Terrestrial sources of summer arctic moisture and the implication for arctic temperature patterns

The study addresses the unresolved question of where summer Arctic atmospheric water vapor originates and how its variability influences Arctic temperatures and sea ice. Although Arctic amplification is strongest in winter, it is tightly linked to prior summer and fall sea-ice extent and ocean heat storage, both modulated by atmospheric moisture and longwave radiation. Prior work using Eulerian and Lagrangian methods has yielded conflicting results about the dominant moisture sources (oceans vs. continents) and may be biased by methodological limitations. The purpose here is to quantify the geographic sources of summer Arctic vapor using online moisture tracers in a climate model, evaluate their variability, and assess how land-sourced vapor relates to Arctic temperature patterns, especially in regions adjacent to Siberia (90°E–150°E).

Previous Eulerian analyses indicate strong poleward moisture transport from the North Atlantic and North Pacific, with weaker land contributions. Lagrangian back-trajectory studies have disagreed on whether oceanic lower latitudes or continental regions (notably North America) dominate summer Arctic vapor. However, back-trajectory and analytical approaches can oversimplify key processes (e.g., vertical precipitation fluxes, turbulence, low temporal resolution; assumptions such as well-mixed columns and arbitrary uptake thresholds), potentially biasing source attribution. Given rapid changes in terrestrial evapotranspiration and land surface properties, a re-evaluation using model-embedded online tracers is warranted to resolve the conflicting estimates of land vs. ocean contributions to Arctic moisture.

The study employs CESM1.3 with CAM5 atmosphere (30 vertical levels; 1.9°×2.5° grid) and CLM4CN land model in a land–atmosphere coupled configuration for 30 years; the final 29 years are analyzed after a 1-year spin-up. Monthly varying preindustrial SSTs and sea-ice concentrations from a fully coupled equilibrated CESM1.3 PI simulation are prescribed to capture variability. CAM5’s online numerical water tracers tag evaporated moisture (including transpiration) from predefined regions (passive tracers undergo identical physical processes as bulk moisture). Tracked source regions include: land (further partitioned), North Pacific (30°N–70°N, 105°E–100°W), North Atlantic (30°N–70°N, 100°W–75°E), Subtropical North Pacific (10°N–30°N, 105°E–100°W), Subtropical North Atlantic (10°N–30°N, 100°W–25°E), and Arctic Ocean (>70°N). Land is divided by latitude bands: HIL (>60°N), HML (60°–45°N), LML (45°–30°N), LOL (30°–0°N), and by longitude sectors between 30°–90°N: WNA (170°W–105°W), ENA (105°W–15°W), WER (15°W–45°E), CER (45°E–105°E), EER (105°E–170°W). Model validation compares summer total integrated vapor (TIV) and 2 m temperatures against ERA5 (1990–2018) regridded to 1.9°×2.5°. Statistical analysis includes: (1) defining anomalously high/low region-specific vapor years using thresholds of mean ± 0.15×mean due to heterogeneous variances and distributions; (2) defining Arctic-wide high/low total land vapor years using 0.90/0.10 quantiles; (3) bootstrap resampling (10,000 random 3-year subsets from 29 years) to assess significance of anomalous contributions (outside 2.5–97.5% of null). Correlations are computed between near-surface (2 m) temperature and TIV, integrated land vapor (by source), cloud amounts, and downward SW/LW fluxes for the Arctic (>69°N) and Sector 5 (90°E–150°E). Composite analyses of SLP and 500 mb geopotential height (GPH), and case-by-case anomalies with 925 mb winds, diagnose circulation controls. The Arctic Dipole (AD) is characterized via EOF of JJA SLP north of 70°N to relate modes of variability to land-vapor transport.

