Adaptation of the Canadian Fire Weather Index to Mediterranean forests

Natural Hazards - The Canadian Fire Weather Index (FWI) is one of the most used tools to forecast forest fire hazard. In this paper, we propose an adaptation of the FWI to take into account the...     

Patterns of sea surface temperature variability on seasonal to sub-annual scales at and offshore the Río de la Plata estuary

SST variability on seasonal to sub-annual scales in the coastal region of South America between 30° and 39°S, largely influenced by the Rio de la Plata estuary’s plume, and its relation to wind variability are explored. Data are six years of daily ensembles of gridded satellite SST and sea surface winds with spatial resolutions of about 11 and 25 km, respectively. Observations from oceanographic cruises are used to validate the results. It is found that the seasonal cycle can be explained in terms of two modes. The first one, characterizing fall-early winter/spring-early summer, is related to the radiative cycle. The second one, corresponding to late summer and winter, displays warm/cold anomalies along the Uruguayan coast forced by the prevailing winds during those seasons. In the upper estuary and the northern part of the area of influence of the freshwater plume, variability in sub-annual scales is significant. A large portion of this variance is related to zonal wind anomalies that force warm/cold SSTs along that coast. Cold anomalies of up to −5 °C occur under anomalously intense easterly winds, indicating upwelling. These events are very frequent and show large persistence, occurring up to one and a half months. They also display a marked seasonal cycle – being more frequent in late spring and summer – large inter-annual variability and seem to be modulated by the continental runoff. When discharge is low, the freshwater plume retracts to the west, reducing the inner-shelf stratification and increasing the likelihood of a full upwelling to the surface. In winter, short time-scale SST variability is mostly due to variability in the atmospheric cold fronts crossing the region. Weaker or less frequent (stronger or more frequent) fronts produce a generalized warming (cooling) over the region. As the estuary heats (colds) faster than the shelf, a warm (cold) anomaly develops in the upper Río de la Plata. On inter-annual time scales, probably because ENSO activity was weak during the studied period, SST variability was not important.     

Upper-level cut-off lows in southern South America

Summary This paper presents a statistical study of the spatial and seasonal distribution and duration of cut-off low systems over the southern South American region based on the NCEP- NCAR reanalysis data for the period 1979–1988. Cut-off lows were first objectively determined as minimum geopotential values at the 250 hPa level and then subjectively imposing a cut-off circulation and a cold core. A total of 171 cut-off low events were detected, being more frequent in austral autumn followed by winter, spring and summer. There is a preferential region of occurrence in spring and autumn located between 68°–80° W and 30°–45° S. The Pacific area showed the greatest frequency of occurrence followed by the Atlantic and the continental areas. Most of the cut-off lows last 2 or 3 days (around 90% of the cases) though there is a tendency of the continental events to be longer. The cut-off low event developed upwind the Andes on 22–28 September 1986 was selected as a case study. Low-level cold air advection was the main forcing of the deepening of the upper level low system.