ABSTRACT
The far eastern tropical Pacific experienced a rapid, marked warming in early 2017, causing torrential rains along the west coast of South America with a significant societal toll in Peru ...and Ecuador. This strong coastal El Niño was largely unpredicted, even a few weeks before its onset, and it developed differently from either central or eastern events. Here we provide an overview of the event, its impacts and concomitant atmospheric circulation. It is proposed that a remotely forced, sustained weakening of the free tropospheric westerly flow impinging the subtropical Andes leads to a relaxation of the southeasterly (SE) trades off the coast, which in turn may have warmed the eastern Pacific throughout the weakening of upwelling in a near‐coastal band and the lessening of the evaporative cooling farther offshore.
As depicted in this GPM radar image, torrential rains afflicted the otherwise arid coast of Ecuador and Peru from January to April 2017, resulting in a death toll of at least 200 and widespread damage to civil works and infrastructure. The storms occurred in connection with a very strong and largely unpredicted coastal El Niño, whose essential features and plausible trigger mechanism are described here.
While it is widely accepted that the global mean atmospheric temperature has increased in recent decades, the spatial distribution of global warming has been complex. In this study we comprehensively ...characterize the spatial pattern, including vertical structure, of temperature trends along the subtropical west coast of South America (continental Chile) for the period 1979–2006 and examine their consistency with expectations based on the CMIP‐3 ensemble of coupled ocean‐atmosphere simulations for the late 20th century. In central and northern Chile (17°–37°S) the most notable feature is a strong contrast between surface cooling at coastal stations (−0.2°C/decade) and warming in the Andes (+0.25°C/decade), only 100–200 km further inland. Coastal radiosonde data imply that the coast‐Andes variation is largely due to strong vertical stratification of temperature trends in the atmosphere west of the Andes. The coastal cooling appears to form part of a larger‐scale, La Niña‐like pattern and may extend below the ocean mixed layer to depths of at least 500 m. Over continental Chile the CMIP‐3 GCM ensemble predicts temperature trends similar to those observed in the Andes. The cooling along the Chilean coast is not reproduced by the models, but the mean SST warming is weaker there than any other part of the world except the Southern Ocean. It is proposed that the intensification of the South Pacific Anticyclone during recent decades, which is also a simulated consequence of global warming, is likely to play a major role in maintaining cooler temperatures off the coast of Chile.
Within large uncertainties in the precipitation response to greenhouse gas forcing, the Southeast Pacific drying stands out as a robust signature within climate models. A precipitation decline, of ...consistent direction but of larger amplitude than obtained in simulations with historical climate forcing, has been observed in central Chile since the late 1970s. To attribute the causes of this trend, we analyze local rain gauge data and contrast them to a large ensemble of both fully coupled and sea surface temperature‐forced simulations. We show that in concomitance with large‐scale circulation changes, the Pacific Decadal Oscillation explains about half of the precipitation trend observed in central Chile. The remaining fraction is unlikely to be driven exclusively by natural phenomena but rather consistent with the simulated regional effect of anthropogenic climate change. We particularly estimate that a quarter of the rainfall deficit affecting this region since 2010 is of anthropogenic origin. An increased persistence and recurrence of droughts in central Chile emerges then as a realistic scenario under the current socioeconomic pathway.
Key Points
Evidence of an anthropogenic contribution to the drying trend in central Chile
Anthropogenic forcing explains about a quarter of the 2010–2014 drought in central Chile
Major PDO effect on the recent large‐scale circulation and South Pacific precipitation trends
Central Chile, home to more than 10 million inhabitants, has experienced an uninterrupted sequence of dry years since 2010 with mean rainfall deficits of 20–40%. The so‐called Mega Drought (MD) is ...the longest event on record and with few analogues in the last millennia. It encompasses a broad area, with detrimental effects on water availability, vegetation and forest fires that have scaled into social and economical impacts. Observations and reanalysis data reveal that the exceptional length of the MD results from the prevalence of a circulation dipole‐hindering the passage of extratropical storms over central Chile—characterized by deep tropospheric anticyclonic anomalies over the subtropical Pacific and cyclonic anomalies over the Amundsen–Bellingshausen Sea. El Niño Southern Oscillation (ENSO) is a major modulator of such dipole, but the MD has occurred mostly under ENSO‐neutral conditions, except for the winters of 2010 (La Niña) and 2015 (strong El Niño). Climate model simulations driven both with historical forcing (natural and anthropogenic) and observed global SST replicate the south Pacific dipole and capture part of the rainfall anomalies. Idealized numerical experiments suggest that most of the atmospheric anomalies emanate from the subtropical southwest Pacific, a region that has experienced a marked surface warming over the last decade. Such warming may excite atmospheric Rossby waves whose propagation intensifies the circulation pattern leading to dry conditions in central Chile. On the other hand, anthropogenic forcing (greenhouse gases concentration increase and stratospheric ozone depletion) and the associated positive trend of the Southern Annular Mode also contribute to the strength of the south Pacific dipole and hence to the intensity and longevity of the MD. Given the concomitance of the seemingly natural (ocean sourced) and anthropogenic forcing, we anticipate only a partial recovery of central Chile precipitation in the decades to come.
