Extreme ocean surface wave heights significantly affect coastal structures and offshore activities and impact many vulnerable populations of low-lying islands. Therefore, better understanding of ...ocean wave height variability plays an important role in potentially reducing risk in such regions. In this study, global impacts of natural climate variability such as El Niño–Southern Oscillation (ENSO), North Atlantic Oscillation (NAO), and Pacific decadal oscillation (PDO) on extreme significant wave height (SWH) are analyzed using ERA-Interim (1980–2014) and ECMWF twentieth-century reanalysis (ERA-20C; 1952–2010) datasets for December–February (DJF). The nonstationary generalized extreme value (GEV) analysis is used to determine the influence of natural climate variability on DJF maxima of SWH (Hmax), wind speed (Wmax), and mean sea level pressure gradient amplitude (Gmax). The major ENSO influence on Hmax is found over the northeastern North Pacific (NP), with increases during El Niño and decreases during La Niña, and its counter responses are observed in coastal regions of the western NP, which are consistently observed in both Wmax and Gmax responses. The Hmax response to the PDO occurs over similar regions in the NP as those associated with ENSO but with much weaker amplitude. Composite analysis of different ENSO and PDO phase combinations reveals stronger (weaker) influences when both variability modes are of the same (opposite) phase. Furthermore, significant NAO influence on Hmax, Wmax, and Gmax is observed throughout Icelandic and Azores regions in relation to changes in atmospheric circulation patterns. Overall, the response of extreme SWH to natural climate variability modes is consistent with seasonal mean responses.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper presents an analysis of changes in global land extreme temperature indices (1951–2015) based on the new global land surface daily air temperature dataset recently developed by the China ...Meteorological Administration (CMA). The linear trends of the gridpoint time series and global land mean time series were calculated by using a Mann–Kendall method that accounts for the lag-1 autocorrelation in the time series of annual extreme temperature indices. The results, which are generally consistent with previous studies, showed that the global land average annual and seasonal mean extreme temperature indices series all experienced significant long-term changes associated with warming, with cold threshold indices (frost days, icing days, cold nights, and cold days) decreasing, warm threshold indices (summer days, tropical nights, and warm days) increasing, and all absolute indices (TXx, TXn, TNx, and TNn) also increasing, over the last 65 years. The extreme temperature indices series based on daily minimum temperatures generally had a stronger and more significant trend than those based on daily maximum temperatures. The strongest warming occurred after the mid-1970s, and a few extreme temperature indices showed no significant trend over the period from 1951 to the mid-1970s. Most parts of the global land experienced significant warming trends over the period 1951–2015 as a whole, and the largest trends appeared in mid- to high latitudes of the Eurasian continent.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This review article enumerates, categorizes, and compares many of the methods that have been proposed to detect undocumented changepoints in climate data series. The methods examined include the ...standard normal homogeneity (SNH) test, Wilcoxon’s nonparametric test, two-phase regression (TPR) procedures, inhomogeneity tests, information criteria procedures, and various variants thereof. All of these methods have been proposed in the climate literature to detect undocumented changepoints, but heretofore there has been little formal comparison of the techniques on either real or simulated climate series. This study seeks to unify the topic, showing clearly the fundamental differences among the assumptions made by each procedure and providing guidelines for which procedures work best in different situations. It is shown that the common trend TPR and Sawa’s Bayes criteria procedures seem optimal for most climate time series, whereas the SNH procedure and its nonparametric variant are probably best when trend and periodic effects can be diminished by using homogeneous reference series. Two applications to annual mean temperature series are given. Directions for future research are discussed.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The projected changes and trends in the regional annual and monthly maxima of the significant wave height (Hs) in the Arctic Ocean are studied using wave simulations derived from the CMIP5 (Coupled ...Model Intercomparison Project Phase 5) climate simulations for 1979–2005 and 2081–2100 periods. Under the RCP8.5 scenario, the regional annual maximal Hs increases on average up to ∼3 cm/year, or >0.5%/year, relative to the 1986–2005 climatological value, in many Arctic areas (and up to 0.8%/year in the east side of the Arctic Ocean). While strong winds need to occur for large waves to develop, the changes in wind speed alone cannot explain the increases in the regional maximal Hs. Sea ice retreat also plays an important role by increasing fetch to promote wave growth, thereby contributing notably to the projected increase in wave height. It also contributes to increasing the probability of strong winds over the widening ice‐free waters.
