4. Changes inferred from T/S observations

In general, the vertical and spatial patterns of hydrographic parameters in the AO and adjacent North Atlantic had undergone considerable changes by IPY although the large-scale distributions of the water masses align with the historic climatology. Readers unfamiliar with AO geography and its bathymetric features are encouraged to follow this discussion with an atlas, e.g. https://geology.com/ articles/arctic-ocean-features/.

#### 4.1 Atlantic waters

Figure 8.

Figure 9.

Figure 10.

12

Summer PW depth of Tmax. IPY (l) and anomaly (r).

Arctic Studies - A Proxy for Climate Change

Summer PW Tmax. IPY (l) and anomaly (r).

Summer PW lower boundary depth. IPY (l) and anomaly (r).

Elevated pan-Arctic heat content due to the extraordinary heat transported to the AO from the North Atlantic is a significant change evident during the IPY period. Advection of relatively warmer AW resulted in anomalous hydrographic state formation over the entire deep Arctic Basin [17, 38]. The temperatures within the core of AW were observed 0.3–1.0°C higher than climatic values; mean changes are 0.65°C over the Eurasian Basin and 0.25°C over Canada and Makarov basins.

Of further note is the warm tongue of AW that appears to be topographically steered by the Lomonosov Ridge; Figure 4 shows a clear 0.5°C core temperature anomalous increase extending from the Laptev Sea toward the Greenland Shelf. This feature resides at a depth of about 275 m, 75 m surfaceward of the historic AW core depth per Figure 3. Over the Makarov Basin, AW expanded 50 m deep into the column [39], while the AW core depth has moved 100–150 m surfaceward with an associated 0.5–1.0 GJ/m<sup>2</sup> increase in associated heat content. Similar changes including the AW moving surfaceward and retaining more heat at depth are present throughout most of the AO indicating stronger potential influence on ice-related processes [40].

By 2007, the intermediate AW layer had deepened and thickened in the Pacific sector [23], but the changes are heterogeneous over the central and Eurasian basins. In particular, the net AW layer thickness appears to have thinned over the Amundsen Basin, which is likely a mass-balance response to the thickened layer observed on the Pacific side of Lomonosov Ridge. Within the western side of Fram Strait, the AW layer has thickened by roughly 70 m, moving 20 m closer to the surface without change in the core depth.

### 4.2 Pacific water

Figure 2 shows another of the most drastic changes in the Arctic—the change in freshwater distribution. As a proxy for the AW-PW upper-ocean front in the central Arctic, the strong FW anomaly gradient illustrates the change from the Lomonosov Ridge to the Alpha-Mendeleev Ridge (AMR) system [22 and references therein, 41]. Further, the boundary marking the extent of present SPW in Figures 9–11 tracks very directly the local bathymetric minimum of the AMR. Estimates shortly after IPY show that FWC in the Eurasian domain decreased by nearly one-quarter, while the American domain increased by the same percentage [16, 42]. The influx of PW through Bering Strait was near a record high in 2007, importing anomalously large FW volume and thermal input [20].

0.1°C per 0.01 PSU so that inaccuracies in background salinity amplify errors in

Changes in Arctic Ocean Climate Evinced through Analysis of IPY 2007–2008 Oceanographic…

Figure 12 illustrates the inaccuracies of these assumptions by examining the relationship between near-surface temperatures observed by 2006–2009 ITP and FT calculated from the associated salinity. Observations are primarily over the Pacific sector and central Arctic. The thick diagonal line shows exact correspondence between observed T and FT. Colors indicate binned values of T + 1.8°C (T-FT) in winter (summer) in the left (right) plot, with dashed lines demarcating percentiles as labeled. In winter months of November–April, all observations correspond to freezing point, but only about 25% of measurements have T ≤ 1.64°C, the freezing temperature associated with 30 PSU. In summer months of May–October, temperatures clearly depart from freezing, but only 25% of measurements differ from freezing by more than 0.05°C. In both summer and winter, the vertical structure of the plots demonstrates inaccuracy of the 1.8°C at 32.86 PSU assumption; surface

