Freshwater Exchange Research Articles

Interannual to interdecadal variability of sea surface salinity in the Atlantic and its link to the atmosphere in a coupled model

The interannual to interdecadal variability of sea surface salinity (SSS) in the Atlantic is investigated using 200 years of a control simulation of the Bergen climate model. After verifying that the model behaves realistically, we show that most centers of SSS variability are located near the mouths of the main rivers of the basin. This variability is due primarily to anomalous runoff, except in the St. Lawrence area, and to anomalous Ekman and geostrophic currents, while fluctuations in the freshwater flux play a somewhat lesser role. Away from the coast, SSS variability is strong in the northern tropics, due largely to anomalous advection and freshwater exchanges. Regression analysis and a Lagrangian technique show that the SSS anomalies are advected by the mean oceanic circulation and can be followed along the gyres for several years. The link between SSS anomalies and the dominant modes of variability of the atmosphere is also investigated. The SSS response to the North Atlantic Oscillation (NAO) is dominated by relatively small scales that are forced by anomalous Ekman currents, but the relative influence of the large‐scale freshwater fluxes increases at long timescales. The NAO‐induced SSS anomalies are also strongly advected and distorted by the mean currents, so that their pattern at the interannual timescale only reflects in part that of the atmospheric forcing. The link between SSS anomalies and the East Atlantic pattern is similar.

Arctic decadal variability: An auto‐oscillatory system of heat and fresh water exchange

This paper presents a mechanism of decadal variability in the Artic Ocean–GIN Sea (Greenland, Iceland and Norwegian Seas) atmosphere‐ice‐ocean system. We hypothesize that Arctic variability is regulated by heat and freshwater exchange between the Arctic Ocean and the GIN Sea. The interaction between basins is weak during anticyclonic circulation regimes (low AO/NAO) and strong during cyclonic circulation regimes (high AO/NAO). Regime shifts are controlled by the system itself through oceanic and atmospheric gradients (dynamic height and surface air temperature) that increase during the anticyclonic regime and decrease during the cyclonic regime. This conceptual mechanism for Arctic decadal variability has been reproduced in a model experiment. Both model results and observational data support the suggested mechanism.

Comparison of the global air-sea freshwater exchange evaluated from independent datasets *

Abstract The implied air-sea freshwater flux is examined in two reanalysis datasets provided respectively by European Center for Medium-range Weather Forecasts and National Center for Environmental Prediction. Not only for annual mean state but also for seasonal variation, two reanalyses agree qualitatively well with each other, and both can reproduce reasonably the global distribution of E-P (i.e. Evaporation minus Precipitation). In the view of quantitative comparison, however, remarkable difference has been found on regional scales, especially over the middle and lower latitudes, with some local disagreement exceeding 100 cm/yr. One important difference between current and previous evaluation is the new found net evaporation over the high latitude North Atlantic, which is demonstrated to result from the transient disturbances during boreal winter.

Freshwater transports in HadCM3

The hydrological cycle can influence climate through a great variety of processes. A good representation of the hydrological cycle in climate models is therefore crucial. Attempts to analyse the global hydrological cycle are hampered by a deficiency of suitable observations, particularly over the oceans. Fully coupled general circulation models are potentially powerful tools in interpreting the limited observational data in the context of large-scale freshwater exchanges. We have looked at large-scale aspects of the global freshwater budget in a simulation, of over 1000 years, by the Hadley Centre coupled climate model (HadCM3). Many aspects of the global hydrological cycle are well represented, but the model hydrological cycle appears to be too strong, with overly large precipitation and evaporation components in comparison with the observational datasets we have used. We show that the ocean basin-scale meridional transports of freshwater come into near balance with the surface freshwater fluxes on a time scale of about 400 years, with the major change being a relative increase of freshwater transport from the Southern Ocean into the Atlantic Ocean. Comparison with observations, supported by sensitivity tests, suggests that the major cause of a drift to more saline condition in the model Atlantic is an overestimate of evaporation, although other freshwater budget components may also play a role. The increase in ocean freshwater transport into the Atlantic during the simulation, primarily coming from the overturning circulation component, which changes from divergent to convergent, acts to balance this freshwater budget deficit. The stability of the thermohaline circulation in HadCM3 may be affected by these freshwater transport changes and this question is examined in the context of an existing conceptual model.

