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General circulation model (GCM) outputs are a primary source of information for climate change impact assessments. However, raw GCM data rarely are used directly for regional-scale impact assessments as they frequently contain systematic error or bias. In this article, we propose a novel extension to standard quantile mapping that allows for a continuous seasonal change in bias magnitude using localized regression. Our primary goal is to examine the efficacy of this tool in the context of larger statistical downscaling efforts on the tropical island of Puerto Rico, where localized downscaling can be particularly challenging. Along the way, we utilize a multivariate infilling algorithm to estimate missing data within an incomplete climate data network spanning Puerto Rico. Next, we apply a combination of multivariate downscaling methods to generate in situ climate projections at 23 locations across Puerto Rico from three general circulation models in two carbon emission scenarios: RCP4.5 and RCP8.5. Finally, our bias-correction methods are applied to these downscaled GCM climate projections. These bias-correction methods allow GCM bias to vary as a function of a user-defined season (here, Julian day). Bias is estimated using a continuous curve rather than a moving window or monthly breaks. Results from the selected ensemble agree that Puerto Rico will continue to warm through the coming century. Under the RCP4.5 forcing scenario, our methods indicate that the dry season will have increased rainfall, while the early and late rainfall seasons will likely have a decline in total rainfall. Our methods applied to the RCP8.5 forcing scenario favor a wetter climate for Puerto Rico, driven by an increase in the frequency of highmagnitude rainfall events during Puerto Rico's early rainfall season (April to July) as well as its late rainfall season (August to November). [ABSTRACT FROM AUTHOR]

At a continental scale, trends in aggregate ablation frequency inform changes in snow cover extent, however the variability and trends in the frequency and magnitude of snow ablation events at regional scales are less well understood. Determining such variability is critical in describing regional hydroclimate, where snow ablation can influence streamflow, soil moisture and groundwater supplies. This study uses a gridded dataset of United States and Canadian snow ablation events derived from 1960 to 2009 surface observations to examine spatial and temporal variations of snow ablation frequency. Here, we show a relatively narrow band of peak ablation frequency seasonally advances and recedes over North America, forced by variations in snow depth and meteorological conditions suitable for ablation. Particularly in more moist regions away from the continent's interior, hydrologically relevant ablation events of at least 10.0 cm occur on an approximately yearly basis. Collectively, ablation events became significantly less frequent with time, where events specifically in the Appalachians and in Great Lakes regions declined by as much as 75% over the 50‐year period. Decreases in ablation frequency across the study region are primarily driven by significant decreases in snow cover, inhibiting the potential for ablation to occur due to a lack of sufficiently deep snowpacks. These results point to important snow cover related changes in the hydrologic cycle in a warming climate and highlight specific areas of interest where more localized analysis of ablation trends and forcing mechanisms would be appropriate. [ABSTRACT FROM AUTHOR]

We provide an updated analysis of instrumental Greenland monthly temperature data to 2019, focusing mainly on coastal stations but also analysing ice‐sheet records from Swiss Camp and Summit. Significant summer (winter) coastal warming of ~1.7 (4.4)°C occurred from 1991–2019, but since 2001 overall temperature trends are generally flat and insignificant due to a cooling pattern over the last 6–7 years. Inland and coastal stations show broadly similar temperature trends for summer. Greenland temperature changes are more strongly correlated with Greenland Blocking than with North Atlantic Oscillation changes. In quantifying the association between Greenland coastal temperatures and Greenland Ice Sheet (GrIS) mass‐balance changes, we show a stronger link of temperatures with total mass balance rather than surface mass balance. Based on Greenland coastal temperatures and modelled mass balance for the 1972–2018 period, each 1°C of summer warming corresponds to ~(91) 116 Gt·yr−1 of GrIS (surface) mass loss and a 26 Gt·yr−1 increase in solid ice discharge. Given an estimated 4.0–6.6°C of further Greenland summer warming according to the regional model MAR projections run under CMIP6 future climate projections (SSP5‐8.5 scenario), and assuming that ice‐dynamical losses and ice sheet topography stay similar to the recent past, linear extrapolation gives a corresponding GrIS global sea‐level rise (SLR) contribution of ~10.0–12.6 cm by 2100, compared with the 8–27 cm (mean 15 cm) "likely" model projection range reported by IPCC in 2019 (SPM.B1.2). However, our estimate represents a lower limit for future GrIS change since fixed dynamical mass losses and amplified melt arising from both melt‐albedo and melt‐elevation positive feedbacks are not taken into account here. [ABSTRACT FROM AUTHOR]

