**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.

![](media/image1.png){width="2.990972222222222in"
height="3.870833333333333in"}

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