Fast Kelvin Research Articles

The quasi-6-day waves in NOGAPS-ALPHA forecast model and their climatology in MLS/Aura measurements (2005–2014)

This study has examined both: (i) the global structure and seasonal variability of the eastward- and westward-travelling quasi 6-day waves (Q6DWs) observed in the geopotential height (GPH) NOGAPS-ALPHA hourly forecast data during the time interval of 14 months (January 2009–February 2010), and (ii) the climatology and interannual variability of the Q6DWs observed in the satellite MLS/Aura GPH data for an extended period of 10 years (January 2005–December 2014). Both data sets are analyzed by using the same approach. The detailed analysis revealed that the westward-travelling Q6DWs have been identified mainly at mid-high latitudes with zonal wave numbers 1 and 2. Two different types of eastward-travelling waves have been found: (i) waves at middle and high latitudes with zonal wave numbers 1 and 2, which are observed in the local winters, and (ii) waves observed over the equator with zonal wave number 1, which maximize mainly between June and September with a secondary enhancement between January–March belonging to the fast Kelvin waves. Some signatures of the solar cycle and SSW impact on the interannual variability as well as ∼2–3-year variability have been distinguished in the different ∼6-day waves. This study draws attention to both the rarely studied winter-time eastward-travelling PWs and the impact of the SSW events, particularly the major ones, on the short- and long-term variability of the PW populations.

On the physical interpretation of the lead relation between Warm Water Volume and the El Niño Southern Oscillation

The Warm Water Volume (WWV), a proxy for the equatorial Pacific heat content, is the most widely used oceanic precursor of the El Nino Southern Oscillation (ENSO). The standard interpretation of this lead relation in the context of the recharge oscillator theory is that anomalous easterlies during, e.g. La Nina, favour a slow recharge of the equatorial band that will later favour a transition to El Nino. Here we demonstrate that WWV only works as the best ENSO predictor during boreal spring, i.e. during ENSO onset, in both observations and CMIP5 models. At longer lead times, the heat content in the western Pacific (WWVW) is the best ENSO predictor, as initially formulated in the recharge oscillator theory. Using idealised and realistic experiments with a linear continuously stratified ocean model, and a comprehensive wave decomposition method, we demonstrate that spring WWV mostly reflects the fast Kelvin wave response to wind anomalies early in the year, rather than the longer-term influence of winds during the previous year. WWV is hence not an adequate index of the slow recharge invoked in the recharge oscillator. The WWVW evolution before spring is dominated by forced Rossby waves, with a smaller contribution from the western boundary reflection. WWVW can be approximated from the integral of equatorial wind stress over the previous ~ 10 months, thus involving a longer-term time scale than WWV main time scale (~ 3 months). We hence recommend using WWVW rather than WWV as an index for the slow recharge before the spring predictability barrier.

Features of 3–7-day planetary-wave-type oscillations in F-layer vertical drift and equatorial spread F observed over two low-latitude stations in China

Abstract. Recent studies on the equatorial atmosphere–ionosphere coupling system have shown that planetary-wave-type oscillations, as an important seeding mechanism for equatorial spread F (ESF), play an important role in ESF irregularity development and its day-to-day variability in the equatorial latitudes. In this study, ionosonde virtual height and ESF measurements over Sanya (18.4° N, 109.6° E; 12.8° N dip latitude) and meteor radar neutral-wind measurements over Fuke (19.5° N, 109.1° E; 14° N dip latitude) during 2013 are used to investigate the features of planetary-wave-type oscillations in both the lower atmosphere and the ionosphere and their possible influences on ESF occurrence under the weak solar maximum year. The ∼ 3-day and ∼ 7-day planetary-wave-type oscillations have been observed in the neutral zonal winds and the time rate of change in F-layer virtual heights. According to the propagation characteristics, the 3-day and 7-day planetary-wave-type oscillations are basically recognized as ultrafast and fast Kelvin waves, respectively. With increasing heights, the 3-day wave oscillations are gradually amplified, while the 7-day wave oscillations are generally constant. By performing a cross-wavelet transform on the onsets of ESF and the vertical drifts of the F layer, we found that there are simultaneously occurring 7-day and 3-day common wave oscillations between them. The 7-day waves are mainly in the inversion phase, while the 3-day waves are mostly in an in-phase state, indicating that the 7-day waves may play a main role in ESF initiation. Approximate delays of 6 days for the 7-day waves and 5 days for the 3-day waves in their propagation upward from the lower atmosphere to the ionosphere are evaluated with wavelet power spectrum analysis. The estimated upward velocities from these time delays provide consistent evidence that the 7-day and 3-day waves propagate vertically upward with typical Kelvin wave characteristics. The results highlight the role of planetary-wave-type oscillations in the initiation and development of ESF in the Chinese low-latitude region.

