Eiji Watanabe

Is the Dipole Anomaly a major driver to record lows in Arctic summer sea ice extent

] Recent record lows of Arctic summer sea ice extentare found to be triggered by the Arctic atmospheric DipoleAnomaly (DA) pattern. This local, second–leading mode ofsea–level pressure (SLP) anomaly in the Arctic produced astrong meridional wind anomaly that drove more sea ice outof the Arctic Ocean from the western to the eastern Arcticinto the northern Atlantic during the summers of 1995,1999, 2002, 2005, and 2007. In the 2007 summer, the DAalso enhanced anomalous oceanic heat flux into the ArcticOceanviaBeringStrait,whichacceleratedbottomandlateralmelting of sea ice and amplified the ice–albedo feedback. Acoupled ice–ocean model was used to confirm the historicalrecord lows of summer sea ice extent.

Beaufort shelf break eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean detected by satellite and modeling analyses

[1]xa0Mesoscale eddies and shelf-basin exchange of Pacific summer water in the western Arctic Ocean are examined using satellite data sets and an eddy-resolving coupled sea ice–ocean model. Several surface eddy-like features along the Beaufort shelf break are detected by the Moderate-Resolution Imaging Spectroradiometer (MODIS) sea surface temperature and the Global Imager (GLI) radiance image. The QuikSCAT sea wind vectors indicate that summertime shelf-wide wind is an important factor for interannual variations in the eddy properties. A realistic numerical experiment reveals that the origin of shelf break warm eddies and timing of their generation can be classified into three types. Type I eddies are frequently produced in the vicinity of the Barrow Canyon throughout the summer season by a combination of outflow of Pacific summer water with a low potential vorticity from the Barrow Canyon and lateral velocity shear of the canyon jet. Surface wind variability modulates the timing of generation. Type II and Type III eddies originate from an eastward current along the Beaufort shelf break, although origin depth and background hydrographic structure differ between them. Type II eddies are spawned from the shelf slope at middepth during early summer. Type III eddies are generated from the surface isopycnal front over the shelf break during late summer. The shift from Type II eddies to Type III eddies is caused by surface buoyancy input in the upstream region. A sensitivity experiment using different atmospheric forcing data suggest that the mechanism controlling Pacific water transport from the Chukchi shelf to the Canada Basin differs depending on surface wind fields in the shelf region. The mesoscale eddies primarily induce the shelf-to-basin transport under weak or westerly wind conditions during summer, while wind-driven Ekman transport is a major driver in the easterly wind regime. Year 2003 (2007) corresponds to the former (latter) case.

Enhanced role of eddies in the Arctic marine biological pump

The future conditions of Arctic sea ice and marine ecosystems are of interest not only to climate scientists, but also to economic and governmental bodies. However, the lack of widespread, year-long biogeochemical observations remains an obstacle to understanding the complicated variability of the Arctic marine biological pump. Here we show an early winter maximum of sinking biogenic flux in the western Arctic Ocean and illustrate the importance of shelf-break eddies to biological pumping from wide shelves to adjacent deep basins using a combination of year-long mooring observations and three-dimensional numerical modelling. The sinking flux trapped in the present study included considerable fresh organic material with soft tissues and was an order of magnitude larger than previous estimates. We predict that further reductions in sea ice will promote the entry of Pacific-origin biological species into the Arctic basin and accelerate biogeochemical cycles connecting the Arctic and subarctic oceans.

Western Arctic primary productivity regulated by shelf-break warm eddies

The response of phytoplankton to the Beaufort shelf-break eddies in the western Arctic Ocean is examined using the eddy-resolving coupled sea ice–ocean model including a lower-trophic marine ecosystem formulation. The regional model driven by the reanalysis 2003 atmospheric forcing from March to November captures the major spatial and temporal features of phytoplankton bloom following summertime sea ice retreat in the shallow Chukchi shelf and Barrow Canyon. The shelf-break warm eddies spawned north of the Barrow Canyon initially transport the Chukchi shelf water with high primary productivity toward the Canada Basin interior. In the eddy-developing period, the anti-cyclonic rotational flow along the outer edge of each eddy moving offshore occasionally traps the shelf water. The primary production inside the warm eddies is maintained by internal dynamics in the eddy-maturity period. In particular, the surface central area of an anti-cyclonic eddy acquires adequate light, nutrient, and warm environment for photosynthetic activity partly attributed to turbulent mixing with underlying nutrient-rich water. The simulated biogeochemical properties with the dominance of small-size phytoplankton inside the warm eddies are consistent with the observational findings in the western Arctic Ocean. It is also suggested that the light limitation before autumn sea ice freezing shuts down the primary production in the shelf-break eddies in spite of nutrient recovery. These results indicate that the time lag between the phytoplankton bloom in the shelf region following the summertime sea ice retreat and the eddy generation along the Beaufort shelf break is an important index to determine biological regimes in the Canada Basin.

