Editing IPCC:AR6/WGI/Chapter-8 (section)

==== 8.3.2.8 Extratropical Cyclones, Storm Tracks and Atmospheric Rivers ====

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===== 8.3.2.8.1 Extratropical cyclones and storm tracks =====

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The AR5 indicated ''low confidence'' in long-term changes in the intensity of extratropical cyclones (ETC) over the 20th century derived from centennial reanalyses and storminess proxies based upon sea level pressure. This was confirmed by the SREX assessment that the main Northern Hemisphere (NH) and Southern Hemisphere (SH) extratropical storm tracks ''likely'' experienced a poleward shift during the last 50 years ( [[#Seneviratne--2012|Seneviratne et al., 2012]] ) with ''low confidence'' , and inconsistencies within reanalysis datasets remain.

Since AR5 there has been considerable progress in quantifying storm track activity using multiple reanalysis products and different methodologies (Hodges et al. , 2011; Neu et al. , 2013; Tilinina et al. , 2013; X.L. Wang et al. , 2016). Over the NH increases in the total number of cyclones from 1979 show a large spread of trends across different estimates ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.4.3|Section 2.3.1.4.3]] ; [[#Neu--2013|Neu et al., 2013]] ; Z. [[#Li--2016|Li et al., 2016]] a; [[#Grieger--2018|Grieger et al., 2018]] ) resulting in ''low confidence'' in any clear increase of in the total number of cyclones. However, starting from the early 1990s, most reanalyses show increases in the total cyclone number by about 2 – 5% per decade (Figure 8.12). Increasing trends in the total number of cyclones are dominated by the increase in the number of shallow and moderate cyclones (which are more dependent on the datasets and identification methods used) than with decreasing number of deep cyclones since the early 1990s ( [[#Tilinina--2013|Tilinina et al., 2013]] ; [[#Chang--2018|Chang, 2018]] ). In the SH the variability of the total number of cyclones is characterized by strong inter-decadal variability preventing a clear assessment of trends. However, in contrast to the NH,there is a significant increasing trend in the number of deep cyclones (about 10% over 1979 – 2018) in ERA5, ERA-Interim, JRA55 and MERRA, and in the CFSR dataset after 2000 (Figure 8.12; [[#Reboita--2015|Reboita et al., 2015]] ; X.L. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ).

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[[File:7df44346fb7c505f3a447802068e15be IPCC_AR6_WGI_Figure_8_12.png]]

'''Figure 8.12 |''' '''Annual anomalies (with respect to the reference period''' '''1979–2018''' ''') of the total number of extratropical cyclones (a, b) and of the number of deep cyclones (&lt;980 hPa) (c, d) over the Northern (a, c) and the Southern (b, d) Hemispheres in different reanalyses (shown in colours in the legend).'''
Note different vertical scales for panels (a, b) and (c, d). Thin lines indicate annual anomalies and bold lines indicate five-year running averages. (e, f) The number of reanalyses (out of five) simultaneously indicating statistically significant (90% level) linear trends of the same sign during 1979–2018 for JFM (January–February–March) over the Northern Hemisphere (e) and over the Southern Hemisphere (f). Updated from [[#Tilinina--2013|Tilinina et al. (2013)]] . Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Changes in the number of deep storms, which are often associated with heavier precipitiation over the North Atlantic and North Pacific, exhibit strong seasonal differences and decadal variability (Colle et al. , 2015; Chang et al. , 2016; Matthews et al. , 2016; Priestley et al. , 2020a). An increase in the number of summer cyclones over the Atlantic-European sector (Tilinina et al. , 2013) is consistent with the increase in the strength of the strongest fronts over Europe (Schemm et al. , 2018). Chang et al. (2016) reported a decrease in the number of strong summer storms in the latitudinal band 40°N – 75°N over the last decades, however, the assessment of seasonal trends in the Atlantic-European sector is complicated by the choice of region, attribution of tracks to the region selected, and thresholds used to identify trajectories, leading to ''low confidence'' on regional seasonal trends. For the SH, [[#Grieger--2018|Grieger et al. (2018)]] reported a growing number of cyclones over sub-Antarctic region in the austral-summer during 1979 – 2010, while statistically significant trends were absent during the austral winter.

Analysis of storm track activity over longer periods suffers from uncertainties associated with changing data assimilation and observations before and during the satellite era, resulting in in homogeneities and discontinuities in centennial reanalyses (Krueger et al. , 2013; X.L. Wang et al. , 2013, 2016; [[#Chang--2016|Chang and Yau, 2016]] ; Varino et al. , 2019). Feser et al. (2015) reviewed multiple storm track records for the Atlantic-European sector and demonstrated growing storm activity north of 55°N from the 1970s to the mid-1990s with declining trend thereafter, sugesting strong inter-decadal variability in storm track activity. This was also confirmed by [[#Krueger--2019|Krueger et al. (2019)]] from the analysis of geostrophic winds derived from sea level pressure gradients.

