Sea Ice Concentration Variability Research Articles

Sea ice concentration variability over the Southern Ocean and its impact on precipitation in southeastern South America

ABSTRACTThis article quantifies the relationship between sea ice cover (SIC) on the Southern Ocean and precipitation and river discharges over southeastern and eastern South America. The period of analysis covers from 1981 to 2008 and includes monthly mean data of SIC over the Ross, Amundsen–Bellingshausen and Weddell Seas, as well as atmospheric variables and discharges of selected rivers in eastern South America. To isolate the effect of known modes of climate variability on SIC, the signals of El Niño‐Southern Oscillation and the Southern Annual Mode are removed from all time series through regression analysis. Composites of precipitation differences over South America reveal a pattern of enhanced rainfall activity over the South Atlantic Convergence Zone (SACZ) in summer (JFM) when SIC is above average particularly over the Weddell Sea area, while winter (ASO) SIC anomalies show negative and significant correlations with rainfall over much of South America. Moisture fluxes patterns in summer reveal increased moisture transport towards the SACZ region and decreased low‐level jet activity (and precipitation) over northern Argentina. In particular a large dry bias over southeastern South America associated to positive SIC anomalies on the Weddell Sea in September is shown to impact the discharges of the Uruguay and Iguazú Rivers, with the largest effects found two months after the SIC anomaly (November). Although less robust, a relationship with the Paraná River is also found. These results suggest that increased SIC over the Weddell Sea during September can help to forecast drier conditions particularly on the Uruguay basin and somewhat wetter conditions in the SACZ region within the following season.

A note on the intraseasonal variability in an Antarctic polynia: Prior to and after the Mertz Glacier calving

During winter, thin sea ice is formed in coastal polynias, areas of open-water within a sea ice pack and important sites of dense shelf water formation. In the Mertz Glacier polynia (138°E–147°E, East Antarctica), we examined the effects of an extreme calving event, the last Mertz glacier calving, on the regional sea ice distribution. The high-frequency variability of sea ice concentration was studied for years 1992 to 2011 from passive microwave satellite data with fine spatial (6.25km and 12.5km) and temporal (1day) resolutions. Our results showed that the last calving of the Mertz glacier tongue in February 2010 greatly modified the size and shape of the polynia with a significant westward shift in the regional ice regimes. We also identified a post-calving transition state characterised by a 70%-decrease in polynia area from the pre-calving mean. In the eastern part of the study area, our findings are in agreement with other studies predicting an important decrease in polynia activity. In the western part, where the main polynia activity has shifted after the calving, the new sea ice distribution is expected to have a major impact on local sea ice production, dense shelf water sinking, and potentially the regional thermohaline circulation. With extreme climatic events predicted to occur more frequently, long-term monitoring of the regional icescape could be used to evaluate the vulnerability of Antarctic physical processes and related ecosystems.

Decadal variability in an OGCM Southern Ocean: Intrinsic modes, forced modes and metastable states

An ocean general circulation model (OGCM) is used to identify a Southern Ocean southeast Pacific intrinsic mode of low frequency variability. Using CORE data a comprehensive suite of experiments were carried out to elucidate excitation and amplification responses of this intrinsic mode to low frequency forcing (ENSO,SAM) and stochastic forcing due to high frequency winds. Subsurface anomalies were found to teleconnect the Pacific and Atlantic regions of the Antarctic Circumpolar Current (ACC) thermocline. The Pacific region of the ACC is characterised by intrinsic baroclinic disturbances that respond to both SAM and ENSO, while the Atlantic sector of the ACC is sensitive to higher frequency winds that act to amplify thermocline anomalies propagating downstream from the Pacific. Non-stationary cluster analysis was used to identify the system’s dynamical regimes and characterise meta-stability, persistence and transitions between the respective states. This analysis reveals significant trends, indicating fundamental changes to the meta-stability of the ocean dynamics in response to changes in atmospheric forcing. Intrinsic variability in sea-ice concentration was found to be coupled to thermocline processes. Sea-ice variability localised in the Atlantic was most closely associated with high frequency weather forcing. The SAM was associated with a circumpolar sea-ice response whereas ENSO was found to be a major driver of sea-ice variability only in the Pacific. This simulation study identifies plausible mechanisms that determine the predictability of the Southern Ocean climate on multi-decadal timescales.

