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.

We present the second update to a data set of gridded land‐based temperature and precipitation extremes indices: HadEX3. This consists of 17 temperature and 12 precipitation indices derived from daily, in situ observations and recommended by the World Meteorological Organization (WMO) Expert Team on Climate Change Detection and Indices (ETCCDI). These indices have been calculated at around 7,000 locations for temperature and 17,000 for precipitation. The annual (and monthly) indices have been interpolated on a 1.875°×1.25° longitude‐latitude grid, covering 1901–2018. We show changes in these indices by examining ”global”‐average time series in comparison with previous observational data sets and also estimating the uncertainty resulting from the nonuniform distribution of meteorological stations. Both the short and long time scale behavior of HadEX3 agrees well with existing products. Changes in the temperature indices are widespread and consistent with global‐scale warming. The extremes related to daily minimum temperatures are changing faster than the maximum. Spatial changes in the linear trends of precipitation indices over 1950–2018 are less spatially coherent than those for temperature indices. Globally, there are more heavy precipitation events that are also more intense and contribute a greater fraction to the total. Some of the indices use a reference period for calculating exceedance thresholds. We present a comparison between using 1961–1990 and 1981–2010. The differences between the time series of the temperature indices observed over longer time scales are shown to be the result of the interaction of the reference period with a warming climate. The gridded netCDF files and, where possible, underlying station indices are available from http://www.metoffice.gov.uk/hadobs/hadex3 and http://www.climdex.org.

Regional patterns of tropical Indo-Pacific climate change are investigated over the last six decades based on a synthesis of in situ observations and ocean model simulations, with a focus on physical consistency among sea surface temperature (SST), cloud, sea level pressure (SLP), surface wind, and subsurface ocean temperature. A newly developed bias-corrected surface wind dataset displays westerly trends over the western tropical Pacific and easterly trends over the tropical Indian Ocean, indicative of a slowdown of the Walker circulation. This pattern of wind change is consistent with that of observed SLP change showing positive trends over the Maritime Continent and negative trends over the central equatorial Pacific. Suppressed moisture convergence over the Maritime Continent is largely due to surface wind changes, contributing to observed decreases in marine cloudiness and land precipitation there.Furthermore, observed ocean mixed layer temperatures indicate a reduction in zonal contrast in the tropical Indo-Pacific characterized by larger warming in the tropical eastern Pacific and western Indian Ocean than in the tropical western Pacific and eastern Indian Ocean. Similar changes are successfully simulated by an ocean general circulation model forced with the bias-corrected wind stress. Whereas results from major SST reconstructions show no significant change in zonal gradient in the tropical Indo-Pacific, both bucket-sampled SSTs and nighttime marine air temperatures (NMAT) show a weakening of the zonal gradient consistent with the subsurface temperature changes. All these findings from independent observations provide robust evidence for ocean-atmosphere coupling associated with the reduction in the Walker circulation over the last six decades.

The authors investigate the effects of tropical cyclones (TCs) on seasonal and interannual rainfall variability over the western North Pacific (WNP) by using rainfall data at 22 stations. The TC-induced rainfall at each station is estimated by using station data when a TC is located within the influential radius (1000 km) from the station. The spatial-temporal variability of the proportion of TC rainfall is examined primarily along the eastwest island chain near 108N (between 78 and 138N) and the north-south island chain near 1258E (between 1208 and 1308E).Along 108N the seasonality of total rainfall is mainly determined by non-TC rainfall that is influenced by the WNP monsoon trough. The proportion of the TC rain is relatively low. During the high TC season from July to December, TC rainfall accounts for 30% of the total rainfall in Guam, 15%-23% in Koror and Yap, and less than 10% at other stations. In contrast, along 1258E where the WNP subtropical high is located, the TC rainfall accounts for 50%-60% of the total rainfall between 188 and 268N during the peak TC season from July to October. In Hualien of Taiwan, TC rainfall exceeds 60% of the total rainfall.The interannual variability of the TC rainfall and total rainfall is primarily modulated by El Niñ o-Southern Oscillation (ENSO). Along 108N, the ratio of TC rainfall versus total rainfall is higher than the climatology during developing and mature phases of El Niñ o (from March to the following January), whereas the ratio is below the climatology during the decaying phase of El Niñ o. The opposite is true for La Niñ a, except that the impact of La Niñ a is shorter in duration. Furthermore, in summer of El Niñ o developing years, the total seasonal rainfall increases primarily because of the increase of TC rainfall. In the ensuing autumn, an anticyclonic anomaly develops over the Philippine Sea and TC rainfall shifts eastward; as a result, the total rainfall over the Philippines and Taiwan decreases. The total rainfall to the east of 1408E, however, changes little, because the westward passage of TCs enhances TC rainfall, which offsets the decrease of non-TC rainfall. Along the meridional island chain between 1208 and 1308E, the total rainfall anomaly is affected by ENSO starting from the autumn to the following spring, and the variation in TC rainfall dominates the total rainfall variation only in autumn (August-November) of ENSO years.The results from this study suggest that in the tropical WNP and subtropical East Asian monsoon regions (east of 1208E), the seasonal and interannual variations of rainfall are controlled by changes in nonlocal circulations. These changes outside the monsoon domain may substantially affect summer monsoon rainfall by changing TC genesis and tracks.

