Net Freshwater Flux Research Articles

Zonal circulation in the North Atlantic ocean at 52°W from WOCE-WHP and CLIVAR sections: 1997, 2003 and 2012

The A20 line is a meridional hydrographic section located at 52°W that cuts through the western North Atlantic Subtropical Gyre (NASG). It encloses the main paths of the Atlantic Meridional Overturning Circulation (AMOC). Using data from three A20 hydrographic cruises carried out in 1997, 2003 and 2012 together with Acoustic Doppler Current Profiler data and the velocities from an inverse box model, the circulation of the western NASG is estimated. The Gulf Stream is the main poleward path of the AMOC, carrying 155.3 ± 11.1, 102.7 ± 13.5 and 181.1 ± 14.9 Sv in 1997, 2003 and 2012, respectively (1 Sv = 106 m3/s ∼ 109 kg/s). In opposite direction, the Deep Western Boundary Current crosses the section at the northern/southern boundaries with a mass transport of −21.2 ± 8.9/29.0 ± 9.1, −14.4 ± 10.8/14.2 ± 8.1 and −37.9 ± 10.2/44.5 ± 9.8 Sv in 1997, 2003 and 2012, respectively. A net heat exchange from the ocean to the atmosphere is estimated to be −0.7 ± 0.1 PW and −0.6 ± 0.1 PW in 1997 and 2012, respectively, but is negligible in 2003 (0.1 ± 0.1 PW). The freshwater flux is significantly lower in 2003 (0.3 ± 0.1 Sv) than in 1997 (0.6 ± 0.1 Sv) and 2012 (0.6 ± 0.1 Sv). Ocean numerical models such as ECCO, GLORYS and MOM are used to address the interannual variability between the three surveys, but only the GLORYS output roughly agrees with the hydrographic data. GLORYS suggests a heat transport from the ocean to the atmosphere throughout the year and a net freshwater flux supported by precipitation plus river runoff higher than evaporation in all seasons except the spring.

A box model to represent estuarine dynamics in mesoscale resolution ocean models

Representing the net freshwater flux at river mouths is challenging for global and regional scale ocean modelling. Although rivers are well known to affect both the coastal and basin-wide circulation and dynamics, coarse resolution ocean models cannot resolve the estuarine dynamics and are usually forced at river outlets in a simplistic way, with climatological runoff and zero or constant salinity values. The aim of this study is to provide a more realistic representation of the estuarine water inputs to a coarse but eddy-resolving regional model.First, the river volume transport and salinity values at the outlets are modelled with three different Estuary Box Models (EBMs) for stratified estuaries: the Knudsen relations model, a published EBM, called UCONN-NCAR EBM, which parameterizes the tidal inflow and mixing inside the estuary, and a new model, called CMCC-EBM. The CMCC EBM has been conceived to represent the estuarine processes coupled to a mesoscale resolving hydrodynamic model that resolves the entering flow field at the estuary mouth and it offers a new representation of the tidal inflow and a new salinity tidal mixing parameterization via horizontal diffusive processes.The Ofanto and Po rivers flowing into the Adriatic Sea (northern part of the central Mediterranean Sea) are selected as case studies. The coupling of the eddy resolving ocean model to the CMCC EBM is found to outperform the one with the UCONN-NCAR EBM in the region of freshwater influence on the shelf areas.

Mediterranean overflow water in the North Atlantic and its multidecadal variability

The Mediterranean overflow water is one of the most important intermediate-depth water masses in the North Atlantic. To investigate its properties a pre-industrial simulation with the earth system model EC-Earth is used. The multidecadal variability of the outflow is analysed by examining the modelled volume and salt transports through the Strait of Gibraltar as well as different atmospheric patterns (such as the wind pattern and the net freshwater fluxes). The salinity evolution in the main core of the outflow in the mid-Atlantic is also taken into account. The leading empirical orthogonal functions for the modelled salinity 900 m coincided with the modelled distribution of outflow water. The associated principal component showed a multidecadal variability of the salinity field. The variability of the net salt transport through the Strait of Gibraltar showed a similar behaviour where the Atlantic-Mediterranean system manifested two clear states. One of these is when the Mediterranean imports salt from the Atlantic and the other is where salt export to the Atlantic predominates. This result indicates that the Mediterranean Sea acts as a storage of salt alternating between the two states. The negative phase of the North Atlantic oscillation appears to play a role driving the variability of the salt transport and its impact on the overturning circulation in the North Atlantic.