Heat and Freshwater Fluxes through the Nordic Seas

Abstract The major exchanges of volume, heat, and freshwater between the Arctic Ocean and the World Ocean occur through the Nordic seas. Here is presented the northernmost estimate for the oceanic transport of these properties that is derived from a set of hydrographic and direct current measurements, using a lowered acoustic Doppler current profiler, between the Greenland and European coasts. By applying box inverse methods to a synoptic section from the summer of 1999, a heat transport of 0.20 ± 0.08 PW toward the Arctic and a freshwater transport of 0.10 ± 0.05 Sv (1 Sv ≡ 106 m3 s−1) away from the Arctic are calculated, with a likely additional freshwater transport on the order of 0.05 Sv near the Greenland coast. Uncertainties associated with how representative the section is of the seasonal mean are included in the error analysis. Large depth-independent components in the currents throughout the section, including the Atlantic inflow, are observed. The increase (decrease) in the heat transport result...

On the interannual variability of surface salinity in the Atlantic

The mechanisms controlling the interannual variability of sea surface salinity (SSS) in the Atlantic are investigated using a simulation with the ECHAM4/OPA8 coupled model and, for comparison, the NCEP reanalysis and an observed SSS climatology. Anomalous Ekman advection is found to be as important as the freshwater flux in generating SSS anomalies, in contrast to sea surface temperature (SST) anomalies which are primarily caused by surface heat flux fluctuations. Since the surface heat flux feedback does not damp the SSS anomalies but generally damps existing SST anomalies, SSS anomalies have a larger characteristic time scale. As a result, they are more influenced by the mean currents and the geostrophic variability, which dominate the SSS changes at low frequency over much of the basin. The link between SSS anomalies and the dominant patterns of atmospheric variability in the North Atlantic sector is also discussed. It is shown that the North Atlantic Oscillation generates SSS anomalies much more by Ekman advection than by freshwater exchanges. At least in the coupled model, there is little one-to-one correspondence between the main atmospheric and SSS anomaly patterns, unlike what is found for SST anomalies.

Comparison of the global air-sea freshwater exchange evaluated from independent datasets

Comparison of the global air-sea freshwater exchange evaluated from independent datasets

Mixing Between Two Canals Connected by an Automated Bi-directional Gated Structure, Gold Coast, Australia

Research was undertaken to examine the mixing and exchange of saltwater between a microtidal estuary and a lake connected by an automated box culvert bi-directional gated structure. The gates open four times each day and remain open for a period of 2h. They are programmed to open once during each semi-diurnal tidal phase, thus allowing alternative and partial exchange between the two water bodies. To determine the mixing dynamics, vertical profiles of temperature and salinity were collected as water flowed from the lake into the estuary and vice versa. The profiles were taken over a number of days and during various tidal ranges to examine the effect of tidal amplitude on the mixing dynamics of the plume. The results showed that the structure permits the exchange of saline and freshwater between the two systems, maintaining the lake system as a saline (brackish) environment. It perpetuated a permanently stratified environment on both sides of the structure under all tidal ranges examined.

Sensitivity of the thermohaline circulation to tropical and high latitude freshwater forcing during the last glacial‐interglacial cycle

Recent studies indicate that the tropical freshwater budget of the Atlantic is modulated by changes in El Niño‐Southern Oscillation (ENSO). If anomalies of the Atlantic freshwater balance persist on the order of decades, these might have a large influence on the North Atlantic thermohaline circulation (THC). Here we present a sensitivity study in which we use a model scenario for ENSO behavior during the last 120 kyr to force a simplified model of the THC with corresponding freshwater exchange between the tropical Atlantic and Pacific. If the steady state strength of the North Atlantic THC is similar to the present‐day, its response to the forcing is of the order of 1–3 Sv (1 Sv = 106 m3 s−1). No mode changes of the THC are simulated for reasonable values of the coupling constant between freshwater exchange and ENSO. If, on the other hand, the steady state overturning is significantly weaker, a collapse of the THC occurs due to forcing from the tropics. The modeled THC variation due to tropical freshwater perturbations are compared to those resulting from middle‐ and high‐latitude freshwater forcing due to long‐term growth and decay of Northern Hemisphere ice sheets. It is found that both forcings are of similar amplitude except during the deglaciation when high latitude forcing dominates. A possible out‐of‐phase relationship between deep water formation in the North Pacific and North Atlantic is also explored.

