An earlier post Arctic Ice Factors discussed how ice extent varies in the Arctic primarily due to the three Ws: Water, Wind and Weather. There are other posts on the details of Water and Wind linked below at the end, but this post looks at some ordinary and repeating Weather events in the Arctic that influence ice formation. An interesting new study prompted this essay, but first some background on heat exchange observations in the Arctic.
One project in particular has provided comprehensive empirical data on the energy interface between Arctic Sea Ice and the atmosphere. The SHEBA project collected heat exchange data on site in the Arctic as described in this article SHEBA: The Surface Heat Budget of the Arctic Ocean by Donald K. Perovich and John Weatherly, U.S. Army Engineer Research and Development Center, Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire; and Richard C. Moritz, Polar Science Center, University of Washington, Seattle.
The combination of the importance of the Arctic sea ice cover to climate and the uncertainties of how to treat the sea ice cover led directly to SHEBA: the Surface Heat Budget of the Arctic Ocean. SHEBA is a large, interdisciplinary project that was developed through several workshops and reports. SHEBA was governed by two broad goals: understand the ice–albedo and cloud–radiation feedback mechanisms and use that understanding to improve the treatment of the Arctic in large-scale climate models. The SHEBA project was sponsored _jointly by the National Science Foundation’s Office of Polar Programs Arctic System Science program and the Office of Naval Research’s High Latitude Dynamics program.
Ice Station SHEBA
On 2 October 1997, the Canadian Coast Guard icebreaker Des Groseilliers stopped in the middle of an ice floe in the Arctic Ocean, beginning the year-long drift of Ice Station SHEBA. For the next 12 months, until 11 October 1998, Ice Station SHEBA drifted with the pack ice from 75°N, 142°W to 80°N, 162°W. At any given time, there were 20–50 researchers at Ice Station SHEBA. During the year over 200 researchers participated in the field campaign, spending anywhere from just a few days to the entire year. Conducting a year-long sea ice experiment provided daunting scientific and logistic challenges: low temperatures, high winds, ice breakup, demanding instruments, and polar bears.
There was an intense measurement program designed to obtain a complete, integrated time series of every possible variable defining the state of the “SHEBA column” over an entire annual cycle. This column is an imaginary cylinder stretching from the top of the atmosphere through the ice into the upper ocean. Observations included longwave and shortwave radiative fluxes; the turbulent fluxes of latent and sensible heat; cloud height, thickness, phase, and properties; energy exchange in the boundary layers of the atmosphere and ocean; snow depth and ice thickness; and upper ocean salinity, temperature, and currents. This year-long, integrated data set provides a test bed for exploring the feedback mechanisms and for model development.
The full set of observations is available in a report entitled Reconciling different observational data sets from Surface Heat Budget of the Arctic Ocean (SHEBA) for model validation purposes
All the detailed measurements are in the report, and the takeaway findings are summarized in Figure 8 below.
Figure 8a shows how the conductive heat flux in winter (October –March) is controlled by the net longwave radiation. The net longwave radiation has large variability. It is generally high for clear sky conditions, and low for cloudy sky, and constitutes a heat loss from the surface throughout the whole year. The net shortwave radiation (Figure 8a) is steadily growing in spring and early summer with a sudden increase in mid-June when the snow cover starts disappearing and the albedo drops to a lower value. When the surface temperature is at the melting point, the energy surplus is used for melting. This heat flux becomes the major counterbalance of the net solar flux during summer (April –September).
The sensible heat flux (Figure 8b) is usually small except in winter during clear sky conditions when the air temperature is relatively higher than the surface and the wind speed is higher [see Walsh and Chapman, 1998] (see Figure 1). In general, the surface is colder than the overlying air and the sensible heat is downward. During the winter,the sensible heat flux and the net longwave radiation are generally anticorrelated (Figures 8a – 8b). That is, the heat loss from the surface to the atmosphere during clear sky conditions leads to a positive temperature gradient in the air and results in a downward sensible heat flux. The coupling between these two fluxes is discussed in more detail by Makshtas et al. . The latent heat flux (Figure 8b) is close to zero except after the onset of the melt season when it has several peaks indicating moisture transport from the surface to the atmosphere. Figure 8a shows most components of the surface energy budget together, and the residual from all fluxes.
The Effects of Polar Weather Intrusions
With this background understanding of the winter heat flux over Arctic ice, let us consider the implications of the recent study.
An interesting paper analyzes intrusive weather and estimates the connection between such events and ice extents in the Arctic. The paper is: The role of moist intrusions in winter Arctic warming and sea ice decline in Journal of Climate 29(12):160314091706008 · March 2016 by Cian Woods and Rodrigo Caballero, Department of Meteorology, and Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
This paper examines the trajectories followed by intense intrusions of moist air into the Arctic polar region during autumn and winter and their impact on local temperature and sea ice concentration. It is found that the vertical structure of the warming associated with moist intrusions is bottom amplified, corresponding to a transition of local conditions from a ‘‘cold clear’’ state with a strong inversion to a ‘‘warm opaque’’ state with a weaker inversion. In the marginal sea ice zone of the Barents Sea, the passage of an intrusion also causes a retreat of the ice margin, which persists for many days after the intrusion has passed. The authors find that there is a positive trend in the number of intrusion events crossing 708N during December and January that can explain roughly 45% of the surface air temperature and 30% of the sea ice concentration trends observed in the Barents Sea during the past two decades.
