Semiannual Time Scales Research Articles

Predrought and Its Persistence Determined the Phenological Changes of Stipa krylovii in Inner Mongolia

Clarifying the response of plant phenology to drought duration is helpful for accurately interpreting and predicting carbon sinks in ecosystems. Based on the response of different phenological periods of the dominant species Stipa krylovii to monthly, seasonal, and semiannual time scale drought in the typical steppe of Inner Mongolia from 1983 to 2018, the results revealed that (1) the start of the growing season (SOS) was characterized by an advance–delay–advance pattern, and the heading stage (HOS), flowering stage (FOS), and end of the growing season (EOS) all showed consistent advanced trends, which provided additional insight into the conclusions of previous studies that found the SOS was advanced in arid and semiarid regions. (2) The response mechanism of the SOS to the timing of drought was not consistent. Among the response mechanism, the SOS was delayed because of the January–February drought at different time scales but advanced because of the April drought at different time scales. The HOS/FOS was delayed by the June–July drought at different time scales, and the EOS was advanced by the August–September drought at different time scales. (3) More importantly, the SOS, HOS, FOS, and EOS were affected by predrought and its persistence, and the effects were greater the closer to the phenological periods that drought occurred. (4) At the monthly scale, droughts in January, June, and August were the critical drought periods affecting the SOS, HOS/FOS, and EOS, while seasonal and semiannual scale droughts in February, June–July, and September were the critical periods affecting the SOS, HOS/FOS, and EOS. The results of this study enrich our understanding of how drought at different time scales affects different phenological periods, providing a basis for improving plant phenological models.

Biophysical modelling of coastal upwelling variability and circulation along the Tanzanian and Kenyan coasts

Ocean circulation, upwelling phenomena and chlorophyll-a concentrations were investigated within the framework of numerical model simulations with 1/12° nested horizontal grid-size, in the tropical western Indian Ocean, along the coasts of Tanzania and Kenya. Ekman driven upwelling exhibited high levels of spatial and temporal variability in the region, characterized by a more vigorous occurrence/intensification during the Northeast than the Southwest Monsoon season. A similar trend was observed for chlorophyll-a distribution, but with an additional strong contribution during the inter-monsoon period from March to April. Trend analysis of a SST-derived coastal upwelling index (CUI) computed over the Pemba Channel and offshore of the East African Coastal Current (EACC), for 24 years (1990 - 2013), revealed a general linear relation of the form CUI(yr) = 2.4x10-7yr – 285, with a steady small annual increase of the upwelling phenomena by 0.0024/year ≃ 4% during the whole period of the simulation, which could be attributed to documented increasing trends of wind intensity and water volume transport in the region. The CUI exhibited the two most dominant peaks of variabilities on the range of annual and semi-annual timescales. The wind-stress southward component and the easting/westing veering of the northward EACC at 6°S revealed that these parameters were moderate and significantly correlated with the CUI (r = - 0.53 and 0.52, p<0.05) respectively, further suggesting its intensification during the Northeast Monsoon season.

Hydro-climatology study of the Ogooué River basin using hydrological modeling and satellite altimetry

Hydrological models are important tools for the simulation of water storage and hydrological fluxes in large basins and complex river systems. The hydrological models can compensate the lack of observed data in ungauged basins. In this study, the hydrological model of large basins MGB (for Model of Large Basins in Portuguese) is used to evaluate the hydrological processes of the Ogooué River Basin (ORB), which has been mostly unmonitored for about three decades. Simulations were carried out over an 18-year period from 1998 to 2015 using TRMM 3B42 daily rainfall data from the Tropical Rainfall Measurement Mission (TRMM) as forcing and in situ and altimetry-based river discharges from Envisat, Saral Altika and Jason-2 for calibration and validation. The results of the model were in good agreement with the flows measured at stations upstream and downstream of the Ogooué basin (Nash-Sutcliffe Efficiency (NSE) > 0.56 for all calibration gauges). The MGB model efficiently describes the seasonal and interannual variations of the flow in the Ogooué River and its major tributaries which were found to be highly correlated to the rainfall (r ranging from 0.72 to 0.90 and 0.56 to 0.87 at seasonal and interannual time-scales respectively). Interannual variations of precipitation and river discharge of the ORB are linked to the El Niño Southern Oscillation (ENSO) in the tropical eastern Pacific Ocean and southeastern tropical Atlantic Niño. Also, the Ogooué River discharge was found to be strongly correlated with Sea Surface Temperature (SST) at annual and semi-annual time-scales.

