Paleoceanography and Paleoclimatology [PP]

PP52A   BCC:325   Friday 

Controls of the Intertropical Convergence Zone Position in Past Climates: Observations and Models III

Presiding: A J Broccoli, Rutgers University; J Chiang, University of California

PP52A-01  

Some Perspectives on the Reconstruction of Past ITCZ Variations from Paleoclimate Data

* Broccoli, A J (broccoli@envsci.rutgers.edu) , Rutgers University, Dept. of Environ. Sciences 14 College Farm Road, New Brunswick, NJ 08901-8551 United States
Yoshimori, M (masa@cep.rutgers.edu) , Rutgers University, Dept. of Environ. Sciences 14 College Farm Road, New Brunswick, NJ 08901-8551 United States

The intertropical convergence zone (ITCZ) is an important feature of the tropical atmospheric circulation and climate. Strictly speaking, the ITCZ is defined as the axis along which the northeast and southeast trade winds converge. But the location of the ITCZ can also be identified from other climatic quantities, as the surface wind convergence induces rising motion in the atmosphere, deep convective clouds, and abundant precipitation. Of the climatic quantities that are indicative of the ITCZ, precipitation is the one that is most commonly reconstructed from paleoclimatic evidence. Other climatic mechanisms also influence precipitation, however, complicating the interpretation of precipitation changes at a given point. We will discuss some of the issues involved in reconstructing past ITCZ variations from paleoclimatic proxies, including other mechanisms of climate response that can influence tropical precipitation. We will also suggest possible strategies for better constraining past variations of the ITCZ from paleoclimatic proxies.

PP52A-02  

The Northward Advance of ITCZ During the Last Deglaciation in the Tropical Western Pacific

* Wei, K (weiky@ntu.edu.tw) , Dept. of Geosciences, National Taiwan University, PO Box 13-318, Taipei, Taiwan, 106 Taiwan
Chen, C (chihwe@ms7.hinet.net) , Dept. of Geosciences, National Taiwan University, PO Box 13-318, Taipei, Taiwan, 106 Taiwan
Mii, H (t44006@cc.ntnu.edu.tw) , Dept. of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan, 106 Taiwan
Chen, M (mtchen@mail.ntou.edu.tw) , Inst. of Applied Geosciences, National Taiwan Ocean University, Keelung, 20224 Taiwan
Shiau, L () , Inst. of Applied Geosciences, National Taiwan Ocean University, Keelung, 20224 Taiwan
Huang, C (hunagcy@mail.ncku.edu.tw) , Dept. of Earth Sciences, National Cheng Kung University, Tainan, Taiwan
Lee, M (monyoung@tmue.edu.tw) , Dept. of Science Education of Education, Taipei Municipal University, Taipei, 100 Taiwan
Zhao, M (maxzhao04@yahoo.com) , School of Ocean and Earth Sciences, Tongji University, Shanghai, China

Marine planktic oxygen isotope stratigraphy and paleo-temperatures (U$^{37}$$_{K}$-SST) of sea surface of the past 25 thousand years were established for four IMAGES piston cores in the South China Sea (MD972142, MD972151), Celebes Sea (MD12388) and Banda Sea (MD12380), respectively. By subtracting the temperature and ice-volume effects from the foraminiferal $\delta$$^{18}$O values, we were able to reconstruct the corrected sea-water $\delta$$^{18}$O values ($\delta$$^{18}$O$_{w-ice}$) for the four sites. The comparison of the time-series of the four sites shows a northward trend of progressive depletion of $\delta$$^{18}$O$_{w-ice}$ values during the last deglaciation. This diachronous depletion trend in $\delta$$^{18}$O$_{w-ice}$ might have resulted from a northward advance of the Inter-Tropical Convergence Zone (ITCZ), which brought in heavy precipitation and therefore contributed meteoritic waters with much depleted oxygen isotopes (i.e., by amount effect). We interpret that the ITCZ advanced progressively from the south to the north (\&6$\deg$S - \&7$\deg$N -- \&9$\deg$N - \&12$\deg$N) during the last deglaciation (15.6 ka -- 14.8 ka -- 14.2 ka -- 13.7 ka).

