Tier 2 Ocean vs Atmosphere Attribution

3 figures · 1990–2014 → 2040–2049

Synthesis

Replacing the structured NEMO ocean with the unstructured FESOM grid significantly alters regional climate sensitivity, mitigating the North Atlantic warming hole and shifting the Pacific ITCZ southward, which amplifies European warming and intensifies Amazonian drying.
The choice of ocean component (unstructured FESOM vs. structured NEMO) within the IFS coupled system drives profound divergences in projected warming patterns under SSP3-7.0, particularly in regions governed by deep convection and boundary currents. The most significant discrepancy occurs in the North Atlantic Subpolar Gyre, where IFS-FESOM projects a robust warming signal (> +1.5 K relative to IFS-NEMO), effectively diminishing the 'warming hole' signature associated with AMOC slowdown seen in the NEMO configuration. This excess oceanic heat is advected downwind, resulting in 0.5–1.0 K greater surface warming over Europe and Northern Asia. Conversely, the Weddell Sea exhibits a strong negative anomaly (< -1.5 K) in IFS-FESOM, suggesting fundamentally different sensitivities in Antarctic deep water formation and stratification between the unstructured and structured grids. These thermal divergences force significant shifts in the global hydrological cycle, most notably a southward displacement of the Intertropical Convergence Zone (ITCZ) in the Pacific sector within IFS-FESOM relative to IFS-NEMO. This shift manifests as a strong meridional precipitation dipole and is teleconnected to a marked drying signal over the Amazon basin and Northeast Brazil. Furthermore, high-gradient regions such as the Gulf Stream and Kuroshio Extension display sharp alternating warm/cool anomalies, indicating that the numerical treatment of western boundary current separation and eddy pathways acts as a primary control on regional climate sensitivity, often outweighing the influence of the shared atmospheric formulation.

Related diagnostics

AMOC_stability_metrics Southern_Ocean_mixed_layer_depth tropical_precipitation_bias

Ocean Model Effect: 2m Temperature

Ocean Model Effect: 2m Temperature
Variables avg_2t
Models ifs-fesom, ifs-nemo
Units K
Baseline 1990-2014
Future 2040-2049
Method Δvar_FESOM − Δvar_NEMO where Δvar = future_mean − hist_mean.

Summary high

The figure illustrates the sensitivity of projected surface warming to the choice of ocean model component (FESOM vs. NEMO) within the IFS coupled system. IFS-FESOM projects significantly stronger warming in the North Atlantic and Arctic compared to IFS-NEMO, while simulating reduced warming (relative cooling) in the Weddell Sea sector of the Southern Ocean.

Key Findings

  • IFS-FESOM exhibits a positive temperature difference (> +1.5 K) relative to IFS-NEMO in the North Atlantic subpolar gyre and Labrador Sea, indicating a weaker 'warming hole' or sustained northward heat transport.
  • A strong negative anomaly (< -1.5 K) is centred on the Weddell Sea, indicating IFS-FESOM warms significantly less than IFS-NEMO in this specific region of deep water formation.
  • Western boundary currents (Gulf Stream extension and Kuroshio) show positive anomalies, suggesting IFS-FESOM projects greater warming along these frontal zones.
  • The excess oceanic heat in the North Atlantic in IFS-FESOM advects downwind, leading to 0.5–1.0 K greater warming over Europe and Northern Asia compared to the NEMO configuration.

Spatial Patterns

The signal is dominated by high-latitude discrepancies. The Northern Hemisphere is broadly warmer in FESOM (red), driven by the North Atlantic hotspot. The Southern Hemisphere shows a complex dipole, with the Weddell Sea showing strong relative cooling (blue) while the broader Southern Ocean shows mild relative warming. Tropical regions show weaker, diffuse anomalies (< 0.5 K).

Model Agreement

The figure explicitly highlights model disagreement. The shared atmosphere (IFS) isolates the ocean component as the source of divergence. The largest disagreements occur in regions of deep convection (Labrador Sea, Weddell Sea) and sea-ice edge zones, suggesting the unstructured grid (FESOM) versus structured grid (NEMO) distinction fundamentally alters convective mixing and deep water formation responses.

Physical Interpretation

The North Atlantic pattern implies that IFS-NEMO likely simulates a stronger AMOC slowdown (Atlantic Meridional Overturning Circulation) and associated cooling trend, which is dampened or absent in IFS-FESOM. In the Southern Ocean, the Weddell Sea anomaly suggests differences in stratification and vertical mixing; IFS-NEMO may be convecting warm deep water to the surface more vigorously in the future scenario (or losing sea ice faster) than IFS-FESOM.

Caveats

  • The figure shows the difference in *change* (future minus historical). It does not disentangle whether the discrepancy arises from the historical baseline climatology or the transient climate response.
  • Without separate AMOC or mixed layer depth diagnostics, the specific convective mechanism (e.g., open ocean polynyas vs. coastal formation) remains inferential.

Ocean Model Effect: Sea Surface Temperature

Ocean Model Effect: Sea Surface Temperature
Variables avg_tos
Models ifs-fesom, ifs-nemo
Units K
Baseline 1990-2014
Future 2040-2049
Method Δvar_FESOM − Δvar_NEMO where Δvar = future_mean − hist_mean.

