Delayed Impact of North Atlantic Oscillation on Biosphere Productivity in Asia
Wang GL, You LZ
Geophysical Research Letters
31, L12210, doi: 10.1029/2004GL019766, 2004

Abstract
This study examines the relationship between the North Atlantic Oscillation (NAO) and vegetation productivity in Asia inferred from both crop yields data in China and satellite-derived Normalized Difference Vegetation Index data. Our finding suggests that vegetation productivity in northern Asia during the main growing season correlates significantly to NAO, with a surprising long delay of 1.5 years. Correlation at shorter time lags, which was the focus of previous studies, is weak and not significant between the NAO index and vegetation activities in Asia. This suggests the existence of a so-far unrecognized mechanism that carries the NAO signal for multiple years.  The lagged vegetation response also provides the potential for NAO to serve as a predictor for crop yields in China. 

A Conceptual Modeling Study on Biosphere-Atmosphere Interactions and its Implications for Physically Based Climate Modeling
Wang GL
Journal of Climate
17, 2572-2583, 2004

Abstract
This paper presents a conceptual modeling study on the behaviors of terrestrial biosphere-atmosphere system as they relate to multiple equilibrium states and climate variability, and emphasizes their implications for physically based climate modeling. The conceptual biosphere-atmosphere model consists of equilibrium responses of vegetation and precipitation to each other, dynamics of the vegetation system, and stochastic forcing of precipitation representing the impact of atmospheric internal variability. Using precipitation as the atmospheric variable in describing the biosphere-atmosphere interactions, this model pertains to regions where vegetation growth is limited by water. Low moisture convergence in the atmosphere combined with high sensitivity of the atmospheric climate to vegetation changes provide the most favorable condition for the existence of multiple equilibrium states. In a coupled biosphere-atmosphere system with multiple equilibria, experiments varying the stochastic forcing indicate that atmospheric internal variability is an important factor in the long term variability of the model climate and in its sensitivity to initial conditions. Specifically, the enhancement of low-frequency rainfall variability by vegetation dynamics is most pronounced with a moderate magnitude of atmospheric internal variability, and is less pronounced if internal variability is either too large or too small; detecting the existence of multiple equilibria by examining the sensitivity of the coupled model climate to initial conditions is not always reliable, since too large an internal variability reduces or even eliminates the model sensitivity to initial conditions. Findings from the conceptual model are confirmed using results from a physically based, synchronously coupled biosphere-atmosphere model. This study provides guidance in interpreting and understanding the model dependence of biosphere-atmosphere interaction studies using complex climate system models.

Decadal variability of rainfall in the Sahel: Results from the coupled GENESIS-IBIS atmosphere-biosphere model
Wang GL, Eltahir EAB, Foley JA, Pollard D, Levis S
Climate Dynamics
22, 625-637, 2004
                                                                      
Abstract

In this study we investigate the impact of large-scale oceanic forcing and local vegetation feedback on the variability of the Sahel rainfall using a global biosphere-atmosphere model, the coupled GENESIS-IBIS model, running at two different resolutions. The observed global sea surface temperature in the 20th century is used as the primary model forcing. Using this coupled global model, we experiment on treating vegetation as a static boundary condition and as a dynamic component of the earth climate system. When vegetation is dynamic, the R30-resolution model realistically reproduces the multi-decadal scale fluctuation of rainfall in the Sahel region; keeping vegetation static in the same model results in a rainfall regime characterized by fluctuations at much shorter time scales, indicating that vegetation dynamics acts as a mechanism for persistence of the regional climate. Even when vegetation dynamics is included, the R15 model fails to capture the main characteristics of the long-term rainfall variability due to the exaggerated atmospheric internal variability in the coarse resolution model.

