Arctic sea ice and climate
Surface snow/ice-albedo feedback mechanism
Sea ice plays an important role in our climate system through the snow/ice-albedo feedback mechanism by moderating global temperatures. Sea ice reflects a significant portion of incoming solar radiation and insulates the ocean from heat and moisture loss by moderating movement and mixing of the upper ocean in response to wind, with greatest impact observed during winter months. As sea ice extent and concentration are reduced, surface reflectivity decreases, allowing more sunlight to be absorbed by the surface and the atmosphere, leading to surface and atmospheric warming. Conversely, as sea ice extent and concentration increases, surface reflectance will increase and reduce the amount of sunlight absorbed by the surface and the atmosphere, leading to an overall atmospheric and surface cooling. Sea ice covered with snow can reflect between 85% and 90% of the solar radiation incident on its surface whereas open water reflects about 10%. Albedo values of a snow/ice covered surface undergoe significant seasonal variation. Reflectance values can be as low as 50% during July and August, and as high as 85% - 90% during the winter (e.g., Curry et al. 1995). The ice-albedo feedback mechanism (Figure 2) affects local surface energy budgets through internal processes that occur within a multiyear pack ice and are related to its thickness, distribution, lead fraction, and melt pond characteristics. The effect exhibits significant seasonal variation with solar insolation such that during winter when the amount of incoming sunlight dramatically decreases, the albedo effect is most suppressed.
Surface heat budget modifications
Its high reflectivity and low thermal conductivity, along with the high amounts of latent heat required to convert ice to liquid water make sea ice an important player in defining the unique local character of the Arctic climate. Sea ice effectively insulates the atmosphere from the underlying warm ocean water. In comparison to the open ocean, for instance, sea ice surfaces moderate surface heat and moisture fluxes, equilibrate relatively quickly, and especially during winter quickly cool and dry the overlying atmosphere. The most direct consequence of ice cover change, is the impact on surface energy fluxes that change the thermal regimes of the local atmosphere summarized in Figure 2.
Sea ice cover removal causes dramatic upward increase in the sensible and latent heat exchange between the ocean and atmosphere and an increases in longwave radiation or infrared radiative flux from the surface, that lead to an increase in the net surface radiation balance and local warming; sea ice expansion coincides with significant decreases all three heat flux exchanges leading to a decline in net surface radiation balance and local cooling. The sensible heat flux is dominant over the latent flux that exceeds the infrared radiative flux. The sensible heat flux anomalies have been estimated at approximately twice the latent anomalies and quadruple the longwave flux anomalies. The greatest increase in the flux anomalies typically occur in areas of greatest ice edge retreat; greatest decreases are observed in areas of largest advances. As well heat flux adjustments are most pronounced in winter when the ocean surface is significantly warmer than the overlying atmosphere producing strong vertical temperature gradients and winds are generally stronger. The geographic extent of the flux anomalies is relatively limited, very much confined to the location of the sea ice anomaly. This occurrence has been attributed to changes in sea ice distribution where a sudden presence of ice isolates the atmosphere from the ocean as the ice encroaches into regions where it was previously absent; the removal of ice exposes the atmosphere to the ocean in regions that were previously covered with ice, quickly changing the surface energy and radiation responses.
The dramatic changes in heat and radiation fluxes associated with the expansion or removal of sea ice influence boundary-layer temperatures (i.e., layers of the atmosphere close to the surface) over the immediate surface. Atmospheric warming is caused by vertical diffusion of increased fluxes of sensible and latent heat and of longwave radiation from the ice-free ocean surface; cooling is associated with a decrease in vertical diffusion of fluxes and radiation when sea ice cover increases. Largest temperature variances typically occur near the largest ice anomalies and coincide with lowered atmospheric pressures just above that surface. In their study, for instance, it was found that upon complete sea ice removal the atmospheric warming reached maximum amplitudes over areas that lost the most ice and in the high latitudes of North America and Asia. As well, projected sea ice loss to the end of the 21st century in the Arctic basin was found to be accompanied by a terrestrial warming found to be largest in coastal regions adjacent to the Arctic Ocean, with maximum change observed over Siberia and northern Canada and Alaska . Some found maximum warming of about 6.5ºC at the surface over land and 16ºC over the ocean as a result of Arctic sea ice loss by the end of the 21st century. The amplitude of temperature change is linearly associated with changes in sea ice cover and are typically much greater when sea ice declines than the amplitude of negative anomalies when sea ice expands. Spatial displacement of the anomalies is limited and emerges and increases vertically only in the upper atmosphere, above the 700 hPa level.
