Extreme wet seasons – their definition and relationship with synoptic-scale weather systems

An extreme aggregation of precipitation on the seasonal timescale, leading to a so-called extreme wet season, can have substantial environmental and socio-economic impacts. This study has a twofold aim: first to identify and statistically characterize extreme wet seasons around the globe and second to elucidate their relationship with specific weather systems. Extreme wet seasons are defined independently at every grid point of ERA-Interim reanalyses as the consecutive 90 d period with the highest accumulated precipitation in the 40-year period of 1979–2018. In most continental regions, the extreme seasons occur during the warm months of the year, especially in the midlatitudes. Nevertheless, colder periods might be also relevant, especially in coastal areas. All identified extreme seasons are statistically characterized in terms of climatological anomalies of the number of wet days and of daily extreme events. Results show that daily extremes are decisive for the occurrence of extreme wet seasons in regions of frequent precipitation, e.g., in the tropics. This is in contrast to arid regions where wet seasons may occur only due to anomalously frequent wet days. In the subtropics and more precisely within the transitional zones between arid areas and regions of frequent precipitation, both an anomalously high occurrence of daily extremes and of wet days are related to the formation of extreme wet seasons. A novel method is introduced to define the spatial extent of regions affected by a particular extreme wet season and to relate extreme seasons to four objectively identified synoptic-scale weather systems, which are known to be associated with intense precipitation: cyclones, warm conveyor belts, tropical moisture exports and breaking Rossby waves. Cyclones and warm conveyor belts contribute particularly strongly to extreme wet seasons in most regions of the globe. But interlatitudinal influences are also shown to be important: tropical moisture exports, i.e., the poleward transport of tropical moisture, can contribute to extreme wet seasons in the midlatitudes, while breaking Rossby waves, i.e., the equatorward intrusion of stratospheric air, may decisively contribute to the formation of extreme wet seasons in the tropics. Three illustrative examples provide insight into the synergetic effects of the four identified weather systems on the formation of extreme wet seasons in the midlatitudes, the Arctic and the (sub)tropics.

Extreme wet seasons – their definition and relationship with synoptic scale weather systems

An extreme aggregation of precipitation on the seasonal timescale, leading to a so-called extreme wet season, can have substantial environmental and socio-economic impacts. In contrast to extreme precipitation events on hourly to daily timescales, which are typically caused by single weather systems, an extreme wet season may be attributed to a combination of different and/or recurring weather systems. In fact, extreme wet seasons may be formed by almost continuously occurring moderate events, or by more frequent and/or more intense short-duration extreme events, or by a combination of these scenarios. This study aims at identifying and statistically characterizing extreme wet seasons around the globe, and elucidating their relationship with specific weather systems. To define extreme wet seasons, we used 40 years (1979–2018) of ERA-Interim reanalyses. Primary extreme seasons were defined independently at every grid point as the consecutive 90-day period with the highest accumulated precipitation. Secondary extreme seasons were also considered, if accumulated precipitation amounts to at least 90 % of the precipitation in the primary season at the same grid point. A high number of secondary extreme seasons was found for instance in the extratropical storm tracks, suggesting that these regions are less likely to experience an exceptional amount of precipitation in a particular 90-day period. In most continental regions, the extreme seasons occur during the warm months of the year, especially in the mid-latitudes. Nevertheless, colder periods might be also relevant to extreme seasons within the same continent, especially in coastal areas. All identified extreme seasons were statistically characterised in terms of anomalies compared to the climatology of the number of wet days and daily extreme events. Results show that daily extremes are decisive for the occurrence of extreme wet seasons in regions of frequent precipitation, e.g. in the tropics. In contrast, e.g., in arid regions where wet days are scarce, extreme seasons may occur only due to anomalously high numbers of wet days. In the subtropics and more precisely within the transitional zones between arid areas and regions of frequent precipitation, both an anomalously high occurrence of daily extremes and wet days are related to the formation of extreme wet seasons. The spatial extent of regions affected by the same extreme wet season is variable and can reach continental scales, although the vast majority of extreme seasons is limited to scales of the order of 20 × 105 km2. Finally, the relationship of extreme seasons to synoptic-scale weather systems was investigated on the basis of four objectively identified weather systems that are known to be associated with intense precipitation: cyclones, warm conveyor belts, tropical moisture exports and breaking Rossby waves. A grid-to-grid association of these weather systems to daily precipitation allows quantifying their role for extreme wet seasons. In particular, cyclones and warm conveyor belts contribute strongly to extreme wet seasons in most regions of the globe. But interlatitudinal influences are also shown to be important: tropical moisture exports, i.e., the poleward transport of tropical moisture, can contribute to extreme wet seasons in the mid-latitudes, while breaking Rossby waves, i.e., the equatorward intrusion of stratospheric air, may decisively contribute to the formation of extreme wet seasons in the tropics. Four illustrative examples provide insight into the synergetic effects of the four identified weather systems on the formation of extreme wet seasons in the Arctic, the mid-latitudes, Australia, and the tropics.