PP52A-03  

On the Control of Zonal-mean Position of the ITCZ in Quasi-equilibrium GCM Experiments

* Yoshimori, M (masa@cep.rutgers.edu) , Department of Environmental Sciences/Center for Environmental Prediction, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901 United States
Broccoli, A J (broccoli@envsci.rutgers.edu) , Department of Environmental Sciences/Center for Environmental Prediction, Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901 United States

Several proxy records and modeling studies suggest that changes in the mean latitude of the ITCZ are associated with variations in temperature contrast between the hemispheres. A new GFDL atmospheric GCM coupled with a mixed layer ocean model (SM2.1z) is used to study the equilibrium response of the ITCZ and associated tropical convective system to variations in interhemispheric temperature contrast induced by a variety of radiative forcing agents. The forcing agents investigated include changes in solar irradiance and changes in atmospheric concentrations of carbon dioxide, tropospheric ozone, black carbon, organic carbon, anthropogenic sulfate aerosols, and volcanic aerosols, each applied separately. These forcings yield interhemispheric temperature contrast ranging from about -1.3 to 1.0 K. It is found that the mean latitude of the ITCZ is more tightly linked to the interhemispheric contrast of the imposed radiative forcing than the surface air temperature contrast. When the same amount of radiative forcing is imposed on each hemisphere, equilibrium is achieved without changes in net heat transport between the hemispheres. On the other hand, when the radiative forcing is stronger in one hemisphere, an anomalous heat exchange between the hemispheres results. This heat exchange is accomplished by an anomalous cross-equatorial Hadley circulation, with the ascending branch in the hemisphere with a more positive (less negative) radiative forcing and the descending branch in the hemisphere with a less positive (more negative) radiative forcing. As the vertical motion in the Hadley cell is accompanied by the low-level mass convergence/divergence, the mean latitude of the ITCZ also moves toward the hemisphere with the more positive (less negative) radiative forcing. Because the hemisphere with the more positive radiative forcing undergoes greater warming, the ITCZ usually, but not always, appears to move toward the warmer hemisphere.

PP52A-04  

The Response of Tropical Australia to Heinrich Events: Evidence in a Terrestrial Paleorecord from Lynch's Crater

* Muller, J (joanne.muller@jcu.edu.au) , School of Earth Sciences, James Cook University, Townsville, QLD 4810 Australia
Kylander, M (malin.kylander@imperial.ac.uk) , Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ United Kingdom
Wust, R (raphael.wust@jcu.edu.au) , School of Earth Sciences, James Cook University, Townsville, QLD 4810 Australia
Weiss, D (dominik.weiss@imperial.ac.uk) , Department of Earth Science and Engineering, Imperial College London, London, SW7 2AZ United Kingdom
Martinez-Cortizas, A (edantxon@usc.es) , Department Edafologia y Quimica Agricola, Fac. de Biologia, Campus Sur, Santiago, Que E- 15782 Spain

A rare terrestrial paleorecord from Lynch's Crater in NE Queensland (Australia) displays evidence of Heinrich cyclicity and the Younger Dryas in the southern hemisphere. The 55 kyr BP peat record may support previous theories of southward migration of the Intertropical Convergence Zone (ITCZ) during Heinrich events and offers further insight into the intensity and extent of these migrations during the late Pleistocene. This is the first terrestrial record of its kind providing an ideal opportunity to study the response of the subtropics and more specifically Australia, to abrupt climate change. Scanning electron microscope identifies abundant growth of sponge spicules and some diatom frustules in high inorganic layers through out the Lynch's Crater peat deposit. Biogenic silica analyses by UV/VIS Spectrometer support the theory of intense growth of sponges and diatoms within these layers. In Lynch's Crater it is proposed that a significantly increased precipitation regime would be required to facilitate such sponge and algae growth. These layers of high Si/Al ratios coincide with the timing of Heinrich events and the Younger Dryas. It has been found that Lynch's Crater is in better agreement with the Younger Dryas and Heinrich events, as recorded in the delta O18 GISP record, than other subtropical records displaying similar wet events. Lynch's Crater gives rise to the first terrestrial record in the southern hemisphere to display these events and here it is possible to observe climate change as it occurs on land, due to the relatively quick response of biogenic silica organisms to wetting. The Lynch's Crater record may provide the missing key in the southern migration of the ITCZ, previously suggested by tropical records from South America and Africa and allows cross latitudinal correlations that further advance our knowledge of this ITCZ response. Identifying the mechanisms that allow changes to be abrupt is essential for understanding the fundamental behaviour of the climate and its sensitivity to forcing.

