The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system. The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided b...

The NCEP/NCAR 40-Year Reanalysis Project

The ice arches that usually develop at the northern and southern ends of Nares Strait play an important role in modulating the export of Arctic Ocean multi-year sea ice. The Arctic Ocean is evolving towards an ice pack that is younger, thinner, and more mobile and the fate of its multi-year ice is becoming of increasing interest. Here, we use sea ice motion retrievals from Sentinel-1 imagery to report on the recent behavior of these ice arches and the associated ice fluxes. We show that the duration of arch formation has decreased over the past 20 years, while the ice area and volume fluxes along Nares Strait have both increased. These results suggest that a transition is underway towards a state where the formation of these arches will become atypical with a concomitant increase in the export of multi-year ice accelerating the transition towards a younger and thinner Arctic ice pack.

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Anomalous collapses of Nares Strait ice arches leads to enhanced export of Arctic sea ice

The Community Climate System Model version 3 (CCSM3) has recently been developed and released to the climate community. CCSM3 is a coupled climate model with components representing the atmosphere, ocean, sea ice, and land surface connected by a flux coupler. CCSM3 is designed to produce realistic simulations over a wide range of spatial resolutions, enabling inexpensive simulations lasting several millennia or detailed studies of continental-scale dynamics, variability, and climate change. This paper will show results from the configuration used for climate-change simulations with a T85 grid for the atmosphere and land and a grid with approximately 1° resolution for the ocean and sea ice. The new system incorporates several significant improvements in the physical parameterizations. The enhancements in the model physics are designed to reduce or eliminate several systematic biases in the mean climate produced by previous editions of CCSM. These include new treatments of cloud processes, aerosol ...

/pdf/the-community-climate-system-model-version-3-ccsm3-i5kdvl8zjx.pdf

The Community Climate System Model Version 3 (CCSM3)

The Community Earth System Model (CESM) is a flexible and extensible community tool used to investigate a diverse set of Earth system interactions across multiple time and space scales. This global coupled model significantly extends its predecessor, the Community Climate System Model, by incorporating new Earth system simulation capabilities. These comprise the ability to simulate biogeochemical cycles, including those of carbon and nitrogen, a variety of atmospheric chemistry options, the Greenland Ice Sheet, and an atmosphere that extends to the lower thermosphere. These and other new model capabilities are enabling investigations into a wide range of pressing scientific questions, providing new foresight into possible future climates and increasing our collective knowledge about the behavior and interactions of the Earth system. Simulations with numerous configurations of the CESM have been provided to phase 5 of the Coupled Model Intercomparison Project (CMIP5) and are being analyzed by the broad com...

/pdf/the-community-earth-system-model-a-framework-for-2zf04g06j7.pdf

The Community Earth System Model: A Framework for Collaborative Research

Salinity stratification is critical to the vertical circulation of the high-latitude ocean. We here examine the control of the vertical circulation in the northern seas, and the potential for altering it, by considering the budgets and storage of fresh water in the Arctic Ocean and in the convective regions to the south. We find that the present-day Greenland and Iceland seas, and probably also the Labrador Sea, are rather delicately poised with respect to their ability to sustain convection. Small variations in the fresh water supplied to the convective gyres from the Arctic Ocean via the East Greenland Current can alter or stop the convection in what may be a modern analog to the halocline catastrophes proposed for the distant past. The North Atlantic salinity anomaly of the 1960s and 1970s is a recent example; it must have had its origin in an increased fresh water discharge from the Arctic Ocean. Similarly, the freshening and cooling of the deep North Atlantic in recent years is a likely manifestation of the increased transfer of fresh water from the Arctic Ocean into the convective gyres. Finally, we note that because of the temperature dependence of compressibility, a slight salinity stratification in the convective gyres is required to efficiently ventilate the deep ocean.

The role of sea ice and other fresh water in the Arctic circulation

Sea ice thickness distribution in the Arctic Ocean

[1] This paper presents a new hypothesis along with supporting evidence that the Beaufort Gyre (BG) plays a significant role in regulating the arctic climate variability. We propose and demonstrate that the BG accumulates a significant amount of fresh water (FW) during one climate regime (anticyclonic) and releases this water to the North Atlantic (NA) during another climate regime (cyclonic). This hypothesis can explain the origin of the salinity anomaly (SA) periodically found in the NA as well as its role in the decadal variability in the Arctic region.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL015847

The role of the Beaufort Gyre in Arctic climate variability: Seasonal to decadal climate scales

The water masses and circulation over the northern Greenland continental shelf are described based on two cruises to the region during the summers of 1979 and 1984. The cold Polar Water over the shelf was found to increase notably in salinity toward the shelf break and the East Greenland Polar Front. A colder, more saline, and relatively isolated fraction of the Polar Water was identified just above the Atlantic Intermediate Water of the front. A middepth layer, stable in time, indicates little vertical heat exchange. Warm Atlantic Intermediate Water was found to intrude close to the coast via a newly defined system of troughs in the shelf. The Polar Water was found to circulate over the shelf in a clockwise gyre. Baroclinic transports, northward near the coast with 0.58-Sv mean flow and southward elsewhere with 1.47-Sv mean flow, yielded a net southward transport in the East Greenland Current of 0.89 Sv.

Circulation and water masses of the East Greenland shelf

A salinity-temperature survey of Fram Strait in September 1985 extending to north of 81°N demonstrated the western branch of the West Spitsbergen Current flowing northwest and then west, mainly south of 81°N. There also was a southwestward recirculation from the eastern branch of the West Spitsbergen Current. Baroclinic flow of warm water northward through 81°N was negligible or zero. Some doubt is cast on past flow diagrams based on core analysis which show the western branch of the West Spitsbergen Current coursing northward to 83°N. One of a probable line of mesoscale eddies is described in some detail.

The westward turning branch of the West Spitsbergen Current

Closely spaced salinity and temperature measurements in the region of the East Greenland Polar Front from 75°N to 79°N in October–November 1981 are presented. The Return Atlantic Current (RAC), having a core of relatively warm and saline Atlantic Intermediate Water (AIW) (T = 0.5° to 3.0°C, S = 34.9 to 35.0), was found everywhere along a steep front separating it from the colder, fresher Polar Water. A narrow frontal jet was found to have velocities greater than 0.80 m/s where the station density was great enough to resolve its concentrated character. Notable fine structure was present, especially in the warm AIW just east of the front. A cold, saline water, forming a knee in the temperature-salinity correlation, was present in the upper margins of the RAC. The knee is formed primarily by warm AIW or Atlantic water flowing under the upper layers of water flowing from the Arctic Ocean. Calculations are presented to show that an initially isothermal underflow could be modified to a thick thermocline by double diffusion. Calculations of the rate of cooling of fine-structure elements by double diffusion indicate that the fine structure would have a limited lifetime (about 12 days) if its waters were not continually replenished.

/pdf/the-east-greenland-polar-front-in-autumn-mpvxcakuld.pdf

Robert H. Bourke

Papers

Sea ice thickness distribution in the Arctic Ocean

The role of the Beaufort Gyre in Arctic climate variability: Seasonal to decadal climate scales

Circulation and water masses of the East Greenland shelf

The westward turning branch of the West Spitsbergen Current

The East Greenland Polar Front in autumn