Significance There is evidence that the 2007−2010 drought contributed to the conflict in Syria. It was the worst drought in the instrumental record, causing widespread crop failure and a mass ...migration of farming families to urban centers. Century-long observed trends in precipitation, temperature, and sea-level pressure, supported by climate model results, strongly suggest that anthropogenic forcing has increased the probability of severe and persistent droughts in this region, and made the occurrence of a 3-year drought as severe as that of 2007−2010 2 to 3 times more likely than by natural variability alone. We conclude that human influences on the climate system are implicated in the current Syrian conflict.
Before the Syrian uprising that began in 2011, the greater Fertile Crescent experienced the most severe drought in the instrumental record. For Syria, a country marked by poor governance and unsustainable agricultural and environmental policies, the drought had a catalytic effect, contributing to political unrest. We show that the recent decrease in Syrian precipitation is a combination of natural variability and a long-term drying trend, and the unusual severity of the observed drought is here shown to be highly unlikely without this trend. Precipitation changes in Syria are linked to rising mean sea-level pressure in the Eastern Mediterranean, which also shows a long-term trend. There has been also a long-term warming trend in the Eastern Mediterranean, adding to the drawdown of soil moisture. No natural cause is apparent for these trends, whereas the observed drying and warming are consistent with model studies of the response to increases in greenhouse gases. Furthermore, model studies show an increasingly drier and hotter future mean climate for the Eastern Mediterranean. Analyses of observations and model simulations indicate that a drought of the severity and duration of the recent Syrian drought, which is implicated in the current conflict, has become more than twice as likely as a consequence of human interference in the climate system.
In recent years, two alarming trends in North Atlantic climate have been noted: an increase in the intensity and frequency of Atlantic hurricanes and a rapid decrease in Greenland ice sheet volume. ...Both of these phenomena occurred while a significant warming took place in North Atlantic sea surface temperatures (SSTs), thus sparking a debate on whether the warming is a consequence of natural climate variations, anthropogenic forcing, or both; and if both, what their relative roles are. Here models and observations are used to detect and attribute long-term (multidecadal) twentieth-century North Atlantic (NA) SST changes to their anthropogenic and natural causes. A suite of Intergovernmental Panel on Climate Change (IPCC) twentieth-century (C20C) coupled model simulations with multiple ensemble members and a signal-to-noise maximizing empirical orthogonal function analysis are used to identify a model-based estimate of the forced, anthropogenic component in NA SST variability. Comparing the results to observations, it is argued that the long-term, observed, North Atlantic basin-averaged SSTs combine a forced global warming trend with a distinct, local multidecadal “oscillation” that is outside of the range of the model-simulated, forced component and most likely arose from internal variability. This internal variability produced a cold interval between 1900 and 1930, followed by 30 yr of relative warmth and another cold phase from 1960 to 1990, and a warming since then. This natural variation, referred to previously as the Atlantic Multidecadal Oscillation (AMO), thus played a significant role in the twentieth-century NA SST variability and should be considered in future, near-term climate projections as a mechanism that, depending on its behavior, can act either constructively or destructively with the region’s response to anthropogenic influence, temporarily amplifying or mitigating regional climate change.
Mediterranean-type climates are defined by temperate, wet winters, and hot or warm dry summers and exist at the western edges of five continents in locations determined by the geography of winter ...storm tracks and summer subtropical anticyclones. The climatology, variability, and long-term changes in winter precipitation in Mediterranean-type climates, and the mechanisms for model-projected near-term future change, are analyzed. Despite commonalities in terms of location in the context of planetary-scale dynamics, the causes of variability are distinct across the regions. Internal atmospheric variability is the dominant source of winter precipitation variability in all Mediterranean-type climate regions, but only in the Mediterranean is this clearly related to annular mode variability. Ocean forcing of variability is a notable influence only for California and Chile. As a consequence, potential predictability of winter precipitation variability in the regions is low. In all regions, the trend in winter precipitation since 1901 is similar to that which arises as a response to changes in external forcing in the models participating in phase 5 of the Coupled Model Intercomparison Project. All Mediterranean-type climate regions, except in North America, have dried and the models project further drying over coming decades. In the Northern Hemisphere, dynamical processes are responsible: development of a winter ridge over the Mediterranean that suppresses precipitation and of a trough west of the North American west coast that shifts the Pacific storm track equatorward. In the Southern Hemisphere, mixed dynamic–thermodynamic changes are important that place a minimum in vertically integrated water vapor change at the coast and enhance zonal dry advection into Mediterranean-type climate regions inland.
