Kohei Mizobata

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.

Bering Sea cyclonic and anticyclonic eddies observed during summer 2000 and 2001

Abstract Using satellite altimeter and ship data, Bering Sea cyclonic and anticyclonic eddies were observed in summer 2000 and 2001 to examine their biological, chemical and physical structures. Results from the ship transect revealed the interactions between the physical and biological conditions of Bering Sea eddies. At the center of a cyclonic (anticlockwise) eddy, upwelling was transporting nutrient (NO3+NO2) rich water (>25 μM) to the surface, which resulted in relatively high chlorophyll a concentrations (>1.0 mg m−3) developing under the pycnocline. In contrast, in the center of an anticyclonic (clockwise) eddy there was downwelling. This downwelling of surface warm water was destroying a cold layer (at about 150 m depth) caused by winter convection. However, around the periphery of the anticyclonic eddy the isopycnals were tilted up and nutrient-rich water was being transported along with them up into the euphotic zone, so that high chlorophyll a concentrations were being developed above the pycnocline inside the anticyclonic eddy.

Wintertime variability of the Beaufort gyre in the Arctic Ocean derived from CryoSat‐2/SIRAL observations

We processed the sea surface height measured by the SAR (Synthetic Aperture Radar)/Interferometric Radar Altimeter (SIRAL) on board CryoSat-2 (CS-2) and successfully estimated the monthly dynamic ocean topography (DOT) of the Arctic Ocean. The CS-2 monthly DOT showed the interannual and monthly variability of the Beaufort Gyre (BG) during winter between 2010/2011 and 2014/2015. The northward flow at the western edge of the BG was primarily estimated over the Chukchi Borderland (CBL). However, the BG extended across the CBL, and the northward flow was estimated over the Mendeleev Ridge in the winter of 2012/2013. Our analyses revealed a significantly variable BG in response to changes in the sea surface stress field. Our analysis indicated that (1) sea ice motion, driven by wind fields, acts as a driving force for the BG when sea ice motion was intensified during winter and (2) sea ice motion can also act as an inhibiting force for the BG when sea ice motion is weakened during winter. In addition, the relationship between the DOT, steric height, and ocean bottom pressure implied that the DOT during winter responded to varying wind stresses through baroclinic and barotropic adjustments. According to a tracer experiment, we inferred that in the winter of 2012/2013, the Pacific-origin water carried into the BG through the Barrow Canyon was transported to the northern shelf and shelf break of the Chukchi Sea rather than the CBL, which is where the Pacific-origin water had been transported in the other years of the observation period.

A modeling study of coastal circulation and landfast ice in the nearshore Beaufort and Chukchi seas using CIOM

This study investigates sea ice and ocean circulation using a 3-D, 3.8 km CIOM (Coupled Ice-Ocean Model) under daily atmospheric forcing for the period 1990–2008. The CIOM was validated using both in situ observations and satellite measurements. The CIOM successfully reproduces some observed dynamical processes in the region, including the Bering-inflow-originated coastal current that splits into three branches: Alaska Coastal Water (ACW), Central Channel branch, and Herald Valley branch. In addition, the Beaufort Slope Current (BSC), the Beaufort Gyre, the East Siberian Current (ESC), mesoscale eddies, and seasonal landfast ice are well simulated. The CIOM also reproduces reasonable interannual variability in sea ice, such as landfast ice, and anomalous open water (less sea ice) during the positive Dipole Anomaly (DA) years, vice versa during the negative DA years. Sensitivity experiments were conducted with regard to the impacts of the Bering Strait inflow (heat transport), onshore wind stress, and sea ice advection on sea ice change, in particular on the landfast ice. It is found that coastal landfast ice is controlled by the following processes: wind forcing, Bering Strait inflow, and sea ice dynamics.

Abrupt Climate Changes and Emerging Ice-Ocean Processes in the Pacific Arctic Region and the Bering Sea

The purpose of this chapter is to reveal several emerging physical ice-ocean processes associated with the unprecedented sea ice retreat in the Pacific Arctic Region (PAR). These processes are closely interconnected under the scenario of diminishing sea ice, resulting in many detectable changes from physical environment to ecosystems. Some of these changes are unprecedented and have drawn the attention of both scientific and societal communities. More importantly, some mechanisms responsible for the diminishing sea ice cannot be explained by the leading Arctic Oscillation (AO), which has been used to interpret most of the changes in the Arctic for the last several decades. The new challenging questions are: (1) What is the major forcing? (2) Is the AO, the DA, or their combination, contributing to the sea ice minima in recent years? How do we use models to investigate the recent changes in the PAR. Is the heat transport through the Bering Strait associated with the DA? What processes accelerate sea ice melting in the PAR?