Regardless how vegetation is treated, conditions in the last three decades of the twentieth century are always drier than normal in the Sahel, suggesting that global oceanic forcing during that period favors the occurrence of a drought. Vegetation dynamics is found to enhance the severity of this drought. However, with both the observed global SST forcing and feedback from dynamic vegetation in the model, the simulated drought is still not as persistent as what has been observed. This indicates that anthropogenic land cover changes, a mechanism missing in the model, may have contributed to the occurrence of the 20th century drought in the Sahel.
Climate Change, Climate Modes, and Climate Impacts
Wang GL, Schimel D
Annual Reviews for Environment and Resources
28, 1-28, November 2003

Abstract
Variability of the atmospheric and oceanic circulations in the earth system gives rise to an array of naturally occurring dynamical modes. Instead of being spatially independent or spatially uniform, climate variability in different parts of the globe is orchestrated by one or a combination of several climate modes, and global changes take place with a distinctive spatial pattern resembling that of the modes-related climate anomalies. Climate impact on the dynamics of terrestrial and marine biosphere also demonstrates clear signals for the mode effects. In this review, we view modes as an important attribute of climate variability, changes, and impact and emphasize the emerging concept that future climate changes may be manifest as changes in the leading modes of the climate system. The focus of this review is on three of the leading modes: the North Atlantic Oscillation, the El Ni˜no-Southern Oscillation, and the Pacific Decadal Oscillation.


Regime shifts in the Sahara and Sahel: Interactions between ecological and climatic systems in northern Africa
Foley JA, Coe MT, Scheffer M, Wang GL
ECOSYSTEMS
6 (6): 524-539 SEP 2003
 
Abstract
The Sahara and Sahel regions of northern Africa have complex environmental histories punctuated by sudden and dramatic "regime shifts" in climate and ecological conditions. Here we review the current understanding of the causes and consequences of two environmental regime shifts in the Sahara and Sahel. The first regime shift is the sudden transition from vegetated to desert conditions in the Sahara about 5500 years ago. Geologic data show that wet environmental conditions in this region-giving rise to extensive vegetation, lakes, and wet-lands-came to an abrupt end about 5500 years ago. Explanations for climatic changes in northern Africa during the Holocene have suggested that millennial-scale changes in the Earth's orbit could have caused the wet conditions that prevailed in the early Holocene and the dry conditions prevalent today. However, the orbital hypothesis, by itself, does not explain the sudden regime shift 5500 years ago. Several modeling studies have proposed that strong, nonlinear feedbacks between vegetation and the atmosphere could amplify the effects of orbital variations and create two alternative stable states (or "regimes") in the climate and ecosystems of the Sahara: a "green Sahara" and a "desert Sahara." A recent coupled atmosphere-ocean-land model confirmed that there was a sudden shift from the "green Sahara" to the "desert Sahara" regime approximately 5500 years ago. The second regime shift is the onset of a major 30-year drought over the Sahel around 1969. Several lines of evidence have suggested that the interactions between atmosphere and vegetation act to reinforce either a "wet Sahel" or a "dry Sahel" climatic regime, which may persist for decades at a time. Recent modeling studies have indicated that the shift from a "wet Sahel" to a "dry Sahel" regime was caused by strong feedbacks between the climate and vegetation cover and may have been triggered by slow changes in either land degradation or sea-surface temperatures. Taken together, we conclude that the existence of alternative stable states (or regimes) in the climate and ecosystems of the Sahara and Sahel may be the result of strong, nonlinear interactions between vegetation and the atmosphere. Although the shifts between these regimes occur rapidly, they are made possible by slow, subtle changes in underlying environmental conditions, including slow changes in incoming solar radiation, sea-surface temperatures, or the degree of land degradation.

How does the partitioning of evapotranspiration and runoff between different processes affect the variability and predictability of soil moisture and precipitation?
Dickinson RE, Wang GL, Zeng XB, Zeng QR
ADVANCES IN ATMOSPHERIC SCIENCES
20(3): 475-478 May 2003

Abstract
Water stored as part of the land surface is lost to evapotranspiration and runoff on different time scales, and the partitioning between these time scales is important for modeling soil water in a climate model. Different time scales are imposed on evapotranspiration primarily because it is derived from different reservoirs with different storage capacities, from the very rapid evaporation of canopy stores to the slow removal by transpiration of rooting zone soil moisture. Runoff likewise ranges in time scale from rapid surface terms to the slower base-flow. The longest time scale losses of water determine the slow variation of soil moisture and hence the longer time scale effects of soil moisture on precipitation. This paper shows with a simple analysis how shifting the partitioning of evapotranspiration between the different reservoirs affects the variability of soil moisture and precipitation. In particular, it is concluded that a shift to shorter time scale reservoirs shifts the variance of precipitation from that which is potentially predictable to unpredictable.