Plain Language Summary
Significant wave height (Hs) is the average of the highest third of wave heights. Using simulations of historical and future wave climates, we reveal that, under the RCP8.5 scenario, regional annual maximal Hs in the Arctic Ocean and surrounding seas is projected to increase at a rate of up to ∼3 cm/year, or equivalently up to 80%, relative to the 1986–2005 climate period, over a 100‐year period. While strong winds need to occur for large waves to develop, changes in wind speed alone cannot explain the increases in wave height. Sea ice retreat also plays an important role by increasing the water distance over which wind can blow, thereby promoting wave growth and thus contributing to the increase in wave height. There is also an increase in the probability of strong winds over the widening ice‐free waters due to sea ice retreat.
Key Points
Changes in surface wind speed alone cannot explain the projected changes in the regional maximum significant wave heights in the Arctic
The inclusion of a wind predictor that accounts for fetch‐limited conditions illustrates that sea ice retreat contributes to wave height increase by increasing fetch
Sea ice retreat also plays an important role in the projected increases in the regional maximum winds over widening ice‐free areas, which in turns affects the regional maximum wave heights
Historical reanalyses that span more than a century are needed for a wide range of studies, from understanding large‐scale climate trends to diagnosing the impacts of individual historical extreme ...weather events. The Twentieth Century Reanalysis (20CR) Project is an effort to fill this need. It is supported by the National Oceanic and Atmospheric Administration (NOAA), the Cooperative Institute for Research in Environmental Sciences (CIRES), and the U.S. Department of Energy (DOE), and is facilitated by collaboration with the international Atmospheric Circulation Reconstructions over the Earth initiative. 20CR is the first ensemble of sub‐daily global atmospheric conditions spanning over 100 years. This provides a best estimate of the weather at any given place and time as well as an estimate of its confidence and uncertainty. While extremely useful, version 2c of this dataset (20CRv2c) has several significant issues, including inaccurate estimates of confidence and a global sea level pressure bias in the mid‐19th century. These and other issues can reduce its effectiveness for studies at many spatial and temporal scales. Therefore, the 20CR system underwent a series of developments to generate a significant new version of the reanalysis. The version 3 system (NOAA‐CIRES‐DOE 20CRv3) uses upgraded data assimilation methods including an adaptive inflation algorithm; has a newer, higher‐resolution forecast model that specifies dry air mass; and assimilates a larger set of pressure observations. These changes have improved the ensemble‐based estimates of confidence, removed spin‐up effects in the precipitation fields, and diminished the sea‐level pressure bias. Other improvements include more accurate representations of storm intensity, smaller errors, and large‐scale reductions in model bias. The 20CRv3 system is comprehensively reviewed, focusing on the aspects that have ameliorated issues in 20CRv2c. Despite the many improvements, some challenges remain, including a systematic bias in tropical precipitation and time‐varying biases in southern high‐latitude pressure fields.
A significant new version of the Twentieth Century Reanalysis data assimilation system, 20CRv3, has been developed. The 20CRv3 dataset will provide an ensemble of sub‐daily global atmospheric conditions spanning at least 180 years by assimilating only surface pressure observations into a coupled atmosphere–land forecast model. The new 20CRv3 system improves upon the previous system in several notable ways, including the use of upgraded data assimilation methods, a newer and higher‐resolution forecast model, and a larger set of available pressure observations.
Based on the homogenized data set, we analyze changes in mean temperature and some extreme temperature indices over China since 1961 and especially during the recent warming hiatus period (1998–2012) ...in a global average context. The result shows that the decrease of annual mean maximum has contributed most to the decreases in overall mean temperature and in diurnal temperature range (DTR) during the warming hiatus period. In most parts of China except the southwest, the summer mean maximum temperature (TxS) shows the largest increase, while the winter mean minimum temperature (TnW) indicates slight cooling trends. These changes have augmented the seasonal cycle and increased the likelihood of extreme warm and cold events. Further analyses reveal that the increases in TxS are significantly correlated with concurrent increases in solar radiation. In southwest China, the annual mean temperature, TxS, TnW, and DTR increased during 1998–2012, possibly related to increased dryness in this region during the hiatus period.
Key Points
China experiences the recent warming hiatus
The temperature changes have increased the likelihood of extreme events
These are correlated with increases in solar radiation and dryness
Abstract
Extreme surface ocean waves are often primary drivers of coastal flooding and erosion over various time scales. Hence, understanding future changes in extreme wave events owing to global ...warming is of socio-economic and environmental significance. However, our current knowledge of potential changes in high-frequency (defined here as having return periods of less than 1 year) extreme wave events are largely unknown, despite being strongly linked to coastal hazards across time scales relevant to coastal management. Here, we present global climate-modeling evidence, based on the most comprehensive multi-method, multi-model wave ensemble, of projected changes in a core set of extreme wave indices describing high-frequency, extra-tropical storm-driven waves. We find changes in high-frequency extreme wave events of up to ∼50%–100% under RCP8.5 high-emission scenario; which is nearly double the expected changes for RCP4.5 scenario, when globally integrated. The projected changes exhibit strong inter-hemispheric asymmetry, with strong increases in extreme wave activity across the tropics and high latitudes of the Southern Hemisphere region, and a widespread decrease across most of the Northern Hemisphere. We find that the patterns of projected increase across these extreme wave events over the Southern Hemisphere region resemble their historical response to the positive anomaly of the Southern Annular Mode. Our findings highlight that many countries with low-adaptive capacity are likely to face increasing exposure to much more frequent extreme wave events in the future.