Semiannual Variability of Middepth Zonal Currents along 5°N in the Eastern Indian Ocean: Characteristics and Causes

AbstractFour-year (2014–17) zonal current data observed by a mooring at (5°N, 90.5°E) in the eastern Indian Ocean show a strong semiannual cycle in the middepth (~1200 m) with distinct vertical structure. This pronounced middepth semiannual variability, however, is inconsistent with the local wind forcing, which shows a predominant annual cycle. The underlying causes for this unique middepth variability along 5°N were elucidated with the addition of a reanalysis product and a continuously stratified linear ocean model. The results suggest that the observed seasonal variability in the middepth zonal flow at 5°N is primarily caused by boundary-reflected Rossby waves forced by the remote semiannual winds along the equator. Contribution from the locally wind-forced Rossby waves is much less. The theoretical Wentzel–Kramers–Brillouin ray paths further verify that the strong semiannual variability of the middepth signals over a moored region in the eastern Indian Ocean is largely a manifestation of the steep angles of propagating energy of the long Rossby waves at semiannual time scale. The annual signals are only significant in the upper and western sections (75°–80°E) as a result of the smooth trajectories of Rossby waves forced by local annual winds. Further analysis reveals that the middepth zonal currents along 5°N are expected to be associated with equatorial symmetric Rossby waves at semiannual period. Consequently, similar zonal flows should also exist in the middepth near 5°S.

Vertical Propagation of Middepth Zonal Currents Associated With Surface Wind Forcing in the Equatorial Indian Ocean

AbstractThe vertical propagation of middepth (200–1,500 m) zonal velocity in the equatorial Indian Ocean (EIO) and its relationship with surface wind forcing are investigated. We focus on semiannual time scales, using in situ mooring observations and an oceanic reanalysis product that can reasonably well reproduce the structure and magnitude of the observed middepth zonal velocity in the EIO. The pronounced semiannual cycle in observed middepth zonal currents indicates a clear upward phase propagation with particularly robust signatures at depths of 500–1,000 m between 3°S to 3°N, 70 to 85°E. The main characteristics of the middepth currents are consistent with vertically propagating first‐meridional‐mode Rossby wave at semiannual period. Theory suggests that equatorial Kelvin waves, forced by zonal wind variations in the western half of the basin, transfer energy downward and eastward, reflecting into Rossby waves at the eastern boundary. These reflected Rossby waves transfer energy downward and westward, contributing greatly to observed phase propagation at middepths in the EIO.

Features of the Equatorial Intermediate Current Associated with Basin Resonance in the Indian Ocean

AbstractThis paper investigates the features of the Equatorial Intermediate Current (EIC) in the Indian Ocean and its relationship with basin resonance at the semiannual time scale by using in situ observations, reanalysis output, and a continuously stratified linear ocean model (LOM). The observational results show that the EIC is characterized by prominent semiannual variations with velocity reversals and westward phase propagation and that it is strongly influenced by the pronounced second baroclinic mode structure but with identifiable vertical phase propagation. Similar behavior is found in the reanalysis data and LOM results. The simulation of wind-driven equatorial wave dynamics in the LOM reveals that the observed variability of the EIC can be largely explained by the equatorial basin resonance at the semiannual period, when the second baroclinic Rossby wave reflected from the eastern boundary intensifies the directly forced equatorial Kelvin and Rossby waves in the basin interior. The sum of the first 10 modes can reproduce the main features of the EIC. Among these modes, the resonant second baroclinic mode makes the largest contribution, which dominates the vertical structure, semiannual cycle, and westward phase propagation of the EIC. The other 9 modes, however, are also important, and the superposition of the first 10 modes produces downward energy propagation in the equatorial Indian Ocean.

