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The North Atlantic Oscillation (NAO) is responsible for the climatic variability in the Northern Hemisphere, in particular, in Europe and is related to extreme events, such as droughts. The purpose of this paper is to study the correlation between precipitation and winter (December-January-February-March (DJFM)) NAO both for the historical period (1951-2000) and two future periods (2001-2050 and 2051-2100). NAO is calculated for these three periods by using sea level pressure, while precipitation data from seven climate models following the representative concentration pathway (RCP) 8.5 are also used in this study. An increasing trend in years with positive DJFM NAO values in the future is defined by this data, along with higher average DJFM NAO values. The correlation between precipitation and DJFM NAO is high, especially in the Northern (high positive) and Southern Europe (high negative). Therefore, higher precipitation in Northern Europe and lower precipitation in Southern Europe are expected in the future. Cross-spectral analysis between precipitation and DJFM NAO time series in three different locations in Europe revealed the best coherence in a dominant cycle between 3 and 4 years. Finally, the maximum drought period in terms of consecutive months with drought is examined in these three locations. The results can be used for strategic planning in a sustainable water resources management plan, since there is a link between drought events and NAO. [ABSTRACT FROM AUTHOR]

Yangtze River Valley (YRV) is the core region for the occurrence of heat wave (HW) in China. HW over YRV has been reported to be associated with intraseasonal oscillations (ISOs) from tropical/extratropical regions. However, the specific roles of these ISOs on the generation of HW cannot be separated in the observational analysis. This study identified the impact of ISOs from tropic/extratropic on the period of HW development through partial lateral forcing (PLF) experiments using the Weather Research and Forecasting model for a typical ISO‐related HW event over YRV in 2012. The observations show that a remarkable eastward/westward extension of South Asian high/western North Pacific subtropical high accompanied the HW occurrence. Accordingly, this anomalous strong high pressure produced the HW through horizontal warm advection and adiabatic heating, which was related to a lower‐level westward propagating meridional dipole anomaly and an upper‐level eastward migrating wave train from the extratropical region. The PLF experiments confirmed that this HW event was caused by the propagation of intraseasonal perturbations from the surrounding regions. The tropical westward propagating meridional dipole anomaly directly generated a lower‐level anomalous anticyclone in the early period of HW development, whereas extratropical intraseasonal forcing primarily contributed to the eastward extension of the South Asian high in the late, which jointly established the strong high pressure and caused the HW. The PLF experiments also indicate plausible tropical–extratropical interaction of intraseasonal perturbations over YRV. Improving the prediction skills of the tropical/extratropical ISOs may facilitate the subseasonal forecast of the local HW events. Key Points: A strong high‐pressure anomaly induced by both tropical and extratropical ISOs is the direct local contributor to the 2012 summer heat wavePartial lateral forcing experiments have identified the different roles of the tropical/extratropical ISOs in the heat wave generationThe westward tropical ISO takes effect in the early period of heat wave development, while the eastward extratropical participates in the late period [ABSTRACT FROM AUTHOR]

Accurate knowledge of the impact of internal atmospheric variability is required for the detection and attribution of climate change and for interpreting glacier records. However, current knowledge of such impacts in high-mountain regions is largely based on statistical methods, as the observational data required for process-based assessments are often spatially or temporally deficient. Using a case study of Kilimanjaro, 12 years of convection-permitting atmospheric modeling are combined with an 8-yr observational record to evaluate the impact of climate oscillations on recent high-altitude atmospheric variability during the short rains (the secondary rain season in the region). The focus is on two modes that have a well-established relationship with precipitation during this season, El Niño--Southern Oscillation and the Indian Ocean zonal mode, and demonstrate their strong association with local and mesoscale conditions at Kilimanjaro. Both oscillations correlate positively with humidity fluctuations, but the association is strongest with the Indian Ocean zonal mode in the air layers near and above the glaciers because of changes in zonal circulation and moisture transport, emphasizing the importance of the moisture signal from this basin. However, it is found the most anomalous conditions during co-occurring positive events because of the combined effects of the (i) extended positive sea surface temperature anomalies, (ii) enhanced atmospheric moisture capacity from higher tropospheric temperatures, (iii) most pronounced weakening of the subsiding branch of the Indian Ocean Walker circulation over East Africa, and (iv) stronger monsoonal moisture fluxes upstream from Kilimanjaro. This study lays the foundation for unraveling the contribution of climate modes to observed changes in Kilimanjaro's glaciers. [ABSTRACT FROM AUTHOR]