Mechanisms and Impacts of a Partial AMOC Recovery Under Enhanced Freshwater Forcing

AbstractThe Atlantic Meridional Overturning Circulation (AMOC) is expected to weaken in the 21st century due to increased surface buoyancy. Such AMOC changes in ocean models are often accompanied by a subsurface reduction in density. Here we perform freshwater perturbation experiments with both a 1° coupled model and an idealized zonally averaged ocean‐only model to demonstrate that slow subsurface property changes (1) introduce a negative feedback that erodes the stratification and partially reinvigorates convection and the AMOC and (2) ensure the meridional heat transport weakens less than the AMOC. In the coupled model with a 0.1‐Sv net freshwater flux introduced around Greenland, an initial 22% AMOC reduction over 40 years is followed by a recovery of almost half the lost strength after 400 years. The final heat transport, however, is weakened by only 7%. Similar responses in the idealized model demonstrate that 2‐D ocean‐only dynamics control the changes.

Transient Response of the Southern Ocean to Changing Ozone: Regional Responses and Physical Mechanisms

The impact of changing ozone on the climate of the Southern Ocean is evaluated using an ensemble of coupled climate model simulations. By imposing a step change from 1860 to 2000 conditions, response functions associated with this change are estimated. The physical processes that drive this response are different across time periods and locations, as is the sign of the response itself. Initial cooling in the Pacific sector is driven not only by the increased winds pushing cold water northward, but also by the southward shift of storms associated with the jet stream. This shift drives both an increase in cloudiness (resulting in less absorption of solar radiation) and an increase in net freshwater flux to the ocean (resulting in a decrease in surface salinity that cuts off mixing of warm water from below). A subsurface increase in temperature associated with this reduction in mixing then upwells along the Antarctic coast, producing a subsequent warming. Similar changes in convective activity occur in the Weddell Sea but are offset in time. Changes in sea ice concentration also play a role in modulating solar heating of the ocean near the continent. The time scale for the initial cooling is much longer than that seen in NCAR CCSM3.5, possibly reflecting differences in natural convective variability between that model (which has essentially no Southern Ocean deep convection) and the one used here (which has a large and possibly unrealistically regular mode of convection) or to differences in cloud feedbacks or in the location of the anomalous winds.

The contrast between Atlantic and Pacific surface water fluxes

The Atlantic Ocean is known to have higher sea surface salinity than the Pacific Ocean at all latitudes. This is thought to be associated with the Atlantic Meridional Overturning Circulation and deep water formation in the high latitude North Atlantic – a phenomenon not present anywhere in the Pacific. This asymmetry may be a result of salt transport in the ocean or an asymmetry in the surface water flux (evaporation minus precipitation; ) with greater over the Atlantic than the Pacific. In this paper, we focus on the surface water flux. Seven estimates of the net freshwater flux ( including run-off, R), calculated with different methods and a range of data sources (atmospheric and oceanic reanalyses, surface flux data-sets, hydrographic sections), are compared. It is shown that over the Atlantic is consistently greater than over the Pacific by about 0.4 Sv (1 Sv m s). The Atlantic/Pacific asymmetry is found at all latitudes between 30S and 60N. Further analysis with ERA-Interim combined with a run-off data-set demonstrates that the basin asymmetry is dominated by an evaporation asymmetry in the northern high-latitudes, but by a precipitation asymmetry everywhere south of 30N. At the basin scale, the excess of precipitation over the Pacific compared to the Atlantic (S N) dominates the asymmetry. Also it is shown that the asymmetry is present throughout the year and quite steady from year to year. Investigation of the interannual variability and trends suggest that the precipitation trends are not robust between data-sets and are indistinguishable from variability. However, a positive trend in evaporation (comparable to other published estimates) is seen in ERA-Interim, consistent with sea surface temperature increases.

