Sea Surface Height Changes Research Articles

Potential positive feedback between Greenland Ice Sheet melt and Baffin Bay heat content on the west Greenland shelf

AbstractGreenland ice sheet meltwater runoff has been increasing in recent decades, especially in the southwest and the northeast. To determine the impact of this accelerating meltwater flux on Baffin Bay, we examine eight numerical experiments using an ocean‐sea ice model: Nucleus for European Modelling of the Ocean. Enhanced runoff causes shoreward increasing sea surface height and strengthens the stratification in Baffin Bay. The changes in sea surface height reduces the southward transport through the Canadian Arctic Archipelago and strengthens the gyre circulation within Baffin Bay. The latter leads to further freshening of surface waters as it produces a larger northward surface freshwater transport across Davis Strait. Increasing the meltwater runoff leads to a warming and shallowing of the west Greenland Irminger water on the northwest Greenland shelf. These warmer waters can now more easily enter fjords on the Greenland coast and thus provide additional heat to accelerate the melting of marine‐terminating glaciers.

An observational study of salt fluxes in Delaware Bay

AbstractAn observational study was conducted in Delaware Bay during the summer of 2011 aiming to quantify the different mechanisms driving the salt flux in this system. Seven moorings, equipped with bottom‐mounted ADCPs and CT sensors at difference depths, were deployed across a section of the estuary. The total area‐averaged and tidal‐averaged salt flux was decomposed in three different contributions: the advective salt flux that represents the flux caused by river input and meteorological‐induced flows, the steady shear dispersion that is the salt flux driven by the estuarine exchange flow, and the tidal oscillatory salt flux that is induced by the tidal currents. The advective salt flux dominated over the steady shear dispersion and tidal oscillatory salt flux, because it was driven mainly by changes in sea surface height associated with wind‐driven setup and setdown. The steady shear dispersion was always positive and presented a spring/neap variability that was consistent with a two‐layer exchange flow. On the other hand, the tidal oscillatory salt flux fluctuated between positive and negative values, but increased around a strong neap tide and decreased on the following spring tide. This variability is contrary to previous parameterizations, whereby the tidal salt flux is proportional to the amplitude of the tidal currents. The observational estimate was compared to a parameterization that relates tidal salt flux as proportional to tidal current amplitude and stratification. The observational estimate agreed with this new parameterization when the river discharge was relatively constant.

Changes in extreme regional sea surface height due to an abrupt weakening of the Atlantic meridional overturning circulation

Abstract. As an extreme scenario of dynamical sea level changes, regional sea surface height (SSH) changes that occur in the North Atlantic due to an abrupt weakening of the Atlantic meridional overturning circulation (AMOC) are simulated. Two versions of the same ocean-only model are used to study the effect of ocean model resolution on these SSH changes: a high-resolution (HR) strongly eddying version and a low-resolution (LR) version in which the effect of eddies is parameterised. The weakening of the AMOC is induced in both model versions by applying strong freshwater perturbations around Greenland. A rapid decrease of the AMOC in the HR version induces much shorter return times of several specific regional and coastal extremes in North Atlantic SSH than in the LR version. This effect is caused by a change in main eddy pathways associated with a change in separation latitude of the Gulf Stream.

A fully coupled 3-D ice-sheet–sea-level model: algorithm and applications

Abstract. Relative sea-level variations during the late Pleistocene can only be reconstructed with the knowledge of ice-sheet history. On the other hand, the knowledge of regional and global relative sea-level variations is necessary to learn about the changes in ice volume. Overcoming this problem of circularity demands a fully coupled system where ice sheets and sea level vary consistently in space and time and dynamically affect each other. Here we present results for the past 410 000 years (410 kyr) from the coupling of a set of 3-D ice-sheet-shelf models to a global sea-level model, which is based on the solution of the gravitationally self-consistent sea-level equation. The sea-level model incorporates the glacial isostatic adjustment feedbacks for a Maxwell viscoelastic and rotating Earth model with coastal migration. Ice volume is computed with four 3-D ice-sheet-shelf models for North America, Eurasia, Greenland and Antarctica. Using an inverse approach, ice volume and temperature are derived from a benthic δ18O stacked record. The derived surface-air temperature anomaly is added to the present-day climatology to simulate glacial–interglacial changes in temperature and hence ice volume. The ice-sheet thickness variations are then forwarded to the sea-level model to compute the bedrock deformation, the change in sea-surface height and thus the relative sea-level change. The latter is then forwarded to the ice-sheet models. To quantify the impact of relative sea-level variations on ice-volume evolution, we have performed coupled and uncoupled simulations. The largest differences of ice-sheet thickness change occur at the edges of the ice sheets, where relative sea-level change significantly departs from the ocean-averaged sea-level variations.