Non-Contact, Sub-Surface Detection of Alloy Segregation in Back-End of Line Copper Dual-Damascene Structures

Alloy seedlayers for copper back-end of line interconnects have become common at technology groundrules of 45 nm and below, due to reliability requirements. The key requirement of the minority alloy component (e.g., Al or manganese, also referred to as the “dopant”) is that it segregates to the copper (Cu)/dielectric cap layer interface in order to promote adhesion between the Cu in the line and the dielectric capping layer. The segregation beneath the dielectric cap has a complex dependence on many process parameters and needs to be monitored across the wafer. In this paper, we present a contactless, in-line, fast, and reliable Kelvin probe technique for the sub-surface detection of alloy segregation in Cu interconnects for sub-22 nm technologies.

Regression Analysis of Zonally Narrow Components of the MJO

Abstract Recent works have demonstrated that eastward-propagating features smaller than zonal wavenumber 3 but with spatial structures similar to those of the Madden–Julian oscillation (MJO) frequently develop over the Indo-Pacific warm pool. These signals are characterized by periods shorter than 4 weeks, but since they occur as part of a spectral peak of the MJO, they might be characterized by similar physics. These zonally narrow features occur at any phase of traditionally defined 30–60-day MJO events, but they occur most frequently in its active convective phase. This work presents a linear regression analysis based on filtering in the wavenumber–frequency domain to compare such signals with traditionally defined MJOs and 15–30 m s−1, convectively coupled Kelvin waves. Results show that the trough collocated with the easterly wind anomaly extends westward into the region of lower-tropospheric westerly wind and deep convection in the zonally narrow slow signals and MJOs. The fast Kelvin waves have a ridge anomaly collocated with the westerly wind anomaly. The zonally narrow slow signals and MJOs include a warm anomaly in the boundary layer west of the deep convection that is absent in fast Kelvin waves. Results suggest that MJO dynamics are not confined to the 30–60-day band and that time scales as short as 2 weeks could be considered in wavenumber–frequency diagnostics for the MJO.

Signatures of ultra fast Kelvin waves in low latitude ionospheric TEC during January 2009 stratospheric warming event

In this study, for the first time, planetary wave signatures in ionospheric Total Electron Content (TEC) retrieved from global positioning service (GPS), mesospheric wind and temperature at low latitude station has been identified during January–February 2009. Our investigations revealed that planetary waves with 3–5 days periodicity characterized as ultrafast Kelvin (UFK) waves caused by stratospheric warming event during January 2009. The UFK waves are observed to be propagated from lower atmosphere to ionosphere. The UFK perturbations during SSW event are discussed in the light of current understanding of role of non linear interaction of planetary waves to modulate low latitude ionosphere.

Analysis of viscoelasticity of POF gratings in the stress sensing

The time-dependent behavior of polymer optical fiber (POF) grating under constant tensile stresses has been studied. We show that the evident time-dependence is due to the viscoelastic nature of POF grating materials that can be described with the Kelvin Model. Based on the Kelvin Model, the time-dependent relationship between the Bragg wavelength shift and stress has been analyzed in detail. The results show that the viscoelasticity has a great impact upon the stress response of POF gratings. With the increment of stress level, the visco response part increases faster than that of elastic response part. Especially, the response and recovery with and without stress can be fitted with dual exponential decay and the time constant of fast response and relaxation is ∼102s and that of the slow is ∼103s, which corresponds to the fast and slow movements of polymer segments, respectively. Experiments and regressions also show that the stress sensitivity is observed to be 369pm/MPa for the immediate response and 598pm/MPa for response balance, showing high stress sensitivity. All the spring stiffness and dashpot viscosity of Kelvin elements are larger than 1010Pa and 1013Pas. The dashpot viscosity of slow Kelvin element (1014Pas) is around one order larger than that of fast Kelvin element (1013Pas) with stress. Further analysis demonstrate the response and recovery of POF gratings with and without the stress displays an evident non-linear viscoelasticity, which will bring more complexity for POF gratings in the mechanical sensing applications.