ABSTRACT: The tropical forests of northeast Puerto Rico (NE PR) and the Luquillo Mountains (LM) are a large repository for biodiversity and have an important role in regional biogeochemical processes. Precipitation is a key driver of the productivity of these sensitive ecosystems. This study analyses historical precipitation variability from 1985 to 2014 at El Verde Field Station (EVFS) at 380 m on the north facing slope of the LM. The primary objective of this study is to identify atmospheric states that lead to extreme wet/dry conditions at EVFS. This study also investigates how those wet/dry atmospheric states change over the study period through an epoch approach on annual and seasonal timescales. Self‐organizing maps (SOMs) are used to produce atmospheric states from ERA‐Interim low‐tropospheric moisture and circulation variables. These atmospheric states are downscaled to precipitation at the EVFS rain gauge. A probability density function of observed precipitation is calculated for each atmospheric state. Changes in node frequency, which is the number of days mapping to a particular node compared to the total number of days in the temporal period, are used to evaluate changes in wet/dry atmospheric states at EVFS. Results indicate that low‐precipitation days at EVFS are associated with atmospheric states with high 1000–700 hPa bulk wind shear and decreased 700 hPa moisture. Wet days in the study region are associated with moist low‐tropospheric environments with low wind shear. Our results indicate an increased frequency of dry season atmospheric states with lower 700 hPa moisture. Over the study period, the dry season has a decrease in median and extreme precipitation during rainy days (days >0 mm). A decrease in early rainfall season median precipitation on rainy days is observed despite an increase in days with measurable precipitation, likely driven by an increase in light rainfall days (<5 mm). [ABSTRACT FROM AUTHOR]

ABSTRACT Ongoing climatic and cryospheric changes observed throughout the greater Alaska region are interconnected and often linked to oceanic and atmospheric patterns and processes that operate on varying spatiotemporal scales. To evaluate the long-term, mid-tropospheric circulation field across Alaska, and possible connections to climate and environmental change in the Pacific sector of the Arctic, the Alaska Blocking Index ( ABI) is developed over the domain (54°-76°N, 125°-180°W) using daily gridded 500 hPa geopotential height fields derived from the ERA-40 (1958-1978) and ERA-Interim (1979-2014) data sets, 1958-2014. Climatological characteristics of the seasonal and annual ABI conditions are evaluated and periods of prevalent blocking conditions are identified and subsequently linked to possible local and large-scale forcing mechanisms. The ABI has exhibited positive trends during all seasons and annually since 1979. Many of the extreme high ABI values occurred since 2000, including the highest annual values in 2013 and 2014. Anomalous blocking patterns in all seasons are associated with diminished snow depth and sea-ice cover, positive near-surface air temperature anomalies, and anticyclonic flows of heat and moisture across the domain. The ABI is also shown to differ from the long-term variability and atmospheric circulation responses associated with phases of the Pacific-North American pattern and Pacific Decadal Oscillation, revealing some notable spatial and temporal disconnects between the region-centric, high-latitude blocking flow and some of the predominant modes of sea surface temperature and middle tropospheric circulation variability in the Northern Hemisphere. [ABSTRACT FROM AUTHOR]

ABSTRACT Atmospheric blocking commonly occurs over the high latitudes of the Northern Hemisphere, resulting from the development of persistent areas of high pressure that lead to warmer-than-average surface temperatures west of the high centre. While the variability and trends in anticyclonic circulation patterns (including blocking) over Greenland have been previously documented, an analysis of the most extreme blocking events within the observational record is lacking. In this study, a historical climatology of extreme Greenland blocking episodes ( GBEs) from 1958 to 2013 is examined within the context of anomalous anticyclonic circulation patterns over the North Atlantic region during recent years. Based on a combination of the ERA-40 (1958-1978) and ERA-Interim (1979-2013) reanalysis data sets, the Greenland Blocking Index ( GBI) is used to quantify 500 hPa geopotential height anomalies for the identification of extreme GBEs. The annual rate of extreme blocking days has doubled since 1958, reaching an average of approximately 20 days per year by 2013. The frequency and, to some extent, duration of extreme GBEs were unprecedentedly high from 2007 to 2013 compared to the 56-year period of record, with a majority of the increase occurring during the spring ( MAM) and summer ( JJA). A multiple linear regression analysis reveals that interannual variability in extreme blocking and the Atlantic Multidecadal Oscillation ( AMO) are the two predominant drivers of surface meltwater production across the entire Greenland ice sheet ( GrIS), but Arctic sea ice extent and North Atlantic cyclone activity can also influence the extent of summer melting over portions of the GrIS. Thus, in addition to the larger-scale atmospheric and oceanic variability, smaller-scale features such as extratropical cyclones can play a significant role in modulating GrIS surface melting each summer. [ABSTRACT FROM AUTHOR]

