Parallel Ocean Climate Model Research Articles

Towards measuring the meridional overturning circulation from space

Abstract. We present a step towards measuring the meridional overturning circulation (MOC), i.e. the full-depth water mass transport, in the North Atlantic using satellite data. Using the Parallel Ocean Climate Model, we simulate satellite observations of ocean bottom pressure and sea surface height (SSH) over the 20-year period from 1979–1998, and use a linear model to estimate the MOC. As much as 93.5% of the variability in the smoothed transport is thereby explained. This increases to 98% when SSH and bottom pressure are first smoothed. We present initial studies of predicting the time evolution of the MOC, with promising results. It should be stressed that this is an initial step only, and that to produce an actual working system for measuring the MOC from space would require considerable future work.

Oceanic excitations on polar motion: a cross comparison among models

Recent studies based on various ocean general circulation models (OGCMs) demonstrate that the oceans are a major contributor to polar motion excitations. In this paper, we analyse and compare observed non-atmospheric polar motion excitations with oceanic angular momentum (OAM) variations determined from four OGCMs, which include the parallel ocean climate model (POCM), a barotropic ocean model (BOM), the Estimating the Circulation and Climate of the Ocean (ECCO) non-data-assimilating model (ECCO-NDA) and the ECCO data-assimilating model (ECCO-DA). The data to be analysed span a 5-yr overlapped period from 1993 to 1997. At annual timescale, considerable discrepancies exist between POCM and the other three models, which result mainly from differences in annual components of the forcing wind fields. At semi-annual timescale, however, POCM shows better phase agreement with observed non-atmospheric polar motion excitation than the other three ocean models. At intraseasonal timescales, ECCO-DA yields better agreement with observations, and reduces the variance of non-atmospheric excitations by similar to 60 per cent, 10-20 per cent more than those explained by the other three models. However, at the very short periods of 4-20 days, the BOM estimates could explain about half of the observed variance, twice as much as that by ECCO-NDA, and also shows considerably better correlation with observations. Due to different modelling schemes and methods, significant discrepancies could arise with respect to the quality of modelling large-scale oceanic mass redistribution and current variation. A complete understanding of global oceanic contributions to polar motion excitation still remains a challenge.

Tropical Instability Waves as a Resonance between Equatorial Rossby Waves*

Abstract To understand the characteristics of sea surface height signatures of tropical instability waves (TIWs), a linearized model of the central Pacific Ocean was developed in which the vertical structures of the state variables are projected onto a set of orthogonal baroclinic eigenvectors. In lieu of in situ current measurements with adequate spatial and temporal resolution, the mean current structure used in the model was obtained from the Parallel Ocean Climate Model (POCM). The TIWs in the linear model have cross-equatorial structure and wavenumber–frequency content similar to the TIWs in POCM, even when the vertical structures of the state variables are projected onto only the first two orthogonal baroclinic eigenvectors. Because this model is able to reproduce TIWs with relatively simple vertical structure, it is possible to examine the mechanism for the formation of TIWs. TIWs are shown to form from a resonance between two equatorial Rossby waves as the strength of the background currents is slowly increased.

Eddy Shedding from the Kuroshio Bend at Luzon Strait

TOPEX/POSEDIENT-ERS satellite altimeter data along with the mean state from the Parallel Ocean Climate Model result have been used to investigate the variation of Kuroshio intrusion and eddy shedding at Luzon Strait during 1992–2001. The Kuroshio penetrates into the South China Sea and forms a bend. The Kuroshio bend varies with time, periodically shedding anticyclonic eddies. Criteria of eddy shedding are identified: 1) When the shedding event occurs, there are usually two centers of high Sea Surface Height (SSH) together with negative geostrophic vorticity in the Kuroshio Bend (KB) area. 2) Between the two centers of high SSH there usually exists positive geostrophic vorticity. These criteria have been used to determine the eddy shedding times and locations. The most frequent eddy shedding intervals are 70, 80 and 90 days. In both the winter and summer monsoon period, the most frequent locations are 119.5°E and 120°E, which means that the seasonal variation of eddy shedding location is unclear.