Influence of the Pacific Decadal Oscillation on regional sea level rise in the Pacific Ocean from 1993 to 2012

The rate of regional sea level rise (SLR) provides important information about the impact of human activities on climate change. However, accurate estimation of regional SLR can be severely affected by sea surface height (SSH) change caused by the Pacific Decadal Oscillation (PDO-SSH). Here, the PDO-SSH signal is extracted from satellite altimeter data by multi-variable linear regression, and regional SLR in the altimeter era is calculated, before and after removing that signal. The results show that PDO-SSH trends are rising in the western Pacific and falling in the eastern Pacific, with the strongest signal confined to the tropical and North Pacific. Over the past 20 years, the PDO-SSH accounts for about 30%-40% of altimeter-observed SLR in the regions 8 degrees-15 degrees N, 130 degrees-160 degrees E and 30 degrees-40 degrees N, 170 degrees-220 degrees E. Along the coast of North America, the PDO-SSH signal dramatically offsets the coastal SLR, as the sea level trends change sign from falling to rising.

Demonstrating the complementarity of observations in an operational ocean forecasting system

We have performed a series of near-real-time observing system experiments with FOAM, the Met Office operational ocean forecasting system. These were conducted in parallel to the operational suite and identical to it except that certain observation types were excluded. At the start of each month the parallel system was reset to the operational restart and a run started with a different observation type excluded: in February –XBT; March –TAO/TRITON; April –Jason-2 altimeter; May –all altimeter; June –AVHRR sea-surface temperature (SST) data; and July –Argo data. We show that the existing ocean observing systems offer a good deal of complementary information. All components of the observing system offer unique, independent information that help initialise, and improve, operational ocean forecasts. Withholding XBTs causes little impact on globally averaged metrics, for example RMS innovations. However, locally we see long-lasting temperature impacts (∼1 °C) near XBT transects. Withholding TAO/TRITON data has a regional impact increasing the tropical Pacific RMS temperature and salinity innovations by 39 and 60%, respectively. Withholding Jason-2 data results in a global 4% increase in the RMS sea-surface height (SSH) innovations, and small-scale changes in temperature and salinity of about 2 °C at 100 m depth, and ∼0.2 psu near the surface. Withholding all altimeter data leads to a 16% increase of the RMS SSH innovations. We also see changes in other model variables similar in magnitude to those from withholding Jason-2, but more widespread. Withholding AVHRR SST data produces significant changes around 1 °C in model temperature at the surface and within the surface mixed layer, but there is little or no effect below this. Withholding Argo data for 1 month leads to a 5% increase in the temperature and salinity innovations, and changes in SSH of up to 5 cm.

Interannual sea level variability in the tropical Pacific Ocean from 1993 to 2006

Three net surface heat flux products, namely from 1) version 2 of Common Ocean Reference Experiment (CORE.2), 2) Objectively Analyzed Air–Sea Fluxes (OAFlux), and 3) the European Centre for Medium-Range Weather Forecasts operational ocean analysis/reanalysis system (ECMWF ORA-S3), and three wind stress products, namely from 1) CORE.2, 2) Simple Ocean Data Assimilation Reanalysis, version 2.1.6 (SODA 2.1.6), and 3) ECMWF ORA-S3 are used to investigate the abilities of four simple oceanic mechanisms in explaining the interannual variance of altimetry-derived sea surface height (SSH) anomalies in the tropical Pacific Ocean over the period 1993–2006. It is found that local response to surface heating plays an important role in sea level rise along the western equatorial Pacific (150°–180°E). The dominant processes affecting interannual variability of observed SSH anomalies vary regionally in the tropical Pacific; local response to surface heating, local Ekman pumping, wind-induced first baroclinic mode Rossby waves and the eastern boundary forcing are all important. Both the local response to surface heating and the eastern boundary forcing are important in explaining the interannual variance of observed SSH anomalies in the northeastern tropical Pacific; while the dominant contribution to interannual sea level variability in the southeastern tropical Pacific is from the eastern boundary forcing, the local Ekman pumping plays a relatively minor role in the interannual SSH change there. The wind-induced first baroclinic mode Rossby waves dominate interannual SSH variability in the western tropical Pacific, excluding the area of 2°–10°N, west of 170°E. Although a large part of the interannual sea level variability in the western tropical Pacific is related to the oceanic remote adjustment to wind stress forcing, the contributions of local responses to surface heating and wind forcing cannot be overlooked.