Freshwater changes throughout the Arctic relate to changes in geostrophic current distributions. Over basins, the strengthened FW gradient between the Pacific and Atlantic sectors led to a very significant sea-surface height (SSH) changes, which in turn gives rise to changes in geostrophic currents [16]. The strengthening of geostrophic currents in the Pacific sector is suspected among the factors for the reduction of multiyear ice over the Canadian Basin [50]. Other factors include deepening AW over the Canada Basin since 2004, enhancing the strength of the BG, and its accumulation of freshwater [23]. A recent study demonstrates that atmospheric modulation of geostrophic boundary currents and SSH quantifiably relates

To analyze the quantitative difference in the mean circulation during the IPY period with respect to the climatological circulation, the IPY dataset was conditioned using the four-dimensional variational (4DVar) data assimilation (DA) approach [52, 53] in two ways. To find a quasi-stationary solution, the process uses 4DVar optimization of an ocean model forced by the corresponding heat, salt, and momentum fluxes inferred from NCEP/NCAR reanalysis and regional Pan-Arctic Ice-Ocean Modeling and Assimilation System (PIOMAS). In the nonstationary reconstructions, all available T/S data were averaged for model grid bins, and these

waters in the western Arctic have salinities in the range 30–32 PSU.

5.1 Quasi-stationary "climatological"circulation

to the Northern Hemisphere annular mode strength [51].

associated freezing temperature.

Shallow ITP-observed temperature. Winter (l) and summer (r).

DOI: http://dx.doi.org/10.5772/intechopen.80926

Figure 12.

5. Changes in circulation

15

The loss of FWC near the pole and in the western sector likely results from cyclonic AO moving more AW toward the eastern Amerasian Basin. Simultaneously, the wind-forced anticyclonic BG stored fresher SPW in the Pacific sector, accumulating an average of 4 m FWC on the Pacific side of the front. Much of this FW had been in place prior to 2007; the IPY FWC in the Beaufort Sea is nearly identical to that found for 2006 [21]. Carmack et al. also find that sea-ice freeze/ melt accounts for a net loss of FWC in the Beaufort Region, with riverine water and PW contributing roughly half of the regional FW [21]. Ge et al. find that the mean annual Yukon River outflow, the most significant meteoric source included in SPW, increased 8% between 1977 and 2006 [43].

An increasing trend in Eurasian catchment outflow also is evident [14] and related to changes in permafrost [44] and temporal changes in continental hydrological cycles [45]. Increased Siberian runoff suggests the apparent decreases in FW volumes adjacent to the Laptev and East Siberian seas arising from changes in seasonal ice and the regional dominance of AW, but these source changes alone do not explain FW accumulation observed in the Beaufort Sea during IPY and beyond [46]. Data-conditioned modeling of the 2008 circulation [29] suggests that this accumulation may be supported by transport from the Lincoln Sea [47] and/or regions north of Greenland.

Changes in the organization of water masses have also affected the outflow of AO through Fram Strait, located between Greenland and Svalbard. The Transpolar Drift mode arising from the cyclonic AOO regime impedes PW from reaching the continental shelf north of Greenland. Consequently PW may only exit the AO via the Canadian Archipelago [19], which has been shown to be a significant but variable route for AO export [5, 48, 49].

#### 4.3 Directly observed from ITP data

The gridded IPY data do not resolve a surface layer. Sea-surface temperature and salinity (SST and SSS, respectively) are temporally variable as they depend on the strongly seasonal Arctic diurnal effects. Additionally SST/S in the AO depends seasonally on sea-ice-related processes such as meltwater strata, brine rejection, rapid wintertime heat loss through sea-ice leads, etc. Models and SST satellite data products often assume a surface freezing temperature (FT) of 1.8°C, which assumes background salinity of 32.86 PSU. At that T/S state, FT sensitivity is

Changes in Arctic Ocean Climate Evinced through Analysis of IPY 2007–2008 Oceanographic… DOI: http://dx.doi.org/10.5772/intechopen.80926

Figure 12.