This study reports on the 1871–2010 trends in significant wave heights (Hs) in the North Atlantic, as statistically reconstructed from the 20th century reanalysis (20CR) ensemble of mean sea level ...pressure (SLP) fields. The 20CR SLP data set for the North Atlantic has been reported to be homogeneous since 1871, although it has discontinuities before 1949 in other regions. A multivariate regression model with lagged dependent variable is used to represent the SLP‐Hsrelationship. It is calibrated and validated using the ERA‐Interim reanalysis ofHs and SLP for the period 1981–2010.Trends in the reconstructed annual mean and maximum Hs are found to be consistent with those derived from two dynamical wave reanalysis data sets (MSC50 and ERA40), which indicates robustness of the trend estimates. The trend patterns of extreme Hsgenerally feature increases in the northeast North Atlantic with decreases in the mid‐latitudes; but there are seasonal variations. The main features of the patterns of trends over the last half century or so are also seen in the last 140‐yr period (1871–2010). However, the trend magnitudes are much greater in the last half century than in the 140 years.
Key Points
The reconstructed trends are consistent with those in wave reanalysis data sets
Trend magnitudes are greater over the last half century than over the 140 years
There are seasonal variations in the trend patterns of extreme Hs
This study proposes an algorithm for constructing pentad precipitation fields by integrating the popularly used Global Precipitation Climatology Project (GPCP) daily precipitation data set, GPCP1dd ...v1.2, with Canadian in situ daily precipitation data. This algorithm consists of two major steps. First, the GPCP data were adjusted to remove biases relative to the gauge data, with consideration of the differences between snowfall and rainfall, and of the gauge density. Then, a blended pentad precipitation field was constructed using the adjusted GPCP precipitation field and the differences between the gauge and adjusted GPCP precipitation fields (residual kriging). The skill of the algorithm is evaluated for three networks of sparse to medium gauge density, with the evaluation data set being much larger than the training data set. The results show that the algorithm produces better representation of pentad precipitation fields than the GPCP precipitation estimates or using the gauge data alone; it has smaller root‐mean‐square errors and higher correlation skill scores. This algorithm was used to produce the first blended pentad precipitation data set for the period of 1997–2007 for Canada (CanBP5dV1). It can be used for other regions around the world.
Key Points
A new algorithm for correcting biases in satellite precipitation estimates
A new algorithm for producing blended precipitation data on a pentad time scale
A high‐quality blended pentad precipitation data set for Canada
Surface wind (U
10) and significant wave height (Hs) response to global warming are investigated using a coupled atmosphere–wave model by perturbing the sea surface temperatures (SSTs) with anomalies ...generated by the Working Group on Coupled Modeling (WGCM) phase 3 of the Coupled Model Intercomparison Project (CMIP3) coupled models that use the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4)/Special Report on Emissions Scenarios A1B (SRES A1B) scenario late in the twenty-first century.
Several consistent changes were observed across all four realizations for the seasonal means: robust increase ofU
10and Hs in the Southern Ocean for both the austral summer and winter due to the poleward shift of the jet stream; a dipole pattern of theU
10and Hs with increases in the northeast sector and decreases at the midlatitude during boreal winter in the North Atlantic due to the more frequent occurrence of the positive phases of the North Atlantic Oscillation (NAO); and strong decrease ofU
10and Hs in the tropical western Pacific Ocean during austral summer, which might be caused by the joint effect of the weakening of the Walker circulation and the large hurricane frequency decrease in the South Pacific.
Changes of the 99th percentileU
10and Hs are twice as strong as changes in the seasonal means, and the maximum changes are mainly dominated by the changes in hurricanes. Robust strong decreases ofU
10and Hs in the South Pacific are obtained because of the large hurricane frequency decrease, while the results in the Northern Hemisphere basins differ among the models. An additional sensitivity experiment suggests that the qualitative response ofU
10and Hs is not affected by using SST anomalies only and maintaining the radiative forcing unchanged (using 1980 values), as in this study.
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BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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