Seasonal and Interannual Variability in the Barrier Layer of the Bay of Bengal

AbstractAn objectively analyzed monthly gridded ocean analysis derived from a combination of satellite and in situ observations is used to describe the seasonal and interannual variability in the barrier layer thickness (BLT) over the maximum variance region in the BLT of the Bay of Bengal (BoB) for the period from 1993 to 2012. The northern BoB acquires a maximum variance in the BLT where a large river discharge flux was observed. The BLT showed strong seasonal variability with minima during April–May and maxima during December–February. An investigation of the variability in the BLT showed large amplitude at annual and semiannual time scales. The relationship between the BLT and the Indian Ocean dipole mode (IOD) reveals that the BLT maximum is relatively high in the negative IOD years when compared to the positive IOD years. A significant relation between the BLT and IOD is inferred with the IOD leading the BLT by 3 months and lagging by 1 month. We show that the 1 month lag in the northern BoB is mainly associated with coastally trapped Kelvin waves that radiate Rossby waves and control the BLT. However, further investigation is required regarding the temporal lead of the IOD with the BLT in the northern BoB.

Persistence of productive surface thermal fronts in the northeast Arabian Sea

In this paper we demonstrate the persistence of sea surface thermal fronts based on the frequency of Sea Surface Temperature gradient satellite data, at seasonal and non-seasonal cycles. Prominent thermal fronts were observed in the northeast Arabian Sea between the 50 m bathymetry contour and continental self break (∼200 metre contour). A total of 45–75% of thermal fronts occurred at the annual time-scales whereas 20–30% occurred at the semi-annual time-scales. Maximum frequency occurrence was observed in the coastal waters south and southwest of Saurashtra peninsula during February and March. Primary productivity co-varies with the frequency of the thermal fronts with a time-lag of 2–4 weeks. This outcome can help on timely and reliable prediction of biologically productive zones.

On seasonal and semi-annual approach for flood frequency analysis

As a supplementary method to the conventional flood frequency analysis based on annual peak flows, we propose an approach in this paper to infer the flood frequency distribution on quarterly and semi-annual time scale, which are then converted to annual time scale to obtain the floods corresponding to return periods in unit of year. Two criteria for test of data independence, namely, minimum 7 and 15-day interval between two consecutive peak flows, are tested. The proposed approach was applied to Des Moines River at Fort Dodge, Iowa, USA using its 62 years of observation daily flows. The results show that the estimated floods for given return periods from quarter-annual data series are in general higher than the corresponding estimated floods from semi-annual data series, which is further larger than estimated floods from annual peak flows. The floods estimated from semi-annual data series agree well with the results of previous US Geological Survey study.

Estimating water storage changes and sink terms in Volta Basin from satellite missions

The insufficiency of distributed in situ hydrological measurements is a major challenge for hydrological studies in many regions of the world. Satellite missions such as the Gravity Recovery and Climate Experiment (GRACE) and the Tropical Rainfall Measurement Mission (TRMM) can be used to improve our understanding of water resources beyond surface water in poorly gauged basins. In this study we combined GRACE and TRMM to investigate monthly estimates of evaporation plus runoff (sink terms) using the water balance equation for the period from January 2005 to December 2010 within the Volta Basin. These estimates have been validated by comparison with time series of sink terms (evaporation plus surface and subsurface runoff) from the Global Land Data Assimilation System (GLDAS). The results, for the period under consideration, show strong agreement between both time series, with a root mean square error (RMSE) of 20.2 mm/month (0.67 mm/d) and a correlation coefficient of 0.85. This illustrates the ability of GRACE to predict hydrological quantities, e.g. evaporation, in the Volta Basin. The water storage change data from GRACE and precipitation data from TRMM all show qualitative agreement, with evidence of basin saturation at approximately 73 mm in the equivalent water column at the annual and semi-annual time scales.