Reassessing the impact of North Atlantic Oscillation on the sub-Saharan vegetation productivity
Wang GL
GLOBAL CHANGE BIOLOGY
9 (4): 493-499 APR 2003
Abstract:
The Northern Atlantic Oscillation (NAO) has been shown to have a significant impact on the terrestrial ecosystem in the Sahelian region of Africa during the 1980s, and it has been strongly suggested that NAO may be a reliable predictor for the response of the Sahelian ecosystem to global climate variability. Using data from an extended period, we provide a reassessment for the impact of NAO on the Sahelian climate and ecosystem, and show that there is no consistent relationship between NAO and the ecosystem over Sahel. Statistical analysis on the NAO, vegetation, and precipitation data indicates that NAO influences the Sahelian vegetation productivity exclusively through its impact on precipitation. However, the relationship between the NAO index and Sahelian precipitation varies substantially with time. The correlation coefficient fluctuates between positive and negative values, and does not pass the 5% significance test during most of the twentieth century. The NAO system, although documented to govern the ecosystem dynamics over many other regions, does not have a consistent impact on the ecosystem over the Sahel. Therefore, the NAO index cannot produce a useful prediction on the ecosystem variability and changes in this region. This study provides an example that correlations based on short climate and ecological records (less than 20 years in this case) can be spurious and potentially misleading.

Role of the biosphere in the mid-Holocene climate of West Africa
Irizarry-Ortiz MM, Wang GL, Eltahir EAB
JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES
108 (D2): Art. No. 4042 JAN 16 2003

Abstract:
[1] In previous studies, a zonally symmetric, synchronously coupled biosphere-atmosphere model ( ZonalBAM), which includes explicit representation of ecosystem dynamics, has been developed and validated based on current conditions over the region of West Africa. Here, we use ZonalBAM to study the response of the coupled biosphere-atmosphere system to changes in the Earth's orbital forcing during the Middle Holocene (6K yrs BP) and the relative contribution of vegetation feedbacks. Simulations in which vegetation conditions were fixed to the current distribution, show that an orbitally induced increased seasonality in insolation for the Middle Holocene, by itself, results in a 1.1degrees northward shift in the location of the southern margin of the Sahara as compared to current solar forcings. When vegetation is allowed to be dynamic, a 2.4degrees northward shift is simulated. However, when dynamic vegetation is initialized to palaeovegetation, a 5.1degrees northward shift is simulated, bringing results more consistent with palaeoevidence. Based on previous studies on the role of the gradient of moist static energy on the dynamics of large-scale tropical circulations, a mechanism for the enhancement of the summer monsoon circulation has been developed. Our results suggest that multiple equilibria could have coexisted over the region of West Africa during the Middle Holocene. Furthermore, based on previous studies on the current climate over the region, we hypothesize that transitions between the different equilibria could have taken place during the Middle Holocene causing the southern desert margin to migrate between 18.1degreesN and 21.4degreesN and shaping climate variability.

Impact of CO2 concentration changes on the biosphere-atmosphere system of West Africa
Wang GL, Eltahir EAB
GLOBAL CHANGE BIOLOGY
8 (12): 1169-1182 DEC 2002

