Prior posts on non-hockey-sticks
|Graph D shows the temperature proxy in the North American southwest has cooled over the past ~7,000 years.|
An enhanced role for the Tropical Pacific on the humid Pleistocene–Holocene transition in southwestern North America
- Division of Earth and Ecosystem Sciences, Desert Research Institute, 2215 Raggio Parkway, Reno, NV 89512, USA
- Tropical Pacific variability is compared to SW N America paleoenvironmental records.
- The Tropical Pacific plays a larger role in landscape evolution of the region.
- Tropical moisture and transport mechanisms relevant for mid-latitude arid regions.
Climate effects on landscape evolution during the Late Pleistocene–Holocene transition (∼14.6–8 ka) in southwestern North America traditionally are linked to the activity of the North American Monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity. We performed an integrated analysis of regional alluvial fan, lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies. Our focus was on the potential role the Tropical Pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region.
Our analysis indicates that the onset of alluvial fan aggradation in most of the region at ∼13.5 ka could have been a response to semi-permanent El Niño-like conditions in the Tropical Pacific, which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region. The North American Monsoon was restricted in extent and intensity until ∼7 ka and probably was not a major factor in alluvial fan aggradation. A second stage of alluvial fan aggradation from 11.5 to ∼9 ka was dominated by hyper-concentrated flows and sheet-flood sedimentation, along with deposition in fluvial settings. Storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra-tropical cyclones with moisture advected directly from the Tropical Pacific. At ∼8 ka, favorable conditions for the occurrence of these storms waned and storm tracks shifted northward.
Analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the Tropical Pacific can affect storm properties enough to explain the observed geomorphic effects, regardless of other factors traditionally considered of large impact like vegetation change. Our results suggest that the Tropical Pacific plays a larger role than currently thought in landscape evolution of the region.
Post a Comment