Geographic and climate features of Central Chile. (a) Topographic map (dark grey: Terrain elevation <500 m asl; light grey: 500–3,000 m asl; white: > 3,000 m asl). Blue dots are rain gauges stations operated by DMC/DGA. Red circles are the location of the six stations used to define the regional precipitation index. Stations that provide records for Figure 4 are also indicated. (b) Station‐based annual mean rainfall (1980–2010) according to latitude.
The eruption of the Hunga Tonga‐Hunga Ha'apai Volcano in January 2022 in the southwest Pacific islands of Tonga triggered a tsunami that was detected beyond the Pacific basin. Here we show its ...spatiotemporal signature as revealed by hundreds of publicly available coastal tide gauge records from around the world. The Tonga tsunami was characterized by a uniformly small leading wave that arrived earlier than theoretically expected for a tsunami wave freely propagating away from the volcano. In contrast, the largest waves, of up to +3 m high, were concentrated in the Pacific and their timing agrees well with tsunami propagation times from the volcano. While the leading waves were caused by a previously reported fast‐moving atmospheric pressure pulse generated in the volcanic explosion, the large waves observed later in the Pacific were likely originated in the vicinity of the volcano although its generation mechanism(s) cannot be identified by the tide gauge data alone.
Plain Language Summary
In January 2022, a submarine volcano in the southwest Pacific islands of Tonga erupted and triggered a tsunami that was detected at many places, even outside the Pacific Ocean. The Tonga tsunami was recorded by hundreds of coastal stations that continuously measure the sea level around the world. Here, we analyze such records to provide a global picture of this tsunami. We found that the up and down movements of the sea surface (tsunami waves) around the Pacific coast were distinctly different than those at other parts of the world. The Pacific measurements feature small tsunami waves before the occurrence of much larger waves. Outside the pacific we mainly see these small first waves that do not grow too much afterward. Because the timing of these first waves coincides with the passing of a previously reported atmospheric pressure wave generated in the volcanic explosion, we think they were caused by interactions between the air and seawater. This is why we also see them outside the Pacific. In turn, the much larger waves at the Pacific were likely originated in the vicinity of the volcano. This is why we do not see them elsewhere.
Key Points
Analysis of 589 tide gauge records reveals the spatiotemporal features of the 2022 Tonga volcanic tsunami around the world oceans and seas
The leading wave was ubiquitously small, and its early arrival suggests it was caused by a moving air pressure wave produced in the explosion
The largest waves were concentrated in the Pacific with occurrence times consistent with an origin in the vicinity of the volcano
This study quantifies the impact of atmospheric rivers (ARs) on precipitation in southern South America. An AR detection algorithm was developed based on integrated water vapor transport (IVT) from ...6-hourly CFSR reanalysis data over a 16-yr period (2001–16). AR landfalls were linked to precipitation using a comprehensive observing network that spanned large variations in terrain along and across the Andes from 27° to 55°S, including some sites with hourly data. Along the Pacific (west) coast, AR landfalls are most frequent between 38° and 50°S, averaging 35–40 days yr−1. This decreases rapidly to the south and north of this maximum, as well as to the east of the Andes. Landfalling ARs are more frequent in winter/spring (summer/fall) to the north (south) of ~43°S. ARs contribute 45%–60% of the annual precipitation in subtropical Chile (37°–32°S) and 40%–55% along the midlatitude west coast (37°–47°S). These values significantly exceed those in western North America, likely due to the Andes being taller. In subtropical and midlatitude regions, roughly half of all events with top-quartile precipitation rates occur under AR conditions. Median daily and hourly precipitation in ARs is 2–3 times that of other storms. The results of this study extend knowledge of the key roles of ARs on precipitation, weather, and climate in the South American region. They enable comparisons with other areas globally, provide context for specific events, and support local nowcasting and forecasting.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Consistent with its high elevation (>4000 m) and subtropical location (15°–25°S), the central Andes are expected to become warmer during the twenty-first century, affecting the population, ...ecosystems, and glaciers on the so-called South American Altiplano. Future changes in regional precipitation (even its sign) have been more difficult to estimate, partly because of the low resolution of current global climate models (GCMs) relative to the cross-mountain scale of the Andes. Nevertheless, summer season rainfall over the Altiplano exhibits a strong dependence on the magnitude of the zonal flow in the free troposphere, as quantified in this work using station data. Since GCMs indicate a consistent increase in westerly flow over the central Andes, hindering moisture transport from the interior of the continent, a simple regression analysis suggests a significant reduction (10%–30%) in Altiplano precipitation by the end of this century under moderate-to-strong greenhouse gas emission scenarios.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