In late October 2012, an extreme area of high pressure centred near Greenland, known as a 'Greenland block', forced Hurricane Sandy to turn westward into the Northeast coast of the United States. In light of this unusual and catastrophic event, the Greenland blocking conditions associated with Sandy and previous North Atlantic hurricane tracks are examined from a climatological perspective. Two primary questions raised by Sandy are investigated: (1) How anomalous were the Greenland blocking conditions observed prior to Sandy's landfall? (2) Were North Atlantic hurricane tracks in the historical record affected by Greenland blocking conditions in a manner similar to Sandy? The measure of blocking strength used to answer these questions is the Greenland Blocking Index (GBI), which is calculated as the spatial average of 500 hPa heights over the Greenland region. The GBI prior to Sandy's landfall was found to be more typical of late June conditions, exceeding the 90th percentile of late October climatology during the entire preceding week and peaking at near-record values (99.8th percentile) on 25 October. Analysis of the GBI in relation to past North Atlantic hurricane tracks shows that above-normal GBI values were associated with a southward displacement of hurricane tracks and an increased concentration of tracks near the Northeast US coast. Additionally, composites of atmospheric conditions for 'anomalous' hurricane tracks (defined as tracks with an initial bearing angle between 90° and 360°) revealed an atypical area of high pressure centred over the Canadian Maritimes during these events, whereas near-climatological conditions of atmospheric pressure over the North Atlantic prevailed for 'normal' hurricane tracks. However, variations in Greenland blocking conditions were not associated with significant changes in the frequency of anomalous North Atlantic hurricane tracks. [ABSTRACT FROM AUTHOR]

ABSTRACT The NASA announcement of record surface melting of the Greenland ice sheet in July 2012 led us to examine the atmospheric and oceanic climatic anomalies that are likely to have contributed to these exceptional conditions and also to ask the question of how unusual these anomalies were compared to available records. Our analysis allows us to assess the relative contributions of these two key influences to both the extreme melt event and ongoing climate change. In 2012, as in recent warm summers since 2007, a blocking high pressure feature, associated with negative NAO conditions, was present in the mid-troposphere over Greenland for much of the summer. This circulation pattern advected relatively warm southerly winds over the western flank of the ice sheet, forming a 'heat dome' over Greenland that led to the widespread surface melting. Both sea-surface temperature and sea-ice cover anomalies seem to have played a minimal role in this record melt, relative to atmospheric circulation. Two representative coastal climatological station averages and several individual stations in south, west and north-west Greenland set new surface air temperature records for May, June, July and the whole (JJA) summer. The unusually warm summer 2012 conditions extended to the top of the ice sheet at Summit, where our reanalysed (1994-2012) DMI Summit weather station summer (JJA) temperature series set new record high mean and extreme temperatures in 2012; 3-hourly instantaneous 2-m temperatures reached an exceptional value of 2.2°C at Summit on 11 July 2012. These conditions translated into the record observed ice-sheet wide melt during summer 2012. However, 2012 seems not to be climatically representative of future 'average' summers projected this century. [ABSTRACT FROM AUTHOR]

The article focuses on a study about the extent and trends in ice sheet surface melt in Greenland from 1960 to 2010. It cites the Greenland ice sheet (GrIS) as the largest reservoir of permanent snow and ice in the Northern Hemisphere, adding that it tends to be highly sensitive to changes in climate and has posted record surface melt extent in 2007 and 2010. Factors behind the importance of the extent and duration of surface melting are given, one is the production of substantial differences in surface albedo and energy and moisture balances as a result of melting. Findings indicate a decline in the melt period from 1960 to 1972 followed by record melt extent in 2010 during which melt duration at the southwestern and western margins exceeded the average.

