contribution (about 50%) to the south-western North America warming since 1980.

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