Tier 2 Wet Bulb Temperature

2 figures · 1990–2014 → 2040–2049

Synthesis

While models agree on critical wet-bulb temperature increases exceeding 2.5°C over Northern Hemisphere continents, IFS-NEMO projects a significant North Atlantic cooling signal absent in IFS-FESOM, indicating divergent ocean circulation responses.
The analysis of wet-bulb temperature ($T_w$) proxies for the 2040-2049 period under SSP3-7.0 reveals a bifurcation between robust thermodynamic agreement over land and significant dynamic divergence in the North Atlantic between IFS-FESOM and IFS-NEMO. Both models consistently identify critical heat-stress hotspots in the Northern Hemisphere subtropics, specifically the Indo-Gangetic Plain, Eastern China, and the Persian Gulf, where absolute JJA $T_w$ values saturate the upper bounds of the diagnostic. The projected change ($\\Delta T_w$) relative to 1990-2014 is dominated by thermodynamic forcing (Clausius-Clapeyron scaling), resulting in widespread increases exceeding +2.5°C over Northern Hemisphere mid-to-high latitude landmasses and strong Arctic amplification driven by sea-ice loss feedbacks. However, a major discrepancy emerges in the North Atlantic sector. IFS-NEMO projects a distinct 'warming hole' with $\\Delta T_w$ anomalies of -1°C to -2°C, strongly suggesting a weakening of the Atlantic Meridional Overturning Circulation (AMOC) and reduced northward heat transport. In contrast, IFS-FESOM predicts moderate warming (+0.5°C to +1.5°C) in the same region. This indicates that while the atmospheric thermodynamic response to greenhouse gas forcing is consistent across ocean discretizations, the regional dynamic ocean response—specifically regarding circulation slowdowns and marginal ice zone evolution in the Southern Ocean—remains highly sensitive to the choice of ocean model (structured NEMO vs. unstructured FESOM).

Related diagnostics

Atlantic Meridional Overturning Circulation (AMOC) strength Northern Hemisphere Sea Ice Concentration

Future JJA Wet Bulb Temperature

Future JJA Wet Bulb Temperature
Variables avg_2t, avg_2d
Models ifs-fesom, ifs-nemo
Units K
Baseline 1990-2014
Future 2040-2049
Method Stull (2011) from future JJA T and Td.

Summary high

The figure illustrates the absolute Wet Bulb Temperature (TW) for the JJA season in the near-future (2040-2049) under SSP3-7.0, comparing IFS-FESOM and IFS-NEMO coupled models. Both models display physically consistent, nearly identical spatial patterns with dangerous heat-humidity combinations concentrated in the Northern Hemisphere subtropics and tropics.

Key Findings

  • Critically high TW values (indicated by deep red saturation, likely >25°C) are observed over the Indo-Gangetic Plain, Eastern China, the Persian Gulf, and the Red Sea, aligning with Northern Hemisphere summer monsoon dynamics.
  • Tropical rainforest basins (Amazon, Congo) and tropical oceans exhibit uniformly high TW, driven by high specific humidity.
  • Pronounced topographic cooling is evident over major mountain ranges (Himalayas, Andes, Rockies) and the Greenland Ice Sheet.
  • Southern Hemisphere winter conditions result in extremely low TW (< -30°C) over Antarctica.

Spatial Patterns

A strong latitudinal gradient exists, consistent with JJA insolation. The highest TW values are found in the Northern Hemisphere subtropics (specifically South and East Asia) and the tropical warm pool. Coastal regions near warm seas (e.g., Gulf of Mexico, Arabian Sea) show elevated TW due to moisture advection. In contrast, high-altitude regions create sharp gradients, particularly visible at the southern edge of the Tibetan Plateau.

Model Agreement

There is exceptional agreement between IFS-FESOM and IFS-NEMO. The spatial distribution and magnitude of TW appear visually indistinguishable at this global scale. This suggests that the difference in ocean discretization (unstructured FESOM vs. structured NEMO) does not significantly perturb the broad-scale atmospheric thermodynamic state (2m temperature and humidity) influencing JJA wet-bulb temperatures in this time horizon.