Ekman Transport Dominates Local Air–Sea Fluxes in Driving Variability of Subantarctic Mode Water

Subantarctic Mode Water (SAMW) is formed by deep convection in winter on the equatorward side of the Antarctic Circumpolar Current. Observations south of Australia show that the SAMW temperature (T) and salinity (S) vary significantly from year to year. The magnitude and density-compensating nature of the temperature and salinity changes cannot be explained by variations in air–sea exchange of heat and freshwater in the subantarctic zone where SAMW is formed. Rather, the T and S variability reflects variations in the equatorward Ekman transport of cool, low salinity water across the subantarctic front. Experiments with a coupled climate model suggest that the observations south of Australia are typical of the subantarctic zone. The model changes in SAMW properties are correlated significantly (at 99% level) with changes in wind stress and northward Ekman transport of cool low-salinity water. In contrast, air–sea heat flux anomalies are mostly a response to changes in SST, and anomalies in precipitation minus evaporation in the subantarctic zone are too small to account for the model SAMW salinity variations. Mode waters provide significant reservoirs of heat and freshwater that extend below the depth of the seasonal thermocline and, hence, can persist from year to year. The fact that wind stress variations can drive changes in mode water properties therefore has implications for climate variability.

Causes of long-term changes in Aegean sea deep water

Recent observations of newly formed deep water in the Aegean sea prompts this analysis of whether such deep water could be formed locally by the combination of an increase in net evaporation and wintertime water mass transformation. River diversion projects in Russia and Egypt since the 1950s have effectively increased the amount of net evaporation over the eastern Mediterranean basin. Historical profiles show that low salinity intermediate waters separated the deep and upper waters in the Aegean in 1961–1962. Within a simple mixed layer model, imposing a small net evaporation of 10 cm·yr –1 on the observed hydrographic conditions in March 1962 results in the gradual erosion of the low salinity intermediate waters. After 25 years, the low salinity intermediate waters are absent in agreement with observations made in September 1987. Continuing to force the model with the small net evaporation and with monthly heat and freshwater exchange anomalies from March 1987, new deep water could have been formed as early as March 1988. In the model, major deep water formation events occurred during the severe winters of 1991–1992 and 1992–1993 resulting in the formation of saltier and notably colder deep waters. The effective increase in net evaporation slowly increases the salinity and decreases the stratification in the Aegean sea until a severe winter leads to deep convection and new bottom water formation.

The Role of Ice–Ocean Interactions in the Variability of the North Atlantic Thermohaline Circulation

The simulated influence of Arctic sea ice on the variability of the North Atlantic climate is discussed in the context of a global coupled ice‐ocean‐atmosphere model. This coupled system incorporates a general circulation ocean model, an atmospheric energy moisture balance model, and a dynamic‐thermodynamic sea ice model. Under steady seasonal forcing, an equilibrium solution is obtained with very little variability. To induce variability in the model, daily varying stochastic anomalies are applied to the wind forcing of the Northern Hemisphere sea ice cover. These stochastic anomalies have observed spatial patterns but are random in time. Model simulations are run for 1000 yr from an equilibrium state and the variability in the system is analyzed. The sensitivity of the system to the ice‐ocean coupling of both heat and freshwater is also examined. Under the stochastic forcing conditions, the thermohaline circulation (THC) responds with variability that is approximately 10% of the mean. This variability has enhanced spectral power at interdecadal timescales that is concentrated at approximately 20 yr. It is forced by fluctuations in the export of ice from the Arctic into the North Atlantic. Substantial changes in sea surface temperature and salinity are related to changes in the overturning circulation and the sea ice coverage in the northern North Atlantic. Additionally, the THC variability influences the Arctic Basin through heat transport under the ice pack. Results from sensitivity studies suggest that the freshwater exchange from the variable ice cover is the dominant process for forcing variability in the overturning. The simulated Arctic ice export appears to provide stochastic forcing to the northern North Atlantic that excites a damped oscillatory ocean-only mode. The insulating capacity of the variable sea ice has a negligible effect on the THC. Ice‐ocean thermal coupling acts to damp THC variability, causing an approximately 25% reduction in the THC standard deviation.

