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Century-long observed gridded land precipitation datasets are a cornerstone of hydrometeorological research. But recent work has suggested that observed Northern Hemisphere midlatitude (NHML) land mean precipitation does not show evidence of an expected negative response to mid-twentieth-century aerosol forcing. Utilizing observed river discharges, the observed runoff is calculated and compared with observed land precipitation. The results show a near-zero twentieth-century trend in observed NHML land mean runoff, in contrast to the significant positive trend in observed NHML land mean precipitation. However, precipitation and runoff share common interannual and decadal variability. An obvious split, or breakpoint, is found in the NHML land mean runoff-precipitation relationship in the 1930s. Using runoff simulated by six land surface models (LSMs), which are driven by the observed precipitation dataset, such breakpoints are absent. These findings support previous hypotheses that inhomogeneities exist in the early-twentieth-century NHML land mean precipitation record. Adjusting the observed precipitation record according to the observed runoff record largely accounts for the departure of the observed precipitation response from that predicted given the real-world aerosol forcing estimate, more than halving the discrepancy from about 6 to around 2 W m-2. Consideration of complementary observed runoff adds support to the suggestion that NHML-wide early-twentieth-century precipitation observations are unsuitable for climate change studies. The agreement between precipitation and runoff over Europe, however, is excellent, supporting the use of whole-twentieth-century observed precipitation datasets here.

There is a growing desire for reliable 21st-century projections of water availability at the regional scale. Global climate models (GCMs) are typically used together with global hydrological models (GHMs) to generate such projections. GCMs alone are unsuitable, especially if they have biased representations of aridity. The Budyko framework describes how water availability varies as a non-linear function of aridity and is used here to constrain projections of runoff from GCMs, without the need for computationally expensive GHMs. Considering a Chinese case study, we first apply the framework to observations to show that the contribution of direct human impacts (water consumption) to the significant decline in Yellow river runoff was greater than the contribution of aridity change by a factor of approximately 2, although we are unable to rule out a significant contribution from the net effect of all other factors. We then show that the Budyko framework can be used to narrow the range of Yellow river runoff projections by 34 %, using a multi-model ensemble and the high end RCP8.5 emissions scenario. This increases confidence that the Yellow river will see an increase in runoff due to aridity change by the end of the 21st century. Yangtze river runoff projections change little, since aridity biases in GCMs are less substantial. Our approach serves as a quick and inexpensive tool to rapidly update and correct projections from GCMs alone. This could serve as a valuable resource when determining the water management policies required to alleviate water stress for future generations. [ABSTRACT FROM AUTHOR]

Recent years have seen considerable advances in the understanding of drivers of climate at seasonal lead times in the extratropics and skilful seasonal forecasts are now possible. However, skill in seasonal forecasts in the Northern Hemisphere extratropics is mostly limited to winter. Drivers of summer circulation anomalies over the North Atlantic-European (NAE) sector are poorly understood. Here, we investigate the role of concurrent North Atlantic sea surface temperatures (SSTs) in driving summer atmospheric circulation changes in the NAE sector. We are interested in drivers of circulation variability at interannual time scales, since these are most relevant to seasonal prediction. Using observations and a reanalysis we show that the summer North Atlantic Oscillation (SNAO), the leading mode of observed summer mean sea level pressure variability in the NAE sector, is correlated with a distinct SST tripole pattern in the North Atlantic, with a predominantly cool (warm) North Atlantic associated with positive (negative) SNAO. An atmospheric general circulation model (AGCM) is used to test whether there are robust atmospheric circulation responses over the NAE sector to this SST tripole pattern. We perform a series of simulations to determine the response to whole North Atlantic SSTs, extratropical North Atlantic SSTs and tropical North Atlantic SSTs. We find that there is no significant NAE circulation response to whole North Atlantic SSTs, but this masks contrasting responses to tropical North Atlantic SSTs and extratropical North Atlantic SSTs. The responses to these domains project onto opposite phases of the summer East Atlantic (SEA) pattern, the second dominant mode of observed summer mean sea level pressure variability in the NAE sector. The tropical to extratropical teleconnection appears to be due to Rossby wave propagation from SST anomalies, and in turn precipitation anomalies, in the Caribbean region. We identify key biases in the model, which cause it to simulate the SEA pattern as the dominant mode of variability in the NAE region (instead of the SNAO), and show that a response to the SST tripole pattern that projects onto the SEA pattern is consistent with these biases. Efforts to eradicate these biases, perhaps achieved by higher resolution simulations, would allow for an improved understanding of the true response to North Atlantic SST patterns. [ABSTRACT FROM AUTHOR]