Acknowledgements

agrees well with moored velocity observations from the Nansen and Amundsen Basins Observational System (NABOS, http://nabos.iarc.uaf.edu/data), which are indicated by red arrows in Figure 14 but were not used to obtain the optimized solution. The DA results immediately provide us with quantitative FWC estimates and permit identification of the regional FW. In particular, the total FWC within the volume bounded within [70.25, 80]°N [140, 170]°W above 400 m depth was

literature [update from 46]. A possible source of this difference is a smaller area of the integration for the 4DVar solution and the offshore displacement of the BG

eastern, southern, and western boundaries were estimated 0.08, 0.005, and 0.075 Sv, respectively (positive-oriented gyreward); the boundaries are shown in the top-right panel of Figure 14, where the eastern boundary abuts the figure boundary and the southern one intersects the Alaska coast. Calculated transports suggest that observed changes in the BG FWC were generally caused by the FW transport changes confined to the latitude band of 72–77°N at the eastern boundary

To assess the FW origin accumulated FWC in the BG, FW transports across the

This work introduces an IPY snapshot ocean climatology and discusses freshwater and thermal changes in two principle water masses to establish, in perspective, subsurface changes over the central AO as well as consequences of surface freshening. It focuses only on the ocean and readily neglected continental shelves where important water mass-forming processes occur [56] but enhanced mixing impedes analysis based on T/S, any resolvable changes in Arctic Bottom Water, and a direct analysis of sea ice which requires an extensive discussion of the atmosphere and its

Changes in the AO are not monotonic as they result from cyclic and quasi-cyclic changes in various superimposed feedback-entangled geophysical components in addition to trends in their background values. Changes may arrive in short bursts or "pulses" and may undergo periods of relaxation toward long-term means. The intensive pan-Arctic IPY survey provides evidence of an AO undergoing significant changes and departure from the longer-term mean of the late twentieth century responding to variations in source content (from the Atlantic, Pacific, and continental waters) and the resulting changes in freshwater and heat distribution; atmospheric forcing, induced SSH gradients, and their associated geostrophic responses; and relative volume and means of exit of various water masses present in the AO. During IPY, many of these components appeared to be establishing new records. In the decade following, 2011–2012 set records for associated components such as river outflow, Bering Strait inflow, sea-ice minimum, and Arctic cyclone strength—some of which may have been surpassed those of 2016–2017. From this perspective, conditions of the AO during IPY 2007–2008 show that the region is in transition toward a "new normal," and a gridded IPY dataset provides a useful reference state

A model-DA system was also applied and may quantify the observed difference in the T/S distribution bought on climatological and seasonal temporal scales. The reconstructed mean 2007–2009 AO circulation clearly identified global shifts in the BG and axis of the transpolar drift. Both results are consistent with other qualitative analyses. Analysis of the reconstructed nonstationary circulation for July–December 2008 allowed quantification of several anomalous circulation features including:

variability [57] which are beyond the scope of this presentation.

for establishing how far that transition has progressed.

, which is slightly less (5%) than that found in

found to be about 20,700 km<sup>3</sup>

Arctic Studies - A Proxy for Climate Change

observed in 2008.

of the model domain.

6. Summary

18

J. N. Stroh thanks the University of Nevada, Reno DeLaMare Library, for document preparation resources. G. Panteleev and M. Yaremchuk were supported by the Office of Naval Research (ONR) project "Arctic data assimilation," Program Element 0602435N. O. Francis was supported by the Coastal Hydraulics Engineering Resilience (CHER) Lab, Department of Civil and Environmental Engineering, and National Sea Grant College Program at the University of Hawaii at Manoa. M. Yaremchuk was also supported by the ONR Program Element 0603207N under work on the Navy Earth System Prediction Capability.

Arctic Studies - A Proxy for Climate Change