Advances in knowledge 10 years after the torrential rains in Madeira Island (Portugal)

<p>On February 20, 2010, Madeira island was affected by a tragic event of extreme precipitation. The event was marked by huge economical damage estimated in millions of euros, and more than 40 deaths. Before the event, there were not many studies about severe precipitation in Madeira, which were highly motivated after 2010. This work intent is to show some advancements in knowledge of heavy precipitation events (HPE) in Madeira found in the last decade. The Meso-NH model was used with a rather complete parametrization package of several physical processes occurring in the atmosphere and configured into different dimensions. In order to explore the meridional water vapour transport, the total precipitable water field was extracted from the Atmospheric Infrared Sounder (AIRS) data products. In the first set of simulations, the experiments were performed with three horizontal nested domains (9 km, 3 km, and 1 km resolution). The results for the winter 2009-2010 raised two questions about the topic. First, associated with the large scale environment, and the second one linked to orographic effects. In the first case, apart from a cyclone affecting the island, it was identified the presence of atmospheric rivers (ARs) coupled to frontal systems transporting tropical moisture toward the island. For the orographic effects, the simulations at 1km resolution showed maximums of accumulated precipitation in the highlands. Subsequently, the analysis of the precipitation in Madeira highlands over a 10-year period showed dry summers and the highest rainfall amounts in the winters, although with some significant events occurring also in autumn and spring seasons. Furthermore, it was found that tropical moisture transported through the ARs may reach the island with different intensities and orientation during the winter seasons. However, for the 10 winter periods, the ARs were not the sole factor producing HPE in Madeira. In the second set of simulations, the model was configured with a larger domain of 2.5 km resolution and an inner domain of 0.5 km resolution. All the significant events in autumn 2012 were simulated confirming the orographic effect in the accumulated precipitation. The most interesting result found was the occurrence of maximums values in different regions over the island. For example, over the highlands in the central peaks and southern/northern slopes, or in the coastal plain at lowlands. From the simulations it was possible to explain the causes for the distinct rainfall patterns, and the atmospheric environments associated. The variations in the configuration of the ambient flow, jointly with the orographic forcing may produce convection in distinct regions of the island, resulting in different rainfall patterns. Ten years later, the advances in the understanding of significant precipitation in the Madeira is evident. The results show how different events may occur, since the formation or enhancement of the precipitation over the island is totally dependent on the geographic aspects and atmospheric conditions associated with each precipitating event.</p>

Evaluation of the moisture sources in two extreme landfalling atmospheric river events using an Eulerian WRF tracers tool

A new 3-D tracer tool is coupled to the WRF model to analyze the origin of the moisture in two extreme atmospheric river (AR) events: the so-called Great Coastal Gale of 2007 in the Pacific Ocean and the Great Storm of 1987 in the North Atlantic. Results show that between 80 and 90 % of moisture advected by the ARs, and a high percentage of the total precipitation produced by the systems have a tropical origin. The tropical contribution to precipitation is in general above 50 % and largely exceeds this value in the most affected areas. Local convergence transport is responsible for the remaining moisture and precipitation. The ratio of tropical moisture to total moisture is maximized as the cold front arrives on land. Vertical cross sections of the moisture content suggest that the maximum in tropical humidity does not necessarily coincide with the low-level jet (LLJ) of the extratropical cyclone. Instead, the amount of tropical humidity is maximized in the lowest atmospheric level in southern latitudes and can be located above, below or ahead of the LLJ in northern latitudes in both analyzed cases.

Global Climatologies of Eulerian and Lagrangian Flow Features based on ERA-Interim

This paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zürich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained from the authors on request. These feature climatologies allow studying the frequency, variability, and trend of atmospheric phenomena and their interrelationships across temporal scales. To illustrate the potential of this dataset, boreal winter climatologies of selected features are presented and, as a first application, the very unusual Northern Hemispheric winter of 2009/10 is identified as the season when most of the considered features show maximum deviations from climatology. The second application considers dry winters in the western United States and reveals fairly localized anomalies in the eastern North Pacific of enhanced blocking and surface anticyclones and reduced cyclones.

