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Towards the Prediction of Decadal to Multi-Century Processes in a High-Throughput Climate System Model

Zhengyu Liu, J. Kutzbach, Univ. Wisconsin-Madison
R. Jacob, Argonne National Lab., C. Prentice, MPI-Jena, Germany

Summary

A fast climate system model, FOAM, is used to continue the study of: 1) the mechanisms of decadal climate variability, 2) the dynamics of global warming, and 3) the interaction of climate and the land biosphere. In studying mid-latitude decadal variability, we found that the response of the mid-latitude atmosphere to SST variability depends critically on ocean-atmosphere coupling. In the study of global warming patterns, we found that change of tropical-extratropical SST contrast is a more robust feature than the change of equatorial west/east SST contrast. Finally, by using the coupled climate-terrestrial biosphere model (FOAM-LPJ) to simulate climate/vegetation changes of the past century, we found a greening of the boreal forest that is consistent with observations.

Combining the work of climate scientists with the computational expertise required to modify climate model code to include partial coupling between atmosphere and ocean, a so-called modeling surgery scheme, we have studied the role of oceanic and atmospheric teleconnections in global change. We also linked climate scientists with ecosystem modelers in building a fully coupled ocean-atmosphere-land biosphere model, which enables us to study the interaction of land vegetation changes and climate changes for past, present, and future scenarios. Most of these initial studies are being planned to take advantage of the new NCAR CCSM3 in the next phase of CCPP.

1. Interdecadal Climate Variability

The coupled ocean-atmosphere model FOAM is used to investigate the mechanisms of interdecadal climate variability. Using various partially coupled configurations, we find that major interdecadal variability modes are caused by multiple mechanisms, both local and remote. Further analyses of FOAM, CSM, and PCM (SciDAC member, G. Meehl) show that tropical decadal variability seems to be associated with higher baroclinic modes in the equatorial region. A new theory is also developed to account for the decadal variability in the tropical ocean. Unifying the classical theories of equatorial waves and planetary waves (SciDAC member, P. Cessi), we show that the longest memory of the tropical ocean is in the planetary wave basin mode. The planetary wave basin mode can be excited resonantly by extratropical stochastic wind to generate spectral peaks at decadal time scales in the tropics.

We also examined the atmospheric response to mid-latitude winter SST anomalies, which is a long- standing issue in climate dynamics. Using ensemble experiments in the coupled FOAM model, we find that the atmospheric response depends critically on details of the ocean-atmosphere coupling. The fully coupled experiment produces a strong warm-ridge flow pattern that agrees with a statistical estimation of the atmospheric response. Further experiments show that the atmospheric response is associated with both the SST and the heat flux. The SST forcing favors a warm-ridge pattern, while the heat flux forcing tends to be associated with a warm-trough flow pattern. The correct atmospheric response is generated in the fully coupled model that produces the correct combination of SST and heat flux naturally. We are in the process of further examining this important and sensitive atmospheric response.

2. Global Warming Dynamics

Our recent studies of global warming dynamics focus on the change of tropical-extratropical SST contrast, which appears to be a more robust feature than the change of the equatorial west/east SST contrast in the Pacific. The SST trend in the last 50 years shows a warming on the equator, and less warming or even slight cooling in the extratropics. This feature has been found in transient (increasing) CO₂ experiments in FOAM, CCSM1, and the majority of models included in IPCC and CMIP. A preliminary analysis of the FOAM simulation suggests that the accentuated equatorial heating results primarily from the emergence of an atmospheric super greenhouse effect, and a greater oceanic Ekman heat convergence near the equator. We will be further exploring the dynamics of this seemingly robust global warming “fingerprint”.

3. Coupled climate-land vegetation system

Using FOAM-LPJ (LPJ denotes the land biosphere model), we have performed the first coupled atmosphere-ocean-land vegetation model study of changes associated with the CO₂ rise since the onset of the industrial era. The total CO₂ effect, simulated in experiment RP , is dominated by a surface warming trend (Fig.1f), a northward expansion of the boreal forest (Fig.1a) in the mid- and high-latitude Northern Hemisphere, and a mean global greening. This mid-high latitude trend is largely consistent with the trend of FPAR (in the last two decades of satellite observations, Fig.1d) and the tree-ring width growth trend (Fig.1e). To understand the CO₂ radiative forcing and physiological forcing separately, the radiation-only effect is examined in experiment R, by increasing the atmospheric CO₂ while maintaining a constant CO₂ level of 280 ppmv (for plant physiology) in LPJ; the physiology-only effect is assessed in experiment P, where the physiological CO₂ level in the LPJ model rises while the atmospheric CO₂ level is fixed at 280 ppmv. The expansion of boreal forest in RP is dominated by the CO₂ radiative effect (R in Fig.1c), because of its dominant effect on air temperature warming. Nevertheless, the physiological effect contributes to substantial increase of boreal forest in the Eurasian continent and western United States (Fig. 1b) and is the dominant forcing behind the mean global greening. Furthermore, the physiological effect of CO₂ on the vegetation also has substantial positive feedback on global warming over the continents during boreal winter (Fig.1g), because of albedo-feedback. This suggests that land vegetation might have exhibited substantial feedback on climate change since the pre-industrial era. This study will be extended to studies of future global warming.

Fig.1: Change in annual FPAR over the last 100 years from the total CO₂ effect in (a) RP, the physiological CO₂ effect in (b) P , and the radiative CO₂ effect in (c) R. The change in annual FPAR during 1982-1999 based on Pathfinder AVHRR data is shown in (d) with the regions that are anthropogenically altered by croplands and pastures masked out based on 1990 HYDE data. (e) shows the trend of tree-ring width (mm) for 1800 -1990 from 212 sites with elevations under 500 meters and within 45ºN-75ºN from the International Tree-Ring Data Bank (NOAA). (Red for increase and blue for decrease; the trend is statistically significant at 90% level for the large dots). Trends in DJF surface air temperature (ºC/century ) over last 100 years in (f) RP, show a strong warming at mid- and high latitudes, with significant contribution from the physiological feedback (g) over the continent. Shading in (f) and (g) represents trends exceeding the 95% significance level.(top to bottom, (a) to (g))

For further information on this subject contact:
Prof. Z. Liu,
Center for Climatic Research,
University of Wisconsin-Madison,
zliu3@wisc.edu

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