An injection event is defined as a vertically integrated northward moisture flux across 708N in excess of 200 Tg day21 deg21 that is sustained for at least 1.5 days and occupies a contiguous zonal extent of at least 98 at all times.
The case study in Fig. 2 shows that the passage of an intrusion can induce local warming of over 20 K in the central Arctic. Here, we examine the typical thermodynamic impact of intrusions, focusing on the fully ice-covered interior of the Arctic basin—specifically, the region where monthly climatological SIC exceeds 90% and shows negligible trend across the data record. This region is shaded gray in Fig. 1.
An example intrusion event is shown in Fig. 2. The injection occurs over the northern tip of Norway and lasts for 1.75 days, yielding seven centroid trajectories. As the injection event progresses, its centroid shifts slowly eastward, giving some zonal spread in centroid trajectories. The flow field during the event features a large-scale dipole straddling the North Pole, with cyclonic circulation over the Atlantic/North American sector and an anticyclone over Eurasia. The trajectories reflect this structure, heading toward the North Pole after injection and then curving cyclonically to exit the Arctic over North America. The intrusion event is associated with large surface air temperature anomalies in the central Arctic and a retreat of the sea ice margin in the Barents Sea, topics we discuss in detail in sections 4 and 5 below.
To focus on intrusions that reach deep into the Arctic basin, events in which fewer than 40% of the trajectory ensemble members reaches 808N over 5 days are discarded. This leaves us with a final dataset of 359 intrusion events from 1990 to 2012, or ;16 per ONDJ season.
It is clear from Fig. 3 that by far the largest fraction of intrusions enters the Arctic through the Atlantic sector, with smaller numbers entering over the Labrador Sea and Greenland and from the Pacific. Interestingly, intrusions entering via the Atlantic and the Barents/Kara sector typically turn cyclonically toward North America—just as in the case study above—while those entering to the east of the Kara Sea typically turn anticyclonically and exit over Siberia. This suggests that moist intrusions into the Arctic are typically associated with cyclonic anomalies over eastern North America and anticyclonic anomalies over western Siberia, consistent with previous work.
A key feature of the warming trend in the Arctic is that it is bottom amplified (i.e., that it is in fact a trend toward a weakening of the climatological temperature inversion that prevails in ice-covered regions of the Arctic basin in winter). This feature has previously been mostly attributed to increased upward turbulent heat flux due to sea ice loss (Serreze et al. 2009; Screen and Simmonds 2010a,b).
Our results suggest a more nuanced view. The passage of an intrusion affects local conditions by inducing a transition from a “cold clear” state with a strong inversion to a “warm opaque” state with a much weaker inversion, in agreement with recent modeling work (Pithan et al. 2014; Cronin and Tziperman 2015). This yields an overall bottom-amplified local temperature perturbation, owing largely to surface heating by increased downwelling longwave radiation.
An increase in the frequency of intrusions can therefore drive bottom-amplified warming trend even in the absence of sea ice loss. In addition, the intrusions themselves drive sea ice retreat in the marginal zone and thus promote the upward turbulent fluxes that help produce bottom-amplified warming.
Our results agree with other recent work showing a strong impact of poleward moisture flux on Arctic sea ice variability and trends (D.-S. R. Park et al. 2015; H.-S. Park et al. 2015a,b). Since most of the moisture flux into the Arctic occurs in a small number of extreme events (Woods et al. 2013; Liu and Barnes 2015), it is natural to take an event-based approach as we do here, which allows us to study the structure of the intrusion events and their link to dynamical processes in the Arctic region and at lower latitudes.
Predicted surface air temperature trends (Fig. 9f) are greatest in the Barents Sea area extending into the central Arctic in agreement with observations (Fig. 9k), with the average trend predicted in the Barents Sea box approximately 45% of that observed. This localization of the trends arises both because intrusion counts have risen most rapidly in that region (Fig. 8b) and because individual intrusions have the greatest impact in that region (Fig. 9a). The predicted trend has a peak amplitude of about 3 K decade-1, about half of the observed value. For SIC the predicted trend (Fig. 9g) again coincides spatially with the observed trend (Fig. 9l) and peaks at about 10% decade, or about 1/3 the observed value at the same location, with the average predicted trend in the Barents Sea box being approximately 30% of that observed.
Current wind patterns over Barents and the Atlantic gateway to the Arctic can viewed at nullschool:
Arctic Shifts between Cyclonic and Anticyclonic Wind Regimes The Great Arctic Ice Exchange