1. Introduction

 

The Asian monsoon has been of interest to researchers for centuries. The water that the monsoon brings is vital to life to all of south Asia. Variability in the monsoon can mean floods or famine to more than one quarter of the world's people. Despite much research over the past 50 years, there are still questions concerning how the different components of the land-atmosphere-ocean system contribute to its variability, both at the global and continental spatial scales. Additionally, there are relationships in the smaller scale response to the global scale Asian monsoon which have yet to be explored, as explained below.

The impact of the global scale circulation forcing on the continental scale depends largely on land surface processes, which mediate the continental response to the larger global scale, particularly in the tropics, subtropics, and summertime extratropics. Global scale forcing of the Asian monsoon is controlled by slowly varying boundary conditions, particularly sea surface temperatures (SSTs) (e.g., Ju and Slingo 1995, Lau and Yang 1996, Webster and Yang 1992), while relatively fast land surface processes involve the surface energy and hydrologic cycles, and their coupling through the surface latent heat flux (e.g., Webster 1983, Srinivasan et. al 1993). The land surface energy and hydrologic balances also impact the overlying atmosphere through modulation of static stability and thus large scale convection, which affects continental-scale dynamical and radiative forcing. Dynamical effects result from the hydrostatic adjustment of the atmosphere to the redistribution of latent heat though the depth of the troposphere, while the radiative effects involve the reflection of incoming solar radiation and absorption of outgoing long wave radiation by convective clouds and absorbing gases, particularly water vapor.

The motivation for this research comes from a series of four general circulation model (GCM) experiments, using prescribed and anomalous SSTs, in combination with prescribed and interactive land surface control over evaporation (Lau and Bua 1998, in press). Details can be found in the referenced paper. To summarize the results therein, we found that the simulated broad scale Asian monsoon anomalies, as measured by the vertical shear in the zonal wind between 850 hPa and 200 hPa over the Asian monsoon region from 40-110 and from 2-18N (the M1* index of Webster and Yang, 1992), were correlated with large scale general circulation anomalies over the tropical Pacific and the Asian continent. Anomalous anticyclonic circulation over Asia and anomalous cyclonic circulation over the equatorial central and eastern Pacific were associated with a strong monsoon, and vice versa for a weak monsoon. These results were consistent with previous research by other investigators. Land surface processes alone had little effect on variability in the large scale Asian monsoon, compared to SST anomaly (SSTA) forcing.

The effect of land surface processes on the East Asian summer monsoon, however, was significant, and dependent on the presence of SSTAs. Without SSTAs, precipitation during the simulated East Asian summer monsoon oscillated chaotically (with no specific period of variability) in a dipole pattern between land and ocean portions of the East Asian monsoon region. A similar precipitation dipole was also found in the observed East Asian summer monsoon. However, with SSTA forcing from 1980 to 1989, land surface processes resulted in a phase locking of the East Asian monsoon region land-sea precipitation dipole to the May/June part of the seasonal cycle, so that one phase was preferred in consecutive years. This result implies that interannual variability in large scale, slowly varying SSTAs can result in a consistent, year to year response from the relatively rapidly varying model land surface, through the anomalous general circulation. The mean monthly time series for simulated hydrologic and energy variables in the East Asian monsoon region confirmed such a relationship between the continental scale land surface and the large scale general circulation. A hypothesis linking remote anomalous boundary forcing to the East Asian monsoon region hydrologic and energy cycles, through anomalous atmospheric circulations, was then proposed. The thrust of the work presented here is on understanding land surface processes and their effect on the variability of the East Asian summer monsoon.

There are some difficulties encountered in modeling land surface processes. While the large-scale Asian monsoon climate and its variability have been fairly well simulated by climate modelers, capturing the essence of continental scale climates and their variability has been more difficult. It is on these continental scales that the economic and social impacts of monsoon anomalies have the most importance. Problems in forecasting climate at these spatial scales are usually attributed to inadequate representation of land surface and atmospheric physical processes (such as vegetation heterogeneity and cumulus convection, respectively) which take place at scales smaller than the models can resolve explicitly. Properly describing the initial land surface state, and simulating its subsequent evolution using realistic land surface models, especially for soil moisture, is also problematic. This is because we lack observational data of surface and particularly sub-surface soil moisture conditions over vast expanses of land. What data exist are of point measurements, which may not be representative of the larger scale soil moisture patterns. This results in a significant challenge to land surface and climate modelers in terms of predictive value for any GCM, since both the physical processes and the sub-grid scale variability must be captured by parameterization in terms of the large-scale variables.

Since this study involves climate system sensitivity to land surface processes, however, the lack of soil moisture observations and imprecision of sub-grid scale parameterizations of physical processes, become less important. Here we are not interested in actually predicting climate behavior based on prescribed boundary or initial conditions, but in understanding how different boundary and initial conditions affect the climate system. This only requires a realistic representation of the dynamics and physics involved, and a reasonable approximation of the climatology of the phenomenon being simulated. While climate prediction is not a primary goal here, the information gleaned from these GCM experiments will contribute to the understanding of the relationship of the global scale circulation to the continental scale coupled land-atmosphere response. Such information may be used to improve our ability to forecast changes in continental climates resulting from external forcing such as greenhouse warming and El Niño/Southern Oscillation (ENSO) fluctuations.

This dissertation examines the sensitivity of the global scale Asian monsoon, and continental scale East Asian summer monsoon, to land surface processes under climatological and anomalously dry soil moisture forcing over the Eurasian land mass. In particular, we examine the behavior of a phase locked land-sea precipitation mode over the East Asian monsoon region, which is found both in observational data sets (e.g., Kang et al., 1997) and a GCM simulation with land and atmospheric coupling, forced with observed SST anomalies. The land-sea East Asian summer monsoon precipitation mode is most prominent during May and June, the onset period of the East Asian summer monsoon. Results from ensemble experiments performed in this study indicate that there is a weakening of the global-scale Asian monsoon, and preference for the ocean phase of the East Asian summer monsoon precipitation mode, when initial Eurasian scale drought is used in ensembles of five month simulations of the Asian monsoon started on May 1. This weakening takes place in spite of the increased Eurasian scale land-sea temperature contrast resulting from the initial drought condition. The degree and duration of the weakening of the Asian monsoon and East Asian summer monsoon are dependent on the degree of drought simulated initially.

The Asian monsoon has been shown to be influenced by both SST variability and land surface processes. However, the main focus of this study will be on the land surface controls on large-scale Asian monsoon and particularly, continental East Asian summer monsoon variability, using a GCM coupled to a realistic land surface model (LSM), with a specific emphasis on initial atmospheric and soil moisture conditions and diurnal land surface variability. The observed behavior of the large-scale Asian monsoon and the regional-scale Asian monsoon will be presented in Chapter 2. A discussion of the models to be used, and some limitations in the interpretation of results, is found in Chapter 3. Effects of a climatological range of initial atmospheric and soil moisture conditions, and of the removal of land surface hydrologic variability on the East Asian summer monsoon at different temporal scales, will be described in Chapter 4. Experiments testing the sensitivity of the global scale Asian monsoon and the continental scale East Asian summer monsoon to the diurnal cycle of incoming solar radiation will be discussed in Chapter 5. Sensitivity of the simulated Asian monsoon and East Asian summer monsoon to experiments initiated with varying degrees of Eurasian drought will be examined in Chapter 6. Implications of the results, further research questions to be addressed, and conclusions based on the present work are presented in Chapter 7.