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The mechanisms underlying the current greenhouse gas (GHG) forced decline in Mediterranean rainfall remain a matter of debate. To inform our understanding of the current and projected drying, we examined extended arid intervals in the late Quaternary, Eastern Mediterranean (EM) Levant indicated by substantial salt deposits in a Dead Sea sediment core covering the past 220 kyr. These arid events occurred during interglacials, when the Earth was at perihelion to the sun in boreal fall and during glacial–interglacial transitions, associated with icesheet melting. Climate models forced with realistic late Quaternary insolation variations show that when the Earth is closest to the Sun in boreal fall, the North Atlantic latitudinal surface temperature gradient in the winter intensifies. In response, the overlying midlatitude North Atlantic jet stream and the extratropical storm track move poleward while sea-level pressure rises in the subtropics. These changes bring about a weakening of the Mediterranean storm track and a decline in rainfall over the entire basin. During glacial–interglacial transitions, meltwater from continental icesheets forced abrupt subpolar North Atlantic cooling. This also strengthened the latitudinal surface temperature gradient, likely causing similar atmospheric response and aridity in the Mediterranean. There is a strong resemblance between this paleoclimate scenario and the climatic changes corresponding to the present and projected GHG drying of the EM. Hence, the late Quaternary palaeohydrology of the Dead Sea indicates an important North Atlantic centered response to external forcing, which leads to Mediterranean drying and is relevant in the present. 

Abstract A sequence of torrential rainstorms pounded Pakistan in the summer of 2022, shattering records by massive margins (7 sigma). The severe socioeconomic damages underscore the urgency of identifying its dynamic drivers and relationship with human-induced climate change. Here, we find that the downpours were primarily initiated by the synoptic low-pressure systems, whose intensity and longevity far exceeded their counterparts in history as fueled by a historically-high cross-equatorial moisture transport over the Arabian Sea. The moisture transport has been trending upward since the 1960s and, in 2022, along with the anomalous easterly moisture influx caused by the combination of La Niña and negative Indian Ocean Dipole events, created a corridor of heavy rainfall extending from central India toward southern Pakistan. While it is not yet established whether the observed trend of the cross-equatorial moisture transport has exceeded natural variability, model-based analysis confirms that it is consistent with the fingerprint of anthropogenic climate warming and will raise the likelihood of such rare events substantially in the coming decades. 

The seasonal rainy phase observed in many places across Earth is shaping the climate and is being changed by global climate trends. 

Abstract Dendrochronology in West Africa has not yet been developed despite encouraging reports suggesting the potential for long tree-ring reconstructions of hydroclimate in the tropics. This paper shows that even in the absence of local tree chronologies, it is possible to reconstruct the hydroclimate of a region using remote tree rings. We present the West Sub-Saharan Drought Atlas (WSDA), a new paleoclimatic reconstruction of West African hydroclimate based on tree-ring chronologies from the Mediterranean Region, made possible by the teleconnected climate relationship between the West African Monsoon and Mediterranean Sea surface temperatures. The WSDA is a one-half degree gridded reconstruction of summer Palmer Drought Severity indices from 1500 to 2018 CE, produced using ensemble point-by-point regression. Calibration and verification statistics of the WSDA indicate that it has significant skill over most of its domain. The three leading modes of hydroclimate variability in West Africa are accurately reproduced by the WSDA, demonstrating strong skill compared to regional instrumental precipitation and drought indices. The WSDA can be used to study the hydroclimate of West Africa outside the limit of the longest observed record and for integration and comparison with other proxy and archaeological data. It is also an essential first step toward developing and using local tree-ring chronologies to reconstruct West Africa’s hydroclimate. 

Abstract Surface winds and precipitation over the tropical oceans are related to sea surface temperature (SST) through multiple mechanisms. Greater SST is associated with greater conditional instability, which in turn is more conducive to deep convection. The associated mass and flow responses can extend to the surface, via associated pressure gradients imprinted on the top of the planetary boundary layer (PBL). SST also influences surface pressure and wind directly through its control over PBL temperature, as explained by Lindzen and Nigam. The authors examine the relative magnitudes of these two influences over the eastern tropical Pacific on subseasonal precipitation variability during northern summer, when and where SST gradients are largest and the direct influence via PBL temperature is expected to be strongest. Geopotential at 1000 hPa is partitioned into two components: the geopotential at the PBL top (the PBL top is chosen to be 850 hPa, supported by an analysis of the vertical structure of geopotential and temperature) and the PBL thickness. These fields are composited on quintiles of daily ITCZ precipitation both with and without a high-pass filter that isolates subseasonal time scales. The PBL thickness varies little between the highest and lowest precipitation quintiles, while the PBL top geopotential varies much more. This supports a view in which the direct contribution of SST to the surface pressure and flow fields, including the associated PBL convergence over sharp SST maxima, can be viewed as a steady forcing on the rest of the column, while free-tropospheric transients contribute most of the variability associated with precipitation on subseasonal time scales.