Tier 1 Bowen Ratio

2 figures · 1990–2014 → 2040–2049

Synthesis

Mid-century projections indicate a robust intensification of hydrological contrasts, characterized by sensible-heat-dominated drying in the subtropics and latent-heat-driven wetting in the Sahel and high-latitude cryosphere.
The analysis of surface energy partitioning via Bowen ratio and Evaporative Fraction (EF) in IFS-FESOM and IFS-NEMO simulations reveals a consistent, thermodynamically driven reorganization of hydrological regimes by the mid-21st century (SSP3-7.0). A distinct 'wet-get-wetter, dry-get-drier' terrestrial pattern emerges: moisture-limited subtropical regions, including the Mediterranean, Southern Africa, the Western United States, and the Amazon, exhibit increased Bowen ratios and negative EF anomalies (<-0.1), signaling a shift toward sensible heat dominance driven by soil moisture depletion. Conversely, energy-limited high-latitude landmasses (Siberia, Northern Canada) display increased EF, consistent with enhanced evapotranspiration rates facilitated by warming and permafrost thaw. A prominent dipole characterizes the North African response, where a strong decrease in the Bowen ratio across the Sahel contrasts with drying along the northern Saharan margins, indicative of a northward shift and intensification of the West African Monsoon. Over the oceans, the retreat of sea ice in the Arctic and Southern Oceans drives a drastic increase in EF (>0.15) and reduced Bowen ratios, as the loss of the insulating ice layer exposes open water to significant latent heat fluxes. The high spatial correlation between IFS-FESOM and IFS-NEMO suggests these surface flux responses are largely governed by the shared IFS atmospheric component and land surface scheme (HTESSEL), with ocean model formulation (unstructured FESOM vs. structured NEMO) playing a secondary role in modulating regional moisture advection.

Related diagnostics

soil_moisture_content precipitation_mean_state sea_ice_concentration

Bowen Ratio Change

Bowen Ratio Change
Variables avg_ishf, avg_slhtf
Models ifs-fesom, ifs-nemo
Units W/m2
Baseline 1990-2014
Future 2040-2049
Method B = SHF/LHF. ΔB = B_future − B_historical.

Summary high

This figure displays the projected change in the Bowen ratio (sensible/latent heat flux) for the mid-21st century (2040–2049, SSP3-7.0) relative to the historical baseline (1990–2014) in the IFS-FESOM and IFS-NEMO coupled models. The maps highlight shifts in surface energy partitioning, serving as a proxy for changes in hydrological regimes and aridity.

Key Findings

  • A prominent dipole is visible in North Africa: a strong decrease in Bowen ratio (blue) across the Sahel indicates wetting ('Sahel greening' and northward ITCZ shift), while the Mediterranean and northern Saharan margins show a strong increase (red), signaling drying.
  • Southern Africa and the Western United States exhibit a marked increase in Bowen ratio, consistent with regional aridification trends and reduced soil moisture availability.
  • Complex, high-magnitude changes occur over the cryosphere (Greenland and Antarctica), where the ratio is sensitive to small changes in sublimation and surface melt regimes.

Spatial Patterns

The most coherent patterns are zonal bands in the tropics/subtropics. The Sahel blue band contrasts sharply with the red drying signals in the subtropics (Mediterranean, Southern Africa, parts of Australia). High-elevation regions like the Himalayas and Andes show distinct decreases in Bowen ratio, likely due to increased snowmelt or precipitation. The Amazon basin shows a moderate increase (drying), more pronounced in the IFS-NEMO simulation.

Model Agreement

There is very high spatial correlation between IFS-FESOM and IFS-NEMO. This is expected as both use the same atmospheric IFS model and land surface scheme (HTESSEL). Minor discrepancies in magnitude (e.g., over the Amazon and interior Australia) likely stem from differences in Sea Surface Temperatures (SSTs) arising from the different ocean models (FESOM vs. NEMO) modulating continental moisture advection.

Physical Interpretation

An increase in Bowen ratio (positive anomalies, red) indicates a shift from latent to sensible heat fluxes, typically driven by soil moisture depletion which limits evapotranspiration (land-atmosphere feedback). Conversely, a decrease (negative anomalies, blue) implies increased moisture availability, allowing more net radiation to be dissipated through evaporation (latent heat), as seen in the intensified West African Monsoon region.

Caveats

  • The Bowen ratio becomes mathematically unstable or sensitive to small absolute errors in regions where Latent Heat Flux is near zero (e.g., arid deserts and polar ice sheets), making high-latitude signals difficult to interpret without separate flux components.
  • The 10-year future averaging period (2040-2049) is relatively short and may contain unremoved internal decadal variability.

Evaporative Fraction Change

Evaporative Fraction Change
Variables avg_ishf, avg_slhtf
Models ifs-fesom, ifs-nemo
Units W/m2
Baseline 1990-2014
Future 2040-2049
Method EF = LHF/(SHF+LHF). ΔEF = EF_future − EF_historical.

Summary high

The figure displays the projected change in Evaporative Fraction (EF) by the 2040s under SSP3-7.0 relative to the historical baseline for IFS-FESOM and IFS-NEMO, highlighting a robust contrast between drying in the subtropics and wetting in high latitudes.

Key Findings

  • Pronounced decreases in EF (browning, <-0.1) over the Amazon, Southern Africa, the Mediterranean, and Western North America indicate a shift towards sensible heat dominance due to soil moisture limitations.
  • Strong increases in EF (teal, >0.15) occur over the Arctic Ocean and marginal seas (e.g., Barents, Kara) and the Southern Ocean near Antarctica, directly linked to sea-ice retreat and exposure of open water.
  • A distinct band of increased EF is visible across the Sahel and Central Africa, suggesting an intensification or northward shift of the West African Monsoon.
  • High-latitude land masses (Siberia, Northern Canada) show widespread EF increases, consistent with enhanced evapotranspiration from warming and permafrost thaw.

Spatial Patterns

The maps reveal a 'wet-get-wetter, dry-get-drier' terrestrial signal: moisture-limited regions (subtropics) show reduced EF, while energy-limited regions (high latitudes) and convective zones (ITCZ) show increased EF. Oceanic patterns are dominated by high-latitude sea-ice edge retreat.

Model Agreement

Agreement is very high between IFS-FESOM and IFS-NEMO, particularly over land. This is expected as they share the same atmospheric component (IFS). Minor discrepancies exist in the Southern Ocean sea-ice zone and North Atlantic subpolar gyre, likely attributable to differences in ocean grid geometry (unstructured FESOM vs. structured NEMO) affecting sea-ice dynamics and SST fronts.

Physical Interpretation

Over land, the pattern reflects the thermodynamic response to warming: in water-limited regions (e.g., Mediterranean), increased evaporative demand depletes soil moisture, shifting energy partitioning to sensible heat (lower EF). In energy-limited high latitudes, warming facilitates greater evapotranspiration (higher EF). Over the oceans, the drastic EF increase at the poles is driven by the loss of insulating sea ice, allowing large latent heat fluxes from the exposed, relatively warm ocean to the cold atmosphere.

Caveats

  • The analysis relies on a ratio; changes in EF do not indicate the absolute magnitude of flux changes, only the partitioning.
  • The 10-year future window (2040-2049) is relatively short and may be influenced by decadal variability superimposed on the climate change signal.
  • Atmospheric dominance over land obscures potential feedback differences from the distinct ocean models.