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Rebecca A. Woodgate

Abstract

Part I has shown that a simple assimilation scheme can have a significant effect on the physics of the model. Part II concentrates on the effects of nudging assimilation in a full primitive equation model, the Free Surface Cox Code, illustrating the value of the previous results in a more realistic scenario and providing guidelines relevant to the assimilation of real data.

Using a 1° resolution, midlatitude baroclinic setup and justifying results with simple physical models, the assimilation of different types of data and the effects of various data restrictions are studied.

It is shown that some assimilations fail, for example, velocity alone is numerically unstable, and that success depends on the relative importance of barotropic–baroclinic modes, for example, sea surface height alone drives a predominantly barotropic response. The preferred type of data depends on length scale relative to the Rossby radius. Interestingly, all are benefited by topography; for example, assimilation of density corrects not only the baroclinic fields but also the barotropic (cf. inverse techniques). Some assimilations illustrate spurious artefacts of the assimilation, for example, sea surface height and density altering the Kelvin wave structure. Identical-twin experiments allow assessment of final errors and times for convergence. A simplistic approach to the problems of limited data coverage indicates that lack of data in time and horizontal space can be easily overcome at this resolution, whereas lack of depth data is critical. A projection scheme for obtaining subsurface fields from surface data is also discussed.

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Rebecca A. Woodgate and Peter D. Killworth

Abstract

Although data assimilation is now an established oceanographic technique, little work has been done on the interaction of the assimilation scheme and the physics of the underlying model. The way in which even a simple assimilation scheme (here nudging) can significantly alter the response of the model to which it is applied is illustrated here.

Using analytic and semianalytic models, the assimilation of sea surface height, density, and velocity is studied. It is shown that the assimilation can act to alter the high inertia–gravity wave frequency to be the order of the Coriolis parameter, a result that is of relevance to the problems of initialization. The theory also predicts an optimum strength of nudging, normally dependent on wavelength, wave speed, and latitude, which can give convergence of the assimilation on a timescale as short as a day. The results are verified by identical twin experiments using a full primitive equation model, the Free Surface Cox Code, both in barotropic spinup (results presented here) and in a more realistic baroclinic situation (results presented in Part II).

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Jinlun Zhang, Rebecca Woodgate, and Richard Moritz

Abstract

A coupled sea ice–ocean model is developed to quantify the sea ice response to changes in atmospheric and oceanic forcing in the Bering Sea over the period 1970–2008. The model captures much of the observed spatiotemporal variability of sea ice and sea surface temperature (SST) and the basic features of the upper-ocean circulation in the Bering Sea. Model results suggest that tides affect the spatial redistribution of ice mass by up to 0.1 m or 15% in the central-eastern Bering Sea by modifying ice motion and deformation and ocean flows. The considerable interannual variability in the pattern and strength of winter northeasterly winds leads to southwestward ice mass advection during January–May, ranging from 0.9 × 1012 m3 in 1996 to 1.8 × 1012 m3 in 1976 and averaging 1.4 × 1012 m3, which is almost twice the January–May mean total ice volume in the Bering Sea. The large-scale southward ice mass advection is constrained by warm surface waters in the south that melt 1.5 × 1012 m3 of ice in mainly the ice-edge areas during January–May, with substantial interannual variability ranging from 0.94 × 1012 m3 in 1996 to 2.0 × 1012 m3 in 1976. Ice mass advection processes also enhance thermodynamic ice growth in the northern Bering Sea by increasing areas of open water and thin ice. Ice growth during January–May is 0.90 × 1012 m3 in 1996 and 2.1 × 1012 m3 in 1976, averaging 1.3 × 1012 m3 over 1970–2008. Thus, the substantial interannual variability of the Bering Sea ice cover is dominated by changes in the wind-driven ice mass advection and the ocean thermal front at the ice edge. The observed ecological regime shifts in the Bering Sea occurred with significant changes in sea ice, surface air temperature, and SST, which in turn are correlated with the Pacific decadal oscillation over 1970–2008 but not with other climate indices: Arctic Oscillation, North Pacific index, and El Niño–Southern Oscillation. This indicates that the PDO index may most effectively explain the regime shifts in the Bering Sea.

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Igor V. Polyakov, Vladimir A. Alexeev, Igor M. Ashik, Sheldon Bacon, Agnieszka Beszczynska-Möller, Eddy C. Carmack, Igor A. Dmitrenko, Louis Fortier, Jean-Claude Gascard, Edmond Hansen, Jens Hölemann, Vladimir V. Ivanov, Takashi Kikuchi, Sergey Kirillov, Yueng-Djern Lenn, Fiona A. McLaughlin, Jan Piechura, Irina Repina, Leonid A. Timokhov, Waldemar Walczowski, and Rebecca Woodgate

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Michael A. Rawlins, Michael Steele, Marika M. Holland, Jennifer C. Adam, Jessica E. Cherry, Jennifer A. Francis, Pavel Ya Groisman, Larry D. Hinzman, Thomas G. Huntington, Douglas L. Kane, John S. Kimball, Ron Kwok, Richard B. Lammers, Craig M. Lee, Dennis P. Lettenmaier, Kyle C. McDonald, Erika Podest, Jonathan W. Pundsack, Bert Rudels, Mark C. Serreze, Alexander Shiklomanov, Øystein Skagseth, Tara J. Troy, Charles J. Vörösmarty, Mark Wensnahan, Eric F. Wood, Rebecca Woodgate, Daqing Yang, Ke Zhang, and Tingjun Zhang

Abstract

Hydrologic cycle intensification is an expected manifestation of a warming climate. Although positive trends in several global average quantities have been reported, no previous studies have documented broad intensification across elements of the Arctic freshwater cycle (FWC). In this study, the authors examine the character and quantitative significance of changes in annual precipitation, evapotranspiration, and river discharge across the terrestrial pan-Arctic over the past several decades from observations and a suite of coupled general circulation models (GCMs). Trends in freshwater flux and storage derived from observations across the Arctic Ocean and surrounding seas are also described.

With few exceptions, precipitation, evapotranspiration, and river discharge fluxes from observations and the GCMs exhibit positive trends. Significant positive trends above the 90% confidence level, however, are not present for all of the observations. Greater confidence in the GCM trends arises through lower interannual variability relative to trend magnitude. Put another way, intrinsic variability in the observations tends to limit confidence in trend robustness. Ocean fluxes are less certain, primarily because of the lack of long-term observations. Where available, salinity and volume flux data suggest some decrease in saltwater inflow to the Barents Sea (i.e., a decrease in freshwater outflow) in recent decades. A decline in freshwater storage across the central Arctic Ocean and suggestions that large-scale circulation plays a dominant role in freshwater trends raise questions as to whether Arctic Ocean freshwater flows are intensifying. Although oceanic fluxes of freshwater are highly variable and consistent trends are difficult to verify, the other components of the Arctic FWC do show consistent positive trends over recent decades. The broad-scale increases provide evidence that the Arctic FWC is experiencing intensification. Efforts that aim to develop an adequate observation system are needed to reduce uncertainties and to detect and document ongoing changes in all system components for further evidence of Arctic FWC intensification.

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