Impact of self‐attraction and loading effects induced by shelf mass loading on projected regional sea level rise

We investigate the effect of self‐attraction and loading (SAL) induced by the projected accumulation of sea water on shallow continental shelf areas. Using output from a climate model, we compute 21st century changes in regional steric sea surface height and find that steric changes are largest over the deep ocean and relatively small on the shallow continental shelves. The resulting redistribution of sea water towards the shelf areas leads to mass accumulation on the shelves and therefore to increased gravitational attraction as well as increased loading on the sea floor. We find that, depending on the scenario and region, SAL effects may result in an additional sea level rise of 1–3 cm on the world's continental shelf areas by the end of the 21st century. These estimates are at most 15% of the combined changes in sea surface height induced by redistribution of water masses and steric expansion.

The role of mindanao dome in the variability of the Pacific North equatorial current bifurcation

The Mindanao Dome (MD) features prominent oceanic variability and is located geographically close to the bifurcation latitude Y (b) of the Pacific North Equatorial Current. In this study, the role of the MD in the variability of Y (b) is examined with 20 years of satellite altimetric sea surface height (SSH) data and a 1.5-layer linear Rossby wave model. It is shown that the seasonal variations of surface Y (b) are related to not only the SSH fluctuations near the bifurcation point (bifurcation box; 125A degrees-130A degrees E, 12A degrees-15A degrees N) but also those outstanding in the MD region (MD box; 127A degrees-132A degrees E, 6A degrees-9A degrees N). The impact of the MD SSH changes is significant when the bifurcation point stays at southerly latitudes during February-September, which hinders (delays) the southward leap (northward retreat) of Y (b) in April-May (July-August) and thus leads to the asymmetry of the mean Y (b) seasonal cycle (with a positive skewness of gamma = +0.64). Such asymmetry also shows year-to-year variations depending on yearly mean Y (b) value. A southerly yearly mean Y (b) involves larger contribution of the MD and thus causes larger asymmetry of Y (b) seasonal cycle. At interannual and longer timescales, the MD acts to amplify the fluctuations of the bifurcation. It is responsible for about 20 % of the total low-frequency Y (b) variances and plays an important role in the 0.12A degrees A year(-1) southward trend of Y (b) in the past two decades. The impact of the MD on Y (b) changes is becoming increasingly significant at various timescales such as the bifurcation point migrating southward in recent years.

Climate trends in southern Africa (with erratum)

The observed and projected changes in the climate of southern Africa in the period 1900–2100 were analysed. Ten observed, reanalysed and model-simulated climate data sets were explored for changes in surface air temperature, rainfall, air pressure, winds, ocean currents and sea surface height. The analysis of spatial and temporal climate trends from historical observations provided a context to assess two coupled model simulations (IPSL, MIROC) based on the A1B emission scenario. Temperatures in the satellite era exhibited upward trends greater than +0.4 °C/year in the MIROC and IPSL A1B model simulations; between +0.02 °C/year and +0.03 °C/year in NCDC, HADCRU, CFS-R and NCEPe data sets; +0.01 °C/year in NCEPr and GHCN observations; and +0.002 °C/year in the ECMWF data set. Although rainfall trends in the satellite era were minimal in many data sets because of drought in the early 1980s, there was a significant downtrend in the IPSL simulation of -0.013 mm/day per year. When averaging the longer data sets together over the 20th century, the southern African rainfall trend was -0.003 mm/day per year. Other key features of the analyses include a poleward drift of the sub-tropical anticyclones and a +1.5 mm/year rise in sea surface height along the coast.