Changes in atmospheric temperature associated with sea ice fluctuations also are limited to the lower half of the troposphere, typically below 700-800 hPa. The positive tropospheric temperature anomaly (i.e., warming) present at the surface decays from the surface to the tropopause, with negative temperature anomalies (i.e. cooling) present in the stratosphere. Static winter stability (i.e., rise in temperature with height, or temperature inversion) of the lower atmosphere significantly decreases as Arctic sea ice melts. Some found the magnitude of the inversion to be reduced to virtually zero over the Ocean and by more than 50% over land, the overall effect leading to a more unstable atmosphere over the Arctic as sea ice continues to melt into the 21st century.
Greatest response of atmospheric temperature to varying sea ice conditions occur in autumn and winter and in areas where ice expands or shrinks, when much of the additional heat gained/lost in the previous summer can be released back to the atmosphere. The substantial impact on winter climate is attributed to the presence of large surface heat fluxes as a result of the enhanced ocean-atmosphere temperature gradients. During summer, changes in surface air temperature associated with reduced summer sea ice cover are much less extreme than observed in winter, even when sea ice cover is significantly reduced in the summer season due to the decreased stability of the atmosphere and increased radiative heating of the Arctic Ocean during the warm months.
The ensued changes in atmospheric temperature may be sustained for several months or seasons after the initial alteration to the sea ice extent. Some found that heavy ice in the Bering and Beaufort seas led to colder temperatures in the Alaskan sector one month later. Most recently it was found that summer ice conditions to be associated with changes in atmospheric temperature during the following autumn and winter. Sea ice decline during the summer months is typically followed by warmer-than-normal surface air temperaturess duirng the following winter throughout the Arctic.
Precipitation and cyclonic activity
Variations in sea ice conditions also affect local precipitation regimes throughout the high latitudes. The removal of ice that warms the sea surface enhances local evaporation and precipitation; the expansion of ice cools the surface reducing local evaporation and precipitation. Greatest modifications are typically observed directly over the areas of ice expansion/removal or downstream of the area that had ice removed or expanded. As with temperature, changes in precipitation are generally consistent with those of surface pressure. When sea ice declines, the associated rise in precipitation and snow fall typically is located in regions of surface pressure decrease associated with increased low-level convergence, upward motion, enhanced water vapor content in the boundary layer (i.e., lower atmosphere), and destabilization of the atmospheric boundary layer over land due to the intensified warming at the surface.
Fluctuations in sea ice coverage also are known to contribute to local changes in the character of cyclones including their path, density, and general activity. The position of storm tracks particularly can be affected by the position of the sea ice boundary, such that the storm track typically follows the new ice edge either poleward or southward depending upon whether sea ice has retreated or expanded, respectively. Others found, for instance, that the ice edge east of Greenland retracted poleward and the storm track expanded northwestward into the Greenland Sea in their low sea ice verses high sea ice composites. Some recent studies have shown the number of storms to typically increase directly over the reduced sea ice surface in the high Arctic as a result of the ice-induced variations in the surface energy fluxes into the atmosphere, whereas others found the displacement in cyclonic activity in the North Pacific a response to an adjustment of the baroclinic zone in the Pacific sector and to the development of a cyclonic circulation anomaly in the region upon sea ice removal. The removal of sea ice in the Arctic also has resulted, in some experiments, in significant decreases in the speeds and intensities of cyclonic systems north of about 55ºN, changes attributed to the moderation of the westerly flow.
Studies of the impact of sea ice cover on atmospheric circulation began with in 1924 with a researcher named Wise and continued into the 1970s with Schell. In 1973 it was discovered that the large heat flux modifications associated with changes in sea ice cover may be suggestive of atmospheric circulation's sensitivity to sea ice variations. Variations in sea ice influence the climate by affecting the exchanges of turbulent fluxes between the atmosphere and the ocean that in turn generate feedbacks to large-scale atmospheric circulations and are expressed through modifications in sea level pressure fields and surface wind patterns. The changes are known to be more significant than those associated with factors such as the albedo effect due to the presence of strong winds and vertical temperature gradients.