Tracking passive drifters in a high resolution ocean model: implications for interannual variability of larval krill transport to South Georgia

A particle tracking scheme that uses velocity output from an interannually varying forced run of a global ocean circulation model (Parallel Ocean Climate Model; POCM_4C) allows variability in the transport pathways across the Scotia Sea to South Georgia to be examined for the first time. The time-variant surface fluxes introduce realistic variability into the model velocity fields. This causes large variations in near-surface, mixed-layer transport from the Antarctic Peninsula region to South Georgia, an island in the southwest Atlantic sector of the Southern Ocean. The variability occurs on a variety of timescales with seasonal and longer periods of variability apparent in the 18 year time series of results. A quasi-four year period of variability is evident across the region in the sea surface temperature fields of POCM_4C and appears in the particle tracking results. This period, noted in other Southern Ocean data sets and ascribed to the Antarctic Circumpolar Wave, has been observed in the reproductive success of higher marine predators breeding on the island. The predicted oceanographic variability is likely to be significant for the South Georgia ecosystem by affecting the influx into the region of Antarctic krill ( Euphausia superba), the main prey of the higher predators.

Tracking passive drifters in a high resolution ocean model: implications for interannual variability of larval krill transport to South Georgia

A particle tracking scheme that uses velocity output from an interannually varying forced run of a global ocean circulation model (Parallel Ocean Climate Model; POCM_4C) allows variability in the transport pathways across the Scotia Sea to South Georgia to be examined for the first time. The time-variant surface fluxes introduce realistic variability into the model velocity fields. This causes large variations in near-surface, mixed-layer transport from the Antarctic Peninsula region to South Georgia, an island in the southwest Atlantic sector of the Southern Ocean. The variability occurs on a variety of timescales with seasonal and longer periods of variability apparent in the 18 year time series of results. A quasi-four year period of variability is evident across the region in the sea surface temperature fields of POCM_4C and appears in the particle tracking results. This period, noted in other Southern Ocean data sets and ascribed to the Antarctic Circumpolar Wave, has been observed in the reproductive success of higher marine predators breeding on the island. The predicted oceanographic variability is likely to be significant for the South Georgia ecosystem by affecting the influx into the region of Antarctic krill (Euphausia superba), the main prey of the higher predators.

Subannual variability of the ocean circulation in the Kuroshio region

[1] Using altimeter data of the Kuroshio region, dominant patterns of variability are determined by applying multichannel singular spectrum analysis. A statistically significant propagating mode of variability with a timescale close to 7 months is found. In addition, output from a high-resolution simulation of the Parallel Ocean Climate Model (POCM) is analyzed, which also displays variability on a timescale of 7 months, although not statistically significant at the 95% confidence level. To explain the physics of the apparent preference for variability at this timescale within a strongly eddying flow, we investigate the stability of barotropic North Pacific flows. Within a shallow-water model, there appear to be three different steady flow paths of the Kuroshio that all become unstable to oscillatory modes. One of these modes has a timescale of 7 months for reasonable values of the parameters and has other features in common with the 7-month statistical modes from the POCM output and from the observations. The connection between results from the stability analysis and from the analysis of the observations and POCM output leads to the conjecture that the preference for the 7-month variability is due to the existence of a large-scale barotropic instability of the mean Kuroshio path.

Oceanic torques on solid Earth and their effects on Earth rotation

We calculate that the oceanic torque vectors (x, y, z components) acting on the solid Earth using the Parallel Ocean Climate Model (POCM) output data at 3‐day intervals for the period 1988–1995. This study examined three distinct types of torque, with different physical mechanisms: pressure, gravitational, and frictional. The pressure torque is further divided into two parts: that due to Earth's ellipticity and that due to the ocean bottom topography. According to resultant time series the ellipticity pressure torque causes the largest effect for the x and y components. The gravitational torque and the ellipticity pressure torque are shown numerically to be exactly proportional by a factor of −0.49. The topographic pressure torque is somewhat smaller in overall amplitude than the ellipticity‐induced torque (the sum of the ellipticity pressure torque and gravitational torque). The ocean bottom frictional torque is found to be negligible in all three components. To validate the torque calculations, we compare the calculated oceanic torques with effective torque derived from the ocean angular momentum (OAM) variation, both from POCM. It is shown that the effective torque associated with the mass term of the OAM agrees remarkably well with the ellipticity‐induced torque as required by theory. For the motion term the effective torque can be explained mostly by the topographic pressure torque in the intraseasonal periods, but the seasonal and longer‐period components show significant differences between the two. The difference should theoretically be attributed to the exclusion of the wind frictional torque on the ocean. However, a comparison based on the NASA GEOS‐1 atmospheric model shows that the wind frictional torque obtained is insufficient in power in the x and y components. Finally, in the application to the Earth rotation excitation it is found that both the ellipticity and topographic torques have comparable contributions to polar motion excitation for both seasonal and nonseasonal variations. For the length of day variation, our torque approach is presently inadequate because the oceanic contribution is relatively too small to be estimated in the presence of data and model errors.

