24The summer rainfall climate of East Asia underwent large and abrupt changes during past 25 climates, in response to precessional forcing, glacial-interglacial cycles as well as abrupt changes 26 to the North Atlantic during the last glacial. However, current interpretations of said changes are 27 typically formulated in terms of modulation of summer monsoon intensity, and do not account 28 for the known complexity in the seasonal evolution of East Asian rainfall, which exhibits sharp 29 transition from the Spring regime to the Meiyu, and then again from the Meiyu to the Summer 30 regime. 31We explore the interpretation that East Asian rainfall climate undergoes a modulation of its 32 seasonality during said paleoclimate changes. Following previous suggestions we focus on role 33 of the westerly jet over Asia, namely that its latitude relative to Tibet is critical in determining 34 the stepwise transitions in East Asian rainfall seasons. In support of this linkage, we show from 35 observational data that the interannual co-variation of June (July-August) rainfall and upper 36 tropospheric zonal winds show properties consistent with an altered timing of the transition to 37 the Meiyu (Summer), and with more northward-shifted westerlies for earlier transitions. 38We similarly suggest that East Asian paleoclimate changes resulted from an altered timing in 39 the northward evolution of the jet and hence the seasonal transitions, in particular the transition 40 of the jet from south of the Plateau to the north that determines the seasonal transition from 41Spring rains to the Meiyu. In an extreme scenario -which we speculate the climate system 42 tended towards during stadial (cold) phases of D/O stadials and periods of low Northern 43Hemisphere summer insolation -the jet does not jump north of the Plateau, essentially keeping 44East Asia in prolonged Spring conditions. 45We argue that this hypothesis provides a viable explanation for a key paleoproxy signature of 46 3 D/O stadials over East Asia, namely the heavier mean δ 18 O of precipitation as recorded in 47 speleothem records. The southward jet position prevents the low-level monsoonal flow -which 48 is isotopically light -from penetrating into the interior of East Asia; as such, precipitation there 49 will be heavier, consistent with speleothem records. This hypothesis can also explain other key 50 evidences of East Asian paleoclimate changes, in particular the occurrence of dusty conditions 51 during North Atlantic stadials, and the southward migration of the Holocene optimal rainfall. 52Earth's orbit. Other records corroborate the sense of large and abrupt change in East Asia; for 60 example, paleoproxy dust records show East Asia to be dustier during cold stadials (and in 61 particular Heinrich stadials) Nagashima et al., 2011], and more generally during 62 glacial periods [An, 2000]. 63The dominant interpretation of variability in the speleothem records is as a record of 64 changes in East Asian summer monsoon intensity, with δ 18 O relatively light when m...
As the world warms, there is a profound need to improve projections of climate change. Although the latest Earth system models offer an unprecedented number of features, fundamental uncertainties continue to cloud our view of the future. Past climates provide the only opportunity to observe how the Earth system responds to high carbon dioxide, underlining a fundamental role for paleoclimatology in constraining future climate change. Here, we review the relevancy of paleoclimate information for climate prediction and discuss the prospects for emerging methodologies to further insights gained from past climates. Advances in proxy methods and interpretations pave the way for the use of past climates for model evaluation—a practice that we argue should be widely adopted.
With CO2 concentrations similar to today (410 ppm), the Pliocene Epoch offers insights into climate changes under a moderately warmer world. Previous work suggested a low zonal sea surface temperature (SST) gradient in the tropical Pacific during the Pliocene, the so‐called “permanent El Niño.” Here, we recalculate SSTs using the alkenone proxy and find moderate reductions in both the zonal and meridional SST gradients during the mid‐Piacenzian warm period. These reductions are captured by coupled climate model simulations of the Pliocene, especially those that simulate weaker Walker circulation. We also produce a spatial reconstruction of mid‐Piacenzian warm period Pacific SSTs that closely resembles both Pliocene and future, low‐emissions simulations, a pattern that is, to a first order, diagnostic of weaker Walker circulation. Therefore, Pliocene warmth does not require drastic changes in the climate system—rather, it supports the expectation that the Walker circulation will weaken in the future under higher CO2.
Coupled ocean-atmosphere processes in the Indian Ocean amplify changes in Indo-Pacific climate under glacial conditions.
Proxy records from the last millennium in Mesoamerica suggest a widespread interval of drought at some point between the 7th and 13th centuries CE. In some records, this time period represents the driest proxy values in the last few millennia. There is currently no clear dynamical explanation for these droughts, nor consensus as to whether they were spatiotemporally coherent over the region. We perform several analyses to develop a novel hypothesis to explain these droughts that is consistent with our knowledge of the dynamics of the climate system. We use Bayesian age modeling techniques and a synthesis of regional proxy records to suggest that there is robust evidence of drying between 800 and 1050 CE, with the sites showing dry conditions clustered in southern central America. By studying control simulations of two general circulation models (GCMs), we suggest that this pattern may be diagnostic of hydroclimate changes associated with multidecadal variability in the Atlantic Basin. Models and instrumental data suggest that cooling of tropical Atlantic SSTsand strengthening of the North Atlantic Subtropical High drives a pattern of multidecadal drought with negative rainfall anomalies in southern central America and positive anomalies in northern Mexico. This process could have resulted in the droughts observed in the proxy record. Our work offers a novel hypothesis about the dynamics of multidecadal drought in Mesoamerica, and builds on previous efforts to synthesize proxy records from the region.
Despite tectonic conditions and atmospheric CO2 levels (pCO2) similar to those of present-day, geological reconstructions from the mid-Pliocene (3.3-3.0 Ma) document high lake levels in the Sahel and mesic conditions in subtropical Eurasia, suggesting drastic reorganizations of subtropical terrestrial hydroclimate during this interval. Here, using a compilation of proxy data and multi-model paleoclimate simulations, we show that the mid-Pliocene hydroclimate state is not driven by direct CO2 radiative forcing but by a loss of northern high-latitude ice sheets and continental greening. These ice sheet and vegetation changes are long-term Earth system feedbacks to elevated pCO2. Further, the moist conditions in the Sahel and subtropical Eurasia during the mid-Pliocene are a product of enhanced tropospheric humidity and a stationary wave response to the surface warming pattern, which varies strongly with land cover changes. These findings highlight the potential for amplified terrestrial hydroclimate responses over long timescales to a sustained CO2 forcing.
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