Climate patterns and phytoplankton dynamics in Antarctic latent heat polynyas

Seasonal coherence between satellite‐derived phytoplankton parameters (chlorophyllaconcentration (Chl) and phytoplankton bloom initiation time (BIT)), environmental variables (sea ice concentration, surface solar radiation, and wind speed), and climate patterns (El Niño 3.4, the Southern Annular Mode, the Pacific South America pattern, the semiannual oscillation, and the wave‐3 stationary pattern) was investigated in four latent heat polynyas (Amundsen Sea, western Ross Sea, Dumont d'Urville, and Prydz Bay) using data corresponding to 1998–2006 phytoplankton growing seasons. In general, polynyas in the western sector (i.e., Amundsen Sea and western Ross Sea) had a greater sea ice cover, lower solar radiation levels, higher and more variable Chl, and more variable and delayed (i.e., high BIT) phytoplankton blooms. Differences in Chl and BIT were mainly attributed to differences in water stratification and interannual variability of sea ice concentration caused by ice shelf calving events in the Ross Sea. Changes in solar radiation reaching the sea surface played an important role in determining phytoplankton blooms in the western Ross Sea and Prydz Bay. Stronger winds tend to benefit development of phytoplankton blooms in polynyas having more stratified waters. Sensitivity of phytoplankton to climate variability in polynyas under investigation was highly influenced by the polynya size during summer (e.g., Chl in Dumont d'Urville and BIT in the western Ross Sea). Also, the response of Chl to the same climate pattern changed with the polynya's location (e.g., correlation between Chl and El Niño 3.4 in Amundsen Sea (positive) and Dumont d'Urville (negative)).

A GIS approach to estimating interannual variability of sea ice concentration in the Dumont d’Urville Sea near Terre Adélie from 2003 to 2009

A Geographic Information System (GIS)-based investigation into the interannual variability of sea ice concentration was conducted in the Dumont d’Urville Sea off the Terre Adélie coastline, south of 65°S and between 139 and 146°E. Sea ice concentration data derived from Advanced Microwave Scanning Radiometer-EOS (AMSR-E) data were analysed for the period 2003 to 2009. Sea ice concentration was found to be least variable in three regions, namely the Buchanan Bay/Watt Bay region (143–145°E), along ∼65.5°S (west of 144.5°E), and the Adélie Bank northeast of Dumont d’Urville near 66°S, 140.5°E. The remaining areas had relatively high interannual variability, in particular the Adélie Basin (∼66°S, ∼140°E) and the outer fringe of the Mertz Glacier Polynya (MGP). In general, higher sea ice concentration conditions were experienced in the west of the study area (i.e., where annual fast ice recurs), and open water dominated in the MGP and in the northeast. The years 2007–2009 experienced greater persistence of higher sea ice concentration than earlier years. This study provides a baseline for assessing changes in the regional sea ice regime that may occur since the calving of the Mertz Glacier in February 2010.

Arctic sea-ice variability revisited

AbstractThis paper explores spatial and temporal relationships between variations in Arctic sea-ice concentration (summer and winter) and near-surface atmospheric temperature and atmospheric pressure using multivariate statistical techniques. Trend, empirical orthogonal function (EOF) and singular value decomposition (SVD) analyses are used to identify spatial patterns associated with covariances and correlations between these fields. Results show that (1) in winter, the Arctic Oscillation still explains most of the variability in sea-ice concentration from 1979 to 2006; and (2) in summer, a decreasing sea-ice trend centered in the Pacific sector of the Arctic basin is clearly correlated to an Arctic-wide air temperature warming trend. These results demonstrate the applicability of multivariate methods, and in particular SVD analysis, which has not been used in earlier studies for assessment of changes in the Arctic sea-ice cover. Results are consistent with the interpretation that a warming signal has now emerged from the noise in the Arctic sea-ice record during summer. Our analysis indicates that such a signal may also be forthcoming during winter.

On the spatiotemporal behavior of sea ice concentration anomalies in the Northern Hemisphere

Reduction in the aerial extent and thickness of sea ice in the Northern Hemisphere is an important element in understanding how the arctic responds to global‐scale climate variability and change. Previous studies have demonstrated a significant reduction in minimum sea ice extent [Serreze et al., 2003] with record reduction in 2005. Observational studies attribute this phenomenon to variations in sea level pressure and large‐scale atmospheric teleconnection patterns such as the Northern Annular Mode (NAM) [Deser et al., 2000; Serreze et al., 2003]. In this study we examine variations in sea ice in the context of spatiotemporal variability of sea ice concentration (SIC) anomalies in the Northern Hemisphere during the onset of ice formation. Examined in particular are timescales associated with coherent regions of persistence in an e‐folding time spatial distribution (EFSD). Annual variations of weekly SIC anomalies are studied and spatial relations between positive and negative SIC anomalies are explored on a hemispheric scale. The results from this investigation demonstrate coherent SIC persistence patterns throughout the Northern Hemisphere, with timescales ranging from 3 to 7 weeks. Spatial correspondence between negative trends in SIC anomalies and the EFSD suggests that the same mechanism may be responsible for both increased ice reduction and persistence in this region. Examination of spatial coherence in SIC anomalies indicates that maximum SIC anomalies prevail near the Kara Sea, Beaufort Sea, and Chukchi Sea regions during late summer/early fall from 1979 to 2004. A latitudinal assessment at 80°, 70°, and 60°N indicates a poleward shift and retreat in SIC anomalies, while also underlining the decadal shift in variability often attributed to the NAM.