The atmospheric response is generally greatest over regions where the largest modifications in sea ice cover occur. A decline in Arctic sea ice cover most often is accompanied by decline in sea level pressure and the strength of polar easterlies along the circum-Arctic margins, and a shallowing of the Polar vortex (i.e., a large-scale cyclonic circulation pattern of the middle and upper atmosphere centered over the Arctic region); the expansion of Arctic sea ice is accompanied by a rise in sea level pressure and the strength of polar easterlies as well as a deepening of the polar vortex. The magnitude of the response scales nearly linearly with respect to the size but non-linearly with respect to the direction of the ice anomaly in the high latitudes. And, negative sea ice extent anomalies typically display larger impact on the general circulation of the atmosphere than do positive sea ice anomalies as noted by the changes observed in the mid-tropospheric heights.
The strength and nature of the atmospheric modifications varies throughout the year. They are strongest in winter and very limited in summer in accord with the responses to the magnitudes of the surface energy fluxes, and exhibit geographic complexity. In early winter, between November and December, the local component of the sea ice decline, for example, exhibits a baroclinic structure (i.e. consisting of negative geopotential height anomalies at the surface and positive anomalies at 500 hPa) over the Arctic basin in the lower troposphere with a surface trough over the ice anomaly and an upper level ridge in the free atmosphere over and slightly downstream of the ice anomaly. Outside of the immediate Arctic basin and more recently found the atmospheric response to be an equivalent barotropic (e.g. geopotential height anomalies amplifying with height) ridge over central and eastern Russia and through the Bering Sea. This change was attributed to adiabatic expansion by amplified ascending motion induced by increased low level convergence in areas of reduced surface pressure. In mid-winter, between January and February, the atmospheric response to sea ice loss becomes equivalent barotropic and resembles the negative phase of the North Atlantic Oscillation (NAO) when the Arctic becomes dominated by an upper-level ridge accompanied by equivalent barotropic troughs over the Atlantic and northeast Pacific. Some attribute the shallow baroclinic atmospheric circulation response over the Arctic in early and winter to a linear dynamical response to enhanced boundary layer heating induced by the loss of sea ice.
Variation in Arctic sea ice can influence local atmospheric circulation for several seasons after the expansion/removal of sea ice. In studying the interannual relationships between Arctic sea ice concentration and atmospheric circulation over Greenland and the Barents Sea, for instance, Slonosky et al. (1997) concluded that the Great Salinity Anomaly (GSA) event and the associated winter ice cover anomalies likely influenced the atmospheric circulation up to one year in advance. Positive winter sea ice anomalies in the North Atlantic and the Canadian Basin were found to be associated with high surface pressure and mid-tropospheric height anomalies over the same region on year later, and vice versa. More recently, it was found that summer sea ice extent conditions influencing large-scale atmospheric features during the following autumn and winter in and outside of the Arctic. Increased mid-tropospheric heights were observed during winter over much of the Arctic Ocean after summers with less ice than normal; decreased heights were observed after summers with increased ice extents.
Alterations in the geographic position of sea ice margin also may influence the level of atmospheric response and/or shift the position of large-scale pressure systems and associated atmospheric flow patterns. In 1974 it was found that a significant displacement of the Aleutian and Icelandic lows and the track of maximum midlatitude cyclone activity with a changing sea ice boundary. The reduction of ice cover in the North Atlantic often coincides with a northward-shifted and weakened Icelandic low and a strengthened Azores high. Others note that if the ice edge is collocated with the position of local storm tracks then sea ice anomalies can influence the low-level baroclinicity and impact the path and intensity of storms.
Distinct atmospheric responses to various characteristics of sea ice have been noted. The wintertime response to sea ice extent verses ice concentration anomalies, for instance, produced different mid-tropospheric circulation patterns. Although similar responses were recorded at the surface, in 2004 it was found that the response to be between 40% and 80% larger in the concentration than in the extent simulations over the Atlantic-Asian portions of the Arctic in the middle troposphere.
Arctic sea ice fluctuations pose significant impact on local climates. Changes in sea ice cover alter surface heat and radiation fluxes, surface and atmospheric temperatures, precipitation characteristics, storm tracks, cyclonic activity, as well as local and synoptic scale atmospheric circulation (Table 1). These modifications are known today to propagate to the lower latitudes with potentially significant consequences for global climates. The importance of this realization is underscored by the rapid decline in sea ice cover across the Arctic region that is expected to continue into the 21st century.
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