A network of cooperative observation stations was used to examine trends in average snow depth across the western United States. This station network includes a greater number of lowto moderate–elevation stations, where the snow is particularly sensitive to temperature changes, than the snow course and SNOTEL datasets used in other studies. Results indicate a widespread decrease in snow depth across the region; 70% of stations with statistically significant trends (p ◀ 0.5 or p ◀ 0.10) exhibit negative trends. Elevation is a key factor in explaining the spatial pattern of snow depth trends across the region. The fraction of stations with significant negative trends is greatest (80%) at the lower–elevation stations (<1000 m) and is somewhat less (62%) for stations between 2000 and 3000 m. Pacific climate indices, including the Pacific Decadal Oscillation (PDO) and North Pacific Index (NPI), are shown to have important but spatially varying influences upon average seasonal snow depths. [ABSTRACT FROM AUTHOR]

This study uses a database consisting of 330 austral warm-season (October–May) mesoscale convective complexes (MCCs) during 1998–2007 to determine the contribution of MCCs to rainfall across subtropical South America (SSA). A unique precipitation analysis is conducted using Tropical Rainfall Measuring Mission (TRMM) 3B42 version 6 data. The average MCC produces 15.7 mm of rainfall across 381 000 km2, with a volume of 7.0 km3. MCCs in SSA have the largest precipitation areas compared to North American and African systems. MCCs accounted for 15%–21% of the total rainfall across portions of northern Argentina and Paraguay during 1998–2007. However, MCCs account for larger fractions of the total precipitation when analyzed on monthly and warm-season time scales. Widespread MCC rainfall contributions of 11%–20% were observed in all months. MCCs accounted for 20%–30% of the total rainfall between November and February, and 30%–50% in December, primarily across northern Argentina and Paraguay. MCCs also produced 25%–66% of the total rainfall across portions of west-central Argentina. Similar MCC rainfall contributions were observed during warm seasons. An MCC impact factor (MIF) was developed to determine the overall impact of MCC rainfall on warm-season precipitation anomalies. Results show that the greatest impacts on precipitation anomalies from MCC rainfall were located near the center of the La Plata basin. This study demonstrates that MCCs in SSA produce widespread precipitation that contributes substantially to the total rainfall across the region. [ABSTRACT FROM AUTHOR]

Abstract El Yunque National Forest, situated in the Luquillo Mountains of northeast Puerto Rico, is home to a wide range of climate-sensitive ecosystems and forest types. In particular, these ecosystems are highly sensitive to changes in the hydroclimate, even on short time scales. Current global climate models (GCMs) predict coarse-scale reductions in precipitation across the Caribbean prompting the need to investigate future fine-scale hydroclimate variability in the Luquillo Mountains. This research downscales coarse-resolution GCM RCP8.5 predictions from the IPCC CMIP5 project to the local scale to better assess future rainfall variability during the most critical period of the annual hydroclimate cycle, the early rainfall season (ERS). An artificial neural network (ANN) is developed using five field variables (1000-, 850-, 700-, and 500-hPa specific humidity and 1000–700-hPa bulk wind shear) and four derived precipitation forecasting parameters from the ERA-Interim reanalysis. During the historical period (1985–2016), the ANN predicts a binary dry (<5 mm) versus wet (≥5 mm) day outcome with 92% percent accuracy. When the historical inputs are replaced with bias-corrected data from four CMIP5 GCMs, the downscaled ensemble mean indicates a 7.2% increase in ERS dry-day frequency by mid-century (2041–2060), yielding an ERS dry-day percentage of 70% by mid-century. The results presented here show that the decrease in precipitation and wet-days is, at least in part, due to an increase in 1000–700 hPa bulk wind shear and a less favorable thermodynamic environment driven by increased mid-tropospheric warming and a stronger trade wind inversion. By regressing ERS total precipitation against dry-day frequency (R2 = 0.95), the predicted mid-century dry-day proportion corresponds to a ~200-mm decrease in seasonal precipitation. In contrast, the ensemble predicts a dry-day frequency recovery back towards the historical climatological mean by end-century (2081–2100). Graphical abstract Unlabelled Image Highlights • Uses an ANN to downscale coarse-resolution GCM data to local scale rainfall in ELNF • The ANN predicts dry day frequency (<5 mm) with 92% percent accuracy. • 7.2% increase in ERS dry-day frequency by mid-century (2041–2060) • ~200-mm decrease in ERS rainfall by mid-century • Dry-day frequency recovers by end-century (2081–2100) back towards climatology. [ABSTRACT FROM AUTHOR]