Subtidal Exchange in Eastern Long Island Sound

AbstractLong Island Sound (LIS) is a large and wide macrotidal estuary with distributed river inputs, including the Connecticut River (the largest freshwater source) that flows into the eastern LIS near the mouth. In 2010, shipboard surveys of salinity, temperature, and currents were collected along an across-estuary transect in eastern LIS. Numerical model results are compared to these observations and used to study the spatial and temporal variability of salinity, velocity, and freshwater and salt fluxes over a 4-yr period. For all low wind conditions, observations and model results indicate an outward-flowing, low-salinity wedge on the south side with an inward-flowing, higher-salinity area underneath and to the north. Observations and model results during the low wind surveys indicate that stratification substantially decreases with increased tidal amplitude and decreased river discharge; the velocity field is less variable among surveys. Model analysis indicates strong sensitivities to both tides and river discharge; with discharge response strongest for salinity and freshwater flux and tidal response larger for velocities, volume flux, and salt flux. The long-term average net freshwater and salt fluxes are outward and inward, respectively. For both flux types, subtidal shear dispersion is twice tidal oscillatory diffusion, and both contributions are in the same direction as the net flux. The uniform flow contribution is small for freshwater flux, yet it is the largest single term for salt flux and partially counters the inward contributions.

Estimating the global oceanic net freshwater flux from Argo and comparing it with satellite-based freshwater flux products

Following the idea that analysis of in situ information in the salt budget could be used as a surrogate for global “ocean rain gauge,” the annual mean oceanic net freshwater flux (E-P) was estimated from the Argo profiles and the wind stress data on a global scale. The comparison between the independent E-P estimation from Argo and the E-P product sets, including the combination of precipitation from TRMM, GPCP, CMAP and evaporation from OAFlux, GSSTF3 and IFREMER and E-P set from NEWS formed from satellite, generally show similar spatial patterns, particularly on the large scale. However, there are differences among the different satellite-based E-P estimates and between satellite estimates and independent in situ estimates. Based on the pattern correlation and the RMSD, the evaporation and precipitation from OAFlux and TRMM agrees best with the E-P estimated from the independent Argo-based estimates.

Baffin Island and West Greenland Current Systems in northern Baffin Bay

Temperature, salinity, and direct velocity observations from northern Baffin Bay are presented from a summer 2003 survey. The data reveal interactions between fresh and cold Arctic waters advected southward along Baffin Island and salty and warm Atlantic waters advected northward along western Greenland. Geostrophic currents estimated from hydrography are compared to measured ocean currents above 600m depth. The Baffin Island Current is well constrained by the geostrophic thermal wind relation, but the West Greenland Current is not. Furthermore, both currents are better described as current systems that contain multiple velocity cores and eddies. We describe a surface-intensified Baffin Island Current seaward of the continental slope off Canada and a bottom-intensified West Greenland Current over the continental slope off Greenland. Acoustic Doppler current profiler observations suggest that the West Greenland Current System advected about 3.8±0.27Sv (Sv=106m3s−1) towards the north-west at this time. The most prominent features were a surface intensified coastal current advecting 0.5Sv and a bottom intensified slope current advecting about 2.5Sv in the same direction. Most of this north-westward circulation turned southward in the Baffin Island Current System. The Baffin Island system was transporting 5.1±0.24Sv to the south-east at the time that includes additional contributions from Nares Strait to the north (1.0±0.2Sv) and Lancaster Sound to the east (1.0±0.2Sv). Net freshwater fluxes were 72 and 187mSv for the West Greenland and Baffin Island Currents, respectively. Empirical uncertainty arises from unknown temporal variations at weekly time scales and pertubations introduced by unresolved eddies. Eddies with 10km horizontal and 400m vertical scales were common and recirculated up to 1Sv. Our 2003 observations represent conditions when the North-Atlantic Oscillation index (NAO) was close to zero. Analysis of historical hydrographic data averaged along isobaths during NAO-positive years reveals a baroclinic circulation in Baffin Bay more intense than 2003 with stronger southward flow of fresher Arctic waters along Baffin Island and stronger northward inflow of saltier Atlantic waters along Greenland. During negative NAO years this cyclonic circulation weakens as evidenced by a 1979 synoptic survey of the hydrography along Baffin Island.

