SEMINAR: REMINDER: SEMINAR Today: GEOTOPICS: Pedro Dinezio(3:15 pm)

[Qiong Zhang <qzhang@rsmas.miami.edu>]

##### G  E  O  T  O  P  I  C  S   #####

 P  r  e  s  e  n  t  s  

3:15 PM, Monday,September 27th, 2010
SLAB Seminar Room, S/A 103

Refreshments 3:00 PM

Pedro Dinezio

RSMAS-MPO

University of Miami

pdinezio@rsmas.miami.edu

    "

Response of the Walker Circulation to LGM Forcing: Implications for Detection in Proxies"

The response of the Walker circulation to Last Glacial Maximum (LGM) climate forcing is analyzed using an ensemble of 6 coordinated coupled climate models. Unlike global warming experiments, in which all six models simulate a robust weakening of the Walker circulation, the models do not agree in the response of the Walker circulation to LGM forcing. Two opposing mechanisms operating over the ascending branch of the Walker circulation explain the inter-model differences in the LGM experiments: 1) The Walker circulation strengthens due to a constraint imposed by changes in the hydrological cycle, consistent and opposite to the mechanism proposed to explain the weakening in response to global warming; 2) The ascending branch of the Walker circulation is weakened by a reduction in convection over areas of the Maritime Continent that are exposed in the LGM experiments due to lowered sea level, including the Gulf of Thailand, the South China Sea, and the Java Sea. The models that simulate the largest cooling over exposed land areas simulate the largest anomalous subsidence over the Maritime continent preventing the Walker circulation from strengthening. This suggests that inter-model differences in land vs. ocean cooling could be the source of lack of agreement in the simulation of the LGM tropical Pacific.

Even those models that simulate a stronger Walker circulation do not simulate a clear La Nina-like ocean cooling opposite to the robust El Niño-like warming simulated in the global warming experiments. Instead, the changes in the Walker circulation have a robust and unambiguous signature on the tilt of the equatorial thermocline as expected from momentum balance between the changes zonal surface wind stress and the zonal pressure gradient due to thermocline tilt. The models also indicate that due to competing dynamical and thermodynamical effects the changes in the thermocline do not agree with the changes in the depth of the 18C isotherm, or any other isotherm that lies the in thermocline in the control experiment. The changes in the Walker circulation also have a clear signature on the precipitation changes, but these changes fail to translate into an unambiguous signature in surface salinity. These results indicate that proxies of thermocline depth can only detect real LGM climate changes in the Walker circulation and are key to improve our understanding of the sensitivity of the Walker circulation to climate change.

Qiong Zhang

Marine Geology and Geophysics
Rosenstiel School of Marine and Atmospheric Science
University of Miami
4600 Rickenbacker Causeway
Miami Fl 33149