In particular, the net AW layer thickness appears to have thinned over the Amundsen Basin, which is likely a mass-balance response to the thickened layer observed on the Pacific side of Lomonosov Ridge. Within the western side of Fram Strait, the AW layer has thickened by roughly 70 m, moving 20 m closer to the

Figure 2 shows another of the most drastic changes in the Arctic—the change in

freshwater distribution. As a proxy for the AW-PW upper-ocean front in the central Arctic, the strong FW anomaly gradient illustrates the change from the Lomonosov Ridge to the Alpha-Mendeleev Ridge (AMR) system [22 and references

therein, 41]. Further, the boundary marking the extent of present SPW in Figures 9–11 tracks very directly the local bathymetric minimum of the AMR. Estimates shortly after IPY show that FWC in the Eurasian domain decreased by nearly one-quarter, while the American domain increased by the same percentage [16, 42]. The influx of PW through Bering Strait was near a record high in 2007,

The loss of FWC near the pole and in the western sector likely results from cyclonic AO moving more AW toward the eastern Amerasian Basin. Simultaneously, the wind-forced anticyclonic BG stored fresher SPW in the Pacific sector, accumulating an average of 4 m FWC on the Pacific side of the front. Much of this FW had been in place prior to 2007; the IPY FWC in the Beaufort Sea is nearly identical to that found for 2006 [21]. Carmack et al. also find that sea-ice freeze/ melt accounts for a net loss of FWC in the Beaufort Region, with riverine water and PW contributing roughly half of the regional FW [21]. Ge et al. find that the mean annual Yukon River outflow, the most significant meteoric source included in SPW,

An increasing trend in Eurasian catchment outflow also is evident [14] and related to changes in permafrost [44] and temporal changes in continental hydrological cycles [45]. Increased Siberian runoff suggests the apparent decreases in FW volumes adjacent to the Laptev and East Siberian seas arising from changes in seasonal ice and the regional dominance of AW, but these source changes alone do not explain FW accumulation observed in the Beaufort Sea during IPY and beyond [46]. Data-conditioned modeling of the 2008 circulation [29] suggests that this accumulation may be supported by transport from the Lincoln Sea [47] and/or

Changes in the organization of water masses have also affected the outflow of AO through Fram Strait, located between Greenland and Svalbard. The Transpolar Drift mode arising from the cyclonic AOO regime impedes PW from reaching the continental shelf north of Greenland. Consequently PW may only exit the AO via the Canadian Archipelago [19], which has been shown to be a significant but

The gridded IPY data do not resolve a surface layer. Sea-surface temperature and salinity (SST and SSS, respectively) are temporally variable as they depend on the strongly seasonal Arctic diurnal effects. Additionally SST/S in the AO depends seasonally on sea-ice-related processes such as meltwater strata, brine rejection, rapid wintertime heat loss through sea-ice leads, etc. Models and SST satellite data products often assume a surface freezing temperature (FT) of 1.8°C, which assumes background salinity of 32.86 PSU. At that T/S state, FT sensitivity is

importing anomalously large FW volume and thermal input [20].

surface without change in the core depth.

Arctic Studies - A Proxy for Climate Change

increased 8% between 1977 and 2006 [43].

variable route for AO export [5, 48, 49].

4.3 Directly observed from ITP data

regions north of Greenland.

14

4.2 Pacific water

Shallow ITP-observed temperature. Winter (l) and summer (r).

0.1°C per 0.01 PSU so that inaccuracies in background salinity amplify errors in associated freezing temperature.