• Model–reanalysis agreement: Summer Arctic-mean TIV is 11.7 kg m−2 in CESM vs 12.2 kg m−2 in ERA5 (difference <5%). CESM underestimates TIV mainly due to lower mean 2 m temperatures (272.2 K vs 275.2 K in ERA5).
• Seasonal source breakdown (Arctic >69°N): Land contributes 9% (DJF), 33% (MAM), 56% (JJA), 33% (SON) of total vapor. In JJA, oceans contribute less: North Atlantic 14%, North Pacific 10%; subtropics and Arctic Ocean each <9% in any season.
• Summer land source by latitude: HML 44%, HIL 35%, LML 18%, LOL 3%; Southern Hemisphere land <1%.
• Summer land source by longitude (30°–90°N): EER 27%, CER 20%, ENA 18%, WER 16%, WNA 16%.
• Variance patterns: Land vapor variance peaks in Sector 5 (90°E–150°E; Laptev/Kara/East Siberian Seas). In Sector 5, total vapor variance ≈ land vapor variance, indicating land vapor dominates variability there.
• Temperature links: Strong positive correlation between downward LW flux and 2 m temperature (adjusted R² ≈ 0.8 Arctic-wide and Sector 5). TIV vs temperature: adjusted R² ≈ 0.7 (Arctic) and 0.8 (Sector 5). Negative correlation with SW flux (R² ≈ 0.4–0.5). Total cloud fraction shows no correlation. Land vapor vs temperature: adjusted R² ≈ 0.3 (Arctic) and 0.7 (Sector 5).
• Sector 5 source-specific links: CER and EER land vapor correlate best with Sector 5 temperatures (adjusted R² ≈ 0.3 and 0.6, respectively); WER/ENA/WNA show weak to no correlation.
• Composite circulation controls: High EER/CER vapor years feature negative SLP on central/eastern Eurasian side of Arctic (near 70°N) with poleward low-level flow; 500 mb GPH aligned along 90°E–90°W. Low vapor years often have positive SLP near the source region or equatorward flow driven by North Pacific lows; opposite GPH alignment (120°E–60°W).
• Arctic Dipole (AD) link: Positive AD phase resembles SLP/GPH patterns that enhance EER/CER vapor export and total Arctic land vapor; negative AD aligns with enhanced ENA contributions. Not all AD extremes yield anomalies due to concurrent modes and ET variability.
• Arctic-wide high vs low land vapor years: Land contribution increases to ~60% in high-vapor summers and decreases to ~54% in low-vapor summers; anomalies >0.5 kg m−2 from climatology. Changes are dominated by EER, CER, HML, and HIL. Ocean anomalies are small and slightly counteract land anomalies.
• Role of ET: Circulation is primary, but anomalously high high-latitude ET can produce strong land-vapor anomalies even without ideal SLP (e.g., EER year 3; CER year 28). Composite ET anomalies along northern Siberia are modest but coherent in high/low land-vapor years.

The analysis directly addresses the source attribution of summer Arctic water vapor by embedding tracers in a climate model, showing that terrestrial sources, particularly from central and eastern Eurasia and from mid-to-high latitudes, dominate summer Arctic vapor. This dominance translates into strong control of near-surface temperatures via enhanced downward longwave radiation, especially in Sector 5 (90°E–150°E) adjacent to Siberia, where land vapor variability largely sets total vapor variability and thus temperature anomalies. Circulation patterns—negative SLP on the Eurasian Arctic flank and 500 mb GPH alignment along 90°E–90°W—facilitate poleward advection of land moisture, frequently realized during positive Arctic Dipole configurations. Opposite circulation patterns suppress Eurasian land vapor but can enhance North American contributions, helping constrain Arctic-wide variability through opposing regional effects. While ET anomalies are generally secondary and spatially heterogeneous, exceptional high-latitude ET can overcome less favorable circulation to generate strong land-vapor anomalies. Overall, the findings elevate terrestrial evapotranspiration and Eurasian–Arctic circulation teleconnections as core regulators of summer Arctic moisture and temperature patterns, with implications for sea-ice melt and seasonal Arctic amplification.

The study demonstrates that land surfaces supply a majority (56%) of summer Arctic atmospheric water vapor, with nearly half of that from central and eastern Eurasia and predominantly from mid-to-high latitudes (>45°N). Land vapor exerts a strong control on summer Arctic temperatures via longwave radiative effects, most pronounced in the Laptev/Kara/East Siberian sector. Circulation patterns, especially those resembling the positive Arctic Dipole phase (low SLP over Eurasian Arctic and favorable 500 mb GPH alignment), govern the poleward transport of land moisture; ET plays a secondary but sometimes decisive role. These results highlight that changes in terrestrial ET and shifts in atmospheric circulation can substantially impact Arctic moisture, temperatures, and sea-ice evolution. Future work should: (1) extend tracer implementations across multiple Earth system models to quantify structural uncertainties; (2) analyze daily synoptic variability and wave activity to explicitly link transient disturbances to moisture flux; (3) explore spring and fall teleconnections; and (4) assess sensitivity to contemporary forcings and evolving land-surface changes.

Findings are derived from a single climate model (CESM1.3) using preindustrial SST/SIC boundary conditions; biases in the simulated climate state (e.g., cooler Arctic 2 m temperatures vs ERA5) can affect absolute moisture amounts. Although online tracers avoid algorithmic assumptions in offline methods, model dependence remains. The analysis spans 29 years, limiting sample sizes for composites and extremes, especially for regions with low variance (e.g., WNA). ET anomalies are evaluated at seasonal means, potentially missing higher-frequency drivers. The study focuses on summer; analogous mechanisms in spring/fall are not fully explored.