Atlantic Multi‐decadal Variability (AMV), also known as the Atlantic Multi‐decadal Oscillation (AMO), is characterized by a sharp rise and fall of the North Atlantic basin‐wide sea surface ...temperatures (SST) on multi‐decadal time scales. Widespread consequences of these rapid temperature swings were noted in many previous studies. Among these are the drying of Sahel in the 1960–70s and change in the frequency and intensity of Atlantic hurricanes on multi‐decadal time scales. Given the short instrumental data records (about a century long) the central question is whether these climate fluctuations are robustly linked with the AMV and to what extent are these connections subject to changes in a changing climate. Here we address this issue by using the CMIP3 simulations for the 20th, 21st, and pre‐industrial eras with 23 IPCC models. While models tend to produce AMV of shorter time scales and less periodic than suggested by the observations, the spatial structures of the SST anomaly patterns, and their association with worldwide precipitation, are surprisingly similar between models (with differing external forcing) and observations. Our results confirm the strong link between AMV and Sahel rainfall and suggest a clear physical mechanism for the linkage in terms of meridional shifts of the Atlantic ITCZ. The results also help clarify influences that may not be robust, such as the impacts over North America, India, and Australia.
Key Points
AMV SST patterns are robust for various radiative forcing in models and data
Rainfall pattern associated with AMV is meridional shifts of Atlantic ITCZ
AMV link to Pacific SST is key to North American Rainfall responses
Variations of the North Atlantic subtropical high (NASH) western ridge and their implication to the Southeastern United States (SE US) summer precipitation were analyzed for the years 1948–2007. The ...results show that the movement of the NASH western ridge regulates both moisture transport and vertical motion over the SE US, especially in the last three decades, during which the ridge moved westward towards the American continent. When the NASH western ridge is located southwest (SW) of its mean climate position, excessive summer precipitation is observed due to an enhanced moisture transport. In contrast, when the western ridge is located in the northwest (NW), a precipitation deficit prevails as downward motion dominates the region. Composite analysis indicates that SW ridging results mainly from the NASH center’s intensification; whereas NW ridging is likely caused by stationary wave propagation from the eastern Pacific/US western coast. In recent decades, both the SW and NW ridge positions have been observed to increase in frequency. Our results suggest that the increase in the SW ridging consistently follows the NASH’s intensification associated with anthropogenic forcing as projected by coupled climate models. However, the increased frequency of NW ridging tends to follow the positive Pacific decadal oscillation (PDO) index. Thus, the enhanced variability in the SE US summer precipitation in recent decades might be a combined result of anthropogenic forcing and internal variability of the climate system. Results suggest that, as anthropogenic forcing continues to increase, the SE US will experience more frequent wet summers and an increase in the frequency of dry summers during positive PDO phases.
The Caribbean seasonal rainfall cycle and its characteristics are heavily relied upon by the region’s inhabitants for their socioeconomic needs; the prediction of its variability would be valuable to ...society. An important way to understand the predictability of the Caribbean rainfall cycle is to study its interannual variability. Previous studies vary as to how and what large-scale climate driver(s) affect the interannual variability of rainfall and its associated dynamical mechanisms in the Caribbean. To address this, this study investigates wet and dry Caribbean early-rainy seasons (ERS; mid-April to mid-June) and late-rainy seasons (LRS; late August to mid-November) by conducting the following: (1) a spatial composite of rainfall from 34 Caribbean rainfall stations using daily data; and, (2) spatial composites of sea-surface temperature, sea-level pressure, and mean flow moisture convergence and transports. The ERS and LRS are impacted in distinctly different ways by two different, and largely independent, dominant large-scale phenomena: the North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO), respectively. Dry ERS years are associated with a persistent dipole of cold and warm SSTs over the Caribbean Sea and Gulf of Mexico, respectively, that were caused by a preceding positive NAO state. This setting involves a wind-evaporation-SST (WES) feedback expressed in enhanced trade winds and consequently, moisture transport divergence over all of the Caribbean, except in portions of the NW Caribbean in May. A contribution from the preceding winter cold ENSO event is also discernible during dry ERS years. Dry LRS years are due to the summertime onset of an El Niño event, developing an inter-basin SLP pattern that fluxes moisture out of the Caribbean, except in portions of the NW Caribbean in November. Both large-scale climate drivers would have the opposite effect during their opposite phases leading to wet years for both seasons. The two rainy seasons are independent because the main drivers of their variability are independent. This has implications for prediction.
