The role of nonlinear drying above the boundary layer in the mid-Holocene African monsoon

The role of nonlinear drying above the boundary layer in the mid-Holocene African monsoon AbstractPaleo-climatic proxies indicate that significant summertime rainfall reached the Sahara region during the mid-Holocene, presumably in response to stronger summertime heating in the northern hemisphere. Climate models generally do not replicate the enhanced precipitation. As a step toward understanding the response and possible role of model errors, we conducted a series of idealized experiments with the Community Earth System Model in which local atmospheric heat sources of increasing magnitude are applied in the boundary layer over the Sahel / Sahara. In response to this local heating, the cold and moist southwesterly monsoon inflow encroaches farther northward. A source strength of roughly 1 K/day produces similar responses as a simulation with mid-Holocene orbital forcing imposed globally, while that of 1.5 K/day produces a precipitation response similar to that from paleoproxies.The precipitation increases non-linearly, with a jump at heating of around 1 K/day, even though the low-level monsoon inflow increases linearly. Competition at low-to-mid levels between drying by a shallow return flow just above the boundary layer and moistening by vertical advection within the layer affects convection and determines the northward extension of precipitation. When the heating becomes 1.5 K/day, the boundary-layer flow encroaches sufficiently northward to weaken the shallow return flow, further aiding precipitation. This novel nonlinear mechanism operates without biogeophysical feedbacks, and suggests that poor representation of the local thermodynamic processes may hamper a models ability to simulate dynamical feedbacks and hence the strength and poleward extension of monsoon rains under forcings like those during the mid-Holocene. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Climate American Meteorological Society

The role of nonlinear drying above the boundary layer in the mid-Holocene African monsoon

Loading next page...
 
/lp/ams/the-role-of-nonlinear-drying-above-the-boundary-layer-in-the-mid-Nz4g8nnRsQ
Publisher
American Meteorological Society
Copyright
Copyright © American Meteorological Society
ISSN
1520-0442
D.O.I.
10.1175/JCLI-D-17-0234.1
Publisher site
See Article on Publisher Site

Abstract

AbstractPaleo-climatic proxies indicate that significant summertime rainfall reached the Sahara region during the mid-Holocene, presumably in response to stronger summertime heating in the northern hemisphere. Climate models generally do not replicate the enhanced precipitation. As a step toward understanding the response and possible role of model errors, we conducted a series of idealized experiments with the Community Earth System Model in which local atmospheric heat sources of increasing magnitude are applied in the boundary layer over the Sahel / Sahara. In response to this local heating, the cold and moist southwesterly monsoon inflow encroaches farther northward. A source strength of roughly 1 K/day produces similar responses as a simulation with mid-Holocene orbital forcing imposed globally, while that of 1.5 K/day produces a precipitation response similar to that from paleoproxies.The precipitation increases non-linearly, with a jump at heating of around 1 K/day, even though the low-level monsoon inflow increases linearly. Competition at low-to-mid levels between drying by a shallow return flow just above the boundary layer and moistening by vertical advection within the layer affects convection and determines the northward extension of precipitation. When the heating becomes 1.5 K/day, the boundary-layer flow encroaches sufficiently northward to weaken the shallow return flow, further aiding precipitation. This novel nonlinear mechanism operates without biogeophysical feedbacks, and suggests that poor representation of the local thermodynamic processes may hamper a models ability to simulate dynamical feedbacks and hence the strength and poleward extension of monsoon rains under forcings like those during the mid-Holocene.

Journal

Journal of ClimateAmerican Meteorological Society

Published: Oct 13, 2017

References

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off