Ocean Freshwater Budgets Research Articles

The Potential Role of Bering Strait in the Dynamics of Multidecadal Variability in the North Atlantic: An Idealized Model Study

Abstract Multidecadal variability on time scales between 20 and 70 years have been observed in the time series of North Atlantic SST. Many mechanisms have been proposed to explain multidecadal variabilities in the Atlantic. Generally, it is the interaction between the meridional overturning circulation (MOC) and North Atlantic surface buoyancy distribution that sustains this variability, with buoyancy anomalies either due to ocean-only processes or to air–sea interactions. In this context, the role of the Arctic Ocean, especially its freshwater flux into the North Atlantic, has been underappreciated. Bering Strait, the only oceanic pathway between the Pacific Ocean and the Arctic Ocean, has been found important in Arctic Ocean freshwater budget and in modulating the time-averaged state and long-term response of the MOC to high-latitude buoyancy forcing anomalies, via freshwater transport between the Pacific and Atlantic Oceans. In this paper, we use idealized configurations that include a Pacific-like wide basin and an Atlantic-like narrow basin. The two basins are connected both in the south and north to longitudinally periodic channels, representing the Southern Ocean and the Arctic Ocean, respectively. The Pacific-like basin is opened to the north only through a shallow and narrow strait, while the Atlantic-like basin is fully open to the north. With the goal of studying the role of Bering Strait in the multidecadal variability, we find that the freshwater transport from the Bering Strait forms a tongue structure along the western boundary of the narrow basin, which enhances the local horizontal density gradient. The western boundary region becomes unstable to large-scale baroclinic anomalies, giving rise to multidecadal variability.

Decadal and Intra-Annual Variability of the Indian Ocean Freshwater Budget

Abstract The global freshwater cycle is intensifying: wet regions are prone to more rainfall, while dry regions experience more drought. Indian Ocean rim countries are especially vulnerable to these changes, but its oceanic freshwater budget—which records the basinwide balance between evaporation, precipitation, and runoff—has only been quantified at three points in time (1987, 2002, 2009). Due to this paucity of observations and large model biases, we cannot yet be sure how the Indian Ocean’s freshwater cycle has responded to climate change, nor by how much it varies at seasonal and monthly time scales. To bridge this gap, we estimate the magnitude and variability of the Indian Ocean’s freshwater budget using monthly varying oceanic data from May 2016 through April 2018. Freshwater converged into the basin with a mean rate and standard error of 0.35 ± 0.07 Sv (1 Sv ≡ 106 m3 s−1), indicating that basinwide air–sea fluxes are net evaporative. This balance is maintained by salty waters leaving the basin via the Agulhas Current and fresher waters entering northward across the southern boundary and via the Indonesian Throughflow. For the first time, we quantify seasonal and monthly variability in Indian Ocean freshwater convergence to find amplitudes of 0.33 and 0.16 Sv, respectively, where monthly changes reflect variability in oceanic, rather than air–sea, fluxes. Compared with the range of previous estimates plus independent measurements from a reanalysis product, we conclude that the Indian Ocean has remained net evaporative since the 1980s, in contrast to long-term changes in its heat budget. When disentangling anthropogenic-driven changes, these observations of decadal and intra-annual natural variability should be taken into account.

Wind-forced depth-dependent currents over the eastern Beaufort Sea continental slope: Implications for Pacific water transport

