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Consensus on the cause of recent midlatitude circulation changes toward a wavier manner in the Northern Hemisphere has not been reached, albeit a number of studies collectively suggest that this phenomenon is driven by global warming and associated Arctic amplification. Here, through a fingerprint analysis of various global simulations and a tropical heating-imposed experiment, we suggest that the suppression of tropical convection along the Inter Tropical Convergence Zone induced by sea surface temperature (SST) cooling trends over the tropical Eastern Pacific contributed to the increased summertime midlatitude waviness in the past 40 years through the generation of a Rossby-wave-train propagating within the jet waveguide and the reduced north-south temperature gradient. This perspective indicates less of an influence from the Arctic amplification on the observed mid-latitude wave amplification than what was previously estimated. This study also emphasizes the need to better predict the tropical Pacific SST variability in order to project the summer jet waviness and consequent weather extremes.

 

We present the first Greenlandic tree ring oxygen isotope record (δ18OGTR), derived from four birch trees collected from the Qinguadalen Valley in southwestern Greenland in 1999. Our δ18O record spans from 1950–1999 and is significantly and positively correlated with winter ice core δ18O from southern Greenland. δ18OGTR records are positively correlated with southwestern Greenland January–August mean temperatures. North Atlantic Oscillation (NAO) reconstructions have been developed from a variety of proxies, but never with Greenlandic tree rings, and our δ18OGTR record is significantly correlated with NAO (r = −0.64), and spatial correlations with sea-level pressure indicate a classic NAO pressure seesaw pattern. These results may facilitate a longer NAO reconstruction based on long time series of tree ring δ18O records from Greenland, provided that subfossil wood can be found in areas vacated by melting glaciers. 

Availability of water for irrigated crops is driven by climate and policy, as moderated by public priorities and opinions. We explore how climate and water policy interact to influence water availability for cannabis (Cannabis sativa), a newly regulated crop in California, as well as how public discourse frames these interactions. Grower access to surface water covaries with precipitation frequency and oscillates consistently in an energetic 11–17 year wet-dry cycle. Assessing contemporary cannabis water policies against historic streamflow data showed that legal surface water access was most reliable for cannabis growers with small water rights (<600 m3) and limited during relatively dry years. Climate variability either facilitates or limits water access in cycles of 10–15 years—rendering cultivators with larger water rights vulnerable to periods of drought. However, news media coverage excludes growers’ perspectives and rarely mentions climate and weather, while public debate over growers’ irrigation water use presumes illegal diversion. This complicates efforts to improve growers’ legal water access, which are further challenged by climate. To promote a socially, politically, and environmentally viable cannabis industry, water policy should better represent growers’ voices and explicitly address stakeholder controversies as it adapts to this new and legal agricultural water user. 

Periods of water surplus and deficit in Northern California follow a pronounced quasi‐decadal cycle. This cycle is largely driven by the frequency of atmospheric rivers (ARs), affecting the region’s wet and dry periods. Our analyses demonstrate that the quasi‐decadal cycle of AR frequency relies on moisture transport associated with the position and intensity of the Aleutian Low. In observations, the Aleutian Low is shown to covary with tropical Pacific sea surface temperature anomalies. A modeling experiment, which incorporates ocean observations from the equatorial Pacific into the fully coupled climate model, provides support that the quasi‐decadal cycle of the Aleutian Low is forced by the tropical Pacific. Subsequently, the tropical Pacific modulates the wet season moisture transport toward California on decadal time scales, affecting AR frequency. These results provide metrics for improving interannual‐to‐decadal prediction of AR activity, which drives hydrological cycles in Northern California.