Jacob O. Sewall

Climate sensitivity to changes in land surface characteristics

Abstract Using a recently developed global vegetation distribution, topography, and shorelines for the Early Eocene in conjunction with the Genesis version 2.0 climate model, we investigate the influences that these new boundary conditions have on global climate. Global mean climate changes little in response to the subtle changes we made; differences in mean annual and seasonal surface temperatures over northern and southern hemispheric land, respectively, are on the order of 0.5°C. In contrast, and perhaps more importantly, continental scale climate exhibits significant responses. Increased peak elevations and topographic detail result in larger amplitude planetary ∼4 mm/day and decreases by 7–9 mm/day in the proto Himalayan region. Surface temperatures change by up to 18°C as a direct result of elevation modifications. Increased leaf area index (LAI), as a result of altered vegetation distributions, reduces temperatures by up to 6°C. Decreasing the size of the Mississippi embayment decreases inland precipitation by 1–2 mm/day. These climate responses to increased accuracy in boundary conditions indicate that “improved” boundary conditions may play an important role in producing modeled paleoclimates that approach the proxy data more closely.

Come a little bit closer: A high-resolution climate study of the early Paleogene Laramide foreland

The early Paleogene greenhouse climate has long captured the attention of researchers. However, due to limited data and the coarse spatial resolution of general circulation model (GCM) results, it has proven difficult to investigate early Paleogene climate at the level of detail characteristic of the actual climate system. We present a high-resolution regional climate modeling study of the North American Laramide foreland. Our simulation depicts early Paleogene climate dynamics in western North America with unprecedented detail. Increased horizontal resolution improves matches between modeled temperatures and data in some basins; however, large-scale cold biases found in prior GCM simulations persist. Our study provides insight into the existence of snow and perennial ice in the Laramide highlands and northwestern Cordillera and describes the summer monsoon along the Rocky Mountain front in great detail. Monsoonal precipitation initiates in the southeastern Rockies in May, penetrates north and west as temperatures warm, and peaks at 1 cm/day along the southern and central mountain front; basins farther to the north and west are drier.

Effect of sea surface temperature configuration on model simulations of “equable” climate in the Early Eocene

Abstract A major challenge in paleoclimate modeling studies is reproducing the “equable” climate conditions recorded by proxy climate data. This challenge has long been a focus in studies of Eocene paleoclimate. Climate models consistently overestimate mean annual temperature range (MATR) relative to proxy data interpretations and produce minimum temperatures that are far lower than proxy estimates. We hypothesize that the lack of accurate sea surface temperatures (SSTs) and a complete annual cycle definition is responsible for some of these model-data discrepancies in Eocene comparisons. To test this hypothesis, we developed two Eocene annual cycle SST data sets as boundary conditions for model experiments and ran full annual cycle and abbreviated cycle (perpetual) cases. When a perpetual case is replaced by a full annual cycle of SST values, the January 0°C isotherm (freezeline) over North America shifts poleward by ∼5° latitude and (MATR is reduced by ∼5°C. A change from one annual SST cycle to another, equally plausible, annual SST cycle results in a further latitudinal migration (∼5°) of the freezeline and another ∼5°C change in continental interior MATR. Overall, the global mean annual and cold month mean temperatures show little sensitivity to the forcing, while MATR shows larger sensitivity in some continental regions. Our results suggest that the annual cycle of SSTs and the actual SST values incorporated into experiments are major sources of the model-data continental temperature discrepancies reported in past paleoclimate modeling studies. We also find that the relatively coarse spatial resolution of the topography incorporated into paleoclimate models is an additional source of the discrepancy.

Latitudinal Gradients in Greenhouse Seawater δ18O: Evidence from Eocene Sirenian Tooth Enamel

Tooth enamel from fossil marine mammals shows that the middle latitudes were wetter in the past than they are today. The Eocene greenhouse climate state has been linked to a more vigorous hydrologic cycle at mid- and high latitudes; similar information on precipitation levels at low latitudes is, however, limited. Oxygen isotopic fluxes track moisture fluxes and, thus, the δ18O values of ocean surface waters can provide insight into hydrologic cycle changes. The offset between tropical δ18O values from sampled Eocene sirenian tooth enamel and modern surface waters is greater than the expected 1.0 per mil increase due to increased continental ice volume. This increased offset could result from suppression of surface-water δ18O values by a tropical, annual moisture balance substantially wetter than that of today. Results from an atmospheric general circulation model support this interpretation and suggest that Eocene low latitudes were extremely wet.

Precipitation Shifts over Western North America as a Result of Declining Arctic Sea Ice Cover: The Coupled System Response

Abstract Changes in Arctic sea ice cover have the potential to impact midlatitude climate. A previous sensitivity study utilizing the National Center for Atmospheric Research’s (NCAR) atmospheric general circulation model [AGCM; Community Climate Model, version 3 (CCM3)] to explore climate sensitivity to declining Arctic sea ice cover suggested that, as Arctic sea ice cover is reduced, precipitation patterns over western North America will shift toward dryer conditions in southwestern North America and wetter conditions in northwestern North America. Here, three complementary lines of research validate and explore the robustness of this possible climate change impact: 1) repetition of the previous sensitivity study (specified constant Arctic sea ice cover and atmospheric CO2) with an updated version of the NCAR AGCM [third Community Atmosphere Model (CAM3)], 2) investigation of the climate response to dynamically reduced Arctic sea ice cover (driven by a quadrupling of atmospheric CO2) in the coupled NCAR...

Less ice, less tilt, less chill: The influence of a seasonally ice-free Arctic Ocean and reduced obliquity on early Paleogene climate

For more than 20 years the paleoclimate modeling community has sought the mechanism for generating a warm, equable early Paleogene climate. Increased greenhouse gas concentrations and warm sea-surface temperatures (SSTs) do much to reproduce the climate indicated by proxy data; however, discrepancies between model output and data remain, and a term in the equable climate equation is clearly missing. Based on proxy data indications of reduced early Paleogene seasonality, we test the climate impact of a reduction in Earths obliquity to 18°. Obliquity is one of the few forcing factors that can directly affect seasonality, and our hypothesis of changing obliquity is supported by large planetary mass redistributions in the past 120 m.y. Such mass redistributions have the potential to change the tilt of Earths axis. In our experiment the combination of reduced obliquity, increased early Paleogene greenhouse gas concentrations, and conservatively warm SSTs reproduces early Paleogene climate with greater accuracy than any previous modeling efforts and provides a plausible solution to the equable climate challenge.

Equable paleogene climates: The result of a stable, positive Arctic Oscillation?

The Early Paleogene is recognized as a particularly warm interval in Earths history. Paleogene proxy climate indicators suggest warm polar and mid-latitude continental interior temperatures, and a reduced latitudinal temperature gradient. Most researchers believe that Early Paleogene climate was driven by forcing fields that act globally (e.g. greenhouse gases). However, modeling work based on this hypothesis has failed to reproduce Paleogene climate as indicated by proxy data. Quite possibly, an ameliorating influence acting directly at the poles would more effectively warm high latitudes, provide an additional heat source to mid-latitude continental interiors, and reduce the latitudinal temperature gradient. Here we present a hypothesis based on the positive phase of the modern Arctic Oscillation; in short, that prolonged low pressure over the Arctic Ocean would have warmed mid-latitude continental interiors and drastically reduced the Arctic Oceans ice cover, thus producing conditions consistent with proxy climate indicators for the Paleogene greenhouse interval.