The International Surface Pressure Databank (ISPD) is the world's largest collection of global surface and sea-level pressure observations. It was developed by extracting observations from established international archives, through international cooperation with data recovery facilitated by the Atmospheric Circulation Reconstructions over the Earth (ACRE) initiative, and directly by contributing universities, organizations, and countries. The dataset period is currently 1768-2012 and consists of three data components: observations from land stations, marine observing systems, and tropical cyclone best track pressure reports. Version 2 of the ISPD (ISPDv2) was created to be observational input for the Twentieth Century Reanalysis Project (20CR) and contains the quality control and assimilation feedback metadata from the 20CR. Since then, it has been used for various general climate and weather studies, and an updated version 3 (ISPDv3) has been used in the ERA-20C reanalysis in connection with the European Reanalysis of Global Climate Observations project (ERA-CLIM). The focus of this paper is on the ISPDv2 and the inclusion of the 20CR feedback metadata. The Research Data Archive at the National Center for Atmospheric Research provides data collection and access for the ISPDv2, and will provide access to future versions.

Slow modulation of interannual variability and its relationship to El Niñ o-Southern Oscillation (ENSO) isinvestigated for the period of 1870-2007 using shipboard surface meteorological observations along a frequently traveled track across the north Indian Ocean (NIO; from the Gulf of Aden through Malacca Strait) and the South China Sea (to Luzon Strait). During the decades in the late nineteenth-early twentieth century and in the late twentieth century, the El Niñ o-induced NIO warming persists longer than during the 1910s-mid-1970s, well into the summer following the peak of El Niñ o. During the epochs of the prolonged NIO warming, rainfall drops and sea level pressure rises over the tropical northwest Pacific in summer following El Niñ o. Conversely, during the period when the NIO warming dissipates earlier, these atmospheric anomalies are not well developed. This supports the Indian Ocean capacitor concept as a mechanism prolonging El Niñ o influence into summer through the persistent Indian Ocean warming after El Niñ o itself has dissipated.The above centennial modulation of ENSO teleconnection to the Indo-northwest Pacific region is reproduced in an atmospheric general circulation model forced by observed SST. The modulation is correlated not with the Pacific decadal oscillation but rather with the ENSO variance itself. When ENSO is strong, its effect in the Indo-northwest Pacific strengthens and vice versa. The fact that enhanced ENSO teleconnections occurred 100 years ago during the late nineteenth-early twentieth century indicates that the recent strengthening of the ENSO correlation over the Indo-western Pacific may not entirely be due to global warming but reflect natural variability.

A dataset of tropical cyclone landfall numbers in the Philippines (TLP) is created from a combination of historical observation records of the Monthly Bulletins of Philippine Weather Bureau and Joint Warning Typhoon Center best‐track data for the period of 1902 to 2005. Interdecadal variability of TLP is found to be related to different phases of the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The annual TLP has an apparent oscillation of about 32 years before 1939 and an oscillation of about 10–22 years after 1945. No long‐term trend is found. During the low PDO phase, the annual TLP decreases (increases) significantly in El Niño (La Niña) years. During high PDO phase, however, the difference in annual TLP between different ENSO phases becomes unclear. These results suggest that natural variability related to ENSO and PDO phases appears to prevail in the interdecadal variability of TLP.

ABSTRACT:The Pacific-Japan (PJ) pattern affects interannual variability in the East Asian and western North Pacific (WNP) summer monsoons. This teleconnection pattern is characterized by a meridional dipole of anomalous circulation and precipitation between the tropical WNP and the midlatitudes. This study develops a long index of the PJ pattern using station-based atmospheric pressure data to track the PJ variability from 1897 to 2013. This index is correlated with a wide array of climate variables including air temperature, precipitation, Yangtze River flow, Japanese rice yield and the occurrence of tropical cyclones over the WNP (especially those that make landfall on the Chinese and Korean coast). For the recent three decades, the PJ index reproduces well-known correlations with El Niño-Southern Oscillation (ENSO) in the preceding boreal winter and Indian Ocean temperature in the concurrent summer. For the 117-year period, this ENSO-PJ relationship varies on interdecadal time scales, with low correlations in the 1920s and from the 1940s to 1970s, and recurrences of significant correlations at the beginning of the 20th century and the 1930s. In accordance with the modulation, the magnitude and regional climate effect of the PJ variability have changed. These results highlight the importance of interdecadal modulations of climate anomalies in the summer WNP and the need of long-term observations to study such modulations.