PP52A-05  

Controls on the ITCZ Position in an Idealized Moist GCM

Kang, S (skang@princeton.edu) , Program in Atmospheric and Oceanic Sciences Princeton University, 201 Forrestal Rd., Princeton, NJ 08540 United States
* Frierson, D M (frierson@geosci.uchicago.edu) , UCAR/University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States
Held, I M (Isaac.Held@noaa.gov) , Geophysical Fluid Dynamics Laboratory, 201 Forrestal Rd., Princeton, NJ 08540 United States
Pierrehumbert, R T (rtp1@geosci.uchicago.edu) , Department of Geophysical Sciences University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States

Many recent paleoclimatic studies, both observational and modeling, have pointed to the influence of high latitudes on the tropical precipitation distribution. In this study, we use an idealized moist general circulation model to develop a better understanding of what determines the position of the ITCZ in a broad sense. The moist GCM consists of the primitive equations on the sphere and various idealized physical parameterizations including gray radiative transfer, so water vapor and other tracers have no effect on radiative heating rates, a zonally symmetric aquaplanet mixed layer ocean, and a simplified Betts-Miller convection scheme. We use the model to study the effect of changes in imposed ocean heat fluxes, both tropical and extratropical, on several important latitudes within the tropics including the ITCZ, the boundary between the Hadley cells, and the thermal equator. We compare with theoretical expectations based on simple Hadley circulation theories. Consideration of extreme limits, such as perfect compensation, provides an additional theoretical tool for understanding the movement of these latitudes. In the high latitude forcing cases, the influence of the extratropics is communicated to the Hadley cell through eddy heat and momentum fluxes.

PP52A-06  

An Exploration of Mechanisms for Mediating the Influence of Extratropical Glaciation on the Tropical Climate

* Pierrehumbert, R T (rtp1@geosci.uchicago.edu) , Dept. of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637 United States
Frierson, D M (frierson@geosci.uchicago.edu) , Dept. of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637 United States

To obtain a better understanding of the basic mechanisms by which the atmosphere transmits extratropical influences into the tropics, we have analyzed a series of general circulation model experiments carried out with idealized continental boundary conditions. These experiments were carried out with the FOAM1.5 model, which is in essence a portable Beowulf-oriented reimplementation of CCM3. In accord with our focus on the atmosphere in this work, the atmospheric model is coupled to a mixed-layer ocean with lateral ocean heat flux set to zero. The continental geometry consists of a pair of zonally symmetric continents, one centered on each pole. The Southern Hemisphere continent extends to 65S, and is kept glaciated in all experiments. The Northern Hemisphere continent extends to 42N, and is glaciated in the NHCOLD experiment but bare land in the NHWARM experiment. Sea ice feedback was suppressed in these simulations, but given the geometry of the Northern Hemisphere continent, the NHCOLD case can be taken as representing the combined forcing due to land glaciation and equatorward advance of sea ice. These experiments allow us to examine, in a very clean way, the response of the tropics to a very large extratropical cooling imposed at the surface, in a model which is energetically closed. Comparison of the two simulations has yielded the following results. The principal means by which the midlatitude glaciation affects the tropics is via a marked increase in poleward NH wintertime sensible heat flux, which is uncompensated by reduction in latent heat flux. The coupling of the storm tracks to the tropics is weak, however, and causes only a moderate cooling in the Northern subtropics and hardly any south of the Equator. The dynamics behind this barrier effect are discussed. The increased sensible heat flux,however, causes a considerable strengthening of the Hadley circulation; this strengthening allows the ITCZ precipitation to remain approximately unchanged between the NHWARM and NHCOLD cases, despite the substantial reduction in atmospheric water vapor in the cold case. The extremely strong midlatitude cooling produces a modest southward shift in the January ITCZ, and none at all in the July ITCZ, indicating that basic Hadley dynamics can make the ITCZ very resistant to moving; we find that the ITCZ position closely follows the tropical temperature maximum. The ITCZ shifts are discussed in terms of theoretical concepts applying to the Hadley circulation. Using an energy balance model (EBM) based on diffusion of moist static energy, Frierson and Held have shown that there is a compensation between changes in latent and sensible heat transport as climate warms, provided the meridional distribution of absorbed solar radiation remains fixed. We have extended this analysis to the case in which the solar forcing gradient is allowed to change, as is the case in our simulations owing to the change in surface albedo between the two simulations. In this case, the EBM does not require strict compensation, and in fact correctly reproduces the fact that tropical heat export increases in the NHCOLD case. However, the EBM over-estimates the penetration of the cooling past the Equator, owing to inadequacies in the diffusive treatment of the Hadley circulation. The EBM also misprepresents the magnitude of midlatitude heat flux changes, owing to the bottom-trapped nature of extratropical cooling seen in the GCM experiments, which is not reflected in the assumptions about the vertical profile of temperature built into the EBM. The implications of incorporating this effect will be discussed.