During the last four decades, the sea level pressure has been decreasing over the Amundsen–Bellingshausen Sea (ABS) region and increasing between 30° and 40°S from New Zealand to Chile, thus forming ...a pressure trend dipole across the South Pacific. The trends are strongest in austral winter and have influenced the climate of West Antarctica and South America. The pressure trends have been attributed to decadal variability in the tropics, expansion of the Hadley cell, and an associated positive trend of the southern annular mode, but these mechanisms explain only about half of the pressure trend dipole intensity. Experiments conducted with two atmospheric models indicate that upper ocean warming over the subtropical southwest Pacific (SSWP), termed the Southern Blob, accounts for about half of the negative pressure trend in the ABS region and nearly all the ridging/drying over the eastern subtropical South Pacific, thus contributing to the central Chile megadrought. The SSWP warming intensifies the pressure trend dipole through warming the troposphere across the subtropical South Pacific and shifting the midlatitude storm track poleward into the ABS. Multidecadal periods of strong SSWP warming also appear in fully coupled preindustrial simulations, associated with a pressure trend dipole and reduction in rainfall over the central tropical Pacific, thus suggesting a natural origin of the Southern Blob and its teleconnection. However, the current warming rate exceeds the range of natural variability, implying a likely additional anthropogenic contribution.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In this paper we present an analysis of the direct impacts of climate change on the hydrology of the upper watersheds (range in elevation from 1,000 to 5,500 m above sea level) of the snowmelt-driven ...Limarí river basin, located in north-central Chile (30° S, 70° W). A climate-driven hydrology and water resources model was calibrated using meteorological and streamflow observations and later forced by a baseline and two climate change projections (A2, B2) that show an increase in temperature of about 3–4°C and a reduction in precipitation of 10–30% with respect to baseline. The results show that annual mean streamflow decreases more than the projected rainfall decrease because a warmer climate also enhances water losses to evapotranspiration. Also in future climate, the seasonal maximum streamflow tends to occur earlier than in current conditions, because of the increase in temperature during spring/summer and the lower snow accumulation in winter.
Extratropical precipitation is primarily produced by cold and warm fronts associated with surface cyclones and upper-level troughs. The growth of these midlatitude storms is partially controlled by ...the dry baroclinicity of the troposphere, which in turn can be roughly quantified by the intensity of the upper-level zonal flow. Orographic rainfall, an important component of the precipitation in several midlatitude regions, is also partially determined by the intensity of the cross-mountain midlevel winds. Thus, at monthly and longer time scales, variations of precipitation and zonal flow aloft (as well as wind shear) at a given location should exhibit some degree of coherence. In this work the local covariability of these variables is documented over intermonthly and interannual time scales, using global precipitation products and atmospheric reanalysis from 1979 to 2004. The spatial correspondence between the precipitation and two indices of synoptic activity in the extratropics is also documented.
The local correlation (r₀) between monthly anomalies of precipitation and upper-level (300 hPa) zonal flow varies in space, from moderately and even highly significant values (r₀ ~ 0.3 to 0.7) over the midlatitude oceans to near zero over the interior of continental areas. Broadly similar results are found when considering the monthly variance of the high-pass-filtered meridional wind (an index of eddy activity) or the midlevel Eady growth rate. The local correlation map between precipitation and low-level (850 hPa) zonal flow is similar to its upper-level counterpart, but the correlations over open ocean are somewhat weaker, while orographic effects show up more clearly. The correlations are positive and large upstream of the major north–south-oriented mountain ranges, as strong westerlies promote upslope rain in addition to storm-related precipitation. In contrast, the correlation tends to be negative downstream of the ranges, as strong westerlies enhance the rain shadow effects over the lee side.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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