Pacific water contributes significantly to the Arctic Ocean freshwater budget. Recent increases in Arctic freshwater flux, also affected by the Pacific-derived Arctic water, impact the Atlantic overturning circulation with implications for global climate. The interannual variability of the Pacific water outflow remains poorly understood, partly due to different branches of the Pacific water flow in the Arctic Ocean. The shelfbreak current over the Beaufort Sea continental slope transports ~50% of the Pacific-derived water eastward along the Beaufort Sea continental slope towards the Canadian Archipelago. The oceanographic mooring deployed over the eastern Beaufort Sea continental slope in October 2003 recorded current velocities through depths of 28–108 m until September 2005. Data analysis revealed that these highly energetic currents have two different modes of depth-dependent behaviour. The downwelling-favourable wind associated with cyclones passing north of the Beaufort Sea continental slope toward the Canadian Archipelago generates depth-intensified shelfbreak currents with along-slope northeastward flow. A surface Ekman on-shore transport and associated increase of the sea surface heights over the shelf produce a cross-slope pressure gradient that drives an along-slope northeastward barotropic flow, in the same direction as the wind. In contrast, the upwelling-favourable wind associated with deep Aleutian Low cyclones over the Alaskan Peninsula and/or Aleutian Island Arc leads to surface-intensified currents with along-slope westward flow. This northeasterly wind generates a surface Ekman transport that moves surface waters offshore. The associated cross-slope pressure gradient drives an along-slope southwestward barotropic flow. The wind-driven barotropic flow generated by upwelling and downwelling is superimposed on the background bottom-intensified shelfbreak current. For downwelling, this flow amplifies the depth-intensified background baroclinic circulation with enhanced Pacific water transport towards the Canadian Archipelago. For upwelling, the shelfbreak current is reversed, which results in surface-intensified flow in the opposite direction. These results are supported by numerical simulations.

Antarctic Bottom Water Warming and Freshening: Contributions to Sea Level Rise, Ocean Freshwater Budgets, and Global Heat Gain*

Abstract Freshening and warming of Antarctic Bottom Water (AABW) between the 1980s and 2000s are quantified, assessing the relative contributions of water-mass changes and isotherm heave. The analysis uses highly accurate, full-depth, ship-based, conductivity–temperature–depth measurements taken along repeated oceanographic sections around the Southern Ocean. Fresher varieties of AABW are present within the South Pacific and south Indian Oceans in the 2000s compared to the 1990s, with the strongest freshening in the newest waters adjacent to the Antarctic continental slope and rise indicating a recent shift in the salinity of AABW produced in this region. Bottom waters in the Weddell Sea exhibit significantly less water-mass freshening than those in the other two southern basins. However, a decrease in the volume of the coldest, deepest waters is observed throughout the entire Southern Ocean. This isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential temperature maximum. The water-mass freshening of AABW in the Indian and Pacific Ocean sectors is equivalent to a freshwater flux of 73 ± 26 Gt yr−1, roughly half of the estimated recent mass loss of the West Antarctic Ice Sheet. Isotherm heave integrated below 2000 m and south of 30°S equates to a net heat uptake of 34 ± 14 TW of excess energy entering the deep ocean from deep volume loss of AABW and 0.37 ± 0.15 mm yr−1 of sea level rise from associated thermal expansion.

Assimilation impacts on Arctic Ocean circulation, heat and freshwater budgets

We investigate the Arctic basin circulation, freshwater content (FWC) and heat budget by using a high-resolution global coupled ice–ocean model implemented with a state-of-the-art data assimilation scheme. We demonstrate that, despite a very sparse dataset, by assimilating hydrographic data in and near the Arctic basin, the initial warm bias and drift in the control run is successfully corrected, reproducing a much more realistic vertical and horizontal structure to the cyclonic boundary current carrying the Atlantic Water (AW) along the Siberian shelves in the reanalysis run. The Beaufort Gyre structure and FWC and variability are also more accurately reproduced. Small but important changes in the strait exchange flows are found which lead to more balanced budgets in the reanalysis run. Assimilation fluxes dominate the basin budgets over the first 10 years (P1: 1987–1996) of the reanalysis for both heat and FWC, after which the drifting Arctic upper water properties have been restored to realistic values. For the later period (P2: 1997–2004), the Arctic heat budget is almost balanced without assimilation contributions, while the freshwater budget shows reduced assimilation contributions compensating largely for surface salinity damping, which was extremely strong in this run. A downward trend in freshwater export at the Canadian Straits and Fram Strait is found in period P2, associated with Beaufort Gyre recharge. A detailed comparison with observations and previous model studies at the individual Arctic straits is also included.

