Inland-penetrating atmospheric rivers (ARs) can affect the southwestern United States and significantly contribute to cool season (November–March) precipitation. In this work, a climatological characterization of AR events that have led to cool season extreme precipitation in the Verde River basin (VRB) in Arizona for the period 1979/80–2010/11 is presented. A “bottom up” approach is used by first evaluating extreme daily precipitation in the basin associated with AR occurrence, then identifying the two dominant AR patterns (referred to as Type 1 and Type 2, respectively) using a combined EOF statistical analysis. The results suggest that AR events in the Southwest do not form and develop in the same regions. Water vapor content in Type 1 ARs is obtained from the tropics near Hawaii (central Pacific) and enhanced in the midlatitudes, with maximum moisture transport over the ocean at low levels of the troposphere. On the other hand, moisture in Type 2 ARs has a more direct tropical origin and meridional orientation with maximum moisture transfer at midlevels. Nonetheless, both types of ARs cross the Baja Peninsula before affecting the VRB. In addition to Type 1 and Type 2 ARs, observations reveal AR events that are a mixture of both patterns. These cases can have water vapor transport patterns with both zonal and meridional signatures, and they can also present double peaks in moisture transport at low- and midlevels. This seems to indicate that the two “types” can be interpreted as end points of a range of possible directions.

The easternmost tropical Pacific. Part II: Seasonal and intraseasonal modes of atmospheric variability

The tropics are characterized by a variety of atmospheric and oceanic systems dominated by multi-scale interaction processes. This is Part I of a two-part review study on climate and climate variability of the Eastern Tropical Pacific (ETP). Here, the mean fields of incoming short wave radiation, surface energy fluxes, sea surface temperature, sea level pressure, salinity, wind patterns at 10m height, wind stress curl, precipitation, and evaporation are analyzed and synthetized using available data including that from the last decade. Given the strong interaction between the ETP and Caribbean Sea-Atlantic Ocean, mean field discussions are presented for two different domains, a relatively large domain from 24° S - 36° N and between 129° W - 17° W, and a smaller domain embedding only the Caribbean Sea and the easternmost part of the ETP. Most results on the climate of these two regions can be used as the base line for climate change studies. Interannual variability of tropical cyclones is also investigated over the domain of the latter two basins in the smaller domain. The study is complemented with a short review of some low frequency modes, such as, El Niño-Southern Oscillation, the Pacific Decadal Oscillation, and the Atlantic Multi-decadal Oscillation. Such low-frequency modes are known to modulate regional systems, including tropical cyclone frequency.

Potential climate change impacts on summer precipitation and subsequent hydrologic responses in the southwestern U.S. are poorly constrained at present due to a lack of studies accounting for high resolution processes. In this investigation, we apply a distributed hydrologic model to the Beaver Creek watershed of central Arizona to explore its utility for climate change assessments. Manual model calibration and model validation were performed using radar-based precipitation data during three summers and compared to two alternative meteorological products to illustrate the sensitivity of the streamflow response. Using the calibrated and validated model, we investigated the watershed response during historical (1990–2000) and future (2031–2040) summer projections derived from a single realization of a mesoscale model forced with boundary conditions from a general circulation model under a high emissions scenario. Results indicate spatially-averaged changes across the two projections: an increase in air temperature of 1.2 °C, a 2.4-fold increase in precipitation amount and a 3-fold increase in variability, and a 3.1-fold increase in streamflow amount and a 5.1-fold increase in variability. Nevertheless, relatively minor changes were obtained in spatially-averaged evapotranspiration. To explain this, we used the simulated hydroclimatological mechanisms to identify that higher precipitation limits radiation through cloud cover leading to lower evapotranspiration in regions with orographic effects. This challenges conventional wisdom on evapotranspiration trends and suggest that a more nuanced approach is needed to communicate hydrologic vulnerability to stakeholders and decision-makers in this semiarid region.This is the final peer-reviewed accepted manuscript. The final article as published is available at http://dx.doi.org/10.1016/j.jaridenv.2015.02.022