A study of the mixed layer of the South China Sea based on the multiple linear regression

Multiple linear regression (MLR) method was applied to quantify the effects of the net heat flux (NHF), the net freshwater flux (NFF) and the wind stress on the mixed layer depth (MLD) of the South China Sea (SCS) based on the simple ocean data assimilation (SODA) dataset. The spatio-temporal distributions of the MLD, the buoyancy flux (combining the NHF and the NFF) and the wind stress of the SCS were presented. Then using an oceanic vertical mixing model, the MLD after a certain time under the same initial conditions but various pairs of boundary conditions (the three factors) was simulated. Applying the MLR method to the results, regression equations which modeling the relationship between the simulated MLD and the three factors were calculated. The equations indicate that when the NHF was negative, it was the primary driver of the mixed layer deepening; and when the NHF was positive, the wind stress played a more important role than that of the NHF while the NFF had the least effect. When the NHF was positive, the relative quantitative effects of the wind stress, the NHF, and the NFF were about 10, 6 and 2. The above conclusions were applied to explaining the spatio-temporal distributions of the MLD in the SCS and thus proved to be valid.

Response of a Coupled Ocean–Atmosphere Model to Greenland Ice Melting

We investigate the transient response of the global coupled ocean–atmosphere system to enhanced freshwater forcing representative of melting of the Greenland ice sheets. A 50-year long simulation by a coupled atmosphere–ocean general circulation model (CGCM) is compared with another of the same length in which Greenland melting is prescribed. To highlight the importance of coupled atmosphere–ocean processes, the CGCM results are compared with those of two other experiments carried out with the oceanic general circulation model (OGCM). In one of these OGCM experiments, the prescribed surface fluxes of heat, momentum and freshwater correspond to the unperturbed simulation by the CGCM; in the other experiment, Greenland melting is added to the freshwater flux. The responses by the CGCM and OGCM to the Greenland melting have similar patterns in the Atlantic, albeit the former having five times larger amplitudes in sea surface height anomalies. The CGCM shows likewise stronger variability in all state variables in all ocean basins because the impact of Greenland melting is quickly communicated to all ocean basins via atmospheric bridges. We conclude that the response of the global climate to Greenland ice melting is highly dependent on coupled atmosphere–ocean processes. These lead to reduced latent heat flux into the atmosphere and an associated increase in net freshwater flux into the ocean, especially in the subpolar North Atlantic. The combined result is a stronger response of the coupled system to Greenland ice sheet melting.

Simulated Response to Recent Freshwater Flux Change over the Gulf Stream and Its Extension: Coupled Ocean–Atmosphere Adjustment and Atlantic–Pacific Teleconnection

Abstract Recent observation has shown that the dominant mode of the net freshwater flux variations over the North Atlantic Ocean is the significant trend of freshwater loss over the Gulf Stream region and its extension. In this paper, the coupled ocean–atmosphere response to this freshwater flux change is investigated based on a series of the Fast Ocean–Atmosphere Model coupled-model experiments. The model demonstrates that the freshwater loss over the Gulf Stream and its extension region directly forces an anomalous cyclonic gyre and triggers a SST dipole with cooling in the western subtropical and warming in the eastern subpolar North Atlantic. The freshwater loss also forces a significant response in the atmosphere with a negative NAO-like response in early winter and a basin-scale ridge resembling the eastern Atlantic mode (EAM) in late winter. The salinification also strengthens the Atlantic meridional overturning circulation and thus the poleward heat transport, leading to tropical cooling. The freshwater loss over the Gulf Stream and its extension also leads to an El Niño–like warming in the tropical Pacific and cooling in the North Pacific, similar to the responses in previous water-hosing experiments with an input of freshwater in the subpolar North Atlantic. The tropical Pacific responses subsequently strengthen the Northern Hemispheric atmospheric anomalies in early winter, but reverse them in late winter through an emanation of Rossby wave trains. Overall, the tropical Pacific air–sea coupling plays a damping role, while local air–sea coupling tends to enhance the ocean and atmospheric responses over the North Atlantic.