Figure 12 illustrates the inaccuracies of these assumptions by examining the relationship between near-surface temperatures observed by 2006–2009 ITP and FT calculated from the associated salinity. Observations are primarily over the Pacific sector and central Arctic. The thick diagonal line shows exact correspondence between observed T and FT. Colors indicate binned values of T + 1.8°C (T-FT) in winter (summer) in the left (right) plot, with dashed lines demarcating percentiles as labeled. In winter months of November–April, all observations correspond to freezing point, but only about 25% of measurements have T ≤ 1.64°C, the freezing temperature associated with 30 PSU. In summer months of May–October, temperatures clearly depart from freezing, but only 25% of measurements differ from freezing by more than 0.05°C. In both summer and winter, the vertical structure of the plots demonstrates inaccuracy of the 1.8°C at 32.86 PSU assumption; surface waters in the western Arctic have salinities in the range 30–32 PSU.

## 5. Changes in circulation

### 5.1 Quasi-stationary "climatological"circulation

Freshwater changes throughout the Arctic relate to changes in geostrophic current distributions. Over basins, the strengthened FW gradient between the Pacific and Atlantic sectors led to a very significant sea-surface height (SSH) changes, which in turn gives rise to changes in geostrophic currents [16]. The strengthening of geostrophic currents in the Pacific sector is suspected among the factors for the reduction of multiyear ice over the Canadian Basin [50]. Other factors include deepening AW over the Canada Basin since 2004, enhancing the strength of the BG, and its accumulation of freshwater [23]. A recent study demonstrates that atmospheric modulation of geostrophic boundary currents and SSH quantifiably relates to the Northern Hemisphere annular mode strength [51].

To analyze the quantitative difference in the mean circulation during the IPY period with respect to the climatological circulation, the IPY dataset was conditioned using the four-dimensional variational (4DVar) data assimilation (DA) approach [52, 53] in two ways. To find a quasi-stationary solution, the process uses 4DVar optimization of an ocean model forced by the corresponding heat, salt, and momentum fluxes inferred from NCEP/NCAR reanalysis and regional Pan-Arctic Ice-Ocean Modeling and Assimilation System (PIOMAS). In the nonstationary reconstructions, all available T/S data were averaged for model grid bins, and these

Figure 13.

Quasi-stationary model reconstruction of SSH and near-surface currents from historical 1900–2006 data (left plots) and the IPY data (right plots).

meaned observations were assimilated through the conventional 4DVar DA approach using a semi-implicit ocean model (SIOM) with resolution of 65 km; a framework of the algorithm is described in [52, 54].

The resulting quasi-stationary SSH maps and near-surface currents are shown in Figure 13. A comparison indicates the essential reorganization of the circulation in the AO evident during IPY. The most notable feature is the strong intensification and shift of the BG toward the Alaska. IPY SSH patterns are characterized by a pronounced BG dome which attains a central height greater than 50 cm, while the typical climatological SSH is only about 40 cm. This difference results from intensified westward flow along the Alaskan and Chukchi Sea continental slope. There is also a clear re-centering of the BG resulting from the shift of the Transpolar Drift axis toward the Canada Basin; this agrees well with the recent analysis of the freshwater content and circulations conducted by [55].

### 5.2 Anomalous 2008 circulation

The application of the more advanced 4DVar reconstruction of nonstationary circulation for July–December 2008 indicates stronger circulation than those directly detected from the in situ IPY dataset.

The SIOM-4DVar reconstructed bimonthly evolution of SSH and circulation at 250 m during July–December 2008 is shown in Figure 14. The SSH patterns are characterized by a pronounced BG dome which gets slightly stronger in November– December (Figure 14, right) attaining a 40 cm central elevation. Compared to the relatively smooth and symmetric SSH derived through optimal interpolation of observations (e.g., [16]), the DA-reconstructed SSH reveals finer features consistent with the observations. During September–October, the SSH pattern is characterized by a secondary SSH maximum at 74°N 140°W, which tends to erode by the end of the year but still persists as a tongue spreading toward Alaska along 140°W. This feature is seen in the AVISO anomalies averaged over the second half of 2008 [29].

