Tier 2 Snowfall Fraction Change

3 figures · 1990–2014 → 2040–2049

Synthesis

Models project a robust, thermodynamically driven retreat of the snow-to-rain transition zone by 2040–2049, with snowfall fraction decreases exceeding 20% in maritime sub-polar regions and high-altitude mountain ranges.
By the 2040–2049 period, high-resolution coupled models project a globally coherent reduction in the fraction of precipitation falling as snow relative to the 1990–2014 baseline. This signal is thermodynamically dominated, tracking the poleward and vertical migration of the 0°C isotherm under SSP3-7.0 warming. The most pronounced reductions (fraction anomalies reaching -0.2) occur in climatic transition zones where surface temperatures currently fluctuate near freezing, specifically the North Atlantic storm tracks, the Southern Ocean between 50°S and 60°S, and major orographic barriers including the Himalayas, Andes, and Rockies. Model agreement is exceptionally high between IFS-FESOM and IFS-NEMO, reflecting the shared atmospheric physics, with only minor oceanic-driven modulation in the marginal ice zones. Regionally, the signal manifests as a distinct annular 'browning' of the Southern Ocean and intense declines in the maritime sub-Arctic—particularly the Norwegian and Barents Seas—where warming dictates a phase shift from solid to liquid precipitation. Conversely, the high-altitude Antarctic interior and the Central Arctic Ocean exhibit negligible change, indicating that despite Arctic Amplification, temperatures in these core polar reservoirs remain sufficiently below the freezing threshold to maintain a snow-dominated regime. This diagnostic highlights a critical transition in the hydrological cycle, implying a shift toward rain-dominated precipitation regimes in sub-polar maritime zones and mid-latitude mountains.

Related diagnostics

2m_temperature total_precipitation sea_ice_concentration snow_water_equivalent

Snowfall Fraction Change (Global)

Snowfall Fraction Change (Global)
Variables avg_tsrwe, avg_tprate
Models ifs-fesom, ifs-nemo
Units kg/m2/s
Baseline 1990-2014
Future 2040-2049
Method SF = avg_tsrwe / avg_tprate; ΔSF = SF_future − SF_hist.

Summary high

Both IFS-FESOM and IFS-NEMO project a widespread, globally coherent reduction in snowfall fraction (ratio of snow to total precipitation) by 2040-2049 relative to the 1990-2014 baseline, consistent with thermodynamic expectations under the SSP3-7.0 warming scenario.

Key Findings

  • A dominant global signal of decreased snowfall fraction (brown values down to -0.2) is observed across both hemispheres.
  • The strongest reductions over land are concentrated in high-elevation regions: the Tibetan Plateau, the Andes, the Rocky Mountains, and the Alps/Scandinavia.
  • Oceanic reductions form distinct zonal bands, most notably in the Southern Ocean (50°S–60°S) and the North Atlantic storm track, tracking the poleward migration of the freezing line.

Spatial Patterns

The maps display a robust negative change (browning) in the transition zones where temperatures currently fluctuate near 0°C. This includes a circumpolar band in the Southern Ocean and broad swathes of the North Atlantic and Pacific. Continental interiors in the Northern Hemisphere (Europe, Western Russia, North America) show widespread moderate declines. High-altitude regions (Himalayas, Andes) show sharp, localized decreases. Sparse, weak positive anomalies (pale teal) appear in the high Arctic and parts of Northern Canada, possibly indicating shifts in seasonal precipitation regimes or internal variability.

Model Agreement

There is exceptionally high agreement between IFS-FESOM and IFS-NEMO. The spatial patterns and magnitudes are nearly identical, which is expected as both models utilize the same IFS atmospheric component. The difference in ocean coupling (unstructured FESOM vs. structured NEMO) appears to have negligible impact on the broad thermodynamic drivers of snowfall fraction at this resolution and timescale.

Physical Interpretation

The reduction in snowfall fraction is primarily driven by the poleward and upward shift of the 0°C isotherm due to global warming. As surface air temperatures rise, precipitation that historically fell as snow in transition zones (mid-latitudes, mountain ranges, and sub-polar oceans) increasingly falls as rain. The localized intensity over mountains reflects the intersection of topography with the rising freezing level.

Caveats

  • The future averaging period (2040-2049) is only 10 years, which may allow internal decadal variability to influence the signal, particularly in the patchy regions of increase.
  • The metric is calculated as the ratio of time-averaged sums (avg_tsrwe / avg_tprate); changes in the seasonal distribution of total precipitation (e.g., drier summers, wetter winters) could artificially alter the annual fraction even if local warming is occurring.

Snowfall Fraction Change (North Polar)

Snowfall Fraction Change (North Polar)
Variables avg_tsrwe, avg_tprate
Models ifs-fesom, ifs-nemo
Units kg/m2/s
Baseline 1990-2014
Future 2040-2049
Method SF = avg_tsrwe / avg_tprate; ΔSF = SF_future − SF_hist.