Simulated Response of the Arctic Freshwater Budget to Extreme NAO Wind Forcing

Abstract The authors investigate the response of the Arctic Ocean freshwater budget to changes in the North Atlantic Oscillation (NAO) using a regional-ocean configuration of the Massachusetts Institute of Technology GCM (MITgcm) and carry out several different 10-yr and 30-yr integrations. At 1/6° (∼18 km) resolution the model resolves the major Arctic transport pathways, including Bering Strait and the Canadian Archipelago. Two main calculations are performed by repeating the wind fields of two contrasting NAO years in each run for the extreme negative and positive NAO phases of 1969 and 1989, respectively. These calculations are compared both with a control run and the compiled observationally based freshwater budget estimate of Serreze et al. The results show a clear response in the Arctic freshwater budget to NAO forcing, that is, repeat NAO negative wind forcing results in virtually all freshwater being retained in the Arctic, with the bulk of the freshwater content being pooled in the Beaufort gyre. In contrast, repeat NAO positive forcing accelerates the export of freshwater out of the Arctic to the North Atlantic, primarily via Fram Strait (∼900 km3 yr−1) and the Canadian Archipelago (∼500 km3 yr−1), with a total loss in freshwater storage of ∼13 000 km3 (15%) after 10 yr. The large increase in freshwater export through the Canadian Archipelago highlights the important role that this gateway plays in redistributing the freshwater of the Arctic to subpolar seas, by providing a direct pathway from the Arctic basin to the Labrador Sea, Gulf Stream system, and Atlantic Ocean. The authors discuss the sensitivity of the Arctic Ocean to long-term fixed extreme NAO states and show that the freshwater content of the Arctic is able to be restored to initial values from a depleted freshwater state after ∼20 yr.

Projected changes in Arctic Ocean freshwater budgets

Arctic Ocean freshwater budgets are examined from 10 models participating in the Intergovernmental Panel on Climate Change Fourth Assessment Report. This includes an analysis of sea ice transport and storage, ocean transport and storage, and net surface flux exchange. Simulated budgets for the late 20th century are compared to available observations, followed by an analysis of simulated changes from 1950 to 2050. The consistent theme over this period is an acceleration of the Arctic hydrological cycle, which is expressed as an increase in the flux of water passing through the hydrologic elements. Increased freshwater inputs to the ocean from net precipitation, river runoff, and net ice melt result. While generally attended by a larger export of liquid freshwater to lower latitudes, primarily through Fram Strait, liquid freshwater storage in the Arctic Ocean increases. In contrast, the export and storage of freshwater in the form of sea ice decreases. The qualitative agreement between models for which the only common forcing is rising greenhouse gas concentrations implicates this greenhouse gas loading as the cause of the change. Although the models perform quite well in their simulations of net precipitation over the Arctic Ocean and terrestrial drainage, they differ significantly regarding the magnitude of the trends and their representation of contemporary mean ocean and sea ice budget terms. To reduce uncertainty in future projections of the Arctic freshwater cycle, the climate models as a group require considerable improvement in these aspects of their simulations.

Simulated Arctic Ocean Freshwater Budgets in the Twentieth and Twenty-First Centuries

Abstract The Arctic Ocean freshwater budgets in climate model integrations of the twentieth and twenty-first century are examined. An ensemble of six members of the Community Climate System Model version 3 (CCSM3) is used for the analysis, allowing the anthropogenically forced trends over the integration length to be assessed. Mechanisms driving trends in the budgets are diagnosed, and the implications of changes in the Arctic–North Atlantic exchange on the Labrador Sea and Greenland–Iceland–Norwegian (GIN) Seas properties are discussed. Over the twentieth and the twenty-first centuries, the Arctic freshens as a result of increased river runoff, net precipitation, and decreased ice growth. For many of the budget terms, the maximum 50-yr trends in the time series occur from approximately 1975 to 2025, suggesting that we are currently in the midst of large Arctic change. The total freshwater exchange between the Arctic and North Atlantic increases over the twentieth and twenty-first centuries with decreases in ice export more than compensated for by an increase in the liquid freshwater export. Changes in both the liquid and solid (ice) Fram Strait freshwater fluxes are transported southward by the East Greenland Current and partially removed from the GIN Seas. Nevertheless, reductions in GIN sea ice melt do result from the reduced Fram Strait transport and account for the largest term in the changing ocean surface freshwater fluxes in this region. This counteracts the increased ocean stability due to the warming climate and helps to maintain GIN sea deep-water formation.