Oceanic and soil moisture contributions to seasonal polar motion

Oceanic contributions to the annual and semi-annual wobble of polar motion have been evaluated by Wünsch (Wünsch, J., 2000. Oceanic influence on the annual polar motion. J. Geodynamics 30, 389–399) using three different ocean circulation models: (a) the Parallel Ocean Climate Model (Semtner, A.J., Chervin, R.M., 1992. Ocean circulation from a global eddy-resolving model. J. Geophys. Res. 97, C4, 5493–5550; (b) the model used by Ponte et al. (Ponte, R.M., Stammer, D., Marshall, J., 1998. Oceanic signals in observed motions of the Earth's pole of rotation. Nature 391, 476–479); (c) the Hamburg Ocean Model for Circulation and Tides used by Thomas and Sündermann [Thomas, M., Sündermann, J., 1998. Zur simultanen Modellierung von allgemeiner Zirkulation und Gezeiten im Ozean und Auswirkungen auf bestimmte Erdrotations parameter. In: Freeden, W. (Ed.), Progress in Geodetic Science. Aachen, pp. 144–151]. That result is extended by considering oceanic refinements as well as time variable soil moisture and snow load. Five soil moisture models were used. The snow load according to Chao et al. (Chao, B.F., O'Connor, W.P., Chang, A.T.C., Hall, D.K.,Foster, J.L., 1987. Snow-load effect on the Earth's rotation andgravitational field 1979-1985. J. Geophys. Res. 92, 9415–9422) was added to this in an attempt to close the balance. The NCEP/NCAR reanalysis atmosphere+Ponte et al. (1998) ocean+Chao and O'Connor (Chao, B. F., O'Connor, W. P., 1988. Global surface-water-inducedseasonal variations in the Earth's rotation and gravitationalfield. Geophys. J. 94, 263–270) (rain+snow) nearly close the annual balance of polar motion excitation, i.e. it is very close to the geodetic excitation functioncomputed from the time series of the IERS (International Earth Rotation Service). The other models of soil moisture differ in theirpolar motion contribution from the Chao and O'Connor (1988) estimates. Further improvements in this work await results from the space gravity missions CHAMP, GRACE and GOCE.

Response of the Mid‐Cretaceous global oceanic circulation to tectonic and CO2 forcings

The mid‐Cretaceous was a period of unusually active tectonism that drove enhanced volcanic outgassing and high seafloor spreading rates. This intense tectonic activity is coincident with dramatic events in the marine environment, including oceanic anoxic events 1 (Aptian‐Early Albian) and 2 (Cenomanian/Turonian boundary), high biological turnover rates, and a thermal maximum. In this study, a series of mid‐Cretaceous ocean general circulation model experiments were completed using the Parallel Ocean Climate Model. These experiments demonstrate the effect of enhanced atmospheric CO2 concentrations and paleogeographic change on the global oceanic circulation. The experiments reveal that paleogeography, specifically the presence/absence of a marine connection between the North Atlantic and South Atlantic basins, may have governed the nature of the mid‐Cretaceous global oceanic circulation. In the absence of this connection, an Albian simulation is characterized by extremely warm, saline conditions throughout the North Atlantic and northern South Atlantic Oceans. With a gateway in a Turonian simulation, Antarctic Bottom Water ventilates the Atlantic basins. In both Albian and Turonian simulations the Pacific‐Indian basins are dominated by thermohaline circulation with deep water sources in the Southern Ocean. While atmospheric CO2 concentrations influence the global temperature and salinity, an increase from present‐day to 4 times present‐day levels alters the global circulation very little. Differences between the Albian and Turonian numerical simulations agree well with aspects of the marine record, supporting speculation that the climatic and oceanographic changes surrounding the Cenomanian‐Turonian boundary were driven by the initiation of a connection between the Atlantic Oceans.

Can Pacific Ocean Thermocline Depth Anomalies Be Simulated by a Simple Linear Vorticity Model?

