Tropical cyclones (TCs) are an important element of the climate dynamics in the tropical Americas. They produce intense precipitation during a few days of the rainy season. The contribution of tropical cyclone precipitation to seasonal accumulated rainfall may be as large as fifty per cent, particularly in the arid and semi-arid regions of northern Mexico. A positive trend in the number of tropical cyclones over the eastern Pacific, has resulted in more of these systems approaching the Baja peninsula and a positive trend in annual precipitation. However, the contribution of TCs to regional accumulated may be positive or negative depending on the trajectory followed by the system. If the TC is not close enough to the coastal region, it may induce atmospheric moisture divergence over land, reducing the chances of tropical convective activity and rainfall. Years of large but “distant to continent” TC activity result in negative anomalies in precipitation for some regions of the tropical Americas. Seasonal regional climate predictions or regional climate change scenarios provide information on TC activity but not on preferred trajectories. By means of TC cluster analysis, the preferred trajectories of TCs around the tropical Americas are explored in relation to quasi-stationary circulations at the steering level. Some ideas on how to estimate preferred TCs trajectories for a season are given.

Using monthly data for the period 1979-2010, we study the dynamics and strength of land surface-atmosphere feedbacks (LAFs) among variables involved in the heat and moisture fluxes, at interannual timescales for Tropical South America (TropSA). The variables include precipitation, surface air temperature, specific humidity at 925 hPa, evaporation, and estimates of volumetric soil water content. Using a dimensional reduction, we apply a Maximum Covariance Analysis (MCA) to rank the relative contributions to LAFs and group the time series into Maximum Covariance States (MCS) with common mechanisms among variables. We estimate linear (Pearson correlations) and non-linear (information transfer and causality) coupling metrics among pairs of variables to configure the structure of linkages. The main MCS associated with LAFs over TropSA are strongly influenced by ENSO, and the meridional and equatorial SSTs modes over the Atlantic and Indian Oceans. ENSO favors a unimodal behavior, with center of action in the Amazon River basin, while SSTs over the Tropical North Atlantic result in a dipole between northern and southern TropSA. Results show that soil moisture plays a leading role in regulating heat and water anomalies, and provides the memory of the atmosphere-driven processes and their subsequent influence. Thus, soil moisture is fundamental and leads up to 9 month-lags whereby ENSO enhances the interannual connectivity and memory of LAFs in 25% with respect to the mode influenced by TNA. Within the identified multivariate structure, evaporation and soil moisture enhance the interannual connectivity of the whole set of variables since both variables exhibit more frequent two-way feedbacks with the remaining variables.

In the last decades many studies have pointed out an increasing number of natural hazards associated with extremes in precipitation and drought conditions. Generally, dry and hot conditions across the Europe impact on the Mediterranean region. The Mediterranean is located at the border between the tropical climate zone and the mid latitude climate belt. Due to its large extension and diverse topography, it shows large climatic differences that make its climate scientifically interesting.  

The aim of this study is to analyze the moisture transport during the 2003 drought episode observed over the surroundings of the Mediterranean. The region was defined according to the 5th Intergovernmental Panel on Climate Change (IPCC) Assessment Report. The episode was identified using Standardized Precipitation Evapotranspiration Index (SPEI), calculated using monthly CRU (TS3.24.01) precipitation and potential evapotranspiration (PET). One of the crucial advantages of the SPEI over the other widely used drought indexes is its multi-scalar characteristics, which enable identification of different drought types. Therefore, the monthly SPEI-1, SPEI-3, SPEI-6, SPEI-12 and SPEI-24 indexes were used to identify the episodes on different time scales. This episode was the most severe during the period 1980-2015 according to the SPEI-1 analysis. Analyses of precipitation, potential evapotranspiration, omega at 500hPa, and vertically integrated moisture flux have been conducted to characterize the anomalous patterns over the region during the event. A Lagrangian approach was then applied in order to investigate possible changes in the moisture transport from and toward the Mediterranean region during the episode. This approach is based on the FLEXPART model integrated with the ERA-Interim data set.

