Semiarid grasslands are water‐limited ecosystems where precipitation (PPT) controls the onset and duration of the growing season; however, this variable does not fully explain interannual variability of productivity at temporal scales. We examined the relationship between PPT and carbon (C) fluxes in a semiarid grassland at both seasonal and interannual scales, as well as the role of lagged effects of PPT and asymmetric sensitivities of net ecosystem carbon exchange to PPT and its components (gross ecosystem exchange [GEE] and ecosystem respiration [ER]). Six years of continuous net ecosystem C exchange data measured with the eddy covariance technique and GEE estimated with 15 years of enhanced vegetation index and the gross primary productivity of Moderate Resolution Imaging Spectroradiometer were used. The semiarid grassland was a C source and a C sink among contrasting PPT years (114 to −107 g C·m⁻²·year⁻¹). At seasonal scale, PPT relationship with the 15 years of GEE derived from enhanced vegetation index and gross primary productivity was sigmoidal. Moreover, PPT legacies of the previous dry season determined the C balance of the grassland by affecting the C uptake and ecosystem respiration of the following growing season, but productivity was more sensitive to PPT changes than respiration. Models of climate change for semiarid grasslands in North America predict up to 30% reduction of winter‐spring PPT and slight summer PPT decrease. Thus, our results suggest that future changes in PPT may have a strong impact on the C uptake capacity of this ecosystem due to weakened legacy effects in summer C uptake.

Infrequent and small precipitation (PPT) events characterize PPT patterns in semiarid grasslands; however, plants and soil microorganisms are adapted to use the unpredictable small pulses of water. Several studies have shown short-term responses of carbon and nitrogen mineralization rates (called the “priming effect” or the Birch effect) stimulated by wet–dry cycles; however, dynamics, drivers, and the contribution of the priming effect to the annual C balance are poorly understood. Thus, we analyzed 6 years of continuous net ecosystem exchange measurements to evaluate the effect of the PPT periodicity and magnitude of individual PPT events on the daily/annual net ecosystem C exchange (NEE) in a semiarid grassland. We included the period between PPT events, previous daytime NEE rate, and previous soil moisture content as the main drivers of the priming effect. Ecosystem respiration (ER) responded within a few hours following a PPT event, whereas it took 5–9 d for gross ecosystem exchange (GEE; where −NEE = GEE + ER) to respond. Precipitation events as low as 0.25 mm increased ER, but cumulative PPT > 40 mm infiltrating deep into the soil profile stimulated GEE. Overall, ER fluxes following PPT events were related to the change in soil water content at shallow depth and previous soil conditions (e.g., previous NEE rate, previous soil water content) and the size of the stimulus (e.g., PPT event size). Carbon effluxes from the priming effect accounted for less than 5 % of ecosystem respiration but were significantly high with respect to the carbon balance. In the long term, changes in PPT regimes to more intense and less frequent PPT events, as expected due to the effects of climate change, could convert the semiarid grassland from a small C sink to a C source.

Tornadoes are violent and destructive natural phenomena that occur on a local scale in most regions around the world. Severe storms occasionally lead to the formation of mesocyclones, whose direction or sense of rotation is often determined by the Coriolis force, among other factors. In the Northern Hemisphere, more than 99% of all tornadoes rotate anticlockwise. The present research shows that, in topographically complex regions, tornadoes have a different probability of rotating clockwise or anticlockwise. Our ongoing research programme on tornadoes in Mexico has shown that the number of tornadoes is significantly higher than previously thought. About 40% of all tornadoes occur in the complex topographic region of the Trans-Mexican Volcanic Belt. Data collected (from Internet videos) on the rotation of tornadoes formed in this region showed that about 50% of them rotated in a clockwise direction, contradicting tornado statistics for most of North America. Time series of the helicity parameter showed that tornadoes formed in topographically complex areas exhibited different behaviours compared to those formed in plains that are related with supercell systems.

In the warm season, El Niño/Southern Oscillation (ENSO) causes periods with more rain in Northern Mexico during its positive phase, while less rainfall is recorded in the southern regions during the negative phase. This research study evaluates the variability of summer (July–September) precipitation and moisture fluxes under different ENSO scenarios in the Mexican Altiplano and coast of the state of Nayarit. The catchment of Rio San Pedro-Mezquital (SPM-RB) connects both regions. Using the Oceanic Niño Index (ONI), the years that signal change from El Niño to La Niña (1998), neutral conditions (2005), and strong (moderate) La Niña (1999) were selected to get an insight of ENSO impact on summer precipitation. For anomalies in the Altiplano, two additional contrasting years were analyzed—2006 (mostly dry) and 2010 (wet)—to determine moisture sources. Summer rainfall conditions in 1998 and 1999 had an opposite behavior between coastal Nayarit (wet) and the Altiplano (dry), while in 2005, rainfall deficits were observed in both regions. The moisture fluxes showed large divergence areas over central Mexico and the Southeastern United States in years of intense drought (1998 and 1999) caused by two high-pressure cells at middle levels of the troposphere (500 hPa). The moisture transport mechanisms into the Altiplano were related to atmospheric circulation at the upper level (200 hPa). The variations of the moisture fluxes from 2006 to 2010 are less strong at middle levels. The Eastern Pacific moisture convergence along the western coast of Mexico favors above-average rainfall anomalies in the coastal region but below-average anomalies in the Altiplano.