Summer minimum Arctic sea ice extent and the associated summer atmospheric circulation

Interrelationships between year‐to‐year variations in September Arctic sea ice extent and summer sea level pressure and surface air temperature at high northern latitudes are examined making use of microwave satellite imagery and atmospheric data for the period 1979–2006. Linear trends and year‐to‐year variability about the linear trend lines are considered separately: the latter gives a clearer indication of the physical linkages between fields. Years with low September sea ice extent tend to be characterized by anticyclonic circulation anomalies over the Arctic, with easterly wind anomalies over the marginal seas where the year‐to‐year variability of sea ice concentration is largest. It is hypothesized that the summer circulation anomalies cause sea ice extent principally by way of the Ekman drift in the marginal seas. The associated surface air temperature anomalies also tend to be largest over the marginal seas, with positive anomalies over the regions of reduced sea ice.

A comparison of Arctic Ocean sea ice concentration among the coordinated AOMIP model experiments

Sea ice concentration is a fundamental property of the Arctic ice‐ocean‐atmosphere system reflecting both dynamics and thermodynamics. Concentration integrates across space and time and is useful for characterizing both observed and numerically simulated systems. Concentration is reasonably well measured by remote sensing, and several high‐quality sea ice concentration data sets exist beginning with the satellite era. In this paper we examine the simulated sea ice concentration from nine ice‐ocean numerical models that are part of the coordinated experiments of the Arctic Ocean Model Intercomparison Project (AOMIP). Spatial patterns of means and differences between models and observations, and among models, are compared for a multiyear record and for the September sea ice minimum. Interannual variations are assessed on data with monthly climatology removed. As a proxy for the annual cycle of open water for each model, the total areas with concentration less than 10% are compared among models. Mean ice statistics are computed for grid points with greater than 1% and greater than 10% concentrations. The results show that the models have similar characteristics for the winter months when 100% cover is produced, and most models reproduce an observed minimum in sea ice concentration for 1990. The compared observational data sets use the NASA Team algorithm (Goddard Space Flight Center data, the adjusted or Walsh data, and the Hadley Centre data) and the Bootstrap algorithm. Variability in sea ice concentration is less among the four observational records than among models.

Arctic Sea Ice Variability on a Timescale of Weeks and Its Relation to Atmospheric Forcing

Northern Hemisphere sea ice concentration, 500-hPa height, sea level pressure, and 1000–500-hPa thickness at 7-day intervals are examined for the period 1972–1989, with emphasis on the winter season. The temporal variability of sea ice concentration is largest along the climatological mean ice edge where its frequency distribution is strongly bimodal with ice-free and ice-covered conditions being observed much more frequently than partial ice cover. These results confirm impressions, based on visual inspection of satellite imagery, that most of the variability in these regions is associated with the advance and retreat of the ice edge. Relationships between large-scale patterns of atmospheric variability and sea ice variability are investigated, making use of singular value decomposition of the temporal covariance matrix. The analysis is conducted separately for the Atlantic and Pacific sectors. In agreement with earlier studies based upon monthly mean data on sea ice concentration, the strongest sea ice pattern is comprised of a dipole with opposing centers of action in the Davis Straits/Labrador Sea region and the Greenland and Barents seas. Its temporal variability is strongly coupled to the atmospheric North Atlantic oscillation (NAO). The relationship between the two patterns is strongest with the atmosphere leading the ocean by two weeks. An analogous dipole pattern is observed in the Pacific sector, with opposing centers of action in the Bering Sea and the Sea of Okhotsk, which is related to a distinctive pattern of atmospheric circulation anomalies in the Pacific sector. One polarity of the NAO and its Pacific counterpart is associated with blocking episodes, during which the influence of the atmosphere is strong enough to temporarily hall the climatological mean advance of the ice edge in some regions and substantially accelerate it in others. The relationships between the fields is indicative of local forcing of sea ice in most regions, with wind stress and thermodynamic fluxes at the air-sea interface both contributing. A possible exception is the Greenland Sea, where it may be necessary to invoke some form of remote forcing in order to explain the observed changes on the interannual time scale.