OceanRAIN—the Ocean Rainfall And Ice-phase precipitation measurement Network—provides in-situ along-track shipboard data of precipitation, evaporation and the resulting freshwater flux at 1-min resolution over the global oceans from June 2010 to April 2017. More than 6.83 million minutes with 75 parameters from 8 ships cover all routinely measured atmospheric and oceanographic state variables along with those required to derive the turbulent heat fluxes. The precipitation parameter is based on measurements of the optical disdrometer ODM470 specifically designed for all-weather shipboard operations. The rain, snow and mixed-phase precipitation occurrence, intensity and accumulation are derived from particle size distributions. Additionally, microphysical parameters and radar-related parameters are provided. Addressing the need for high-quality in-situ precipitation data over the global oceans, OceanRAIN-1.0 is the first comprehensive along-track in-situ water cycle surface reference dataset for satellite product validation and retrieval calibration of the GPM (Global Precipitation Measurement) era, to improve the representation of precipitation and air-sea interactions in re-analyses and models, and to improve understanding of water cycle processes over the global oceans.

Modes of climate variability strongly impact our climate and thus human society. Nevertheless, the statistical properties of these modes remain poorly known due to the short time frame of instrumental measurements. Reconstructing these modes further back in time using statistical learning methods applied to proxy records is useful for improving our understanding of their behaviour. For doing so, several statistical methods exist, among which principal component regression is one of the most widely used in paleoclimatology. Here, we provide the software ClimIndRec to the climate community; it is based on four regression methods (principal component regression, PCR; partial least squares, PLS; elastic net, Enet; random forest, RF) and cross-validation (CV) algorithms, and enables the systematic reconstruction of a given climate index. A prerequisite is that there are proxy records in the database that overlap in time with its observed variations. The relative efficiency of the methods can vary, according to the statistical properties of the mode and the proxy records used. Here, we assess the sensitivity to the reconstruction technique. ClimIndRec is modular as it allows different inputs like the proxy database or the regression method. As an example, it is here applied to the reconstruction of the North Atlantic Oscillation by using the PAGES 2k database. In order to identify the most reliable reconstruction among those given by the different methods, we use the modularity of ClimIndRec to investigate the sensitivity of the methodological setup to other properties such as the number and the nature of the proxy records used as predictors or the targeted reconstruction period. We obtain the best reconstruction of the North Atlantic Oscillation (NAO) using the random forest approach. It shows significant correlation with former reconstructions, but exhibits higher validation scores.

Atlantic multidecadal variability is a coherent mode of natural climate variability occurring in the North Atlantic Ocean, with strong impacts on human societies and ecosystems worldwide. However, its periodicity and drivers are widely debated due to the short temporal extent of instrumental observations and competing effects of both internal and external climate factors acting on North Atlantic surface temperature variability. Here, we use a paleoclimate database and an advanced statistical framework to generate, evaluate, and compare 312 reconstructions of the Atlantic multidecadal variability over the past millennium, based on different indices and regression methods. From this process, the best reconstruction is obtained with the random forest method, and its robustness is checked using climate model outputs and independent oceanic paleoclimate data. This reconstruction shows that memory in variations of Atlantic multidecadal variability have strongly increased recently—a potential early warning signal for the approach of a North Atlantic tipping point.

Abstract. Instruments on satellites for Earth observation on polar orbits usually employ a two-point calibration technique, in which deep space and an onboard calibration target provide two reference flux levels. As the direction of the deep-space view is in general close to the celestial equator, the Moon sometimes moves through the field of view and introduces an unwelcome additional signal. One can take advantage of this intrusion, however, by using the Moon as a third flux standard, and this has actually been done for checking the lifetime stability of sensors operating at visible wavelengths. As the disk-integrated thermal emission of the Moon is less well known than its reflected sunlight, this concept can in the microwave range only be used for stability checks and intercalibration. An estimate of the frequency of appearances of the Moon in the deep-space view, a description of the limiting factors of the measurement accuracy and models of the Moon's brightness, and a discussion of the benefits from complementing the naturally occurring appearances of the Moon with dedicated spacecraft maneuvers show that it would be possible to detect photometric lifetime drifts of a few percent with just two measurements. The pointing accuracy is the most crucial factor for the value of this method. Planning such observations in advance would be particularly beneficial, because it allows observing the Moon at welldefined phase angles and putting it at the center of the field of view. A constant phase angle eliminates the need for a model of the Moon's brightness when checking the stability of an instrument. With increasing spatial resolution of future microwave sensors another question arises, viz. to what extent foreground emission from objects other than the Moon will contaminate the flux entering the deep-space view, which is supposed to originate exclusively in the cosmic microwave background. We conclude that even the brightest discreet sources have flux densities below the detection limit of microwave sensors in a single scan.

