Winter thermodynamic vertical structure in the Arctic atmosphere linked to large-scale circulation

Tiina Nygård,Tuomas Naakka,Michael Tjernström

doi:10.5194/wcd-2-1263-2021

Abstract

Abstract. Thermodynamic profiles are affected by both the large-scale dynamics and the local processes, such as radiation, cloud formation and turbulence. Based on ERA5 reanalysis, radiosoundings and cloud cover observations from winters 2009–2018, this study demonstrates manifold impacts of large-scale circulation on temperature and specific humidity profiles in the circumpolar Arctic north of 65∘ N. Characteristic wintertime circulation types are allocated using self-organizing maps (SOMs). The study shows that influence of different large-scale flows must be viewed as a progressing set of processes: (1) horizontal advection of heat and moisture, driven by circulation, lead to so-called first-order effects on thermodynamic profiles and turbulent surface fluxes, and (2) the advection is followed by transformation of the air through various physical processes, causing second-order effects. An example of second-order effects is the associated cloud formation, which shifts the strongest radiative cooling from the surface to the cloud top. The temperature and specific humidity profiles are most sensitive to large-scale circulation over the Eurasian land west of 90∘ E and the Arctic Ocean sea ice, whereas impacts over North America and Greenland are more ambiguous. Eurasian land, between 90 and 140∘ E, occasionally receives warm and moist air from the northern North Atlantic, which, with the support of radiative impacts of clouds, weakens the otherwise strong temperature and specific humidity inversions. Altitudes of maximum temperature and specific humidity in a profile and their variability between the circulation types are good indicators of the depth of the layer impacted by surface–atmosphere processes interacting with the large-scale circulation. Different circulation types typically cause variations of a few hundred metres to this altitude, and the layer impacted is deepest over north-eastern Eurasia and North America.

Highlights

The spatial distributions of temperature and humidity in the wintertime Arctic atmosphere are to a large extent controlled by the predominantly negative surface net radiation budget and poleward energy transport by the atmosphere; in an average annual sense, the poleward energy transport is necessary to balance the radiation loss to the space at the top of the atmosphere for a stable climate
As an outcome of the selforganizing maps (SOMs) analysis, 12 characteristic wintertime atmospheric mean sea level pressure (MSLP) patterns were identified and organized in a 2-D array according to their similarities (Fig. 1)
High pressure over northern Eurasia and low pressures over the northern North Atlantic and Pacific are present in all circulation types, whereas other features only occur in some of the circulation types, with intensity, extent and location varying between the 12 circulation types (Fig. 1)

Summary

Introduction

The spatial distributions of temperature and humidity in the wintertime Arctic atmosphere are to a large extent controlled by the predominantly negative surface net radiation budget and poleward energy transport by the atmosphere; in an average annual sense, the poleward energy transport is necessary to balance the radiation loss to the space at the top of the atmosphere for a stable climate. A meridionally oriented circulation pattern, or cyclone track, especially favours intrusions of warm and moist air from the mid-latitudes into the Arctic (Messori et al, 2018; Woods et al, 2013; Fearon et al, 2021; Pithan et al, 2018).

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Conclusion