7. Concluding discussions

 

7.1 Review of results

The previous chapters have discussed the role of land surface processes in the variability of the East Asian summer monsoon. Observations indicate that a dominant intraseasonal mode of variability for this region is a land-sea precipitation dipole. It is currently thought that this dipole is responsible for periods of flood and drought over East Asia, and therefore important to study. A hypothesis is presented which relates the large scale general circulation to the regional land surface energy and hydrologic cycles, and the feedbacks among them.

We began with examination of the temporal behavior of East Asian precipitation and its relationship to the regional land surface energy and hydrology, using a coupled atmosphere-land surface model, with realistic vegetation control over land surface processes. A series of five-month summer monsoon simulations is performed using four different land-atmosphere model generated, May 1 initial land surface and atmospheric states. From these simulations, we find that, under the observed soil moisture conditions, the East Asian precipitation dipole is an intrinsic mode of variability in the East Asian summer monsoon system. The dipole behavior is chaotic, and seems to result from interaction between the convective heating generated by the monsoon and the regional atmospheric circulation. When the soil moisture is near saturation, the land surface hydrologic cycle does not play an active role in affecting the dipole oscillation because it is uncoupled from the atmosphere, by virtue of vegetation being able to transpire at the maximum possible rate, and because of significant evaporation from the wetted vegetation canopy itself.

Then, similar simulations were run, but with the diurnal cycle of solar radiation removed, since convective instability and precipitation are strongly diurnal before actual monsoon onset. In these non-diurnal simulations, we find that the regional monsoons are delayed (India) or weakened (East Asia) during the May 15-June 15 onset period. Subsequent to onset, monsoon precipitation differences in the non-diurnal ensemble means are insignificant.

Paradoxically, the mean moist static energy over the East Asian monsoon region is higher without diurnal forcing. This is because without the diurnal forcing, the mean moist static energy in the boundary layer can build up further, without being released into the free atmosphere through convection. Since convection is a 'binary' type (on/off) process, it is highly sensitive to variability in the moist static energy in the planetary boundary layer. As a result, the delay (India) or reduction (East Asia) in precipitation is attributable to the delay in reaching of the convective threshold resulting from removal of diurnal variability in convective instability in the planetary boundary layer.

Additionally, a pair of simulations was run with initial Eurasian (0°-180°E, 2°-74°N) soil moisture set to zero and to the wilting level of the model vegetation, respectively. The result was an increase in the land-sea thermal contrast necessary to drive the monsoon circulation over Asia, because of decreased cloudiness and reduced evapotranspiration. In spite of the increased land-sea thermal contrast, there is less precipitation over the northern part of the Asian monsoon region (northern India and China). The broad scale monsoon circulation, as measured by the vertical wind shear of lower to upper tropospheric wind (Webster and Yang, 1992), is also weaker. Examination of the land surface energy and hydrologic data over East Asian land indicate a preference for the hot, dry phase of the East Asian land-sea precipitation mode. These effects were greater in magnitude for the zero initial soil moisture case than for the wilted initial soil moisture case, and during the May to July period.

Preference for the hot, dry phase of the precipitation dipole indicates that the negative feedbacks on land surface temperature from increased upward sensible heat flux and long wave radiation are insufficient to fully compensate for the positive feedback from decreased upward latent heat flux over East Asia. Furthermore, the resulting increased land-sea thermal contrast, is not alone sufficient to restore normal monsoon rainfall. Because of the deficient East Asian soil moisture, moistening of the planetary boundary layer can only take place through advection of moisture from elsewhere. Until enough moisture accumulates in the planetary boundary layer from advective processes, convection over land will be inhibited. Under such conditions, the onset of monsoonal convection could be highly sensitive to processes which would either augment or inhibit the advection of moisture from other regions (e.g., remotely forced circulation anomalies).

In the case of initial Eurasian drought, the mechanism for reduction in East Asian monsoon precipitation is twofold. First, evaporation is reduced locally because of lack of East Asian soil moisture, so that the planetary boundary layer, from which convection originates, is drier. This results in a more convectively stable atmosphere over the East Asian monsoon region, and less diurnal convection. An additional effect of the initial Eurasian soil moisture deficit is the large scale heating of the Eurasian extratropical land surface. As a result, the July circulation develops one month early over the extratropics. Because of this premature July development, the regional circulation over East Asia also undergoes an abrupt transition from the pre-monsoon phase to its July, rather than June (Mei-Yu) configuration. This results in a reduction in the planetary boundary layer and lower tropospheric moisture convergence which would accompany the Mei-Yu phase of the East Asian monsoon advance. Thus, moist static energy in the planetary boundary layer is less, not only because there is less local soil moisture for evaporation, but because of reduced moisture convergence by the regional circulation as well.

Figure 7.1 shows a schematic at the usual time of monsoon onset when the soils are relatively moist in the East Asian monsoon region, all other conditions being equal. The effect of moist simulated initial regional soil moisture is as follows:

1. The initial soil moisture is sufficient to allow diurnal convection to begin over the East Asian land, through destabilization of the planetary boundary layer from local latent heat flux.

2. As a result of condensational heating from diurnal convection, the middle and upper troposphere become warmer, and upward motion results to compensate for the heating.

3. The atmosphere adjusts to the upward motion through enhancement of upper tropospheric divergence and lower tropospheric convergence. The lower tropospheric convergence brings in additional moisture from the adjacent oceans to fuel more convection.

4. Condensation heating continues to be enhanced as a result, as lower tropospheric moisture convergence continues to maintain unstable moist static energy profiles over the East Asian land mass.

