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

The Arctic is warming at a rate significantly faster than the global average, a phenomenon known as Arctic amplification. This amplification is strongly influenced by summer conditions, as decreased summer sea ice extent (SIE) allows for increased absorption of solar radiation and ocean heat content. This accumulated heat then impacts winter conditions by limiting sea ice growth and enhancing oceanic evaporation, increasing atmospheric water vapor. Increased water vapor leads to increased downward longwave flux, further warming the air and creating a positive feedback loop. Understanding the sources of summer Arctic moisture is crucial for attributing and predicting Arctic warming patterns. Previous studies using Eulerian or Lagrangian approaches have yielded conflicting results regarding the relative contributions of oceanic and terrestrial moisture sources. Some studies highlight the North Atlantic and Pacific as primary sources, while others emphasize North America. The limitations of these methods—such as oversimplification of model physics and low temporal resolution in back-trajectory analyses—motivate the need for a more sophisticated approach. This study utilizes a climate model with advanced online water tracking capabilities to address these limitations and provide a more comprehensive understanding of the sources and variability of summer Arctic moisture.

Existing research on Arctic moisture sources employs various techniques, including Eulerian and Lagrangian approaches. Eulerian methods, analyzing moisture transport patterns, have identified strong moisture transport from the North Atlantic and North Pacific, with weaker contributions from landmasses. Studies using Lagrangian techniques, such as back-trajectory analysis, offer conflicting results. Some studies highlight lower-latitude oceanic regions as the dominant moisture source, while others point to continental regions, particularly North America. However, the reliance on reanalysis data and simplifications in back-trajectory methods introduce uncertainties, particularly regarding the contribution of land surfaces. This study aims to resolve these discrepancies by using a climate model with online water tracking capabilities, providing a more comprehensive and nuanced understanding of terrestrial and oceanic contributions to Arctic moisture.

This study employs the Community Earth System Model version 1.3 (CESM1.3), configured with the Community Atmosphere Model version 5 (CAM5) and Community Land Model version 4 with carbon-nitrogen model activated (CLM4CN). The model utilizes a 1.9° x 2.5° grid with 30 active atmospheric levels. A 30-year land-atmosphere coupled simulation was run, using prescribed monthly varying sea-surface temperatures (SSTs) and sea-ice concentrations (SICs) from a preindustrial simulation. The last 29 years of data were used for analysis after a one-year spin-up period. A key aspect of the methodology is the use of online numerical water tracers within CAM5. These tracers follow the movement and phase changes of evaporated moisture from tagged regions (land and ocean areas) until precipitation occurs. Multiple tracers were used, allowing for the quantification of contributions from various land and ocean regions. The land surface was divided into latitude bands (high, high-mid, low-mid, and low latitudes) and longitude regions (western and eastern North America, western, central, and eastern Eurasia) to identify specific source regions. The model's performance was evaluated by comparing simulated vertically integrated water vapor and temperature to ERA5 reanalysis data. Statistical significance of anomalous contributions was assessed using a bootstrap resampling method. The Arctic was divided into six longitudinal sectors for regional analysis of vapor concentration and its correlation with near-surface temperature, downward longwave (LW) and shortwave (SW) fluxes, and total integrated cloud.

The study's key findings include: 1. Land surface contributes 56% of total summer Arctic vapor, significantly more than previously estimated by some studies. 2. Central and eastern Eurasia are the dominant sources of this land-based moisture (47% of total Arctic vapor). 3. Near-surface temperatures in the Arctic, particularly in Sector 5 (90°E-150°E encompassing the Laptev Sea), strongly correlate with land-based vapor concentrations. 4. Years with high land-based vapor concentrations, particularly in Sector 5, exhibit enhanced poleward near-surface flow from Siberia, often associated with the Arctic Dipole anomaly. 5. The variability of land vapor is not uniform, with Sector 5 showing the largest variations. 6. In Sector 5, near-surface temperatures strongly correlate with downward LW flux, total integrated vapor, and land vapor from central and eastern Eurasia, but weakly correlate with SW flux and total integrated cloud. 7. Arctic-wide temperatures show a weaker correlation with land vapor compared to Sector 5, suggesting that anomalous fluxes from multiple regions might be needed to impact Arctic-wide temperatures. 8. Analysis of atmospheric conditions associated with anomalous land vapor transport reveals that lower-than-normal sea level pressure (SLP) on the Eurasian side of the Arctic promotes enhanced poleward flow from central and eastern Eurasia, while higher-than-average SLPs inhibit this flow. 9. The 500 mb geopotential height (GPH) fields also play a key role in regulating land moisture transport. 10. The Arctic Dipole anomaly shows a strong relationship with land vapor transport; the positive phase promotes enhanced flow from eastern Eurasia, while the negative phase enhances flow from eastern North America. 11. While atmospheric circulation is the primary driver of land vapor anomalies, extreme evapotranspiration (ET) events can also significantly influence Arctic vapor concentrations. 12. The model slightly underestimates total integrated vapor compared to ERA5 reanalysis data, likely due to differences in model atmospheric constituents and temperature.

This study's findings significantly advance our understanding of the sources and variability of summer Arctic moisture. The substantial contribution of land-based vapor, particularly from central and eastern Eurasia, challenges previous assumptions that emphasized oceanic sources. The strong correlation between land vapor and near-surface temperatures in Sector 5 highlights the regional sensitivity to atmospheric circulation patterns influencing moisture transport from these Eurasian regions. The link between the Arctic Dipole anomaly and land vapor transport provides further insight into the mechanisms driving Arctic variability. The study's emphasis on the combined influence of atmospheric circulation and ET events offers a more complete picture of the complex interactions shaping Arctic moisture and temperature patterns. These findings have important implications for predicting future Arctic climate change, as changes in land surface characteristics and atmospheric circulation patterns could significantly alter Arctic moisture and warming trends.

This study demonstrates the crucial role of terrestrial moisture sources, particularly from central and eastern Eurasia, in shaping summer Arctic vapor and temperature patterns. The strong correlation between land-based vapor, atmospheric circulation (SLP and GPH), and the Arctic Dipole anomaly highlights the importance of these factors in regulating Arctic climate variability. Future research should focus on improving the representation of ET processes in climate models and investigating the complex interactions between different modes of atmospheric variability and their impacts on Arctic moisture transport. Further investigations into the role of changing land surface characteristics and their influence on Arctic warming are also warranted.

The study relies on a climate model simulation, which may have inherent biases that could affect the accuracy of moisture sourcing estimates. Future studies should compare results with other climate models and observational data to evaluate uncertainties. The relatively low temporal resolution of the model may limit the ability to capture short-term variations in moisture transport. The study focuses primarily on summer conditions; further analysis is needed to investigate the role of terrestrial sources in other seasons. While the study explores the relationship between the Arctic Dipole and land vapor transport, other modes of variability could also play a significant role.