Tier 2 Precipitation Concentration Index CMIP6

1 figure · 1990–2014 → 2040–2049

CMIP6 Multi-Model Mean Context

Comparison with CMIP6 conventional-resolution ensemble mean (up to 8 models under SSP3-7.0, regridded to 0.25°).

Contributing models: MPI-ESM1-2-LR, GISS-E2-1-G, IPSL-CM6A-LR, ACCESS-ESM1-5, EC-Earth3, CNRM-CM6-1, AWI-CM-1-1-MR, CNRM-ESM2-1, FGOALS-g3, INM-CM5-0, MRI-ESM2-0

Synthesis

DestinE models confirm a global trend toward intensified precipitation seasonality but exhibit distinct, high-magnitude deviations from CMIP6 in arid regions, with IFS-NEMO projecting significantly sharper increases in rainfall concentration over North Africa and the Middle East.
The projected change in the Precipitation Concentration Index (PCI) for the 2040–2049 period reveals a widespread intensification of precipitation seasonality, consistent with the thermodynamic strengthening of the hydrological cycle under SSP3-7.0. Both DestinE models (IFS-FESOM and IFS-NEMO) and the CMIP6 multi-model mean exhibit a dominant positive signal across land monsoon regions—particularly the Sahel and South Asia—and tropical oceans, indicating that annual rainfall is becoming increasingly condensed into fewer months. However, the DestinE simulations provide added value by resolving fine-scale kinematic structures absent in the coarser CMIP6 ensemble, such as distinct zonal banding in the tropical Pacific ITCZ and sharper gradients along extratropical storm tracks. Substantial divergence from the CMIP6 envelope occurs in hyper-arid regions, highlighting the sensitivity of high-resolution physics in moisture-limited environments. IFS-NEMO projects a drastic, spatially coherent increase in seasonality extending from North Africa to Central Asia, significantly exceeding the magnitude of the CMIP6 mean. Conversely, IFS-FESOM displays a complex dipole pattern over the Sahara and Arabian Peninsula, including areas of decreased concentration. These discrepancies suggest that while the broad thermodynamic response is robust, the specific temporal distribution of rare precipitation events in arid zones is highly model-dependent. Consequently, the DestinE models likely exhibit high spatial fractions of 'outside envelope' behavior (f_out) in these regions, driven by structural differences in convective parameterization and land-surface coupling rather than just internal variability.

Related diagnostics

global_monsoon_dynamics extreme_precipitation_indices aridity_and_drought_metrics

Precipitation Concentration Index Change

Precipitation Concentration Index Change
Variables avg_tprate
Models ifs-fesom, ifs-nemo, CMIP6-MMM
Units kg/m2/s
Baseline 1990-2014
Future 2040-2049
Method PCI = 12 × Σ(P_i²) / (ΣP_i)² from climatological monthly means. PCI=1 = uniform; PCI>1 = concentrated.

Summary medium

This figure illustrates the projected change in Precipitation Concentration Index (PCI) for the 2040–2049 period relative to 1990–2014, comparing two high-resolution DestinE models (IFS-FESOM, IFS-NEMO) against the CMIP6 multi-model mean. While all models indicate a general global trend towards increased precipitation seasonality (green), significant regional discrepancies exist, particularly over North Africa and the Arabian Peninsula where IFS-NEMO predicts a drastic increase in concentration unlike IFS-FESOM.

Key Findings

  • A widespread positive ΔPCI signal (green) across the tropics and subtropics indicates that precipitation is projected to become more concentrated in fewer months (intensified seasonality).
  • IFS-NEMO shows a much stronger and more spatially coherent increase in precipitation concentration over North Africa, the Middle East, and Central Asia compared to both IFS-FESOM and the CMIP6 mean.
  • IFS-FESOM exhibits a distinct pattern over the Sahara and Arabian Peninsula, showing areas of decreased concentration (brown) where IFS-NEMO shows strong increases.
  • Both high-resolution models resolve fine-scale filament structures in the ITCZ and storm tracks that are absent in the smoothed CMIP6-MMM.

Spatial Patterns

The dominant signal is an increase in PCI (green) over land monsoon regions (Sahel, South Asia) and the tropical oceans. IFS-NEMO displays a particularly intense band of increased concentration stretching from West Africa to Central Asia. In contrast, IFS-FESOM shows a complex dipole pattern in the Sahara/Arabian desert with both strong positive and negative changes. The tropical Pacific shows zonal banding in the high-resolution models, reflecting ITCZ shifts or narrowing.

Model Agreement

All models agree on the sign of change (positive/green) over the South Asian monsoon region, indicating intensified wet seasons. There is significant disagreement over North Africa and the Arabian Peninsula: IFS-NEMO agrees with the sign of CMIP6 (positive) but exceeds its magnitude, while IFS-FESOM diverges with negative anomalies in these arid regions. The CMIP6 mean is notably smoother and lacks the sharp gradients seen in the DestinE simulations.

Physical Interpretation

The broad increase in PCI is consistent with the thermodynamic intensification of the hydrological cycle, where wet seasons become wetter and dry seasons drier. In monsoon regions, this manifests as a sharper seasonal peak. The discrepancy in arid regions (Sahara/Arabia) between IFS-FESOM and IFS-NEMO likely stems from differences in how rare rainfall events are handled; in hyper-arid zones, PCI is sensitive to whether trace precipitation is spread out (decreasing PCI) or concentrated in rare convective bursts (increasing PCI).

Caveats

  • PCI calculations in hyper-arid regions (e.g., Sahara) can be numerically unstable, where small absolute changes in rainfall timing cause large index fluctuations.
  • The analysis is based on a relatively short 10-year future window (2040–2049), meaning internal decadal variability may influence the fine-scale patterns compared to the forced climate response.