12. Januar 2011

Role of tropical Pacific SSTs in global medieval hydroclimate

Role of tropical Pacific SSTs in global medieval hydroclimate: A modeling study (Pdf-Format, 2,1 MB) ist eine interessante Studie von Burgman et al., aus dem Jahre 2010.

Ihre Arbeit behandelt folgende Fragestellung:
The current research seeks to test the influence of persistent La Nina‐like tropical Pacific SSTs [Sea surface temperature, W.v.B.] alone in forcing medieval hydroclimate using a state of the art climate model. A detailed examination of the North American hydroclimatic response is discussed by Seager et al. [2008][1], and here we focus on the global response. A high‐resolution paleo‐proxy coral‐based SST record from the central Pacific is used to reconstruct tropical Pacific SSTs for the period 1320–1462 A.D. The resulting SST reconstruction is used to force an ensemble of climate model simulations. Modeled estimates of medieval hydroclimate are compared to simulations forced by observed SSTs and the differences are verified using paleo‐proxy records of medieval hydroclimate from around the world.

Für uns von Interesse sind natürlich die langfristigen Proxydaten. Burgman et al. führen hier 30 verschiedene Proxies, aus unterschiedlichen Erdteilen an. Die meisten Proxies kannte ich; u.a. neu waren für mich die Arbeiten von Gupta et al. auf der "Arabian peninsula" (21)(Pdf-Format, 376 KB) aus dem Jahre 2003 und Bar-Mathews et al's. Arbeit in der "Soreq cave"/Israel (29), aus dem Jahre 1998[2]. Neueste Arbeiten zur Soreq cave werden auf Bar-Mathews homepage erwähnt.

Unter Fig. 1a sind die Ergebnisse der Modellierung angeführt:
Figure 1. (a) Long term mean differences of vertically integrated soil wetness (shaded, (mm3H2O)/(mm3soil)) and SST (contour, 0.10 C) between POGA‐1L [model simulation, W.v.B.] (1320–1462 A.D.) and GOGA simulation (1856–2005 A.D.) overlaid with records of medieval hydroclimate and SST. SST records are plotted as red (warm) or blue (cool) dots relative to modern observations. Green and brown dots indicate proxy records of wet or dry medieval hydroclimate. The numbers cross reference the records to the references in the text and Table S1 (see auxiliary material). The medieval period is taken to be, approximately, from 800 A.D. to 1500 (adapted from Seager et al. [2007]). (b) The long term mean difference in Precipitation minus Evaporation (P‐E is shaded, units are mm/month, significance levels of 95% stippled) and 200mb geopotential height (contours, units are meters) for the winter half year between the POGA‐1L simulation (1320–1462 A.D.) and the modern GOGA simulation (1886–1998A.D.). The difference in the zonal average P‐E is offset to the right.

Erklärend erfahren wir:
Figure 1a shows the difference in the long‐term mean soil moisture (shaded) and SST (contours) with the estimates from the paleo‐proxy records displayed as colored circles. The colored circles illustrate the SST and hydroclimate conditions of the MCA with respect to modern observations as described by available paleo‐proxy data from around the world (adapted from Seager et al. [2007])(Pdf-Format, 1,5 MB)[3]. Red (blue) dots represent warmer (cooler) SSTs while green (tan) dots represent wetter (dryer) conditions. The numbers by the dots correspond to those in Table S1 (see auxiliary material).*
The specifics of these records and their limitations are covered in some detail by Herweijer et al. [2007](Pdf-Format, 3,4 MB)[4] and Seager et al. [2007] and, for some, also by Graham et al. [2007](Pdf-Format, 1,9 MB)[5]. In order to compare modeled hydroclimate to the various paleo‐proxy records we analyze the  difference in model simulated vertically integrated soil wetness of the root zone (top 0.7 meters), which is significantly correlated with PDSI [Palmer Drought Severity Index, W.v.B.] in observations [Dai et al., 2004][6]. The differing natures of the various proxy reconstructions of hydroclimate, varying from tree rings to flood records, do not allow a quantitative comparison with the model simulation. Instead we are looking for a one sided agreement: conditions either wetter or drier than modern conditions consistently through the medieval period.

Fazit: Eine weitere, wichtige Arbeit in welcher Proxies zur Bestimmung der klimatischen Bedingungen im Mittelalter angeführt werden.

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[1] Seager, R., R. Burgman, Y. Kushnir, A. Clement, E. Cook, N. Naik, and J. Miller (2008), Tropical Pacific forcing of North American medieval megadroughts: Testing the concept with an atmosphere model forced by coral‐reconstructed SSTs, J. Clim., 21, 6175–6190, doi:10.1175/2008JCLI2170.1.
[2] Bar-Matthews, M., Ayalon, A., Kaufman, A., 1998. Middle to late Holocene (6,500 Yr. Period) paleoclimate in the eastern Mediterranean region from the stable isotopic composition of speleothems from Soreq Cave, Israel. In: Issar, A.S., Brown, N. (Eds.), Water, Environment and Society in Times of Climatic Change. Kluwer Academic Publishers, Dordrecht, pp. 203–214.
[3] Fußnoten 3, 4 und 5 werden mit Verweis auf das downloadbare Pdf-Format angeführt. Grund dafür ist, dass in diesen Arbeiten die verschiedensten Langzeitproxies enthalten sind. Seager, R., N. Graham, C. Herweijer, A. L. Gordon, Y. Kushnir, and E. Cook (2007), Blueprints for medieval hydroclimate, Quat. Sci. Rev., 26(19–21), 2322–2336, doi:10.1016/j.quascirev.2007.04.020.
*Auxiliary materials are available in the HTML. doi:10.1029/2009GL042239.
[4] Herweijer, C., R. Seager, E. R. Cook, and J. Emile‐Geay (2007), North American droughts of the last millennium from a gridded network of tree ring data, J. Clim., 20, 1353–1376, doi:10.1175/JCLI4042.1.
[5] Graham, N., et al. (2007), Tropical Pacific‐mid latitude teleconnections in medieval times, Clim. Change, 83, 241285, doi:10.1007/s10584-007-9239-2.
[6] Dai, A., K. E. Trenberth, and T. Qian (2004), A global dataset of Palmer drought severity index for 1870–2002: Relationship with soil moisture and effects of surface warming, J. Hydrometeorol., 5, 1117–1130, doi:10.1175/JHM-386.1.