The Effect of the Sea Ice Freshwater Flux on Southern Ocean Temperatures in CCSM3: Deep-Ocean Warming and Delayed Surface Warming

Abstract This study explores the role of sea ice freshwater and salt fluxes in modulating twenty-first-century surface warming in the Southern Ocean via analysis of sensitivity experiments in the Community Climate System Model, version 3 (CCSM3). In particular, the role of a change in these fluxes in causing surface cooling, expanding sea ice, and increasing deep oceanic storage of heat in the Southern Ocean is investigated. The results indicate that in response to the doubling of CO2 concentrations in the atmosphere in CCSM3, net freshwater input from sea ice to the ocean increases south of 58°S (owing to less growth) and decreases from 48° to 58°S (owing to less melt). The freshwater source from changing precipitation in the model is considerably less than from sea ice south of 58°S, but it serves to compensate for the reduction in sea ice melt near the ice edge, leaving almost no net freshwater flux change between about 48° and 58°S. As a result, freshwater input principally from sea ice reduces ocean convection, which in turns reduces the entrainment of heat into the mixed layer and reduces the upward heat transport along isopycnals below about 1000 m. The reduced upward heat transport (from all sources) causes deep-ocean heating south of 60°S and below 500-m depth, with a corresponding surface cooling in large parts of the Southern Ocean in the model. These results indicate that changing sea ice freshwater and salt fluxes are a major component of the twenty-first-century delay in surface warming of the Southern Ocean and weak reduction in Antarctic sea ice in model projections.

Volume, Freshwater, and Heat Fluxes through Davis Strait, 2004–05*

Abstract Davis Strait volume [−2.3 ± 0.7 Sv (1 Sv ≡ 106 m3 s−1); negative sign indicates southward transport], freshwater (−116 ± 41 mSv), and heat (20 ± 9 TW) fluxes estimated from objectively mapped 2004–05 moored array data do not differ significantly from values based on a 1987–90 array but are distributed differently across the strait. The 2004–05 array provided the first year-long measurements in the upper 100 m and over the shelves. The upper 100 m accounts for 39% (−0.9 Sv) of the net volume and 59% (−69 mSv) of the net freshwater fluxes. Shelf contributions are small: 0.4 Sv (volume), 15 mSv (freshwater), and 3 TW (heat) from the West Greenland shelf and −0.1 Sv, −7 mSv, and 1 TW from the Baffin Island shelf. Contemporaneous measurements of the Baffin Bay inflows and outflows indicate that volume and freshwater budgets balance to within 26% and 4%, respectively, of the net Davis Strait outflow. Davis Strait volume and freshwater fluxes nearly equal those from Fram Strait, indicating that both are significant Arctic freshwater pathways.

Sea surface freshwater flux estimates from GECCO, HOAPS and NCEP

Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over the 50 yr period 1951–2001, are compared to purely satellite-based HOAPS freshwater flux estimates and to the NCEP atmospheric re-analysis net surface freshwater flux fields to assess the quality of all flux products and to improve our understanding of the time-mean surface freshwater flux distribution as well as its temporal variability. Surface flux fields are adjusted by the GECCO state estimation procedure together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. The entirely independent HOAPS net surface freshwater flux fields result from the difference between SSM/I based precipitation estimates and fields of evaporation resulting from a bulk aerodynamic approach using SSM/I data and the Pathfinder SST. All three products agree well on a global scale. However, overall GECCO seems to have moved away from the NCEP/NCAR first guess surface fluxes and is often closer to the HOAPS data set. This holds for the time mean as well as for the seasonal cycle.