Abstract This study attempts to reproduce the salient features of the variability in the depth of the thermocline in the marginally eddy-resolving Parallel Ocean Climate Model (POCM) of Semtner and Chervin, using a simple linear vorticity model that only permits local Ekman pumping and the propagation of long Rossby waves. The dynamic upper-ocean variability in the POCM is examined in response to changes in daily European Centre for Medium-Range Weather Forecasts wind stresses across the tropical and subtropical Pacific Ocean (31°S–31°N) between 1983 and 1989. The POCM provides a complete and physically consistent representation of the state of the Pacific Ocean, with the phase of the thermocline depth anomalies being consistent with the observed El Nino/La Nina variations in the near-equatorial zone and southwest Pacific during the decade. A series of vorticity model sensitivity experiments, incorporating scaled Rossby wave speeds based on recent observations from the TOPEX/Poseidon satellite altimeter, ...

On the annual cycle characteristics of the sea surface height in south china sea

The annual cycle characteristics of the SSH in the South China Sea (SCS) are analyzed based on the Sea Surface Height (SSH) anomaly data from the TOPEX / POSEIDON-ERS altimeter data and the Parallel Ocean Climate Model (POCM) prediction. The results show that the distributions of the SSH anomalies of the SCS in January, March and May, are opposite to those in July, September and November respectively; In January (July) there is the SSH negative (positive) anomaly in the deep water basin and at the Luzon Strait, while there is positive (negative) anomaly on the most of continental shelves in the west and south of South China Sea; In March (September) the SSH anomalies are similar to those in January (July), although their magnitudes have decreased and a small positive (negative) anomaly appears in the center of the South China Sea; The amplitude of the SSH annual cycle reaches its maximum in the Northwest of the Luzon Is-land; The seasonal variability of the wind stress is dominant in the formation of the SSH seasonal variability.

Nontidal oceanic contributions to gravitational field changes: Predictions of the Parallel Ocean Climate Model

This study examines the nontidal contributions of the oceans to the Earth's gravitational field variations as predicted by a global ocean general circulation model: the Parallel Ocean Climate Model. Such variations in the gravitational field Stokes coefficient are determined up to spherical harmonic degree and order 20 and compared with satellite laser ranging (SLR) data from LAGEOS I and LAGEOS II. For most Stokes coefficients except the lowest‐degree ones, this investigation indicates that the application of sea level adjustment to reduce the effects of the model's lack of mass conservation due to the Boussinesq approximation has a negligible effect on timescales less than a few years. Predicted gravitational changes show strong seasonal variability and account for a portion of the variations estimated from SLR. We conclude that, in addition to the atmosphere, the oceans are an important contributor to the temporal variations in the Earth's gravitational field. The Stokes coefficients are useful in examining oceanic mass transport between hemispheres and ocean basins. The estimated oceanic power spectrum has a spectral shape similar to the atmosphere and is well above the noise level of planned satellite missions like the Gravity Recovery and Climate Experiment (GRACE).

The Kuroshio East of Taiwan: Modes of Variability and Relationship to Interior Ocean Mesoscale Eddies

Abstract Observations from the World Ocean Circulation Experiment PCM-1 moored current meter array in the East Taiwan Channel are analyzed and combined with TOPEX/Poseidon altimetry data and the Parallel Ocean Climate Model simulation to study Kuroshio variability and relationships to westward propagating sea surface height anomalies in the Philippine Sea. Approximately 60% of the total subinertial velocity and temperature variance in the Kuroshio east of Taiwan is associated with so-called “transport” and “meandering” modes revealed from empirical orthogonal function analysis. The transport mode is dominated by a 100-day peak, while the most coherent energetic meandering signals are found in three limited frequency bands centered near periods of 100 days, 40 days, and 18 days. The detailed structure of the meanders is studied by frequency domain EOF analysis, which also reveals a higher frequency meander centered near 10 days confined to the western side of the channel. On the 100-day timescale, the Kuro...

Annual vertical crustal motions predicted from surface mass redistribution and observed by space geodesy

Temporal variations of surface mass redistribution among atmosphere, oceans, and continental water reservoirs deform the Earth's crust, in particular in the vertical direction. These displacements can now be measured by space geodesy and predicted from climatic loading data. In this study we first compute globally theoretical vertical displacements of the Earth's crust caused by the main annual surface mass redistributions (atmosphere and ocean mass, soil moisture, and snow load). For that purpose we consider atmospheric pressure data from the National Centers for Environment Prediction (NCEP), soil moisture data from Huang et al. [1996] and from the Global Soil Wetness Project (GSWP), snow data from the International Satellite and Surface Climatology Project (ISLSCP) and GSWP, and ocean mass data from the Parallel Ocean Climate Model (POCM) and from TOPEX‐Poseidon satellite altimetry after correcting for steric effects. Annual vertical displacements are computed for each load individually as well as for the total climatic contribution on global 2.5°×2.5° grids. We then present space geodesy‐derived annual variations of vertical coordinates of 16 Doppler orbitography and radiopositioning integrated by satellite (DORIS) stations globally distributed around the globe. A comparison is then performed for each station between observed (by DORIS) and predicted (from climatology) results.