Physical Interpretation

The patterns are driven by the combination of high summertime temperatures and high humidity. In South Asia, the summer monsoon advects moisture inland, preventing evaporative cooling and raising TW. In the Persian Gulf, high SSTs drive evaporation, creating extreme TW despite arid surroundings. The Clausius-Clapeyron relationship under the SSP3-7.0 warming scenario likely enhances these values compared to historical baselines by increasing the atmosphere's water vapor capacity.

Caveats

  • The colorbar labels stop at 20°C, causing saturation in tropical and subtropical regions. This makes it impossible to visually assess whether values approach the critical 35°C survivability threshold mentioned in the metadata.
  • The figure presents a seasonal mean (JJA); peak daily extremes during heatwaves would be significantly higher than the averages shown here.
  • There is a discrepancy between the metadata units (K) and the plot axis labels (°C); the analysis assumes °C based on the visual labels.

JJA Wet Bulb Temperature Change

JJA Wet Bulb Temperature Change
Variables avg_2t, avg_2d
Models ifs-fesom, ifs-nemo
Units K
Baseline 1990-2014
Future 2040-2049
Method Tw from Stull (2011) approximation using T and RH (RH from Magnus formula via T and Td).

Summary high

The figure displays the projected change in JJA wet bulb temperature ($T_w$) for 2040-2049 relative to 1990-2014 under SSP3-7.0 in IFS-FESOM and IFS-NEMO. Both models predict widespread increases in $T_w$ driven by thermodynamic warming and humidity scaling, with the most severe increases over Northern Hemisphere landmasses, but they diverge significantly in their representation of North Atlantic and Southern Ocean surface changes.

Key Findings

  • IFS-FESOM and IFS-NEMO both project extensive $\Delta T_w$ increases exceeding +2.5°C over Northern Hemisphere mid-to-high latitudes, particularly Eastern Europe, Siberia, and Northern Canada.
  • A major discrepancy exists in the North Atlantic: IFS-NEMO exhibits a distinct 'warming hole' with $\Delta T_w$ decreases of -1°C to -2°C, whereas IFS-FESOM shows moderate warming (+0.5°C to +1.5°C) in the same region.
  • Tropical regions, specifically the Sahel and Amazon basin, show elevated $\Delta T_w$ (+1.5°C to +2.5°C) relative to the surrounding oceans, indicating combined heat and moisture increases.
  • Southern Ocean patterns vary: IFS-FESOM shows strong warming patches (>2°C) near the Antarctic coast, while IFS-NEMO displays localized cooling patches (< -1°C) in the Weddell and Ross Sea sectors.

Spatial Patterns

There is a strong land-sea contrast in the Northern Hemisphere summer, with continental interiors warming significantly more than adjacent oceans. The Arctic region shows the strongest positive anomalies (>3°C saturation), likely driven by sea-ice loss feedbacks. A zonal band of high $\Delta T_w$ stretches across the Sahel, consistent with a northward shift or intensification of the West African Monsoon.

Model Agreement

The models show high agreement on the thermodynamic response over continents and the tropical oceans, driven by the Clausius-Clapeyron relationship. Significant disagreement is confined to regions dominated by ocean dynamics: the subpolar North Atlantic and the marginal ice zones of the Southern Ocean. This suggests differing sensitivities in the ocean components (FESOM vs. NEMO) regarding AMOC slowdown and Antarctic sea-ice evolution.

Physical Interpretation

The widespread increase in $T_w$ is physically consistent with global warming causing concurrent increases in dry-bulb temperature and specific humidity. The 'warming hole' in IFS-NEMO suggests a weakening Atlantic Meridional Overturning Circulation (AMOC), reducing northward heat transport, a feature absent in IFS-FESOM's projection for this timeframe. The high latitude warming in both models reflects the ice-albedo feedback, increasing both temperature and evaporation from newly open waters.

Caveats

  • The analysis relies on a relatively short 10-year averaging period (2040-2049), meaning internal decadal variability could exaggerate differences between models, particularly in the North Atlantic and Southern Ocean.
  • The Stull (2011) approximation for $T_w$ is valid for standard meteorological ranges but may have reduced accuracy at the extreme combinations of T and RH found in future climate scenarios.