the area north of the Bering Strait (upper panels in Figure 14), the heights of which are estimated to be 11, 10, and 6 cm, respectively. This is consistent with the seasonal decline of the Bering Strait inflow from 1.1 Sv in July–August to 0.5 Sv in

Bimonthly averaged fields for SSH in cm (upper panels) and velocity at 250 m depth in cm/s (lower panels). NABOS mooring-observed velocities are shown by insets. The boundaries used for calculating the total FW flux around BG are shown by the thick gray line in the top-right panel; the gray line adjacent to the figure panel frame is the "eastern" boundary around BG through which FW transport is positive (gyreward).

Changes in Arctic Ocean Climate Evinced through Analysis of IPY 2007–2008 Oceanographic…

DOI: http://dx.doi.org/10.5772/intechopen.80926

The effect of the abovementioned SSH decrease on the transport pattern in the region of the AW inflow is of particular interest. During July–August 2008, the negative SSH anomaly is closely attached to the coastline, creating a positive crossshelf SSH gradient and a westward geostrophic transport of 2.9 Sv along the shelf break (lower-left panels in Figure 14). The effect becomes less visible by the end of the year as the negative SSH anomaly detaches from the continental slope; the total transport relaxes to eastward values of 0.8 and 1.0 Sv, respectively, for the September–October and November–December periods. This identified flow reversal

November–December 2008 [20].

Figure 14.

17

Another prominent feature is a zonally spreading trough in the region between 72°N and 80°N from Severnaya Zemlya to the Bering Strait. The emergence of this depression could be one of the causes of intensification of the Bering Strait transport due to the increase of the large-scale sea level difference between the Chukchi and Bering Seas. This is supported by the analysis of Woodgate et al., who estimated the force balance controlling the flow through the Bering Strait and found a significant increase of the pressure head in 2007–2011 with respect to the 1997–2006 period when the Bering Strait transport was smaller [20 and Figure 1h therein]. The behavior of the SSH lowering is shown in the bimonthly SSH fields averaged over

Changes in Arctic Ocean Climate Evinced through Analysis of IPY 2007–2008 Oceanographic… DOI: http://dx.doi.org/10.5772/intechopen.80926

#### Figure 14.

meaned observations were assimilated through the conventional 4DVar DA approach using a semi-implicit ocean model (SIOM) with resolution of 65 km; a

The resulting quasi-stationary SSH maps and near-surface currents are shown in Figure 13. A comparison indicates the essential reorganization of the circulation in the AO evident during IPY. The most notable feature is the strong intensification and shift of the BG toward the Alaska. IPY SSH patterns are characterized by a pronounced BG dome which attains a central height greater than 50 cm, while the typical climatological SSH is only about 40 cm. This difference results from intensified westward flow along the Alaskan and Chukchi Sea continental slope. There is also a clear re-centering of the BG resulting from the shift of the Transpolar Drift axis toward the Canada Basin; this agrees well with the recent analysis of the

Quasi-stationary model reconstruction of SSH and near-surface currents from historical 1900–2006 data (left

The application of the more advanced 4DVar reconstruction of nonstationary circulation for July–December 2008 indicates stronger circulation than those

The SIOM-4DVar reconstructed bimonthly evolution of SSH and circulation at 250 m during July–December 2008 is shown in Figure 14. The SSH patterns are characterized by a pronounced BG dome which gets slightly stronger in November– December (Figure 14, right) attaining a 40 cm central elevation. Compared to the relatively smooth and symmetric SSH derived through optimal interpolation of observations (e.g., [16]), the DA-reconstructed SSH reveals finer features consistent with the observations. During September–October, the SSH pattern is characterized by a secondary SSH maximum at 74°N 140°W, which tends to erode by the end of the year but still persists as a tongue spreading toward Alaska along 140°W. This feature