Abstract:
Vegetation dynamics plays a critical role in causing the decadal variability of precipitation over the Sahel region of West Africa. However, the potential impact of changes in CO2 concentration on vegetation dynamics and precipitation variability of this region has not been addressed by previous studies. In this paper, we explore the role of CO2 concentration in the regional climate system of West Africa using a zonally symmetric, synchronously coupled biosphere-atmosphere model. We first document the response of precipitation and vegetation to incremental changes of CO2 concentration; the impact of CO2 concentration on the variability of the regional biosphere-atmosphere system is then addressed using the second half of the twentieth century as an example. An increase of CO2 concentration causes the regional biosphere-atmosphere system to become wetter and greener, with the radiative effect of CO2 and improved plant-water relation dominant in the Sahelian grassland region and the direct enhancement of leaf carbon assimilation dominant in the tree-covered region to the south. Driven by the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1950-97 and with CO2 concentration prescribed at a pre-industrial level 300ppmv, the model simulates a persistent Sahel drought during the period of 1960s-1990s. The simulated drought takes place in the form of a transition of the coupled biosphere-atmosphere system from a wet/green regime in the 1950s to a dry/barren regime after the 1960s. This climate transition is triggered by SST forcing and materialized through vegetation-climate interactions. The same SST forcing does not produce such a persistent drought when a constant modem CO2 concentration of 350ppmv is specified, indicating that the biosphere-atmosphere system at higher CO2 level is more resilient to drought-inducing external forcings. This finding suggests that the regional climate in Sahel, which tends to alternate between dry and wet spells, may experience longer (or more frequent) wet episodes and shorter (or less frequent) dry episodes in the future than in the past. Our study has significant implications regarding the impact of climate change on regional socio-economic development.

Modeling the biosphere-atmosphere system: The impact of the subgrid variability in rainfall interception
Wang GL, Eltahir EAB
JOURNAL OF CLIMATE
13 (16): 2887-2899 AUG 15 2000

Abstract:
Subgrid variability in rainfall distribution has been widely recognized as an important factor to include in the representation of land surface hydrology within climate models. In this paper, using West Africa as a case study, the impact of the subgrid variability in rainfall interception on the modeling of the biosphere-atmosphere system is investigated. According to the authors' results, when neglecting the rainfall spatial variability, even if the impact on the total evapotranspiration is negligible, significant errors may result in the representation of surface hydrological processes and surface energy balance. These findings are consistent with the results of previous studies. However, in this paper, this issue is further explored and it is demonstrated that the extent of the resulting errors is not limited to the land surface processes. They extend to the atmosphere via the low-level cloud feedback to impact solar radiation, boundary layer energy, atmospheric circulation, and the distribution of precipitation. The same errors also propagate into the biosphere through vegetation dynamics and can eventually lead to a significantly different biosphere-atmosphere equilibrium state. This study provides a good example for the need to have physical realism in modeling the subgrid variability and most other details of the complex biosphere-atmosphere-ocean system.


Biosphere-atmosphere interactions over West Africa. I: Development and validation of a coupled dynamic model
Wang GL, Eltahir EAB
QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
126 (565): 1239-1260 Part B APR 2000

Abstract:
In this study we develop a zonally symmetric, synchronously coupled biosphere-atmosphere model including ecosystem dynamics, and apply this model to study biosphere-atmosphere interactions in the region of West Africa. The atmospheric model is zonally symmetric, and includes representation of atmospheric dynamics, a radiation scheme, a moist convection scheme, a boundary-layer scheme, and a cloud-parametrization scheme. The biospheric model is the Integrated BIosphere Simulator (IBIS), which includes representation of the water, energy, momentum, and carbon balance, vegetation phenology, and vegetation dynamics. We modified the representation of canopy hydrology in IBIS to account for the impact of rainfall subgrid variability. The biospheric model and atmospheric model are separately tested against observations. The synchronously coupled model is then used to simulate the biosphere-atmosphere: system of West Africa. A study on the role of biosphere-atmosphere interactions, including ecosystem dynamics, in the climate variability over West Africa using this model will be presented in a companion paper.


Biosphere-atmosphere interactions over West Africa. II: Multiple climate equilibria
Wang GL, Eltahir EAB
QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
126 (565): 1261-1280 Part B APR 2000

Abstract:
This paper presents both theoretical and numerical analyses on the multiple-equilibrium nature of the regional climate system in West Africa. Based on simple analyses on how the coupled biosphere-atmosphere system responds to vegetation perturbations within the scope of a dynamic ecosystem, we propose that the regional climate system may have multiple equilibrium states coexisting under the same precessional forcing. Using a synchronously coupled biosphere-atmosphere model which includes explicit representation of ecosystem dynamics, we show that the equilibrium state of the model is sensitive to initial vegetation distribution. This modelling result supports the existence of multiple climate equilibria. Using the same model, further experiments are carried out to investigate how the coupled system responds to non-permanent vegetation perturbations. Our results demonstrate how transitions between different climate equilibria can take place when governed by the two-way biosphere-atmosphere feedback. These findings advance our understanding regarding the mechanisms of climate variability over West Africa.