Poleward deflection of mostly oceanic winter storm tracks since 1979 was reported in both the North Atlantic and North Pacific ( [[#Tilinina--2013|Tilinina et al., 2013]] ; J. [[#Wang--2017|]] [[#Wang--2017|]] [[#Wang--2017|Wang et al., 2017]] ). This large-scale tendency has regional variations and may be seasonally dependent. [[#Wise--2017|Wise and Dannenberg (2017)]] reported a southward shift in the east Pacific storm track from the 1950s to mid-1980s followed by northward deflection in the later decades. ( [[#King--2019|King et al., 2019]] ) reported an association of Atlantic storm track migrations with SSW events with Central and South European precipitation anomalies. Over centennial time scales, [[#Gan--2014|Gan and Wu (2014)]] reported an intensification of storm tracks in the poleward and downstream regions of the North Pacific and North Atlantic upper troposphere using the NOAA–CIRES–DOE Twentieth Century Reanalysis. Poleward migration of the SH storm tracks ( [[#Grise--2014|Grise et al., 2014]] ; X.L. [[#Wang--2016|]] [[#Wang--2016|]] [[#Wang--2016|Wang et al., 2016]] ; [[#Dowdy--2019|Dowdy et al., 2019]] ) was identified during the austral summer and is closely associated with cyclone-associated frontal activity ( [[#Solman--2014|Solman and Orlanski, 2014]] , 2016) and cloud cover ( [[#Bender--2012|Bender et al., 2012]] ; [[#Norris--2016|Norris et al., 2016]] ).

The representation of ETCs in both climate models and reanalyses is resolution-dependent, hence changes must be assessed with caution ( [[IPCC:Wg1:Chapter:Chapter-3#3.3.3.3|Section 3.3.3.3]] ). In particular, CMIP5 models show a systematic underestimation of the intensity of ETCs ( [[#Zappa--2014|Zappa et al., 2014]] ), a feature that is partially related to their relatively coarse resolution or other possible deficiencies such as an excess of dissipation ( [[#Chang--2013|Chang et al., 2013]] ). The best representation of ETCs and their intensity in the North Atlantic are provided by relatively high horizontal resolution CMIP5 models ( [[#Zappa--2014|Zappa et al., 2014]] ). Using a single high-resolution climate model, ( [[#Hawcroft--2016|Hawcroft et al., 2016]] ) showed that precipitation amount associated with ETCs was generally well simulated, though with too much precipitation during the strongest ECTs compared with observed estimations.

In summary, there is ''low confidence'' in recent changes in the total number of extratropical cyclones over both hemispheres. It is ''as likely as not'' that the number of deep cyclones over the NH has decreased after 1979 and it is ''likely'' that the number of deep extratropical cyclones increased over the same period in the SH. It is ''likely'' that extratropical cyclone activity in the SH has intensified during austral summer with no significant changes in austral winter. There is ''medium confidence'' that boreal-winter storm tracks during the last decades experienced poleward shifts over the NH and SH oceans. There is ''low'' ''confidence'' of changes in extratropical cyclone activity prior 1979 due to inhomogeneities in the intrumental records and modern reanalyses.

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===== 8.3.2.8.2 Atmospheric rivers =====

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Atmospheric rivers (ARs) are long, narrow (up to a few hundred kilometres wide), shallow (up to few kilometres deep) and transient corridors of strong horizontal water vapour transport that are typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone ( [[#Ralph--2018|Ralph et al., 2018]] ). Atmospheric rivers were not assessed in AR5. ARs are associated with atmospheric moisture transport from the tropics to the mid- and high latitudes ( [[#Zhu--1998|Zhu and Newell, 1998]] ), although the drivers of moisture transport relative to the different airstreams within extratropical cyclones remains a subject of current study ( [[#Dacre--2019|Dacre et al., 2019]] ). While much previous research has focused on the west coast of North America, ARs occur throughout extratropical and polar regions (e.g., [[#Guan--2015|Guan and Waliser, 2015]] ) and are often associated with locally-heavy precipitation, including a substantial fraction of all mid-latitude extreme precipitation events (e.g., [[#Waliser--2017|Waliser and Guan, 2017]] ). ARs also affect East Asia strongly during the period from late spring to summer ( [[#Kamae--2017|Kamae et al., 2017]] ). ARs can be related to warming/melt events trough the intrusions of warm and moist air in Antarctica, Greenland and New Zealand ( [[#Bozkurt--2018|Bozkurt et al., 2018]] ; [[#Mattingly--2018|Mattingly et al., 2018]] ; [[#Little--2019|Little et al., 2019]] ), contributing about 45 – 60% of total annual precipitation in subtropical South America ( [[#Viale--2018|Viale et al., 2018]] ). They also '''''transport moisture from South America to the western and central South Atlantic, feeding the ARs that reach the west coast of South Africa''''' ( [[#Ramos--2019|Ramos et al., 2019]] ). However, the estimation of precipitation rate from ARs can have large uncertainties, especially as ARs hit topographically complex coastal regions ( [[#Behrangi--2016|Behrangi et al., 2016]] ), which can cause complexities in quantifying AR-related precipitation.

Analysis of observed trends in the characteristics of ARs has been limited. [[#Gershunov--2017|Gershunov et al. (2017)]] and Sharma and Déry (2019) have shown a rising trend in land-falling AR activity over the west coast of North American since 1948. ( [[#Gonzales--2019|Gonzales et al., 2019]] ) have also documented a seasonally-asymmetric warming of ARs affecting the West Coast of the USA since 1980, which has hydrological implications for the timing and magnitude of regional runoff. Longer-term paleoclimate analysis of ARs is even more limited, although Lora et al. (2017) reported that in the last glacial maximum, AR landfalls over the North American west coast were shifted southward compared to the present conditions.

In summary, it is ''likely'' that there was an increasing trend in the AR activity in the eastern North Pacific since the mid-20th century. However, there is ''low confidence'' in the magnitude of this trend and no formal attribution, although such an increase in activity is consistent with the expected and observed increase in precipitable water associated with human-induced global warming.

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