The dynamics of the mesosphere and lower thermosphere: a brief review

The dynamics of the mesosphere-lower thermosphere (MLT) (60 to 110 km) is dominated by waves and their effects. The basic structure of the MLT is determined by momentum deposition by small-scale gravity waves, which drives a summer-to-winter pole circulation at the mesopause. Atmospheric tides are also an important component of the dynamics of the MLT. Observations from extended ground-based networks, satellites as well as numerical modelling show that non-migrating tidal modes in the MLT are more important than previously thought, with evidence for directly coupling into the thermosphere/ionosphere. Major disturbances lower in the atmosphere, such as wintertime sudden stratospheric warmings, temporarily disrupt the circulation pattern and thermal structure of the MLT. In the equatorial mesosphere, gravity wave driving leads to oscillations in the zonal wind on semiannual time scales, although variability on quasi-biennial time scales is also apparent. Planetary-scale waves such as the quasi-two-day wave temporarily dominate the dynamics of the summertime MLT, especially in the southern hemisphere. Impacts may include short-term changes to the thermal structure and physics of the high-latitude MLT. Here, we briefly review the dynamics of the MLT, with a particular emphasis on developments in the past decade.

Spatial–temporal patterns and driving mechanisms of semiannual variations in the Philippine Sea

Satellite altimetric sea surface height (SSH) measurements from 1992 through 2010 are used to explore the oceanic semiannual variations in the Philippine Sea (PS). Pronounced semiannual SSH variations are detected within two zonal bands. One lies east of Luzon Strait (19°–22°N) in the northern PS, while the other is southeast of Mindanao coast (4°–7°N) in the southern PS. In the two near-coast boxes where semiannual harmonic amplitude exceeds 4cm, the northern box (127°–133°E, 19°–22°N) and the southern box (127°–133°E, 4°–7°N), semiannual changes contribute considerably to the total annual SSH variance by 12% and 17%, respectively. Despite prominent SSH variability, the bifurcation latitude of the North Equatorial Current (NBL) exhibits weak fluctuations with a peak-to-peak difference of only 0.3° on semiannual time scale. While the in-phase annual SSH variations between the two boxes work together to enhance annual NBL changes, their out-of-phase semiannual SSH variations offset each other in driving the NBL displacements. Further analysis with a 11/2-layer reduced-gravity model forced by ECMWF wind stress reveals that the observed semiannual SSH variations are primarily driven by local wind forcing in the far western Pacific. Rossby wave signals propagating from the eastern/central Pacific contribute much less due to along-path dispersion and cancellation. Semiannual signals of wind field in the northern PS reflect mainly the semiannual changes of the Asian Monsoon system, while those in the southern PS arise from the combined effects of Monsoon transition and the annual migration of the intertropical convergence zone (ITCZ).

Correlation Analysis of the TEC at Wuhan Station With Solar Activities and Geomagnetic Activities During 23rd Solar Cycle

利用武汉电离层观测站1997-2007年电离层TEC资料, 采用连续小波变换和交叉小波以及小波相干方法, 分析了该站电离层TEC的周期变化特征以及与太阳和地磁活动之间的关系. 分析结果表明, 武汉站TEC变化的长期趋势主要随太阳活动的强弱而变化; 在局部时域上分别存在128～256d, 256～512d和512d～1024d的周期尺度, 且与同时期的太阳黑子数和地磁<I>Dst</I>指数的周期特征存在很好的对应关系; 太阳黑子数在512～1024d周期尺度上超前TEC变化约1/6个周期; 在准半年的周期尺度上武汉站TEC与地磁<I>Dst</I>指数几乎呈反相位变化, 但TEC对$Dst$指数的这种响应仅在太阳活动高年存在, 具体机理尚需进一步分析研究.

Variation of ionospheric total electron content at crest of equatorial anomaly in China from 1997 to 2004

The periodic variation of TEC data at Xiamen station (geographic coordinate: 24.4°N, 118.1°E; geomagnetic coordinate: 13.2°N, 187.4°E) at crest of equatorial anomaly in China from 1997 to 2004 is analyzed. The characteristic of TEC association with solar activity and geomagnetic activity are also analyzed. The method of continuous wavelet, cross wavelet and wavelet coherence transform methods have been used. Analysis results show that long-term variations of TEC at Xiamen station are mainly controlled by the variations of solar activities. Several remarkable components including 128–256 days, 256–512 days and 512–1024 days exist in TEC variations. The TEC data at Xiamen station is in anti-phase with geomagnetic Dst index in semiannual time-scale, but this response only exists during high solar activity. Diurnal variation of TEC is studied for different seasons. Some features like the semiannual anomaly and winter anomaly in TEC have been reported.