Summary high

This figure illustrates the discrepancy in projected Sea Surface Temperature (SST) change (SSP3-7.0, 2040s vs 1990s) between two coupled models sharing an atmosphere (IFS) but using different ocean components (FESOM vs. NEMO). The comparison reveals that IFS-FESOM generally projects stronger warming in key dynamic regions, particularly the North Atlantic and North Pacific western boundary currents.

Key Findings

  • IFS-FESOM projects significantly higher SSTs (differences > 1.5 K) in the North Atlantic Subpolar Gyre and Nordic Seas compared to IFS-NEMO.
  • The Kuroshio Extension region exhibits a strong positive anomaly, indicating IFS-FESOM simulates much stronger warming or a different current path than IFS-NEMO.
  • A distinct dipole pattern (cooling/warming) appears in the Gulf Stream separation region, highlighting sensitivity in Western Boundary Current dynamics between the models.
  • The Mediterranean Sea warms notably more in the IFS-FESOM projection.

Spatial Patterns

The most prominent patterns are concentrated in high-energy dynamical regions. The North Atlantic shows a broad region of relative warming in FESOM north of 50°N, contrasting with a sharp negative (blue) anomaly just off the US East Coast. The Southern Ocean displays complex, zonally elongated filaments of alternating sign, consistent with shifts in the Antarctic Circumpolar Current fronts. The tropics show a diffuse, mild relative warming in FESOM, while the subtropical gyre centers show high agreement (near-zero difference).

Model Agreement

Models agree most closely in the quiescent centers of subtropical gyres and the eastern tropical basins. Disagreement is maximized in regions of deep water formation (North Atlantic) and eddy-rich western boundary currents. The divergence in the North Atlantic suggests the models disagree on the magnitude of the 'warming hole' associated with AMOC slowdown.

Physical Interpretation

The positive anomaly in the North Atlantic Subpolar Gyre suggests that IFS-FESOM simulates a weaker decline in the Atlantic Meridional Overturning Circulation (AMOC) or less stratification-induced cooling compared to IFS-NEMO. The sharp dipoles in the Gulf Stream and Kuroshio regions result from differences in the latitudinal separation and extension of western boundary currents, likely driven by the differing numerical discretization (unstructured finite-volume/element in FESOM vs. structured finite-difference in NEMO) affecting effective viscosity and eddy pathway resolution.

Caveats

  • Differences in eddy-rich regions (Southern Ocean, WBCs) may partly reflect unsuppressed internal decadal variability rather than purely forced climate change signal differences.
  • The figure shows the 'delta of deltas' (difference in change); it does not indicate which model has a smaller bias relative to observations.

Ocean Model Effect: Total Precipitation Rate

Ocean Model Effect: Total Precipitation Rate
Variables avg_tprate
Models ifs-fesom, ifs-nemo
Units kg/m2/s
Baseline 1990-2014
Future 2040-2049
Method Δvar_FESOM − Δvar_NEMO where Δvar = future_mean − hist_mean.

Summary high

This figure isolates the impact of the ocean model component (unstructured FESOM vs. structured NEMO) on projected precipitation changes under SSP3-7.0, revealing that the choice of ocean model significantly alters the spatial distribution of tropical convection.

Key Findings

  • A pronounced dipole in the Tropical Pacific indicates a southward shift of the Intertropical Convergence Zone (ITCZ) in IFS-FESOM relative to IFS-NEMO.
  • IFS-FESOM projects significantly drier conditions over the Amazon basin and Northeast Brazil compared to IFS-NEMO.
  • The Indian Ocean exhibits a dipole pattern, with enhanced precipitation in the western basin and reduction in the northeast in the FESOM configuration.

Spatial Patterns

The dominant feature is a zonal band of negative anomalies (brown, < -1.5e-5 kg/m2/s) north of the Equator and positive anomalies (teal, > 1.5e-5 kg/m2/s) south of the Equator in the Pacific. Similar but less zonal dipole structures appear in the Atlantic and Indian Oceans. High-frequency wave-like patterns appear in the mid-latitude storm tracks, likely representing shifts in frontal positioning.

Model Agreement

The plot explicitly visualizes model disagreement. The greatest divergence occurs in the deep tropics, particularly the Pacific ITCZ/SPCZ complex and the Maritime Continent, indicating these regions are most sensitive to the underlying ocean formulation's handling of SSTs.

Physical Interpretation

The meridional dipole in the Pacific suggests that FESOM generates different Sea Surface Temperature (SST) gradients than NEMO—likely a cooler North Equatorial Pacific or warmer South Equatorial Pacific relative to NEMO—which pulls the atmospheric convergence zone southward. The drying over the Amazon suggests FESOM simulates a stronger or shifted Walker Circulation descending limb over South America or reduced Atlantic moisture transport compared to NEMO.

Caveats

  • As these are single realizations of high-resolution models, some smaller-scale signals in the mid-latitudes may represent internal variability (noise) rather than structural model differences.
  • The analysis does not distinguish whether FESOM is correcting a bias in NEMO or introducing a new bias without validation against observations.