The Mean Along-Isobath Heat and Salt Balances over the Middle Atlantic Bight Continental Shelf

Abstract The mean heat and salt balances over the Middle Atlantic Bight continental shelf are investigated by testing the hypothesis that surface fluxes of heat or freshwater are balanced by along-isobath fluxes resulting from the mean, depth-averaged, along-isobath flow acting on the mean, depth-averaged, along-isobath temperature or salinity gradient. This hypothesized balance is equivalent in a Lagrangian frame to a column of water, for example, warming because of surface heating as it is advected southward along isobath by the mean flow. Mean depth-averaged temperatures increase from north to south along isobath at a rate of 2°C (1000 km)−1 at midshelf, which is consistent with the hypothesized balance and mean surface heat flux estimates from the 50-yr NCEP Reanalysis. However, mean surface heat flux estimates from the higher-resolution 20-yr Objectively Analyzed Air–Sea Fluxes (OAFlux) reanalysis are too small to balance the along-isobath heat flux divergence implying a cross-shelf heat flux convergence. It is unclear which surface heat flux estimate, NCEP or OAFlux, is more accurate. The cross-shelf heat flux convergence resulting from the mean cross-shelf circulation is too small to balance the along-isobath heat flux divergence. Mean depth-averaged salinities increase from north to south along isobath at a rate of 1 (psu) (1000 km)−1 at midshelf. Mean precipitation and evaporation rates nearly balance so that the net freshwater flux is too small by more than an order of magnitude to account for the observed along-isobath increase in salinity. The cross-shelf salt flux divergence resulting from the mean cross-shelf circulation has the wrong sign to balance the divergence in the along-isobath salt flux. These results imply there must be an onshore “eddy” salt flux resulting from the time-dependent current and salinity variability. The along-isobath temperature and salinity gradients compensate for each other so that the mean, depth-averaged, along-isobath density gradient is approximately zero. This suggests that there may be a feedback between the along-isobath density gradient and the onshore salt and heat fluxes that maintains the density gradient near zero.

Sea surface freshwater flux estimates from GECCO, HOAPS and NCEP

Surface net freshwater flux fields, estimated from the GECCO ocean state estimation effort over the 50 yr period 1951–2001, are compared to purely satellite-based HOAPS freshwater flux estimates and to the NCEP atmospheric re-analysis net surface freshwater flux fields to assess the quality of all flux products and to improve our understanding of the time-mean surface freshwater flux distribution as well as its temporal variability. Surface flux fields are adjusted by the GECCO state estimation procedure together with initial temperature and salinity conditions so that the model simulation becomes consistent with ocean observations. The entirely independent HOAPS net surface freshwater flux fields result from the difference between SSM/I based precipitation estimates and fields of evaporation resulting from a bulk aerodynamic approach using SSM/I data and the Pathfinder SST. All three products agree well on a global scale. However, overall GECCO seems to have moved away from the NCEP/NCAR first guess surface fluxes and is often closer to the HOAPS data set. This holds for the time mean as well as for the seasonal cycle.

Atmospheric Control on the Thermohaline Circulation

Abstract In an attempt to elucidate the role of atmospheric and oceanic processes in setting a vigorous ocean overturning circulation in the North Atlantic but not in the North Pacific, a comparison of the observed atmospheric circulation and net surface freshwater fluxes over the North Atlantic and Pacific basins is conducted. It is proposed that the more erratic meridional displacements of the atmospheric jet stream over the North Atlantic sector is instrumental in maintaining high surface salinities in its subpolar gyre. In addition, it is suggested that the spatial pattern of the net freshwater flux at the sea surface favors higher subpolar Atlantic salinity, because the geographical line separating net precipitation from net evaporation is found well south of the time-mean gyre separation in the North Pacific, whereas the two lines tend to coincide in the North Atlantic. Numerical experiments with an idealized two-gyre system confirm that these differences impact the salinity budget of the subpolar gyre. Further analysis of a coupled climate model in which the Atlantic meridional overturning cell has been artificially weakened suggests that the more erratic jet fluctuations in the Atlantic and the shift of the zero [net evaporation minus precipitation (E − P)] line are likely explained by features independent of the state of the thermohaline circulation. It is thus proposed that the atmospheric circulation helps “locking” high surface salinities and an active coupling between upper and deep ocean layers in the North Atlantic rather than in the North Pacific basin.