This paper investigates the connection of precipitation organization to the frequency distribution of precipitation intensity, including extremes. The organization of precipitating systems, for example isolated thunderstorms and mesoscale convective systems, is an expression of the influence of the large-scale environment on precipitation. A recent climatology of precipitation organization in the southeastern United States (Rickenbach et al., 2015, QJRMS) demonstrated that a simple framework of identifying the scale of precipitation organization from a radar precipitation dataset was able to capture important differences of the seasonal evolution in precipitation organization. The present study will focus on the question of whether the heaviest daily precipitation values are associated with isolated or mesoscale precipitation organization, and whether this association changes seasonally.

The analysis employs a four-year dataset of daily precipitation values across the southeastern United States to examine the association of heavy precipitation extremes with isolated versus mesoscale organization.  Daily precipitation data covering the four-year period 2009-2012 is derived from the National Mosaic and Multi-sensor Quantitative Precipitation Estimation (NMQ) radar-based dataset on a 1 km x 1 km grid that extends 100 km offshore (see Rickenbach et al. 2015 for details). Each pixel is associated with either mesoscale precipitation features (> 100 km in spatial scale) or isolated precipitation features (< 100 km in spatial scale). Preliminary results will be presented at the online conference.

Reference: Rickenbach, T. M., Nieto-Ferreira, R., Zarzar, C. and Nelson, B. (2015), A seasonal and diurnal climatology of precipitation organization in the southeastern United States. Q.J.R. Meteorol. Soc., 141: 1938–1956. doi:10.1002/qj.2500

Western Iberia is frequently struck by intense mid-latitude cyclones coming from the North Atlantic basin and often impinging extreme weather over large swaths of the Iberian Peninsula (IP). The spatial distribution and characterization of past floods and landslides with important social consequences in Portugal for the period 1865-2015 was performed within the context of the DISASTER project (Zezere et al., 2014). From this database, a major hydro-geomorphologic event was selected, the February 1979, in order to study its atmospheric forcings and to analyze its societal impacts.

The February 1979 event is a top ranked event in the DISASTER database regarding the total number of affected (18578), displaced (14322) and evacuated (4244) people in Portugal and in the Tagus basin (7677, 4816 and 2834, respectively).

Most of the days considered in this event produced daily precipitation values over or within the 90^th-95^th percentile of the corresponding long term daily precipitation series (available at high resolution between 1950 and 2008). Most of the event precipitation occurred in days characterized by wet Circulation Weather Types, i.e. cyclonic (C), west (W) or southwest (SW) types, which agrees with the assessment of wet days obtained by Trigo and DaCamara (2000) and Ramos et al. (2014) for the IP domain.

Also, throughout this period, the North Atlantic Ocean is crossed several times by narrow and prolonged bands of high moisture concentration, with cores above 9 g/kg, that originate near the Caribbean islands and move towards extratropical latitudes by the influence of southwestern low-level jets of medium or high intensity. These are mostly persistent Atmospheric Rivers (ARs) reaching the western IP coast and affecting most of the month of February until the 16^th.

Overall, regarding the large-scale circulation, a deep low-pressure system located over the North Atlantic and reaching western IP, allowed for the frequent passage of frontal systems over the territory which was responsible for this precipitation event. In addition, local convective instabilities and strong moisture transport from the Tropical Atlantic produced an extremely intense 15-day precipitation event over western IP, that establishes as the meteorological trigger of the February 1979 Disaster event.

Acknowledgements: This work was financed by national funds through FCT - Portuguese Foundation for Science and Technology, I.P., under the framework of the project FORLAND Hydro-geomorphologic risk in Portugal: driving forces and application for land use planning (PTDC/ATPGEO/1660/2014). A. M. Ramos was also supported by a FCT postdoctoral grant (FCT/DFRH/ SFRH/BPD/84328/2012).