The understanding of length-of-day variations from satellite gravity and laser ranging measurements

P>The change in the rate of the Earth's rotation, length-of-day (LOD), is principally the result of movement and redistribution of mass in the Earth's atmosphere, oceans and hydrosphere. Numerous studies on the LOD excitations have been made from climatological/hydrological assimilation systems and models of the general circulation of the ocean. However, quantitative assessment and understanding of the contributions to the LOD remain unclear due mainly to the lack of direct global observations. In this paper, the total Earth's surface fluids mass excitations to the LOD at seasonal and intraseasonal timescales are investigated from the Jet Propulsion Laboratory Estimating Circulation and Climate of the Ocean (ECCO) model, the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis and the European Center for Medium-Range Weather Forecasts (ECMWF) Re-analysis (ERA)-Interim, GRACE-derived surface fluids mass and the spherical harmonics coefficient C-20 from the satellite laser ranging (SLR) as well as combined GRACE+SLR solutions, respectively. Results show that the GRACE and the combined GRACE and SLR solutions better explain the geodetic residual LOD excitations at annual and semi-annual timescales. For less than 1 yr timescales, GRACE-derived mass is worse to explain the geodetic residuals, whereas SLR agrees better with the geodetic residuals. However, the combined GRACE and SLR results are much improved in explaining the geodetic residual excitations at intraseasonal scales.

Time–Frequency Characteristics of Regional Climate over Northeast China and Their Relationships with Atmospheric Circulation Patterns

Abstract The time–frequency characteristics of the variations of temperature and precipitation over the city of Changchun in northeast China and their associations with large-scale atmospheric and oceanic conditions are analyzed. It is found that the variations of the regional climate are characterized by strong semiannual signals. For precipitation, the amplitude of semiannual signal is about half of that of the annual cycle. The relationships of the Changchun temperature and precipitation with local winds and large-scale patterns of atmospheric circulation and sea surface temperature are also strongest on annual and semiannual time scales. These strong semiannual signals are potentially helpful for improving the prediction of the regional climate. On the annual time scale, the northeast China climate is affected by both the thermal contrast between the Asian continent and the tropical Indo-Pacific Oceans and that between the continent and the extratropical North Pacific. These effects are manifested by the cyclonic (anticyclonic) pattern over the Asian continent (North Pacific) and the strong southerly flow over East Asia and northwestern Pacific associated with increases in temperature and precipitation. On the semiannual time scale, the northeast China climate is mainly related to the large-scale circulation pattern centered over the North Pacific, with its western portion over northeast China, North and South Korea, and Japan. While temperature signals are related to extratropical atmospheric process more apparently, both extratropical and tropical influences are seen in the semiannual variation of precipitation. There exist strong relationships between Changchun temperature and precipitation and the North Pacific Oscillation (NPO) in the frequency band up to 7 months. Temperature increases and precipitation decreases when NPO is positive. The relationships were weak before 1980 but became stronger afterward, associated with the strengthening of the East Asian trough.

Minima of interannual sea-level variability in the Indian Ocean

Wavelet analysis of altimeter sea level in the Indian Ocean shows regions of high variability (maxima) and low variability (minima) at all time scales. At interannual time scales, i.e., at periods of 17 months or more, minima are seen at several places: in the central equatorial Indian Ocean; in the Arabian Sea along the south and west coasts of India and Sri Lanka, along the northern boundary, in the Gulf of Aden, and in patches along the coast of Oman; and in the Bay of Bengal along the east coasts of Sri Lanka and India south of ∼10°N, and in the southern bay east of the Sri Lanka thermal dome. We investigate the cause of these interannual minima using a linear, continuously stratified numerical model, which is able to simulate the observed minima. We separate the forcing into a set of processes: direct forcing by winds in the interior ocean, forcing by winds blowing along continental boundaries, and forcing by Rossby waves generated by the reflection of equatorial Kelvin waves at the eastern boundary. At interannual periods, minima (maxima) of interannual variability occur where the direct wind forcing and reflected Rossby waves interfere destructively (constructively). At interannual periods within the tropics, the adjustment time scale of the system is less than that of the forcing, leading to a quasi-steady balance, a property that distinguishes the interannual minima from those at annual and semiannual time scales. Idealized solutions show that the presence of India causes the minimum along the Indian west coast, and that it extends around the perimeter of the Arabian Sea into the Gulf of Aden.