Estimating weights for the use of time‐dependent gravity recovery and climate experiment data in constraining ocean models

Using Gravity Recovery And Climate Experiment (GRACE) data to constrain ocean general circulation models requires quantitative knowledge of the errors in GRACE‐derived estimates of ocean bottom pressure (pb) change, which for our purposes include not only instrument noise but also variability not represented in the models (e.g., post‐glacial rebound and self‐gravitation effects). We attempt a spatial mapping of these errors by comparing several GRACE data products to pb simulations from an ocean model. Uncertainties in the global ocean mean , partly related to the net freshwater flux into the ocean, and in the regional pb anomalies about that mean are considered separately. The resultant regional error estimates (∼1–3 cm), when zonally averaged, are comparable to the calibrated errors provided by the GRACE processing centers, except for enhanced errors near some continental regions with high seasonal hydrology signals or large mass trends. Errors in the GRACE‐derived values estimated from model‐data differences (∼0.2 cm) are also comparable with those from the calibrated errors. For both pb and estimates, accounting for the effects of geocenter noise is important. Replacing the C20 harmonic term in the GRACE data with estimates derived from satellite laser ranging results in significantly lower errors in the Southern Ocean. We also find lower errors at high latitudes when the variability of the atmospheric pressure over the land is removed from the data. Given the estimated errors and model‐data comparisons, GRACE data should be useful for constraining estimates of , particularly at interannual periods, but less so when considering regional pb variability.

On the climate response of the low-latitude Pacific Ocean to changes in the global freshwater cycle

Under global warming, the predicted intensification of the global freshwater cycle will modify the net freshwater flux at the ocean surface. Since the freshwater flux maintains ocean salinity structures, changes to the density-driven ocean circulation are likely. A modified ocean circulation could further alter the climate, potentially allowing rapid changes, as seen in the past. The relevant feedback mechanisms and timescales are poorly understood in detail, however, especially at low latitudes where the effects of salinity are relatively subtle. In an attempt to resolve some of these outstanding issues, we present an investigation of the climate response of the low-latitude Pacific region to changes in freshwater forcing. Initiated from the present-day thermohaline structure, a control run of a coupled ocean–atmosphere general circulation model is compared with a perturbation run in which the net freshwater flux is prescribed to be zero over the ocean. Such an extreme experiment helps to elucidate the general adjustment mechanisms and their timescales. The atmospheric greenhouse gas concentrations are held constant, and we restrict our attention to the adjustment of the upper 1,000 m of the Pacific Ocean between 40°N and 40°S, over 100 years. In the perturbation run, changes to the surface buoyancy, near-surface vertical mixing and mixed-layer depth are established within 1 year. Subsequently, relative to the control run, the surface of the low-latitude Pacific Ocean in the perturbation run warms by an average of 0.6°C, and the interior cools by up to 1.1°C, after a few decades. This vertical re-arrangement of the ocean heat content is shown to be achieved by a gradual shutdown of the heat flux due to isopycnal (i.e. along surfaces of constant density) mixing, the vertical component of which is downwards at low latitudes. This heat transfer depends crucially upon the existence of density-compensating temperature and salinity gradients on isopycnal surfaces. The timescale of the thermal changes in the perturbation run is therefore set by the timescale for the decay of isopycnal salinity gradients in response to the eliminated freshwater forcing, which we demonstrate to be around 10–20 years. Such isopycnal heat flux changes may play a role in the response of the low-latitude climate to a future accelerated freshwater cycle. Specifically, the mechanism appears to represent a weak negative sea surface temperature feedback, which we speculate might partially shield from view the anthropogenically-forced global warming signal at low latitudes. Furthermore, since the surface freshwater flux is shown to play a role in determining the ocean’s thermal structure, it follows that evaporation and/or precipitation biases in general circulation models are likely to cause sea surface temperature biases.