Mode water ventilation and subtropical countercurrent over the North Pacific in CMIP5 simulations and future projections

Seventeen coupled general circulation models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) are analyzed to assess the dynamics and variability of the North Pacific Subtropical Countercurrent (STCC). Consistent with observations, the STCC is anchored by mode water to the north. For the present climate, the STCC tends to be stronger in models than in observations because of too strong a low potential vorticity signature of mode water. There are significant variations in mode water simulation among models, i.e., in volume and core layer density. The northeast slanted bands of sea surface height (SSH) anomalies associated with the STCC variability are caused by variability in mode water among models and the Hawaii islands are represented in some models, where the island‐induced wind curls drive the Hawaiian Lee Countercurrent (HLCC) located to the south of STCC. Projected future changes in STCC and mode water under the Representative Concentration Pathways (RCP) 4.5 scenario are also investigated. By combining the historical and RCP 4.5 runs, an empirical orthogonal function analysis for SSH over the central subtropical gyre (160°E–140°W, 15°–30°N) is performed. The dominant mode of SSH change in 17 CMIP5 models is characterized by the weakening of the STCC because of the reduced formation of mode water. The weakened mode water is closely related to the increased stratification of the upper ocean, the latter being one of the most robust changes as climate warms. Thus the weakened STCC and mode water are common to CMIP5 future climate projections.

On the relation between Meridional Overturning Circulation and sea-level gradients in the Atlantic

Abstract. On the basis of model simulations, we examine what information on changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) can be extracted from associated changes in sea surface height (SSH), specifically from a broad Atlantic north–south gradient as has been suggested previously in the literature. Since a relation between AMOC and SSH changes can only be used as an AMOC diagnostic if it is valid independently of the specific forcing, we consider three different forcing types: increase of CO2 concentration, freshwater fluxes to the northern convection sites and the modification of Southern Ocean winds. We concentrate on a timescale of 100 yr. We find approximately linear and numerically similar relations between a sea-level difference within the Atlantic and the AMOC for freshwater as well as wind forcing. However, the relation is more complex in response to atmospheric CO2 increase, which precludes this sea-level difference as an AMOC diagnostic under climate change. Finally, we show qualitatively to what extent changes in SSH and AMOC strength, which are caused by simultaneous application of different forcings, correspond to the sum of the changes due to the individual forcings, a potential prerequisite for more complex SSH-based AMOC diagnostics.

A statistical analysis of Gulf Stream variability from 18+ years of altimetry data

Altimetry data that bracket the core of the Gulf Stream (GS) between 32–39°N and 68–72°W are analyzed from a combined data set spanning the duration of the Topex, Jason-1, and Jason-2 missions. Absolute sea surface height, SSH, referenced to a geoid model and sea surface height anomaly, SSHA, referenced to a global mean sea surface model are calculated using pass 50 data from Topex cycle-002 (04-October-1992) through Jason-2 cycle-098 (02-Mar-2011). Five different indices of GS variability: (i) ΔSSH—the change in absolute height across the GS; (ii) ΔSSHA—the change in sea surface height anomaly across the GS; (iii) a barotropic velocity index; (iv) a baroclinic transport index; and (v) the position of the GS, are estimated from the 18+ year time series via seasonal time series, power spectral density functions, auto-covariance functions, and a principal component analysis (PCA). The average annual distribution of all the variables except velocity index increases from a spring/summer minimum to a fall maximum, then decreases during the winter to minimum values in spring and early summer. The average signal in barotropic velocity index reaches a maximum in summer, whereas the maximum baroclinic transport occurs in the fall. The seasonal signal in GS position is better approximated by a seasonal sinusoid than the dynamical variables, with minimum (southerly) positions in spring and maximum (northerly) positions in the fall. Variations in height difference across the current are in phase with changes in the width of the current, leading to smaller seasonal variations in the velocity which are out of phase with the other variables. None of the seasonal time series except GS position resemble a pure sinusoid; they more closely resemble the signal of a nonlinear oscillator.The three dominant periods for ΔSSH are 15–21 months, 7 months, and 4 years, with an integral time scale of 32 days. The dominant periods for ΔSSHA are 9 months, 21 months, and 5–6 months, with an integral time scale of 38 days. The spectral and PCA analyses of both ΔSSH and ΔSSHA are “pink”, indicating an energetic broad-band spectra. The dominant PCA modes explain less than 30% of the variability, indicative of highly nonlinear processes. The dominant periods for the barotropic velocity index are 9 months, 21 months, and 5 months, with an integral time scale of 14 days. The dominant periods for the baroclinic transport index are 11 months, 2–3 months, and monthly, with an integral time scale of 24 days. The dominant PCA modes of the velocity and transport indices explain 52% and 48% of the variability of their respective time series. The dominant periods for GS position are 9, 11, 13 months, and 1.5–6 years, with an integral time scale of 35 days. The first GS position PCA mode explains 50% of position variability. All of the time series of GS variability and the correlation between the North Atlantic Oscillation and GS position exhibit non-stationary behavior.

