We investigate wintertime extreme sea ice loss events on synoptic to subseasonal time scales over the Barents–Kara Sea, where the largest sea ice variability is located. Consistent with previous studies, extreme sea ice loss events are associated with moisture intrusions over the Barents–Kara Sea, which are driven by the large-scale atmospheric circulation. In addition to the role of downward longwave radiation associated with moisture intrusions, which is emphasized by previous studies, our analysis shows that strong turbulent heat fluxes are associated with extreme sea ice melting events, with both turbulent sensible and latent heat fluxes contributing, although turbulent sensible heat fluxes dominate. Our analysis also shows that these events are connected to tropical convective anomalies. A dipole pattern of convective anomalies with enhanced convection over the Maritime Continent and suppressed convection over the central to eastern Pacific is consistently detected about 6–10 days prior to extreme sea ice loss events. This pattern is associated with either the Madden–Julian oscillation (MJO) or El Niño–Southern Oscillation (ENSO). Composites show that extreme sea ice loss events are connected to tropical convection via Rossby wave propagation in the midlatitudes. However, tropical convective anomalies alone are not sufficient to trigger extreme sea ice loss events, suggesting that extratropical variability likely modulates the connection between tropical convection and extreme sea ice loss events.
According to baroclinic adjustment theory, the isentropic slope maintains its marginal state for baroclinic instability. However, the recent trend of Arctic warming raises the possibility that there could have been a systematic change in the extratropical isentropic slope. In this study, global reanalysis data are used to investigate this possibility. The result shows that tropospheric isentropes north of 50°N have been flattening significantly during winter for the recent 25 years. This trend pattern fluctuates at intraseasonal time scales. An examination of the temporal evolution indicates that it is the planetary-scale (zonal wavenumbers-1–3) eddy heat fluxes, not the synoptic-scale eddy heat fluxes, that flatten the isentropes; synoptic-scale eddy heat fluxes instead respond to the subsequent changes in isentropic slope. This extratropical planetary-scale wave growth is preceded by an enhanced zonal asymmetry of tropical heating and poleward wave activity vectors. A numerical model is used to test if the observed latent heating can generate the observed isentropic slope anomalies. The result shows that the tropical heating indeed contributes to the isentropic slope trend. The agreement between the model solution and the observation improves substantially if extratropical latent heating is also included in the forcing. The model temperature response shows a pattern resembling the warm-Arctic–cold-continent pattern. From these results, it is concluded that the recent flattening trend of isentropic slope north of 50°N is mostly caused by planetary-scale eddy activities generated from latent heating, and that this change is accompanied by a warm-Arctic–cold-continent pattern that permeates the entire troposphere.
more » « less- NSF-PAR ID:
- 10403801
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 78
- Issue:
- 9
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- p. 2879-2894
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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