15:30 〜 15:45
[AOS18-13] Triggering Mechanism of the 2014 Rapid Subsurface Warming in the Bering Sea: A Numerical Study
キーワード：ベーリング海、亜表層温暖化、冬混合層、渦許容の全球海洋大循環モデル
The Bering Sea is a nutrient-rich marginal sea with seasonal sea ice variations, providing key habitats for subarctic species across the Pan-Pacific-Arctic region. Long-term observations (2007–2022) reveal a basin-wide subsurface warming in the main basin of the Bering Sea since 2014, associated with significant transition of the ocean state from a cold (2007-2013) to a warm period (2014-2022). To investigate the mechanisms triggering this warming, an eddy-permitting global ocean general circulation model is used in the present study.
The model has a 0.25° × 0.25° horizontal resolution with 63 vertical layers, and includes tidal mixing parameterization to enhance vertical mixing along the Aleutian Straits. Reanalysis data JRA55-do is utilized as atmospheric forcing. The hindcast simulation (1960–2019) successfully reproduces the observed onset of subsurface warming from 2014, consistent with observations. Tracing the warming in the model, it started from winter 2013/2014, with temperature increases confined to 170–220 m along the 1026.50–1026.65 kg/m³ isopycnals. Comparison of March 2014 temperature at the 1026.50 kg/m³ surface with previous cold years shows a ~40% reduction in outcrop area, indicating a weakened wintertime mixed layer (WML) development. This weakening of WML development reduces the supply of cold water to the subsurface layer, triggering subsurface warming.
To separate the influence of local atmospheric forcing from North Pacific inflow, a nudging experiment was conducted. By restoring 2007–2012 climatological temperature and salinity around the Pacific side of the Aleutian Straits, results show no significant difference in the outcrop area in March 2014 between the nudging and hindcast simulations. This suggests that local atmospheric forcing, rather than Pacific inflow, is the primary driver of weakening of WML development and subsequent subsurface warming.
The model has a 0.25° × 0.25° horizontal resolution with 63 vertical layers, and includes tidal mixing parameterization to enhance vertical mixing along the Aleutian Straits. Reanalysis data JRA55-do is utilized as atmospheric forcing. The hindcast simulation (1960–2019) successfully reproduces the observed onset of subsurface warming from 2014, consistent with observations. Tracing the warming in the model, it started from winter 2013/2014, with temperature increases confined to 170–220 m along the 1026.50–1026.65 kg/m³ isopycnals. Comparison of March 2014 temperature at the 1026.50 kg/m³ surface with previous cold years shows a ~40% reduction in outcrop area, indicating a weakened wintertime mixed layer (WML) development. This weakening of WML development reduces the supply of cold water to the subsurface layer, triggering subsurface warming.
To separate the influence of local atmospheric forcing from North Pacific inflow, a nudging experiment was conducted. By restoring 2007–2012 climatological temperature and salinity around the Pacific side of the Aleutian Straits, results show no significant difference in the outcrop area in March 2014 between the nudging and hindcast simulations. This suggests that local atmospheric forcing, rather than Pacific inflow, is the primary driver of weakening of WML development and subsequent subsurface warming.