Simulating the ice‐thickness distribution in a coupled climate model

Climate simulations in a global coupled model are investigated using a dynamic‐thermodynamic sea ice and snow model with sophisticated thermodynamics and a subgrid scale parameterization for multiple ice thicknesses. In addition to the sea ice component, the model includes a full primitive‐equation ocean and a simple energy‐moisture balance atmosphere. We introduce a formulation of the ice thickness distribution that is Lagrangian in thickness‐space. The method is designed to use fewer thickness categories because it adjusts to place resolution where it is needed most and it is free of diffusive effects that tend to smooth Eulerian distributions. Experiments demonstrate that the model does reasonably well in simulating the mean Arctic climate. We find the climate of the Arctic and northern North Atlantic is sensitive to resolving the ice‐thickness distribution when comparing the model results to a simulation with a two‐level sea ice model. The ice‐thickness distribution causes ice export through Fram Strait to be more variable and more strongly linked to meridional overturning in the North Atlantic Ocean.The Lagrangian formulation of the ice‐thickness distribution allows for the inclusion of a vertical temperature profile with relative ease compared to an Eulerian method. We find ice growth rates and ocean surface salinity differ in our model with a well‐resolved vertical temperature profile in the ice and snow and an explicit brine‐pocket parameterization compared to a simulation with Semtner zero‐layer thermodynamics. Although these differences are important for the climate of the Arctic, the effects of an ice thickness distribution are more dramatic and extend into the northern North Atlantic. Sensitivity experiments indicate that five ice‐thickness categories with ∼50‐cm vertical temperature resolution capture the effects of the ice‐thickness distribution on the heat and freshwater exchange across the surface in the presence of sea ice in these simulations.

The coupling procedure of air-sea freshwater exchange in climate system models

A coupling procedure of air-sea freshwater exchange in climate system models is reported in this note. The first stage of the procedure is to force OGCM to equilibrium under strong restoring surface condition on salinity, then increase the relaxing coefficient and get another steady state. The second stage is to switch the forcing on salinity from the weak restoring condition to the flux condition, and then finish a long-term spinning-up integration. After finishing these OGCM spinning-up stages, the last stage is to couple the OGCM with an active atmosphere, i.e. AGCM. Verification with the Global-Ocean-Atmosphere-Land-System model developed at the State Key Laboratory of Atmospheric Sciences and Geophysical Fluid Dynamics (LASG) shows that the preferred procedure is successful in including the air-sea freshwater exchange process.

Response of IAP/ LASG GOALS model to the coupling of air-Sea fresh water exchange

The process of air—sea fresh water exchange is included successfully in the Global— Ocean—Atmosphere Land—System model developed at the State Key Laboratory of Atmospheric Sciences and Geophysical Fluid Dynamics (LASG). The results of the coupled integration show that the climate drift has been controlled successfully. Analyses on the responses of ocean circulation to the changes of surface fresh water or salinity forcing show that the ocean spin-up stage under flux condition for salinity is the key to the implementation of air-sea fresh water flux coupling. This study also demonstrates that the Modified—Monthly—Flux—Anomaly coupling scheme (MMFA) brought forward by Yu and Zhang (1998) is suitable not only for daily air—sea heat flux coupling but also for daily fresh water flux coupling.

Role of sea ice in global climate change pondered

Large changes in Arctic sea ice cover have occurred in the last few years, though whether they are part of a long‐term natural variability or are related to increased radiative forcing from anthropogenic greenhouse gases is unclear. Knowing how sea ice changes have affected, and will continue to affect, climate (through changes to the surface albedo and through alterations to the ocean‐atmospheric exchange of freshwater and heat) may be crucial for making future climate predictions.Processes in the Arctic that affect climate include melt‐ponds, lead fractions, brine rejection, and ice‐ice interactions. Quite a number more may exist. But, however many, they operate at relatively small scales that cannot be directly incorporated into current climate models. The high latitudes are expected to be the most sensitive regions to possible anthropogenically forced climate changes, yet observations of polar climate and understanding of polar processes have not proceeded as fast as our appreciation of their importance.