5. The Mei-Yu phase of monsoon advance over East Asia further enhances the moisture convergence necessary for large scale convective precipitation over China.




Figure 7.1: A schematic of the East Asian monsoon region at usual time of monsoon onset, with the relatively moist land surface found in the control experiments. Broad arrows indicate the direction of the anomalous circulation. Colors within arrows indicate moisture content, with green being moist. The size of the broad arrows indicates the magnitude of the circulation. The number and length of the thin vertical arrows over land indicate magnitude of evaporation flux. The dashed lines indicate lines of constant pressure, and the heavy dashed line represents the top of the planetary boundary layer, from which convection is generated.

 


The situation for dry initial soils is shown in Figure 7.2. The planetary boundary layer moist static energy is reduced by lack of evaporation flux from the ground. Vertical heat transport from the surface to the middle and upper troposphere is therefore mainly of sensible rather than latent heat. The result is a relatively shallow monsoon circulation, even though the land surface is hotter, because the increased sensible heat flux at the land surface does not transport heat as well vertically as latent heat flux. Boundary layer moisture convergence is inhibited as a result of reduced convective heating in the middle and upper troposphere, and the resulting lack of compensating upward motion. The dryness continues until the boundary layer can be moistened by advective processes.

Advective moistening of the planetary boundary layer through moisture convergence is inhibited not only by the lack of local convection over the East Asian monsoon region, but also because of the early transition of the Eurasian extratropical circulation to its July configuration. This results in a reduction of moisture convergence over the East Asian monsoon region during June, a time when strong moisture convergence normally takes place, which reinforces the initial East Asian drought condition.

 

Figure 7.2: Same as Figure 7.1, but with the dry soil moistures found in the experiments initialized with drought. Increasingly yellow colors indicate increasing dryness relative to moist initial soils case.



From these results, we can conclude that East Asian summer monsoon onset and variability will be sensitive to processes which impact moisture in the planetary boundary layer over land. On the regional scale, such processes include land surface effects such as vegetation resistance to evaporation, sub-grid scale precipitation variability and its effects on the amount of wetted canopy, and the precipitation process itself. The land surface hydrologic and energy states can also have significant impact through communication and reinforcement of remotely forced general circulation effects. In this case, the remote forcing was provided by initial Eurasian drought.

 

7.2 Limitations in interpretation of results

In discussing the applicability of the results to the observed East Asian summer monsoon, some caveats need to be kept in mind. These caveats relate to model error, model resolution, and the spatial scale of initial drought. We noted in Chapters 3 and 6 that the character of the sudden northward shifts of zonal circulation features in the upper troposphere, seems to be reasonably depicted the East Asian summer monsoon circulation. These circulation transitions seem to be associated with rain belt transitions, consistent with the observations. However, the model circulation features advance further north than observed. The possible effect on the interpretation of results is through the reduction of baroclinic processes in the model over East Asia, relative to the observations. Such a reduction would lead to local land surface processes having more influence on East Asian summer monsoon variability than observed. It is possible that the variability in the modeled East Asian summer monsoon may have too much emphasis on the initial soil moisture compared to observations.

Another concern is that the simulated initial Eurasian drought was too broad in spatial scale to be realistic. This breadth of scale was used because of the relatively coarse model resolution. Recent observational studies have shown possible sensitivity of Indian monsoon rainfall, for example, to subcontinental scales of anomalous soil moisture over Russia (Matsuyama and Masuda, 1998). The regional variability of the East Asian summer monsoon has also been shown to be sensitive to snow cover anomalies over Eurasia (Yang and Xi, 1994), suggesting East Asian summer monsoon sensitivity to the resulting soil moisture state as well.



7.3 Further study and experiments

During the 1990s, a number of soil moisture data sets from Eurasia and elsewhere have become available from the former Soviet Union, Mongolia, China, and India (Robock et al., 1995, 1996). These data sets are now being checked for spatial and temporal consistency, so that they may be used for climate research. Preliminary research using the data set from the former Soviet Union indicates some relationship between Russian soil moisture and Indian monsoon rainfall (Matsuyama and Masuda, 1998). As these and other soil moisture data sets are quality checked and made consistent among each other, additional studies with observed, rather than model-derived, soil moisture data will help validate and/or modify the assumptions about the relationship between the Asian and regional monsoons and antecedent soil moisture conditions.

Additional ensemble experiments could also be performed to further clarify the results obtained here. With the existing model configuration, it is possible to run experiments where the continental extent soil moisture anomalies are dependent on latitude (e.g., limited to north of 50°, 30°-50°N, or south of 30°N). This would more accurately simulate soil moisture anomalies caused by meridional anomalies in storm tracks, which typify Eurasian winter and snow mass variability precipitation patterns.

Another issue is the relative contribution of the regional initial soil moisture states alone to the subsequent behavior of the East Asian summer monsoon. For example, if the initial East Asian soil moisture is dry, can the moisture convergence generated by a relatively normal seasonal cycle driven by normal soil moisture elsewhere, compensate for the lack of local latent heat flux? Vice versa, if the initial soil is moist in East Asia and dry elsewhere in Eurasia, would the early transition to the July circulation over East Asia still take place? To examine these questions requires finer spatial resolution in the GCM/ LSM, either by increasing the resolution of the global model, or by nesting a finer grid model of East Asia within the 4° x 5° climate model which was used in this study. Indian and East Asian monsoon simulations have been done recently where a finer grid was nested inside a coarse global grid, with promising results (e.g., Hirakuchi and Giorgi, 1995; Ji and Vernekar, 1997; Liu et al., 1994).