Sea surface height and steric height increases in the Southern Hemisphere

Sea surface height has increased by 3 millimeters per year, globally averaged, since 1993. Some fraction of sea surface height change is due to added water from melting glaciers, for instance, and some is due to increasing heat and salinity changes (steric effects). Focusing on the Southern Hemisphere, Sutton and Roemmich analyzed temperature and salinity data from the Argo float array in relation to earlier data from the World Ocean Circulation Experiment (WOCE) to estimate the steric changes. These were compared with the total sea surface height changes over the same period seen in satellite altimetric data. They found that on decadal time scales, about half of the rise in sea surface height in the Southern Ocean is due to steric effects, with the proportion increasing southward. The accompanying increase in ocean heat content south of 30°S can account for most of the global heat content change during this period. (Geophysical Research Letters, doi:10.1029/2011GL046802, 2011)

Decadal steric and sea surface height changes in the Southern Hemisphere

[1] Sea surface height (SSH) changes result from changes in steric height (SH) and mass. We investigate total SH and mass from co-located measurements of SSH and SH in the upper 1500 dbar (SH0–1500). SSH changes are decomposed into SH0–1500 and ‘other’ contributions, where ‘other’ includes SH changes below 1500 dbar and mass changes. This is done using satellite altimeter measurements of SSH available since late 1992 in combination with WOCE-era hydrography and Argo. A hemispheric analysis of co-located WOCE and Argo profiles gives robust ΔSH/ΔSSH relationships, varying with latitude. The ΔSH/ΔSSH ratio together with satellite SSH yields an estimate of decadal SH increase. It is found that ∼0.5 of the hemispheric decadal SSH rise is steric, with this proportion increasing southwards. The relatively large rate of SSH increase south of 30°S, the high proportion attributable to SH (i.e., ocean warming) and the great area of the southern ocean, mean the total heat gain south of 20°S is comparable to estimates of global 0-700 m heat gain for this period.

An Enhancement of Low-Frequency Variability in the Kuroshio–Oyashio Extension in CCSM3 owing to Ocean Model Biases

Abstract Enhanced decadal variability in sea surface temperature (SST) centered on the Kuroshio Extension (KE) has been found in the Community Climate System Model version 3 (CCSM3) as well as in other coupled climate models. This decadal peak has higher energy than is found in nature, almost twice as large in some cases. While previous analyses have concentrated on the mechanisms for such decadal variability in coupled models, an analysis of the causes of excessive SST response to changes in wind stress has been missing. Here, a detailed comparison of the relationships between interannual changes in SST and sea surface height (SSH) as a proxy for geostrophic surface currents in the region in both CCSM3 and observations, and how these relationships depend on the mean ocean circulation, temperature, and salinity, is made. We use observationally based climatological temperature and salinity fields as well as satellite-based SSH and SST fields for comparison. The primary cause for the excessive SST variability is the coincidence of the mean KE with the region of largest SST gradients in the model. In observations, these two regions are separated by almost 500 km. In addition, the too shallow surface oceanic mixed layer in March north of the KE in the subarctic Pacific contributes to the biases. These biases are not unique to CCSM3 and suggest that mean biases in current, temperature, and salinity structures in separated western boundary current regions can exert a large influence on the size of modeled decadal SST variability.

Monitoring coastal sea level using reflected GNSS signals

A continuous monitoring of coastal sea level changes is important for human society since it is predicted that up to 332 million people in coastal and low-lying areas will be directly affected by flooding from sea level rise by the end of the 21st century. The traditional way to observe sea level is using tide gauges that give measurements relative to the Earth’s crust. However, in order to improve the understanding of the sea level change processes it is necessary to separate the measurements into land surface height changes and sea surface height changes. These measurements should then be relative to a global reference frame. This can be done with satellite techniques, and thus a GNSS-based tide gauge is proposed. The GNSS-based tide gauge makes use of both GNSS signals that are directly received and GNSS signals that are reflected from the sea surface. An experimental installation at the Onsala Space Observatory (OSO) shows that the reflected GNSS signals have only about 3 dB less signal-to-noise-ratio than the directly received GNSS signals. Furthermore, a comparison of local sea level observations from the GNSS-based tide gauge with two stilling well gauges, located approximately 18 and 33 km away from OSO, gives a pairwise root-mean-square agreement on the order of 4 cm. This indicates that the GNSS-based tide gauge gives valuable results for sea level monitoring.

