more on my research on monsoons

Rainfall seasonality and monsoons

Simulations from most of the state-of-the-art GCMs agree on a reduction in the rainfall in the subtropical arid and semi-arid regions and increased rainfall in the equato- rial and high-latitude regions by the end of the twenty-first century under medium-to-high emission scenarios. Apart from knowing how the mean annual rainfall (Fig. 1) amount will change in the future, a more complete description of the modifications of the hydroclimate also requires the knowledge of how precipitation seasonality will be affected by global warming, particularly for monsoonal regimes.

Fig.1. Climatology of annual mean precipitation (mm/day) for GPCC (1950-2010) and CMIP5 historical simulations. From Pascale et al. (2015).

New rainfall metrics have been recently introduced (Feng et al. 2013; Pascale et al. 2014), based on a probabilistic interpretation of rainfall fractions and the concept of relative entropy (RE). The RE (Fig.2) is used to quantify the diversity, for each given year, between the actual monthly fractions of precipitation and a reference, uniform monthly precipitation sequence. The RE provides a threshold-independent metric to quantify the concentration of annual rainfall and the number of dry months. Furthermore, there exists a direct linkage between RE and the effective number of wet/dry days, and thus for arid subtropical and semi-arid Mediterranean-type regions the RE can be used as an objective drought index. Regions with the largest RE (Fig. 2) are those placed in the arid and semi-arid subtropical subsidence belt (southern Africa, eastern Brazil, north Australia, western India, eastern Siberia, eastern Mediterranean sea, western Americas, Middle-East and central Asia) where the little yearly rainfall is concentrated in a few months.

Fig.2. As in Fig.2 but for the associated relative entropy (RE). From Pascale et al. (2015).

The dimensionless seasonality index (DSI) takes into account both RE and the mean annual rainfall (Feng et al. 2013). The DSI (Fig. 3) has its largest values over the global monsoon region. Large DSI over these areas are due to intermediate-to-high levels of annual rainfall and large relative entropy (i.e. precipitation concentrated in a few months). Therefore the DSI and is an effective index of monsoonality (Pascale et al. 2014).

Fig. 3. The Dimensionless Seasonality Index of rainfall (DSI). From Pascale et al. (2015).

The probabilistic interpretation of the monthly rainfall fractions also allows us to estimate circular statics moments associated describing the annual distribution of rainfall and, in particular, the center of mass (centroid) of the annual precipitation, which for seasonal rainfall regimes is a measure of the timing of the wet season (Fig. 4). The centroid is threshold-independent, integral measure of the timing of the wet season and it is related to time of the year around which most of the annual rainfall distributed. The centroid clearly shows the timing of the wet season in the monsoonal regions (e.g. July in Northern Africa or January in Northern Australia) and Mediterranean climates (e.g. Middle East). Areas of transition from monsoonal summer precipitation regimes to Mediterranean-type winter precipitation regimes (e.g. Indus valley and Southwest U.S.) are characterized by a sharp transition from summer to winter months.

Fig. 4. Rainfall centroid for observations and CMIP5 simulations. Values of the centroid re shown only for those area having RE>0.3. From Pascale et al. (2015).

Projected 21st century changes

In spite of model uncertainty and intermodel disagreement affecting the CMIP5 projections for precipitation, some coherent changes of rainfall seasonality in the Tropics can be identified. In areas featuring arid seasonal precipitation regimes such as tropical Southern Africa and Western Mexico and for wetter monsoonal regions like tropical Latin America (Fig. 2-3) most of the examined models indicate a significant increase of RE by the end of the twenty-first century (Fig. 5). According to models projections, these regions will also experience either a slight reduction or no significant changes in their annual rainfall amounts. The implications of such projected changes in RE may be that drier and more prolonged dry seasons will be more likely in the South African, South American and North American monsoon regions, with increased probability of drought as well as flood. This result agree fairly well with analysis of rainfall “monsoonality” projected changes for less extreme scenarios.

Fig. 5. RE changes between 2070-2100 and 1970-2000 and time series (10-year running means) of the area-averaged RE anomalies relative to the period 1930-2000.

Regarding the timing of the precipitation, we find that a delay signal is particularly consistent across models over the monsoonal regions of North and South Africa with 22 out of 24 of the examined models predicting a delay and less consistent over South and North America monsoon regions, where only 17 out of 24 models agree on a forward shift. No coherent centroid delay is projected over North Australia and Asia, where models show very contradictory responses, thus making projections hard to be interpreted over these regions.

Fig. 6. Delay (in days) of the rainfall centroid (2070-2100 and 1970-2000, RCP8.5 scenario). Time series (10- year running means) show the area-averaged centroid delays for the NAM, SAM, NAF, SAF, SAS, AUS monsoonal regions. and AUS regions. In the time- series, the purple line

The physical mechanisms behind these responses, and why they seem to occur more strongly in certain monsoonal regions than others, is not yet fully understood. After this assessment, future research has therefore to focus on the basic dynamical mechanism regulating the monsoon.

References

• Pascale S., V. Lucarini, X. Feng, A. Porporato, S. Hasson (2014), Analysis of rainfall seasonality from observations and climate models, Climate Dynamics, doi: 10.1007/s00382-014-2278-2
• Pascale S., V. Lucarini, X. Feng, A. Porporato, S. Hasson (2015), Projected changes of rainfall seasonality and dry spells in a high greenhouse gas emissions scenario, Climate Dynamics, doi: 10.1007/s00382-015-2648-4