On the observed annual gravity variation and the effect of sea surface height variations

Based on two datasets of sea surface height (SSH) variations, the Parallel Ocean Climate Model (POCM) [J. Geophys. Res. 101 (C10) (1996) 25779] and the TOPEX/POSEIDON (T/P) altimeter, we have analyzed the effect of SSH variations on gravity observations. For this purpose, we first estimated the steric component of SSH variations which we characterized by a thermal steric coefficient relating the sea surface temperature (SST) to SSH. The steric coefficient was estimated on each ocean mesh by assuming a simple linear relation ship between the time variations in the SSH and SST fields. We obtained a value of 0.60×10 −2 m/°C averaged over the central parts of the Pacific and Atlantic Oceans. We then estimated the annual gravity change by calculating the effects of the solid tide, ocean tide, polar motion and SSH variations. The predicted values at the three observation sites (i.e. Esashi in Japan, Canberra in Australia and Syowa Station in Antarctica) were compared with the actual data obtained from the superconducting gravimeters installed at these three sites. The results of the comparison indicate that the predictions agree with the observations within 20% in amplitude (i.e. within 0.2 μGal, where 1 μGal=1×10 −8 ms −2) and 20° in phase at each observation site for both SSH datasets of the POCM and T/P. We have also tested other values for the steric coefficient, i.e. 0.0×10 −2 and 1.0×10 −2 m/° C , but find that the fit-to-gravity observations made at the mid-latitudes is clearly better at 0.60×10 −2 m/°C. It is noted that our gravity observations point to a value of steric coefficient similar to that independently determined from the relationship between the SSH and SST data. We have also tried to investigate the effects of SSH on the gravity observations in other frequency bands. Among these effects, one of the interesting results is the gravity changes induced by ENSO-like ocean oscillations. Our computations suggest that the oscillations contribute 2–3 μGal in peak-to-peak amplitude to gravity variations in the equatorial Pacific at the maximum.

Antarctic Circumpolar Current response to zonally averaged winds

Coherence analysis techniques are used to compare Southern Ocean wind forcing with Antarctic Circumpolar Current transport. Winds are derived from five different products: ERS winds that have been bin‐averaged, weekly gridded ERS winds produced by the Centre ERS d'Archivage et de Traitement, 5 day winds from the Special Sensor Microwave Imager, analysis winds from the European Centre for Medium‐Range Weather Forecasts, and reanalysis winds from the National Centers for Environmental Prediction. Barotropic transport is estimated from the pressure difference between bottom pressure gauges deployed on either side of Drake Passage by Proudman Oceanographic Laboratory as part of the World Ocean Circulation Experiment. Surface transport is estimated from TOPEX altimetry. Results indicate that transport and wind forcing are coherent over a broad range of frequencies, corresponding to time periods of roughly 10–256 days. Highest coherences occur for winds at latitudes on the south side of Drake Passage. Barotropic ocean transport lags wind forcing not by a constant time interval but by a constant phase lag of about one eighteenth of a cycle at a broad range of frequencies, suggesting that the oceanic response to wind is controlled by both the tendency term and a frequency‐dependent viscous process. Surface transport lags by a longer phase interval. Wind stress curl north of Drake Passage is more coherent with transport than is wind stress curl in the latitudes of Drake Passage. Ocean transport lags wind stress curl, suggesting that transport fluctuations are not governed by a simple Sverdrupian vorticity balance. Like the observations, general circulation model transports from the Parallel Ocean Program and from the Parallel Ocean Climate Model are coherent with wind stress from the south side of Drake Passage and with wind stress curl from latitudes north of Drake Passage. Unlike the observations, model transport and bottom pressure vary almost simultaneously with the wind and do not replicate the observed phase lags, implying that the effective model viscosity may be too large.