Another prominent feature is a zonally spreading trough in the region between 72°N and 80°N from Severnaya Zemlya to the Bering Strait. The emergence of this depression could be one of the causes of intensification of the Bering Strait transport due to the increase of the large-scale sea level difference between the Chukchi and Bering Seas. This is supported by the analysis of Woodgate et al., who estimated the force balance controlling the flow through the Bering Strait and found a significant increase of the pressure head in 2007–2011 with respect to the 1997–2006 period when the Bering Strait transport was smaller [20 and Figure 1h therein]. The behavior of the SSH lowering is shown in the bimonthly SSH fields averaged over

is seen in the AVISO anomalies averaged over the second half of 2008 [29].

framework of the algorithm is described in [52, 54].

freshwater content and circulations conducted by [55].

directly detected from the in situ IPY dataset.

5.2 Anomalous 2008 circulation

Figure 13.

16

plots) and the IPY data (right plots).

Arctic Studies - A Proxy for Climate Change

Bimonthly averaged fields for SSH in cm (upper panels) and velocity at 250 m depth in cm/s (lower panels). NABOS mooring-observed velocities are shown by insets. The boundaries used for calculating the total FW flux around BG are shown by the thick gray line in the top-right panel; the gray line adjacent to the figure panel frame is the "eastern" boundary around BG through which FW transport is positive (gyreward).

the area north of the Bering Strait (upper panels in Figure 14), the heights of which are estimated to be 11, 10, and 6 cm, respectively. This is consistent with the seasonal decline of the Bering Strait inflow from 1.1 Sv in July–August to 0.5 Sv in November–December 2008 [20].

The effect of the abovementioned SSH decrease on the transport pattern in the region of the AW inflow is of particular interest. During July–August 2008, the negative SSH anomaly is closely attached to the coastline, creating a positive crossshelf SSH gradient and a westward geostrophic transport of 2.9 Sv along the shelf break (lower-left panels in Figure 14). The effect becomes less visible by the end of the year as the negative SSH anomaly detaches from the continental slope; the total transport relaxes to eastward values of 0.8 and 1.0 Sv, respectively, for the September–October and November–December periods. This identified flow reversal 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.

a. A reversal of the total transport in the AW inflow region of 2.9 Sv in July– August which later relaxed to an eastward transport of 0.8–1.0 Sv. This reversal of a long-slope current is confirmed by independent observations

Changes in Arctic Ocean Climate Evinced through Analysis of IPY 2007–2008 Oceanographic…

b.Formation of a prominent SSH trough extending from the eastern Laptev Sea to the Bering Strait. A similar and even stronger structure was obtained in the PIOMAS solution and is indirectly evidenced by two NABOS moorings located

c. The aforementioned SSH depression near the Chukchi Sea tends to increase the large-scale sea level difference between the Bering Sea and the AO. This contributes to the 25% increase in the Bering Strait transport at that time and agrees with the regional force balance suggesting an increased role of the pressure head between the Bering Sea and AO during 2007–2011 [20].

d.A significant total FWC of 20,700 km3 in the BG during 2008. The FW accumulation agrees with estimates from in situ hydrographic observations [46]. Analysis of the FW transports across model boundaries around the BG indicates that FW accumulation in 2008 was mainly caused by the anomalous

inflow through the eastern section. The DA model estimate of 0.8 Sv qualitatively agrees with other works [58, 59] that suggest FW sources may

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

from NABOS moorings.

DOI: http://dx.doi.org/10.5772/intechopen.80926

include from near Greenland.

work on the Navy Earth System Prediction Capability.

Acknowledgements

19

on the continental slope of the Laptev Sea.

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 found to be about 20,700 km<sup>3</sup> , which is slightly less (5%) than that found in 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 observed in 2008.

To assess the FW origin accumulated FWC in the BG, FW transports across the 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 of the model domain.

## 6. Summary

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

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 for establishing how far that transition has progressed.

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:

Changes in Arctic Ocean Climate Evinced through Analysis of IPY 2007–2008 Oceanographic… DOI: http://dx.doi.org/10.5772/intechopen.80926

