A Lagrangian Climatology of Wintertime Cold Air Outbreaks in the Irminger and Nordic Seas and Their Role in Shaping Air–Sea Heat Fluxes

Abstract

Understanding the climatological characteristics of marine cold air outbreaks (CAOs) is of critical importance to constrain the processes determining the heat flux forcing of the high-latitude oceans. In this study, a comprehensive multidecadal climatology of wintertime CAO air masses is presented for the Irminger Sea and Nordic seas. To investigate the origin, transport pathways, and thermodynamic evolution of CAO air masses, a novel methodology based on kinematic trajectories is introduced. The major conclusions are as follows: (i) The most intense CAOs occur as a result of Arctic outflows following Greenland’s eastern coast from the Fram Strait southward and west of Novaya Zemlya. Weak CAOs also originate in flow across the SST gradient associated with the Arctic Front separating the Greenland and Iceland Seas from the Norwegian Sea. A substantial fraction of Irminger CAO air masses originate in the Canadian Arctic and overflow southern Greenland. (ii) CAOs account for 60%–80% of the wintertime oceanic heat loss associated with few intense CAOs west of Svalbard and in the Greenland, Iceland, and Barents Seas and frequent weak CAOs in the Norwegian and Irminger Seas. (iii) The amount of sensible heat extracted by CAO air masses is set by their intensity and their pathway over the underlying SST distribution, whereas the amount of latent heat is additionally capped by the SST. (iv) Among all CAO air masses, those in the Greenland and Iceland Seas extract the most sensible heat from the ocean and undergo the most intense diabatic warming. Irminger Sea CAO air masses experience only moderate diabatic warming but pick up more moisture than the other CAO air masses.

On the Thermodynamic Preconditioning of Arctic Air Masses and the Role of Tropopause Polar Vortices for Cold Air Outbreaks From Fram Strait

“…The distributions of the 10‐day average diabatic cooling rates in the two groups of trajectories, however, are nearly identical, which for the median trajectory amounts to about −2.0 K/day (Figure b). Also, the medians of the 10‐day average diabatic heating rates are similar in each category (Figure c), yielding a total θ tendency of about −1.3 K/day in close agreement with typical rates found for air masses involved in CAOs in the Greenland Sea (Papritz & Spengler, ). Yet 25% of the NO‐CAO trajectories also experience diabatic heating of more than 1.4 K/day, which is less for ALL‐CAO trajectories with 1.1 K/day.…”

“…This can lead to vigorous convective overturning and the release of latent heat during cloud formation (see review by Pithan et al, , and references therein). At the same time, the surface heat fluxes cool the ocean's mixed layer; in fact, CAOs deliver the bulk of the wintertime heat flux forcing of the Nordic Seas (Papritz & Spengler, ) and are the key driver for ocean convection at the northernmost extremity of the Atlantic Meridional Overturning Circulation (Buckley & Marshall, ; Marshall & Schott, ). Due to their important role in the climate system, CAOs in the Nordic Seas have garnered increasing interest in recent years and—as we outline below—substantial progress has been made in terms of the mechanistic understanding of CAO formation in this region and the associated air mass transformations.…”

“…These two processes operate on different timescales. Specifically, Papritz and Spengler () found average cooling rates of about 1.2 K/day in the inner Arctic along kinematic backward trajectories from CAO air masses in the Greenland Sea. In comparison, average heating rates amount to more than 9 K/day when the same air masses are exposed to surface fluxes during a CAO.…”

Quantifying the Impacts of Cold Airmass on Aerosol Concentrations Over North China Using Isentropic Analysis

“…Iwasaki et al () developed an isentropic analysis method for the polar cold airmass, defining it as the airmass below a threshold potential temperature. Because potential temperature is a conservative parameter (in the Lagrangian sense) under adiabatic processes, it can be used to reflect both the extent and trajectory of the cold airmass (Iwasaki et al, ; Papritz & Spengler, ). This advanced method allows us to quantitatively measure the amount, horizontal flow, strength, loss, and generation rates of a cold airmass.…”

Synoptic environments and characteristics of cold air outbreaks in the Irminger Sea

“…In this study, I focus on a particular type of wintertime weather event that plays a key role in controlling air‐sea heat exchanges: marine cold air outbreaks (CAOs). In fact, CAOs account for more than 80% of the wintertime oceanic heat loss over most parts of the Irminger Sea (Papritz and Spengler, ). Previously, Irminger Sea CAOs have been investigated in the context of climatological studies addressing CAOs over the entire north‐east Atlantic (Kolstad et al , ; Papritz and Spengler, ); still fundamental questions regarding the synoptic conditions leading to their formation, as well as their characteristics remain, as I aim to outline in the following.…”