Formation of maritime convergence zones within cold air outbreaks due to the shape of the coastline or sea ice edge

Abstract

Maritime cold air outbreaks often feature convergence zones that provide a conducive environment for the development of polar mesoscale cyclones and polar lows. This study examines the formation mechanisms of these convergence zones in cold air outbreaks downstream of a coastline or sea-ice edge. A simplified configuration in which the coastline or sea-ice edge is approximated by a line featuring a bend with an angle is examined using analytic solutions and idealised numerical simulations. The bend of the coastline causes differences in the fetch over which air parcels travel, causing a warm wedge of air downstream of the bend due to differential airmass transformations. The warm wedge is associated with a pressure trough that leads to convergence in the presence of surface friction. The analytic model captures this mechanism and compares well with the idealised numerical simulations. Condensational heating associated with moist convection enhances vertical motions and thus intensifies the horizontal convergence. The idealised numerical simulations also reproduce an asymmetry in the vertical shear of the horizontal wind across the convergence zone, which explains the transverse cloud streets downstream to the left of the convergence zone.

Analysis of the Development Mechanisms of a Polar Low over the Norwegian Sea Simulated with the Canadian Regional Climate Model

“…The main limitation of this study is that, since the CRCM6/GEM4 does not include either an ocean or a sea ice component, the atmosphere-ocean-sea ice interactions were not explicitly represented, despite the fact that these interactions modulated the PL development during its life cycle. In effect, the shape of the sea ice edge, which is affected by near surface winds and surface oceanic currents, affects the formation of convergence zones within MCAO [14] and influences PL genesis [15]. As suggested in Renfrew et al [56], coupled interactions between the atmosphere and the oceanic features play a significant role in shaping surface fluxes and impact on the ocean, which in turn modify the low-level atmospheric conditions.…”

“…MCAOs lead to convergence zones that are favourable for the formation of PLs (e.g., [13]). The shape of the sea ice edge and the coastline has an impact on the generation and intensification of the convergence zones associated with MCAOs, due to the resulting asymmetry in the air mass transformation [14] or due to local discontinuities in surface diabatic fluxes from land or sea ice toward open water areas (e.g., [15]). These convergence zones are more intense the smaller the bend angle (a simple representation of the sea ice edge or coastline consists of two straight lines that meet at a certain angle-the bend angle), the higher the sea surface temperature (SST) and the weaker the stability of the atmosphere.…”

Polar low research: recent developments and promising courses of research

“…Thus, strong baroclinicity in addition to low static stability are key ingredients for PL development (Boyd et al, 2022;Wang et al, 2023). There are other factors that contribute to PL development, such as the generation of convergence zones when MCAOs occur downstream of a sea ice edge with a certain shape (Watanabe et al, 2022), and the occurrence of weather events that can trigger PL formation, such as katabatic storms (e.g., Gutjahr et al, 2022). Although baroclinic eddies are the main responsible for poleward energy transport in mid-latitudes (Holton and Hakim, 2013), Stoll et al (2023) have found that the meridional energy transport by mesoscale eddies-such as PLs-is negligible on annual and seasonal timescales.…”