THERMOHALINE CIRCULATION: High-Latitude Phenomena and the Difference Between the Pacific and Atlantic

▪ Abstract Deepwater formation, the process whereby surface water is actively converted into deep water through heat and freshwater exchange at the air-sea interface, is known to occur in the North Atlantic but not in the North Pacific. As such, the thermohaline circulation is fundamentally different in these two regions. In this review we provide a description of this circulation and outline a number of reasons as to why deep water is formed in the North Atlantic but not in the North Pacific. Special emphasis is given to the role of interactions with the Arctic Ocean. We extend our analysis to discuss the observational evidence and current theories for decadal-interdecadal climate variability in each region, with particular focus on the role of the ocean. Differences between the North Atlantic and North Pacific are highlighted.

A Generalized Depth-Integrated Model of the Oceanic Mixed Layer

Abstract A generalized depth-integrated model of the oceanic mixed layer is developed by considering the heat and energy budgets of the upper ocean. Unlike the Kraus–Turner-type bulk models, the assumptions of an a priori well mixed layer and a positive density discontinuity at the base of the layer are not required in the present formulation. Relaxation of these restrictions is achieved by dealing directly with time-integrated equations, thus eliminating inaccuracies introduced by forming a differential equation prior to time discretization. The formulation involves a careful accounting of the effects of stratification below the mixed layer and the change in the density of the mixed layer produced by heat absorption and freshwater exchange. Scale analysis of the equation governing changes in mixed layer depth is used to reveal the conditions under which the present model reduces to a standard Kraus–Turner-type bulk model. Model results are compared with observations at OWS Papa in order to confirm the ut...

Global Thermohaline Circulation. Part II: Sensitivity with Interactive Atmospheric Transports

Abstract A hybrid coupled ocean–atmosphere model is used to investigate the stability of the thermohaline circulation (THC) to an increase in the surface freshwater forcing in the presence of interactive meridional transports in the atmosphere. The ocean component is the idealized global general circulation model used in Part I. The atmospheric model assumes fixed latitudinal structure of the heat and moisture transports, and the amplitudes are calculated separately for each hemisphere from the large-scale sea surface temperature (SST) and SST gradient, using parameterizations based on baroclinic stability theory. The ocean–atmosphere heat and freshwater exchanges are calculated as residuals of the steady-state atmospheric budgets. Owing to the ocean component’s weak heat transport, the model has too strong a meridional SST gradient when driven with observed atmospheric meridional transports. When the latter are made interactive, the conveyor belt circulation collapses. A flux adjustment is introduced in which the efficiency of the atmospheric transports is lowered to match the too low efficiency of the ocean component. The feedbacks between the THC and both the atmospheric heat and moisture transports are positive, whether atmospheric transports are interactive in the Northern Hemisphere, the Southern Hemisphere, or both. However, the feedbacks operate differently in the Northern and Southern Hemispheres, because the Pacific THC dominates in the Southern Hemisphere, and deep water formation in the two hemispheres is negatively correlated. The feedbacks in the two hemispheres do not necessarily reinforce each other because they have opposite effects on low-latitude temperatures. The model is qualitatively similar in stability to one with conventional “additive” flux adjustment, but quantitatively more stable.

Laboratory simulation of exchange through Fram Strait

Laboratory experiments and theory were conducted to observe the flow patterns and transport in both buoyancy‐driven and wind‐driven rotating fluids. In “lock‐exchange” experiments, water with one density flows into a second basin after a sliding gate is removed. Water of a second density flows back into the first basin. The size and location of the currents for various values of density difference, rotation rate, and assorted sidewall geometries was recorded. Volume flux of the fluid was also measured and compared with a theory for lock‐exchange flow of a rotating fluid. In a separate group of experiments with a passive upper layer, easterly winds (like those in the Arctic Ocean) drive the upper level water into the Arctic Ocean and therefore oppose the buoyant exchange. Westerly winds would drive the water out of the Arctic Ocean. This indicates that the exchange between the Arctic Ocean and the Greenland‐Norwegian Sea is likely to be driven by buoyancy rather than by driven by wind. Crude estimates of the volumetric and fresh water exchange rate from the lock‐exchange formulas are compared with observed ocean fluxes, and approximate agreement is found.