Variability in river runoff distribution in the Eurasian Basin of the Arctic Ocean

The distribution of freshwater within the Arctic Ocean and its export from it are intimately involved in climate and climate change processes both within and outside the Arctic Ocean. River runoff in the Arctic Ocean constitutes a major part of the Arctic Ocean freshwater budget. Within the Arctic Ocean, variability in the distribution of river runoff will be reflected in the location of the cold halocline that isolates the sea ice from the warm Atlantic Layer. Outside the Arctic Ocean, such variability will impact on the salinity of North Atlantic waters (Great Salinity Anomaly) and on deep convection areas of the North Atlantic Ocean, and thereby potentially on global thermohaline circulation. Rivers entering the Arctic Ocean have high levels of total alkalinity that contribute significantly to the total alkalinity of the surface Polar Mixed Layer. We exploit total alkalinity data to trace river runoff in the surface Polar Mixed Layer and to observe variability in the river runoff distribution in the Eurasian Basin over the period 1987–2001. The river runoff front changed from a position over the Gakkel Ridge in 1987 and 1991 to over the Lomonosov Ridge in 1996, and returned to a midpoint between the two ridges in 2001. Wind field changes as characterized by the Arctic Oscillation index are considered to be a major factor in determining ice and surface water flow. We note a correlation with 4–6 years delay between changes in river runoff distribution and the Arctic Oscillation index. We show that the delay can be inferred from a geostrophic flow calculation.

Modelling the climate evolution from the last interglacial to the start of the last glaciation: The role of Arctic Ocean freshwater budget

The Earth's climate has alternated between glacial and interglacial stages, but we still do not understand how the changes in insolation have been amplified by the climate system to lead to such a large climatic response. Here we use a climate model of intermediate complexity to show that the orbitally induced shift from an interglacial to a pre‐glacial climate is highly sensitive to the high northern latitudes moisture budget. Using the insolation forcing alone, the model produces a first order response of sea surface temperatures and thermohaline circulation evolution which agrees with the available geological data. Under the same gradual insolation evolution, adding a small freshwater input into the Arctic Ocean induces a rapid climate switch from an interglacial to a pre‐glacial mode. These results suggest a simple connection between orbital forcing and the thermohaline circulation through a freshwater threshold within the ocean‐atmosphere‐sea‐ice system.

Preconditioning the Greenland Sea for deep convection: Ice formation and ice drift

The role of sea ice in preconditioning the mixed layers of the central Greenland Sea for deep convection is investigated, with particular emphasis on the formation of the “Nordbukta.” The opening of the ice free bay in late January 1989 indicated that the upper layer was well preconditioned for deep convection which reached down to 1500 m depth in March 1989. We propose that the ice free bay occurred due to diminishing new ice formation without extensive ice melt. A key process is wind‐driven ice drift to the southwest, as observed by upward looking acoustic Doppler current profilers, which will alter the upper ocean freshwater budget when an ice volume gradient along the ice‐drift direction exists. We investigated the importance and effects of such an ice‐drift‐induced freshwater loss on upper ocean properties using an ice‐ocean mixed‐layer model. Observed temperature and salt profiles from December 1988 served as initial conditions, and the model was integrated over the winter season. Given the one‐dimensional physics and climatological surface fluxes, the model was not able to produce a reasonable ice and mixed‐layer evolution. However, allowing ice drift to reduce the local ice thickness improved the ice‐ocean model performance dramatically. An average ice export of 5–8 mm d−1 was needed to be consistent with the observed evolution of mixed‐layer properties and ice cover. Using the same fluxes and ice export, but initial conditions from the “Is Odden” region, yielded ice cover throughout the winter over a shallow mixed layer, both of which are consistent with the observations from the Odden region.