Variations of U.S. Regional Precipitation and Simulations by the NCEP CFS: Focus on the Southwest

Abstract Variations of U.S. regional precipitation in both observations and free-run experiments with the NCEP Climate Forecast System (CFS) are investigated. The seasonality of precipitation over the continental United States and the time–frequency characteristics of precipitation over the Southwest (SW) are the focus. The differences in precipitation variation among different model resolutions are also analyzed. The spatial distribution of U.S. precipitation is characterized by high values over the East and the West Coasts, especially over the Gulf Coast and southeast states, and low values elsewhere except over the SW in summer. A large annual cycle of precipitation occurs over the SW, northern plains, and the West Coast. Overall, the CFS captures the above features reasonably well, except for the SW. However, it overestimates the precipitation over the western United States, except the SW in summer, and underestimates the precipitation over the central South, except in springtime. It also overestimates (underestimates) the precipitation seasonality over the intermountain area and Gulf Coast states (SW, West Coast, and northern Midwest). The model using T126 resolution captures the observed features more realistically than at the lower T62 resolution over a large part of the United States. The variability of observed SW precipitation is characterized by a large annual cycle, followed by a semiannual cycle, and the oscillating signals on annual, semiannual, and interannual time scales account for 41% of the total precipitation variability. However, the CFS, at both T62 and T126 resolution, fails in capturing the above feature. The variability of SW precipitation in the CFS is much less periodic. The annual oscillation of model precipitation is much weaker than that observed and it is even much weaker than the simulated semiannual oscillation. The weakly simulated annual cycle is attributed by the unrealistic precipitation simulations of all seasons, especially spring and summer. On the annual time scale, the CFS fails in simulating the relationship between the SW precipitation and the basinwide sea surface temperature (SST) and the overlying atmospheric circulation. On the semiannual time scale, the model exaggerates the response of the regional precipitation to the variations of SST and atmospheric circulation over the tropics and western Atlantic, including the Gulf of Mexico. This study also demonstrates a challenge for the next-generation CFS, at T126 resolution, to predict the variability of North American monsoon climate.

Semiannual variability in temperature and salinity observed by Triangle Trans‐Ocean Buoy Network (TRITON) buoys in the eastern tropical Indian Ocean

The time series of temperature and salinity obtained by Triangle Trans‐Ocean Buoy Network buoys in the eastern Indian Ocean was used to investigate the characteristics of their variations and possible mechanisms of the semiannual ocean variability. Spectral analysis indicates that the dominant temperature signals in the surface layer and below the thermocline have intraseasonal and semiannual timescales, respectively. The semiannual isotherm deepening in the thermocline appears clearly during the monsoon transition periods, and indicates the vertical phase shift below the thermocline. The characteristic speed of the wave estimated from the vertical phase shift corresponds to a value between the characteristic speeds of the second and third baroclinic modes. The correlation between the zonal surface wind stress on the equator and the depth of 20°C isotherm variation shows that the forcing region of the semiannual isotherm variation is located around 68°E to 78°E longitude. The estimation of characteristic speed from the lag between these variations also shows a value between the second and third vertical‐mode gravity waves. The salinity variations also indicate that the intraseasonal and semiannual signals are dominant in the surface layer and the thermocline, respectively. The appearance of salty water around the upper thermocline lags the subsurface zonal current variation by about 45 days on the semiannual timescale, suggesting the advection of salty water by the subsurface currents along the equator. There is a possibility that the contact between the subsurface salty water and surface fresh water results in the formation of a barrier layer in the eastern basin.

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.