Abstract. The response of evapotranspiration to anthropogenic warming is of critical importance to the water and carbon cycle. Con-flicting observations about changes of evapotranspiration stem mostly from the brevity of observations in time and space as well as a high degree of internal variability. Here we present the first gridded reconstruction of the European summer vapour pressure deficit (VPD) for the past four centuries. The gridded reconstruction is based on 26 European tree-ring oxygen iso-tope records and is performed using a Random Forest approach. Based on our reconstruction, we show that from the mid-1700s a trend towards higher VPD occurred in Central Europe and the Mediterranean region which is based on the simulta-neous increase in temperature and decrease in precipitation. This increasing VPD trend continues throughout the observation-al period and recent times. Climate model projections show this increase in VPD for the Mediterranean region continuing until the end of the 21st century, whereby the extent depends on the amount of greenhouse gas emissions. In contrast, pro-jected VPD in North and Central Europe shows a tendency towards higher VPD only in the highest emission scenario (the produced data is available here: https://doi.org/10.5281/zenodo.5958837 (Balting, D. F. et al., 2022).

El Niño Southern Oscillation (ENSO) events yield precipitation deficits and ensuing droughts, often damaging regional forests, in many parts of the world. The relative roles of ENSO, other natural climate changes, and anthropogenic factors on the forest clearing of Easter Island over the last millennium are still debated. Here, we analyze Easter Island precipitation changes using in situ, satellite‐derived and reanalysis products spanning the last 4–7 decades, and 46 monthly 156‐year‐long (1850–2014) simulations derived from 25 CMIP5 and 21 CMIP6 (Coupled Model Intercomparison Project phases 5 and 6) General Circulation Models. Our analysis shows that La Niña events, the cold phases of ENSO, cause precipitation deficits of −0.2 to −0.3 standard deviation (relative to long‐term mean) in all analyzed data types. ENSO‐like events are further examined over the last millennium (850–1981). A new multiproxy reconstruction of the NINO3.4 index based on proxy records from the Past Global Changes 2k database and Random Forest method is produced. Our reconstruction reveals unusual high recurrences of La Niña‐like situations during the fifteenth to seventeenth centuries, which likely induced significant precipitation deficits on the island. These situations are compared to published vegetation reconstructions based on pollen analyses derived from sedimentary cores collected in three island sites. We conclude the environmental consequences of cumulative precipitation deficits over long‐lasting La Niña‐like situations reconstructed here over the fifteenth to seventeenth centuries were likely favoring drought and forest flammability. La Niña events should be better accounted for among the causes of forest clearing on Easter Island.

The mid-Holocene (6,000 years before present) was a warmer period than today in summer in most of the Northern Hemisphere. In winter, over Europe, pollen-based reconstructions show a dipole of temperature anomalies as compared to present-day, with warmer conditions in the north and colder in the south. It has been proposed that this pattern of temperature anomaly could be explained by a persisting positive phase of the North Atlantic Oscillation during this period, which was, however, not reproduced in general by climate models. Indeed, PMIP3 models show a large spread in their response to the mid-Holocene insolation changes, the physical origins of which are not understood. To improve the understanding of the

The quantitative distribution of atmospheric vapor mixture, AVM, into three distinguished vapor endmembers is lacking in the literature. This work lls such a gap. The isotope ratio, d 18 O L , of rainwater in Winter, and arti cial condensates in Summer, gave the 18 O V contents of local AVMs at temperature-dependent equilibrium, downtown Cairo city, Nile Delta apex. We used our models, TIMAM, CLAW, and SIGNALS to process the d 18 O V and the commensurate speci c humidity, S, values in several AVM data sets for determining the percent and mass contributions of three moisture origins and their temporal waveforms. The proportions and masses revealed Marine vapor dominance, followed by evapotranspiration. By far, the free Troposphere source showed a slight input. The quota of each constituent manifests a delayed waveform vs. AVM d 18 O in ux, which shows a diurnal peak and a nocturnal tunnel. The moderate ET percent inputs in Winter, and by daytime, impose signi cant AVM 18 O enrichment. In contrast, the high Maritime vapor inputs in Summer, and by night, stand behind the depleted AVM 18 O content. The relationships between the mass input of each source and the AVM isotope ratio show signi cant dispersion for the negative trend of the diurnal-nocturnal Marine vapor in the two seasons. Such a high scattering is due to the mingling of northern wind-gust diurnal convection (marked by low Marine vapor input) and northern steady nocturnal advection (characterized by high Marine vapor input). Marine vapor waveform has a 12-hour time-lag by the intertwining of turbulent diurnal transmission, and steady nocturnal transport, through the long trajectory (180 km) from the Mediterranean coast to Cairo. In contrast, the relationships between ET mass input and AVM isotope ratio, on the one hand, and between the Troposphere vapor mass input and AVM isotope ratio, on the other hand, manifest low-dispersion positive and negative regressions, respectively. Such a low dispersion is due to the short transport pathway, the narrow range of the biological input (that increases only by daytime), and sharp Troposphere downdraft (moving northward in Winter but southward in Summer). Also, the ET waveform has a Zero-hour time-lag, like that of the Tropospheric vapor. Albeit the low S value of the Troposphere vapor pole, its impact on the AVM isotopic depletion is signi cant due to its extremely shallow 18 O content. The increase of the Tropospheric input at low AVM S values is related to regional drought, as expected. The high S values, of Marine and biotic origins, usually go with temperature apogees, especially in Summertime, as anticipated. The used models help in improving the time-series simulation of evaporation runs, since using seasonal d 18 O V and S markers is better than using a snapshot. The ternaryvapor-source allocation procedure is a breakthrough in isotope hydrology. This thoroughly useful procedure will prove its ultimate bene ts when the users get CRDS laser-controlled devices for the continuous measurements of the isotopic ra...