Low Primary Productivity in the Chukchi Sea Controlled by Warm Pacific Water: A Data-Model Fusion Study

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) has identified a broad low chlorophyll-a (chl-a) area in the Chukchi Sea since 2002. High sea surface temperature from 2002 (more than 5°C), which resulted in a long duration of open water, was also detected by satellite. An intensified ocean color front at the southwest Chukchi Sea near the Siberian Coast indicates nutrient depletion in the Alaska Coastal Current and its branches. A low chl-a area started to emerge in the Hope Valley in June, and then expanded to the Herald Shoal and Hanna Shoal during July and August. The evolution pattern of low chl-a area is consistent with the variability of the pathway of the Pacific water simulated by a Coupled Ice-Ocean Model (CIOM). These results suggest that the summer phytoplankton bloom from 2002 to 2005 was suppressed by the dominance of warm nutrient-poor water from the Pacific, and by the deepening of the surface mixed layer by strong wind stress. During the summer of 2004, a phytoplankton bloom was detected at the ice edge when the sea surface wind field was relatively calm. Our results imply that the ice-edge bloom was induced due to weak wind speeds, which produce shallower upper mixed layer, favoring the ice-edge bloom.

Characteristics of cyclonic and anticyclonic eddies in the southeastern Bering Sea (1998-2000) using TOPEX/POSEIDON

During 1998 - 2000, sea surface height anomalies (SSHA) calculated from TOPEX/POSEIDON Merged Geophysical Data Record and The TOPEX/ERS2 SSHA maps (extracted from CCAR web site, http://ccar.colorado.edu/~realtime/bering/) were analyzed to understand the mesoscale eddy field along and across the shelf edge in the southeastern Bering Sea. The trace of the mesoscale eddy using SSHA maps showed the number and the occurrence of the mesoscale eddy. In 1998 to 1999, the number of the mesoscale eddy was low (244 ~ 256) and positive SSHA was prominent along the shelf edge. Especially, the eddy kinetic energy along the shelf edge was quite low inthe summer of 1999. In 2000 and 2001, however, the number of the mesoscale eddy relatively high (312 ~ 324) and the eddy kinetic energy was high in summer. TOPEX/POSEIDON SSHA ground track D-79 implies the change of the variability and the velocity of the positive SSHA field meaning anticyclonic eddy field along the shelf edge from 2000. These results indicate that the variability of the Bering Slope Current flux resulted in the occurrence of the mesoscale eddy field that has an influence on nutrient supply following high chlorophyll α distribution into the euphotic zone along the shelf edge. Mesoscale eddy field in the oceanic region migrated offshore or remained near the shelf break, not moved onto the shelf. A drifting buoy tracking Bering Sea eddies, however, observed the onshelf flow, in 2000. Results indicate that the horizontal mixing of mesoscale eddies along the shelf edge will play an important role in the Shelf-Slope exchange.

Phytoplankton distribution as observed from bio-optical drifters and SeaWiFS images in the Bering Sea green belt

Bering Sea green belt is characterized by high productive region, with interactions of cyclonic and anti-cyclonic eddies. To study temporal and spatial variability of phytoplankton, we deployed two bio-optical drifters in a clockwise eddy in 2001 and 2002. The drifters were equipped with a spectroradiometer to measure upwelling radiance as same as Sea Wide Field-of-view Sensor (SeaWiFS) wavelength. SeaWiFS images were employed to monitor the spatial pattern of chlorophyll-a (chl-a). The drifter also measured sea surface temperature (SST). We compared SST with TOPEX/POSEIDON sea surface anomaly data and analyzed time and scale correlation of SST and phytoplankton distribution in some eddies. In 2001, a clockwise eddy trapped the drifter for 10 days. It represented relatively high chl-a concentration (about 1.0 mg m-3) and low SST (about 8.5°C). After that it drifted to the Bering Sea slope current region along the shelf edge, and observed among 20 days. In 2002 we deployed a drifter in center of clockwise eddy, it was staying in this eddy for 14 days. There represented relatively low chl-a concentration (about 0.4 mg m-3) and high SST (about 9.5°C). After that it drifted along Bering slope current similarly. The phytoplankton distribution was deferent from two years in spite of same kind of anti-cyclonic eddy. These deferent might be depending on deference of eddys life stage.