Progress and Challenges in Biogeochemical Modeling of the Pacific Arctic Region

At this early stage of modeling marine ecosystems and biogeochemical cycles in the Pacific Arctic Region (PAR), numerous challenges lie ahead. Observational data used for model development and validation remain sparse, especially across seasons and under a variety of environmental conditions. Field data are becoming more available, but at the same time PAR is rapidly changing. Biogeochemical models can provide the means to capture some of these changes. This study introduces and synthesizes ecosystem modeling in PAR by discussing differences in complexity and application of one-dimensional, regional, and global earth system models. Topics include the general structure of ecosystem models and specifics of the combined benthic, pelagic, and ice PAR ecosystems, the importance of model validation, model responses to climate influences (e.g. diminishing sea ice, ocean acidification), and the impacts of circulation and stratification changes on PAR ecosystems and biogeochemical cycling. Examples of modeling studies that help place the region within the context of the Pan-Arctic System are also discussed. We synthesize past and ongoing PAR biogeochemical modeling efforts and briefly touch on decision makers’ use of ecosystem models and on necessary future developments.

Does Arctic sea ice reduction foster shelf–basin exchange?

The recent shift in Arctic ice conditions from prevailing multi-year ice to first-year ice will presumably intensify fall-winter sea ice freezing and the associated salt flux to the underlying water column. Here, we conduct a dual modeling study whose results suggest that the predicted catastrophic consequences for the global thermohaline circulation (THC), as a result of the disappearance of Arctic sea ice, may not necessarily occur. In a warmer climate, the substantial fraction of dense water feeding the Greenland-Scotland overflow may form on Arctic shelves and cascade to the deep basin, thus replenishing dense water, which currently forms through open ocean convection in the sub-Arctic seas. We have used a simplified model for estimating how increased ice production influences shelf-basin exchange associated with dense water cascading. We have carried out case studies in two regions of the Arctic Ocean where cascading was observed in the past. The baseline range of buoyancy-forcing derived from the columnar ice formation was calculated as part of a 30-year experiment of the pan-Arctic coupled ice-ocean general circulation model (GCM). The GCM results indicate that mechanical sea ice divergence associated with lateral advection accounts for a significant part of the interannual variations in sea ice thermal production in the coastal polynya regions. This forcing was then rectified by taking into account sub-grid processes and used in a regional model with analytically prescribed bottom topography and vertical stratification in order to examine specific cascading conditions in the Pacific and Atlantic sectors of the Arctic Ocean. Our results demonstrate that the consequences of enhanced ice formation depend on geographical location and shelf-basin bathymetry. In the Pacific sector, strong density stratification in slope waters impedes noticeable deepening of shelf-origin water, even for the strongest forcing applied. In the Atlantic sector, a 1.5x increase of salt flux leads to a threefold increase of shelf-slope volume flux below the warm core of Atlantic water. This threefold increase would be a sufficient substitute for a similar amount of dense water that currently forms in the Greenland, Iceland, and Norwegian (GIN) seas but is expected to decrease in a warming climate.

Seasonal changes in the population structure of dominant planktonic copepods collected using a sediment trap moored in the western Arctic Ocean

Winter ice cover of the Arctic Ocean makes year-round zooplankton sampling by plankton net a difficult task. Therefore, the collection of copepods with a sediment trap can be a powerful tool. In the present study, we analysed the seasonal changes in the population structures of five dominant planktonic copepods (Oncaea parila, Calanus hyperboreus, Metridia longa, Paraeuchaeta glacialis and Heterorhabdus norvegicus), which were collected using a sediment trap rotated at 10–15 day intervals moored at 184–260 m in the Northwind Abyssal Plain (75°00′N, 162°00′W) of the western Arctic Ocean from October 2010 to September 2012. Oncaea parila C6F with egg sacs occurred throughout the year, and the total abundance and composition of early copepodid stages (C1−C3) had two peaks each year. Calanus hyperboreus was dominated by C6F throughout the year, and their maturation was observed during February to May. Metridia longa C6F had a clear seasonality in lipid accumulation and gonad maturation: high lipid accumulation was observed from October to February, whereas gonad maturation occurred from March to September. Paraeuchaeta glacialis C6F also showed seasonality in lipid accumulation and gonad maturation, although their seasonal patterns varied from those of M. longa: high lipid individuals were abundant from February to April and mature individuals dominated from October to November. Heterorhabdus norvegicus showed seasonal changes in population structure as well: C1, C5, and C6M dominated from April to May, November to February and August to October, respectively. The life cycle patterns of these species are compared with those reported from other areas. While the results obtained by a sediment trap are inevitably subject to collection bias (i.e. passive collection at a fixed depth), a sediment trap should be considered as a powerful tool for the evaluation of the life cycle of planktonic copepods, especially in ice-covered oceans.