Role of vegetation dynamics in enhancing the low-frequency variability of the Sahel rainfall
Wang GL, Eltahir EAB
WATER RESOURCES RESEARCH
36 (4): 1013-1021 APR 2000

Abstract:
Rainfall observations in the Sahel region of West Africa show significant variability at the timescale of decades. Here we explore the mechanisms of this low-frequency variability using a coupled biosphere-atmosphere model which includes explicit representation of vegetation dynamics. By forcing the model with the observed sea surface temperature (SST) of the tropical Atlantic Ocean during the period 1898-1997, numerical experiments on the climate variability of West Africa have been carried out. The results of these experiments suggest that vegetation dynamics is a significant process in shaping the natural variability of the Sahel rainfall. The response of the regional climate system to large-scale forcings is significantly regulated by vegetation dynamics. The relatively slow response of vegetation to changes in the atmosphere acts to enhance the low-frequency rainfall variability. The regional climate system over West Africa has several climate regimes coexisting under the current precessional forcing. Climate transitions between different regimes act as another mechanism contributing to the low-frequency rainfall variability. Climate persistence at one regime and climate transition toward another collectively compose a distinct type of multidecadal variability.

Ecosystem dynamics and the Sahel drought
Wang GL, Eltahir EAB
GEOPHYSICAL RESEARCH LETTERS
27 (6): 795-798 MAR 15 2000

Abstract:
The Sahel region in West Africa has been experiencing a persistent drought throughout the last three decades. Here, we present a new perspective on the underlying physical mechanism behind this phenomenon. We use a coupled biosphere-atmosphere model including explicit representation of ecosystem dynamics to demonstrate that, regardless of the nature of the initial forcing, the natural response of the local grass ecosystem to the dry conditions of the late 1960s played a Critical role in maintaining the drought through the following decades. The onset of the drought has been marked by, a forced shift from a self-sustaining wet climate equilibrium to a similarly self-sustaining but dry climate equilibrium.


Use of ENSO information in medium- and long-range forecasting of the Nile floods
Wang GL, Eltahir EAB
JOURNAL OF CLIMATE
12 (6): 1726-1737 JUN 1999

Abstract:
The natural variability in the annual flow of the Nile is significantly regulated by the El Nino-Southern Oscillation (ENSO). In this paper, several sources of information are combined, including ENSO, rainfall over Ethiopia, and the recent history of river flow in the Nile, in order to obtain accurate forecasts of the Nile hood at Aswan. The Bayesian theorem is used in developing the discriminant forecasting algorithm. Conditional categoric probabilities are used to describe the flood forecasts, and a synoptic index is defined to measure the forecasts' skill. The results presented show that ENSO information is the only valuable predictor for the long-range forecasts (lead time longer than the hydrological response timescale, which is 2-3 months in this study). However, the incorporation of the rainfall and river flow information in addition to the ENSO information significantly improves the quality of the medium-range forecasts (lead time shorter than the hydrological response timescale).


Nilometers, El Nino, and climate variability
Eltahir EAB, Wang GL
GEOPHYSICAL RESEARCH LETTERS
26 (4): 489-492 FEB 15 1999

Abstract:
Nilometers have been used for gauging the level of water in the Mile river for more than five millennia. The written records describing some of these measurements represent the longest written records for any hydrological phenomenon. They describe interannual fluctuations in the Mile river flow which are closely associated with El Nino phenomenon. Here, we use information about long-term variability in El Nino occurrences that has been extracted from the Nilometers records to test the significance of the recent-trend in the frequency of El Nine years. We show that the observed frequency of EI Nino years during the last two decades is rather high compared to the long-term statistics that are computed from about a thousand years of Nilometers data; however similar levels of activity have been observed during the first millennium.