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Abstract The midwinter minimum in North Pacific storm‐track intensity is a perplexing phenomenon because the associatedlocalbaroclinity in the North Pacific is maximum during midwinter. Here, a new mechanism is proposed wherein the midwinter minimum occurs in part because global planetary‐scale waves consume the zonal available potential energy, limiting its availability for storm‐track eddy growth. During strong midwinter suppression years, the midwinter minimum is preceded by anomalously large planetary‐scale eddy kinetic energy and subsequent reduction in zonal available potential energy andglobalbaroclinity. Consistent with previous studies, this large planetary‐scale eddy kinetic energy takes place after enhanced Pacific warm pool convection, which peaks during winter. These results indicate that the midwinter minimum is in part caused by heightened warm pool convection, which, through excitation of planetary‐scale waves, leads to a weaker storm‐track. This finding also helps explain the existence of the midwinter North Atlantic storm‐track minimum.
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Midwinter Suppression of Storm Tracks in an Idealized Zonally Symmetric Setting
The midwinter suppression of eddy activity in the North Pacific storm track is a phenomenon that has resisted reproduction in idealized models that are initialized independently of the observed atmosphere. Attempts at explaining it have often focused on local mechanisms that depend on zonal asymmetries, such as effects of topography on the mean flow and eddies. Here an idealized aquaplanet GCM is used to demonstrate that a midwinter suppression can also occur in the activity of a statistically zonally symmetric storm track. For a midwinter suppression to occur, it is necessary that parameters, such as the thermal inertia of the upper ocean and the strength of tropical ocean energy transport, are chosen suitably to produce a pronounced seasonal cycle of the subtropical jet characteristics. If the subtropical jet is sufficiently strong and located close to the midlatitude storm track during midwinter, it dominates the upper-level flow and guides eddies equatorward, away from the low-level area of eddy generation. This inhibits the baroclinic interaction between upper and lower levels within the storm track and weakens eddy activity. However, as the subtropical jet continues to move poleward during late winter in the idealized GCM (and unlike what is observed), eddy activity picks up again, showing that the properties of the subtropical jet that give rise to the midwinter suppression are subtle. The idealized GCM simulations provide a framework within which possible mechanisms giving rise to a midwinter suppression of storm tracks can be investigated systematically.
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- Award ID(s):
- 1760402
- PAR ID:
- 10141028
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 77
- Issue:
- 1
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- 297 to 313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JAS-D-18-0353.1
