Lin et al. (2025) The impact of polar lows on the underlying ocean varies significantly by location

Identification

• Journal: Communications Earth & Environment
• Year: 2025
• Date: 2025-11-21
• Authors: Ting Lin, Lichuan Wu
• DOI: 10.1038/s43247-025-02972-0

Research Groups

• Department of Earth Sciences, Uppsala University, Uppsala, Sweden

Short Summary

This study assesses the global impact of Polar Lows (PLs) on the underlying ocean using sensitivity experiments, revealing that their effects vary significantly by region depending on PL frequency and ocean structure, with PL-associated wind and precipitation often having distinct and opposing influences.

Objective

• To quantify the influence of Polar Low-associated perturbations on deep ocean ventilation and large-scale ocean circulation.
• To understand how the deficiency of PL-associated energy and mass fluxes in climate models limits confidence in their predictions.

Study Configuration

• Spatial Scale: Global ocean, with a focus on subpolar seas including the Northeast Atlantic, Labrador and Irminger Seas, Northwest Pacific, and Southern Ocean. The ocean model has a mean horizontal grid size of 29 km in the Arctic Ocean, with a highest resolution of 9 km.
• Temporal Scale: Simulations covered the period from 2000 to 2020. Diagnostic analysis focused on the last 5 years of the simulations (2016-2020).

Methodology and Data

• Models used:
  • NEMO (Nucleus for European Modeling of the Ocean) with ORCA05 configuration.
  • SI3 (Sea Ice Modeling Integrated Initiative) interactively coupled with the ocean component.
  • Eddy parameterization scheme (Gent & Mcwilliams, 1990).
  • Turbulent kinetic energy closure scheme for vertical eddy viscosity and diffusivity.
  • ECMWF bulk algorithm for air-sea turbulent fluxes.
• Data sources:
  • ERA5 reanalysis data for atmospheric forcing (surface wind, sea surface pressure, precipitation).
  • Polar Low tracks and centers from Stoll (2018) for identifying and removing PL-associated anomalies.
  • Initial temperature and salinity fields derived from a millennium-long ORCA2 simulation.

Main Results

• Regional Variability: The impact of PLs on the ocean varies significantly by location, governed by PL frequency, ocean structure, and the relative contributions of PL-associated wind and precipitation.
• Northeast Atlantic Ocean:
  • Exhibits the most pronounced response.
  • PL-associated wind enhances oceanic heat loss (up to 13 W/m²) and vertical mixing, deepening the mixed layer (up to 18 m in the open ocean) and causing cooling (approximately 0.1 °C in the upper 800 m).
  • PL-associated precipitation has a minor opposing effect.
  • Overall, the oceanic response is primarily driven by PL-associated wind forcing, leading to a decrease in upper ocean heat content by 4 × 10⁸ GJ.
• Labrador and Irminger Seas:
  • Mixed-layer depth is most significantly affected.
  • PL-associated precipitation shoals the mixed layer by up to 52 m, while PL-associated wind deepens it by up to 76 m.
  • These opposing effects largely counterbalance each other when both are considered, resulting in a maximum mixed layer depth increase of 47 m.
  • Both wind and precipitation influence horizontal heat transport, with opposing effects that largely cancel out. PL-associated wind increases heat content (2.5 × 10⁸ GJ) due to enhanced horizontal heat transport (additional 0.07 PW), while precipitation decreases it (2 × 10⁸ GJ) due to reduced transport (reduction of 0.03 PW).
  • Overall, PLs cause a slight cooling (around 0.03 °C in the upper 600 m) and a decrease in upper ocean heat content by 1.25 × 10⁸ GJ, largely replenished by horizontal heat transport (additional 0.03 PW).
• Northwest Pacific and Southern Ocean:
  • Minimal impact, with mixed-layer depth changes generally less than 5 m and weak thermal effects.
  • In the Northwest Pacific, high average surface heat fluxes reduce the ocean's sensitivity to PLs.
  • In the Southern Ocean, strong background westerly winds diminish the influence of PLs.
• Subpolar Gyre (50°N–65°N, 40°W–10°W):
  • Shows pronounced sensitivity, leading to a notable accumulation of heat, with water at 200 m warming by approximately 0.2 °C, a magnitude comparable to effects caused by global warming.

Contributions

• This study provides the first global assessment of Polar Low impacts on the ocean, expanding beyond previous regional studies.
• It uniquely quantifies the separate influences of PL-associated wind and precipitation, demonstrating their distinct and often opposing effects on ocean properties (e.g., mixed-layer depth, heat content, stratification).
• Utilizes higher-resolution ERA5 reanalysis data and a direct filtering approach for PL-associated fluxes, improving upon previous parameterization methods.
• Highlights the significant role of oceanic horizontal advection in modulating PL impacts, particularly in the Labrador and Irminger Seas.
• Emphasizes the critical need for explicitly resolving PLs in climate simulations to improve their reliability, especially in regions like the subpolar gyre where PL-induced warming is comparable to global warming signals.

Funding

• Swedish Research Council (no. 2020-03190; 2024-04209)
• Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (no. 2024-00375)
• National Academic Infrastructure for Supercomputing in Sweden (NAISS) (grant agreement no. 2022-06725)

Citation

@article{Lin2025impact,
  author = {Lin, Ting and Wu, Lichuan},
  title = {The impact of polar lows on the underlying ocean varies significantly by location},
  journal = {Communications Earth & Environment},
  year = {2025},
  doi = {10.1038/s43247-025-02972-0},
  url = {https://doi.org/10.1038/s43247-025-02972-0}
}