Summary high

The figure demonstrates a widespread reduction in the snowfall fraction (ratio of snow to total precipitation) across the North Polar region by 2040-2049 under SSP3-7.0 compared to the 1990-2014 baseline. The decline is most intense over the maritime sub-Arctic, particularly the North Atlantic and Barents Sea sectors.

Key Findings

  • Widespread decrease in snowfall fraction (brown shading) across the sub-Arctic and Arctic margins, with reductions exceeding 0.15 (15%) in key maritime regions.
  • The strongest reductions coincide with the Atlantic inflow pathways: the Norwegian Sea, Barents Sea, and the waters surrounding Southern Greenland.
  • The Central Arctic Ocean shows near-zero change (white), indicating that despite warming, temperatures remain sufficiently below freezing to maintain snow as the dominant precipitation type.
  • Continental regions (Northern Canada, Scandinavia, Siberia) exhibit a moderate decrease in snowfall fraction (roughly -0.05 to -0.10).

Spatial Patterns

The most prominent feature is a band of strong negative anomalies (dark brown) tracking the sub-polar distinct maritime zones: the Labrador Sea, south of the Denmark Strait, and extending into the Barents and Kara Seas. Orographic features are visible, with localized intensification of the signal along coastal mountain ranges (e.g., Scandinavia, Southern Greenland coast). Conversely, the high Arctic basin remains neutral.

Model Agreement

There is very strong agreement between IFS-FESOM and IFS-NEMO regarding the spatial structure and magnitude of the changes. This is expected as both coupled systems utilize the same IFS atmospheric component. Minor discrepancies exist in the magnitude of the anomaly in the eastern Barents/Kara Sea and parts of continental Siberia (where IFS-NEMO shows slight positive anomalies likely due to internal variability), reflecting differences in sea-surface conditions driven by the distinct ocean models (FESOM vs. NEMO).

Physical Interpretation

The pattern is driven by the poleward and upward migration of the $0^{\circ}\text{C}$ isotherm due to Arctic Amplification. As surface temperatures warm, precipitation events in the 'shoulder seasons' (spring/autumn) and in maritime regions previously near the freezing point transition from snow to rain. The lack of change in the Central Arctic reflects that projected warming by 2049 is not yet sufficient to push winter/mean temperatures above the freezing threshold for precipitation formation in the high latitudes.

Caveats

  • The 10-year averaging period (2040-2049) is relatively short and may be influenced by decadal internal variability, particularly over continental interiors.
  • The analysis does not distinguish between seasons; the signal is likely dominated by changes in autumn and spring, as summer is already rain-dominated and deep winter remains snow-dominated in the high Arctic.

Snowfall Fraction Change (South Polar)

Snowfall Fraction Change (South Polar)
Variables avg_tsrwe, avg_tprate
Models ifs-fesom, ifs-nemo
Units kg/m2/s
Baseline 1990-2014
Future 2040-2049
Method SF = avg_tsrwe / avg_tprate; ΔSF = SF_future − SF_hist.

Summary high

Both IFS-FESOM and IFS-NEMO project a robust, circumpolar reduction in snowfall fraction over the Southern Ocean by 2040–2049 under SSP3-7.0, indicating a shift from solid to liquid precipitation regimes.

Key Findings

  • A continuous ring of negative snowfall fraction change (values between -0.10 and -0.20) encircles Antarctica, situated roughly over the Southern Ocean storm tracks (approx. 50°S–65°S).
  • The Antarctic continent itself shows negligible change (values near 0), indicating that despite warming, temperatures remain sufficiently low to maintain a 100% snowfall regime.
  • The most intense reductions in snowfall fraction occur in the Indian and Pacific sectors of the Southern Ocean.

Spatial Patterns

The dominant spatial feature is the annular band of 'browning' (decreased snow fraction) surrounding the continent. This band is sharply delimited by the Antarctic coastline to the south, where the change drops to zero. To the north, the signal fades as the baseline snowfall fraction was already low.

Model Agreement

There is high agreement between IFS-FESOM and IFS-NEMO, likely due to the shared IFS atmospheric component. However, IFS-FESOM exhibits a slightly broader and more intense region of decrease in the Pacific sector compared to IFS-NEMO, potentially reflecting differences in sea surface temperature evolution or sea ice retreat in the underlying ocean models.

Physical Interpretation

The observed pattern is a direct thermodynamic response to warming. As atmospheric and sea surface temperatures rise, the freezing level (0°C isotherm) migrates poleward. Consequently, precipitation events in the Southern Ocean that historically fell as snow are increasingly falling as rain. The continent remains unaffected due to its high elevation and thermal inertia maintaining sub-freezing surface conditions.

Caveats

  • The metric shows fraction change, not absolute mass change; total snowfall could theoretically increase if total precipitation increases sufficiently, even if the fraction drops.
  • The exact latitudinal position of the rain-snow transition zone is sensitive to the specific sea-ice edge retreat in each ocean model.