Dynamics of Intermediate Water Circulation in the Subtropical South Atlantic*

The circulation of the low-salinity Antarctic Intermediate Water in the South Atlantic and the associated dynamical processes are studied, using recent and historical hydrographic profiles, Lagrangian and Eulerian current measurements as well as wind stress observations. The circulation pattern inferred for the Antarctic Intermediate Water supports the hypothesis of an anticyclonic basinwide recirculation of the intermediate water in the subtropics. The eastward current of the intermediate anticyclone is fed mainly by water recirculated in the Brazil Current and by the Malvinas Current. An additional source region is the Polar Frontal zone of the South Atlantic. The transport in the meandering eastward current ranges from 6 to 26 Sv (Sv [ 106 m3 s21). The transport of the comparably uniform westward flow of the gyre varies between 10 and 30 Sv. Both transports vary with longitude. At the western boundary near 288S, in the Santos Bifurcation, the westward current splits into two branches. About three-quarters of the 19 Sv at 408W go south as an intermediate western boundary current. The remaining quarter flows northward along the western boundary. Simulations with a simple model of the ventilated thermocline reveal that the wind-driven subtropical gyre has a vertical extent of over 1200 m. The transports derived from the simulations suggest that about 90% of the transport in the westward branch of the intermediate gyre and about 50% of the transport in the eastward branch can be attributed to the wind-driven circulation. The structure of the simulated gyre deviates from observations to some extent. The discrepancies between the simulations and the observations are most likely caused by the interoceanic exchange south of Africa, the dynamics of the boundary currents, the nonlinearity, and the seasonal variability of the wind field. A simulation with an inflow/outflow condition for the eastern boundary reduces the transport deviations in the eastward current to about 20%. The results support the hypothesis that the wind field is of major importance for the subtropical circulation of Antarctic Intermediate Water followed by the interoceanic exchange. The simulations suggest that the westward transport in the subtropical gyre undergoes seasonal variations. The transports and the structure of the intermediate subtropical gyre from the Parallel Ocean Climate Model (Semtner‐ Chervin model) agree better with observations.

Physics of the 9-Month Variability in the Gulf Stream Region: Combining Data and Dynamical Systems Analyses

Using nonseasonal altimeter data and SST observations of the North Atlantic, and more specifically the Gulf Stream region, dominant patterns of variability are determined using multivariate time series analyses. A statistically significant propagating mode of variability with a timescale close to 9 months is found, the latter timescale corresponding to dominant variability found in earlier studies. In addition, output from a high resolution simulation of the Parallel Ocean Climate Model (POCM) is analyzed, which also displays variability on a timescale of 9 months, although not statistically significant at the 95% confidence level. The vertical structure of this 9-month mode turns out to be approximately equivalent barotropic. Following the idea that this mode is due to internal ocean dynamics, steady flow patterns and their instabilities are determined within a barotropic ocean model of the North Atlantic using techniques of numerical bifurcation theory. Within this model, there appear to be two different mean flow paths of the Gulf Stream, both of which become unstable to oscillatory modes. For reasonable values of the parameters, an oscillatory instability having a timescale of 9 months is found. The connection between results from the bifurcation analysis, from the analysis of the observations, and from the analysis of the POCM output is explored in more detail and leads to the conjecture that the 9-month variability is related to a barotropic instability of the wind-driven gyres.

Oceanic angular momentum variability estimated from the Parallel Ocean Climate Model, 1988–1998

This paper describes the use of a global numerical model of the oceans to estimate variations in both the equatorial and axial components of angular momentum resulting from oceanic mass redistribution and circulation. The Parallel Ocean Climate Model, driven by daily wind fields and monthly heat fluxes from the European Centre for Medium‐Range Weather Forecasts for 1988–1998, provides insight into variations in the transport and exchange of angular momentum, a quantity which is essentially conserved within the Earth system. Exchange of angular momentum between the oceans and the solid Earth should be manifest as changes in the Earth's rotation (both polar motion and length of day), and it is possible to compare predicted Earth rotation changes with actual geodetic observations. Using an inverted barometer assumption for oceans, the numerical model predictions of rotation change agree in sign and magnitude and are significantly correlated with observed polar motion and length of day variations after subtracting the dominant atmospheric contributions. The correlation has a seasonal variation which suggests that the role of the oceans in the excitation polar motion is more important during the Northern Hemisphere summer. Our results indicate that the oceans, to a different extent, account for a significant part of the nonatmospheric angular momentum budget for the Earth, and its fluid envelopes on interannual to submonthly timescales.