Tropical Moisture Exports, Extreme Precipitation and Floods in Northeastern US

A statistically and physically based framework is put forward to investigate the relationship between Tropical Moisture Exports (TMEs), extreme precipitation and floods in the Northeastern United States (NE-US). We found that the NE-US floods in the four seasons are closely related to TMEs and four major moisture sources of TMEs in the tropics account for approximately 85% of all the TMEs that enter the NE-US. The seasonality and interannual variation of the birth processes in the four source regions determine their contribution to the NE-US. Moisture born in Gulf of Mexico (GP) and Gulf stream (GS) are the year-around sources, with some winter contribution from Pineapple Express (PE) region, and West Pacific (WP) region contributes the least. The overall order of their contribution to NE-US is GP>GS>PE>WP. Seasonal association between TMEs birth and ENSO are also found. The seasonal and interannual variations in atmospheric circulation patterns also play an important role in determining the TMEs’ entrance to NE-US. Strong influence of active TMEs periods on the occurrence of extreme rainfall is also identified. We show that the extreme daily precipitation events are dominated by extreme TMEs’ entering the NE-US in every season.

Evaluation of the Moisture Sources in two Extreme Landfalling Atmospheric River Events using an Eulerian WRF-Tracers tool

A new 3D Tracer tool is coupled to the WRF model to analyze the origin of the moisture in two extreme Atmospheric River (AR) events: the so-called Great Coast Gale of 2007 in the Pacific Basin, and the Great Storm of 1987 in the North Atlantic. Results show that between 80 % and 90 % of the moisture advected by the ARs, as well as between 70 % and 80 % of the associated precipitation have a tropical or subtropical origin. Local convergence transport is responsible for the remaining moisture and precipitation. The ratio of tropical moisture to total moisture is maximized as the cold front arrives to land. Vertical cross sections of the moisture suggest that the maximum in humidity does not necessarily coincide with the Low-Level Jet (LLJ) of the extratropical cyclone. Instead, the amount of tropical humidity is maximized in the lowest atmospheric level in southern latitudes, and can be located above, below or ahead the LLJ in northern latitudes in both analyzed cases.

Hydroclimate of the Last Glacial Maximum and deglaciation in southern Australia's arid margin interpreted from speleothem records (23–15 ka)

Terrestrial data spanning the Last Glacial Maximum (LGM) and deglaciation from the southern Australian region are sparse and limited to discontinuous sedimentological and geomorphological records with relatively large chronological uncertainties. This dearth of records has hindered a critical assessment of the role of the Southern Hemisphere mid-latitude westerly winds on the region's climate during this time period. In this study, two precisely dated speleothem records for Mairs Cave, Flinders Ranges, are presented, providing for the first time a detailed terrestrial hydroclimatic record for the southern Australian drylands during 23–15 ka. Recharge to Mairs Cave is interpreted from the speleothem record by the activation of growth, physical flood layering, and δ18O and δ13C minima. Periods of lowered recharge are indicated by 18O and 13C enrichment, primarily affecting δ18O, argued to be driven by evaporation of shallow soil/epikarst water in this water-limited environment. A hydrological driver is supported by calcite fabric changes. These include the presence of laminae, visible organic colloids, and occasional dissolution features, related to recharge, as well as the presence of sediment bands representing cave floor flooding. A shift to slower-growing, more compact calcite and an absence of lamination is interpreted to represent reduced recharge. The Mairs Cave record indicates that the Flinders Ranges were relatively wet during the LGM and early deglaciation, particularly over the interval 18.9–15.8 ka. This wetter phase ended abruptly with a shift to drier conditions at 15.8 ka. These findings are in agreement with the geomorphic archives for this region, as well as the timing of events in records from the broader Australasian region. The recharge phases identified in the Mairs Cave record are correlated with, but antiphase to, the position of the westerly winds interpreted from marine core MD03-2611, located 550 km south of Mairs Cave in the Murray Canyons region. The implication is that the mid-latitude westerlies are located further south during the period of enhanced recharge in the Mairs Cave record (18.9–16 ka) and conversely are located further north when greater aridity is interpreted in the speleothem record. A further comparison with speleothem records from the northern Australasian region reveals that the availability of tropical moisture is the most likely explanation driving enhanced recharge, with further amplification of recharge occurring during the early half of Heinrich Stadial 1 (HS1), possibly influenced by a more southerly displaced Intertropical Convergence Zone (ITCZ). A rapid transition to aridity at 15.8 ka is consistent with a retraction of this tropical moisture source.