Regional Patterns of Sea Level Change Related to Interannual Variability and Multidecadal Trends in the Atlantic Meridional Overturning Circulation*

Abstract Some studies of ocean climate model experiments suggest that regional changes in dynamic sea level could provide a valuable indicator of trends in the strength of the Atlantic meridional overturning circulation (MOC). This paper describes the use of a sequence of global ocean–ice model experiments to show that the diagnosed patterns of sea surface height (SSH) anomalies associated with changes in the MOC in the North Atlantic (NA) depend critically on the time scales of interest. Model hindcast simulations for 1958–2004 reproduce the observed pattern of SSH variability with extrema occurring along the Gulf Stream (GS) and in the subpolar gyre (SPG), but they also show that the pattern is primarily related to the wind-driven variability of MOC and gyre circulation on interannual time scales; it is reflected also in the leading EOF of SSH variability over the NA Ocean, as described in previous studies. The pattern, however, is not useful as a “fingerprint” of longer-term changes in the MOC: as shown with a companion experiment, a multidecadal, gradual decline in the MOC [of 5 Sv (1 Sv ≡ 106 m3 s−1) over 5 decades] induces a much broader, basin-scale SSH rise over the mid-to-high-latitude NA, with amplitudes of 20 cm. The detectability of such a trend is low along the GS since low-frequency SSH changes are effectively masked here by strong variability on shorter time scales. More favorable signal-to-noise ratios are found in the SPG and the eastern NA, where a MOC trend of 0.1 Sv yr−1 would leave a significant imprint in SSH already after about 20 years.

Circumpolar structure and distribution of the Antarctic Circumpolar Current fronts: 2. Variability and relationship to sea surface height

In Part 1 of this study, we showed that the Antarctic Circumpolar Current (ACC) consisted of multiple fronts, each of which was consistently associated with a particular contour of sea surface height (SSH) or approximate streamline. In Part 2 we have used maps of SSH to examine the variability of the ACC fronts between 1992 and 2007. The SSH label associated with each frontal branch is nearly constant around the circumpolar belt. The front labels are also nearly constant in time: the bands of enhanced SSH gradient (i.e., fronts) occur along the same streamlines throughout the 15 year period of observations. Both short‐ and long‐period changes of the SSH frontal labels of the ACC are small. Based on a tight relationship between dynamic height and cumulative baroclinic transport of the ACC, the baroclinic transport variability of the individual branches of the ACC is also expected to be small. The major change in the total ACC baroclinic transport occurs in the Drake Passage. The streamline associated with the northern branch of the SAF (SAF‐N) does not pass through Drake Passage and the waters carried by this branch are not observed there. Instead, the transport of the SAF‐N turns north in the Pacific to supply the export of water to the Indian Ocean north and south of Australia. Strong eddy activity in the southeast Pacific acts to dissipate the hydrographic signature of the SAF‐N there. In the Atlantic, the SAF‐N reappears as the same streamline is again associated with enhanced SSH gradients to the east of the Brazil – Malvinas confluence zone. While the large changes in SSH have occurred in the Southern Ocean between 1992 and 2007, there are strong regional differences. Because the ACC fronts are robustly associated with particular SSH contours, the changes in SSH reflect shifts in the position of the ACC fronts. In the circumpolar average, each of the ACC fronts has shifted to the south by about 60 km. The changes in SSH in the Southern Ocean are largely due to changes in ocean circulation, rather than warming and freshening by atmospheric fluxes. Much larger changes in SSH are observed in some locations of the Southern Ocean, particularly where the fronts interact with large‐scale topography. The northern branch of the PF (PF‐N) near the Kerguelen Plateau is an extreme example, where the PF‐N followed a path around the northern end of Kerguelen Plateau between 1992 and 1997, passed through the Fawn Trough after 2003, and oscillated between the two paths between 1997 and 2003.

Using altimetry to help explain patchy changes in hydrographic carbon measurements

Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O2) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O2 inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short‐timescale (6 months to 2 years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first‐order contribution to DIC and O2 variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O2 column inventory variations and sea surface height (SSH) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSH and both DIC and O2 column inventories for many regions suggests that SSH changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from Climate Variability and Predictability's CO2/Repeat Hydrography program).