This study evaluates the role of the interdecadal Pacific oscillation (IPO) in modulating the El Niño-Southern Oscillation (ENSO)-precipitation relationship. The standard IPO index is described together with several alternatives that were derived using a low-frequency ENSO filter, demonstrating that an equivalent IPO index can be obtained as a low-frequency version of ENSO. Several statistical artifacts that arise from using a combination of raw and smoothed ENSO indices in modeling the ENSO-precipitation teleconnection are then described. These artifacts include the potentially spurious identification of low-frequency variability in a response variable resulting from the use of smoothed predictors and the potentially spurious modulation of a predictor-response relationship by the low-frequency version of the predictor under model misspecification. The role of the IPO index in modulating the ENSO-precipitation relationship is evaluated using a global gridded precipitation dataset, based on three alternative statistical models: stratified, linear, and piecewise linear. In general, the information brought by the IPO index, beyond that already contained in the Niño-3.4 index, is limited and not statistically significant. An exception is in northeastern Australia using annual precipitation data, and only for the linear model. Stratification by the IPO index induces a nonlinear ENSO-precipitation relationship, suggesting that the apparent modulation by the IPO is likely to be spurious and attributable to the combination of sample stratification and model misspecification. Caution is therefore required when using smoothed climate indices to model or explain low-frequency variability in precipitation. [ABSTRACT FROM AUTHOR]

ABSTRACT Trends of annual maximum 1-day precipitation in three major Hawaiian Islands are investigated using a nonparametric Mann-Kendall method and Sen's test ( MKS). The records are from 24 stations on Oahu, Maui, and Hawaii, and the period of analysis ranges from 1960 to 2009. To complement the MKS method, a non-stationary three-parameter generalized extreme value ( GEV) distribution is also used to detect trends in precipitation extremes. Both methods demonstrate that negative trends prevail for Oahu and Maui but positive trends dominate the Island of Hawaii. The influence of the location and the scale parameter in the GEV model on different return levels (2-year, 20-year, and 100-year) are explicitly described. The return-level threshold values are found to change with time considerably. As a result, a rare storm with daily precipitation of 300 mm (20-year return period) in 1960 has become a rather common storm event (3-5-year return period) in 2009 on the Island of Hawaii. The opposite trend behavior in extreme events is observed on Oahu and Maui, where rainfall extremes have become less frequent in the last five decades. A positive relationship is found between the precipitation extremes and Southern Oscillation Index ( SOI), implying greater extreme events during La Niña years and the opposite for El Niño years. [ABSTRACT FROM AUTHOR]

The South America low-level jet (SALLJ) on the eastern slopes of the Andes is a unique climatological feature in the continent. The SALLJ transports large amounts of moisture and controls the spatiotemporal variability of precipitation in southeast South America. This study shows a remarkable influence of the Atlantic Multidecadal Oscillation (AMO) on decadal-to-multidecadal variability of the SALLJ. The results show a consistent pattern in which active SALLJ days during negative AMO phases are associated with negative precipitation anomalies over northern Amazon and the Atlantic Intertropical Convergence Zone (ITCZ). Increased cross-equatorial flow over northwestern South America combined with the outflow associated with the atmospheric subsidence over the negative precipitation anomalies enhances northerly winds along the eastern slopes of the Andes and Amazon. This atmospheric circulation response, which is more prevalent in the austral winter, intensifies the SALLJ. In the exit region of the SALLJ over southern Brazil, Uruguay and northern Argentina, an anomalous low-level cyclonic circulation is associated with enhanced precipitation. The influence of the AMO on the SALLJ is consistent with paleo-proxy studies showing multidecadal changes in precipitation over the La Plata River drainage basin. The analysis shows that secular trends reinforce the SALLJ and precipitation patterns. Moreover, the study highlights the importance of natural variability (i.e., decadal-multidecadal variations) occurring within long-term trends in the mean state, possibly associated with global warming, and significant changes in the SALLJ and precipitation over South America. Climatic cycling: from the North Atlantic to South America The Atlantic Multidecadal Oscillation (AMO), a climate-driven water temperature cycle in the North Atlantic, has a remarkable influence on a wind system on the eastern slopes of the Andes called the South American low-level jet (SALLJ). The SALLJ transports large amounts of moisture to greatly influence the variability in rainfall in south-east South America. Using records spanning more than 100 years, Charles Jones and Leila Carvalho at the University of California, Santa Barbara, USA, identified a consistent correlation linking the levels of precipitation under the influence of SALLJ and the phase of the AMO. This cyclical influence, which repeats with a periodicity of 60-80 years, seems driven by the AMO affecting atmospheric circulation in the equatorial Atlantic. The results may help predict the influence of natural climate variability in South America. [ABSTRACT FROM AUTHOR]