PP52A-07  

Correlation Between the Equatorial and the High-Latitude Climatic Variations and its Implications

* Tiwari, M (tmanish@prl.res.in) , Planetary & Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad, 380009 India
Ramesh, R (r.ramesh@prl.res.in) , Planetary & Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad, 380009 India
Somayajulu, B (soma@prl.res.in) , Planetary & Geosciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad, 380009 India
Jull, A (jull@u.arizona.edu) , NSF Arizona AMS Facility, The University of Arizona, Tucson, AZ 85721 United States
Burr, G (burr@u.arizona.edu) , NSF Arizona AMS Facility, The University of Arizona, Tucson, AZ 85721 United States

The abrupt climate changes recorded in polar ice sheets have elicited various explanations relating to either thermohaline circulation changes by ice-rafting or natural greenhouse gas concentrations modulated by tropics. If these abrupt changes are recorded in equatorial/tropical records as well, it indicates that there is a rapid correspondence between the tropical and high-latitude climate that is possibly achieved via natural greenhouse gas concentrations in atmosphere. The earlier work done in this direction used sediment cores from highly productive ocean regions (e.g., western Arabian Sea) so as to obtain high sedimentation rate, which is comparable with polar ice-core records having high time resolution. Necessarily such regions give the {\it wind} records, as in the tropics high productivity is associated with upwelling driven by winds. Our study is the first attempt in correlating equatorial {\it precipitation} record with the polar record. Moreover, various modelling and observational studies have demonstrated that Intertropical Convergence Zone (ITCZ), which controls the south Asian monsoonal precipitation, shifted southward during periods of global cooling. Here we present $\delta$$^{18}$O data (a proxy for precipitation) of three planktonic species of Foraminifera {\it viz.} {\it Globigerinoides ruber}, {\it Globigerinoides sacculifer} and {\it Globorotalia menardii} from a sediment core namely SS3827G (spanning the past 35 kyr) from the equatorial Indian Ocean ($3.7\deg$N, $75.9\deg$E, water depth: 3118 m), which falls under the southwest monsoon (SWM) realm. Results show that minimum SWM precipitation occurred at Last Glacial Maximum with a subsequent increase at Termination IA. During the Holocene, SWM precipitation intensified uniformly upto the core top ($\sim$2.2 ka BP) as revealed by the decreasing trend exhibited by the $\delta$$^{18}$O values. Variations in {\it precipitation} are consistent with climate changes recorded in polar ice sheets: abrupt cooling/warming events appear to be accompanied by sudden reduction/enhancement in (SWM) rainfall. Thus, mechanisms with time scales much less than a millennium, such as natural greenhouse warming (e.g., CH$_{4}$ concentration), controlled by emissions from the tropics, could have played a major role in high-latitude climate change. This study further reinforces the southward shift of the ITCZ (hence reduced monsoonal precipitation) during periods of Northern Hemisphere cooling.