Using observations and reanalysis, we develop a robust statistical approach based on canonical correlation analysis (CCA) to explore the leading drivers of decadal and longer-term Mediterranean ...hydroclimate variability during the historical, half-year wet season. Accordingly, a series of CCA analyses are conducted with combined, multi-component large-scale drivers of Mediterranean precipitation and surface air temperatures. The results highlight the decadal-scale North Atlantic Oscillation (NAO) as the leading driver of hydroclimate variations across the Mediterranean basin. Markedly, the decadal variability of Atlantic-Mediterranean sea surface temperatures (SST), whose influence on the Mediterranean climate has so far been proposed as limited to the summer months, is found to enhance the NAO-induced hydroclimate response during the winter half-year season. As for the long-term, century scale trends, anthropogenic forcing, expressed in terms of the global SST warming (GW) signal, is robustly associated with basin-wide increase in surface air temperatures. Our analyses provide more detailed information than has heretofore been presented on the sub-seasonal evolution and spatial dependence of the large-scale climate variability in the Mediterranean region, separating the effects of natural variability and anthropogenic forcing, with the latter linked to a long-term drying of the region due to GW-induced local poleward shift of the subtropical dry zone. The physical understanding of these mechanisms is essential in order to improve model simulations and prediction of the decadal and longer hydroclimatic evolution in the Mediterranean area, which can help in developing adaptation strategies to mitigate the effect of climate variability and change on the vulnerable regional population.
Abstract
There is little scientific consensus on the importance of external climate forcings—including anthropogenic aerosols, volcanic aerosols, and greenhouse gases (GHG)—relative to each other and ...to internal variability in dictating past and future Sahel rainfall. We address this query by relating a 3-tiered multi-model mean (MMM) over the Climate Model Intercomparison Project phase 5 “twentieth century” and pre-Industrial control simulations to observations. The comparison of single-forcing and historical simulations highlights the importance of anthropogenic and volcanic aerosols over GHG in generating forced Sahel rainfall variability in models. However, the forced MMM only accounts for a small fraction of observed variance. A residual consistency test shows that simulated internal variability cannot explain the residual observed multidecadal variability, and points to model deficiency in simulating multidecadal variability in the forced response, internal variability, or both.
The dynamical mechanisms associated with the impact of year-to-year variability in tropical North Atlantic (TNA) sea surface temperatures (SSTs) on North American precipitation, during the cold and ...warm halves of the hydrological year (October–September) are examined. Observations indicate that during both seasons warmer-than-normal TNA SSTs are associated with a reduction of precipitation over North America, mainly west of ∼90°W, and that the effect can be up to 30% of the year-to-year seasonal precipitation RMS variability. This finding confirms earlier studieswith observations and models. During the cold season (October–March) the North American precipitation variability associated with TNA fluctuations is considerably weaker than its association with ENSO. During the warm season (April–September), however, the Atlantic influence, per one standard deviation of SST anomalies, is larger than that of ENSO.
The observed association between TNA SST anomalies and global and North American precipitation and sea level pressure variability is compared with that found in the output of an atmospheric general circulation model (AGCM) forced with observed SST variability, both globally and in the tropical Atlantic alone. The similarity between model output and observations suggests that TNA SST variability is causal. The mechanisms of the ‘‘upstream’’ influence of the Atlantic on North American precipitation are seasonally dependent. In the warm season, warmer-than-normal TNA SSTs induce a local increase in atmospheric convection. This leads to a weakening of the North Atlantic subtropical anticyclone and a reduction in precipitation over the United States and northern Mexico, associated with the anomalous southward flow there. In the cold season, a response similar to the warm season over the subtropical Atlantic is identified, but there is also a concomitant suppression of convection over the equatorial Pacific, which leads to a weakening of the Aleutian low and subsidence over western North America, similar to the impact of La Niñaa although weaker in amplitude. The impact of TNA SST on tropical convection and the extratropical circulation is examined by a set of idealized experiments with a linear general circulation model forced with the tropical heating field derived from the full AGCM.
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