Signatures of the 3‐day wave in the low‐latitude and midlatitude ionosphere during the January 2010 URSI World Day campaign

Recent studies of the equatorial ionosphere have found evidence of forcing by atmospheric Ultra Fast Kelvin (UFK) waves. This study investigates the quasi‐3‐day UFK wave and its effects on the variations of the ionosphere at low latitudes and midlatitudes using coordinated observations of both the atmosphere and ionosphere during the January 2010 URSI World Day campaign. The global maps of TEC from the IGS ground‐based GPS product demonstrate a 3‐day periodic variation during January 15–25. This variation has the largest amplitude at 15° magnetic latitude and extends into lower latitudes. Simultaneously, a 3‐day wave is observed in the mesosphere in the zonal wind measurements by a meteor radar at the magnetic equator. The latitudinal range of the TEC variation (20°S–20°N) is also consistent with that of the 3‐day wave. The Incoherent Scatter Radar (ISR) observations show a 3‐day signature in vertical ion drifts over Jicamarca (11.9°S, 76°W) and in the electron densities in the top side of ionosphere measured from Millstone Hill (42.6°N, 71.5°W). This signature is consistent with the fountain effect in the equatorial region, and shows the impact on the topside ionosphere at midlatitudes. The UFK wave is trapped within ±30° geographic latitude, but this study shows that the effects of the wave could reach the ionosphere at the higher latitude even as high as 40°N (50°N magnetic latitude).

Sensitivity of the North Atlantic Ocean Circulation to an abrupt change in the Nordic Sea overflow in a high resolution global coupled climate model

[1] The sensitivity of the North Atlantic Ocean Circulation to an abrupt change in the Nordic Sea overflow is investigated for the first time using a high resolution eddy-permitting global coupled ocean-atmosphere model (GFDL CM2.5). The Nordic Sea overflow is perturbed through the change of the bathymetry in GFDL CM2.5. We analyze the Atlantic Meridional Overturning Circulation (AMOC) adjustment process and the downstream oceanic response to the perturbation. The results suggest that north of 34°N, AMOC changes induced by changes in the Nordic Sea overflow propagate on the slow tracer advection timescale, instead of the fast Kelvin wave timescale, resulting in a time lead of several years between subpolar and subtropical AMOC changes. The results also show that a stronger and deeper-penetrating Nordic Sea overflow leads to stronger and deeper AMOC, stronger northward ocean heat transport, reduced Labrador Sea deep convection, stronger cyclonic Northern Recirculation Gyre (NRG), westward shift of the North Atlantic Current (NAC) and southward shift of the Gulf Stream, warmer sea surface temperature (SST) east of Newfoundland and colder SST south of the Grand Banks, stronger and deeper NAC and Gulf Stream, and stronger oceanic eddy activities along the NAC and the Gulf Stream paths. A stronger/weaker Nordic Sea overflow also leads to a contracted/expanded subpolar gyre (SPG). This sensitivity study points to the important role of the Nordic Sea overflow in the large scale North Atlantic ocean circulation, and it is crucial for climate models to have a correct representation of the Nordic Sea overflow.

Response of the thermosphere and ionosphere to an ultra fast Kelvin wave

Ultra Fast Kelvin (UFK) waves are eastward propagating planetary waves with periods between 3 and 5 days, which are capable of penetrating into the thermosphere and ionosphere where they may modulate phenomena occurring in this region. A sensitivity study has been conducted to examine the effect of an Ultra Fast Kelvin wave on the thermosphere and ionosphere using the NCAR Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME‐GCM) under June solstice solar minimum conditions. It is found that realistic ultra fast Kelvin waves with amplitudes in the MLT region of approximately 20–40 m s−1 in zonal wind fields and 10–20 K in temperature fields, can result in approximately 8–12% perturbations in hourly neutral density at 350 km, as well as hourly total electron content (TEC) perturbations of 25–50% in regions corresponding to the equatorial ionization anomalies (EIAs), with the largest relative changes resolved during the nighttime due to the lower electron densities. The electrodynamical calculations in the model were then disabled to identify the relative importance of ionospheric electrodynamics and direct wave propagation in generating the aforementioned changes. The subsequent results show that changes in thermospheric neutral density are relatively insensitive to the presence of the dynamo electric field, while UFK wave modulation of the dynamo accounts for most of the TEC perturbations due to changes of ionospheric vertical plasma drift.

Signatures of ultra fast Kelvin waves in the equatorial middle atmosphere and ionosphere

In the equatorial atmosphere, oscillations with periods of 3 to 4 days have been observed in the meteor radar zonal wind at Cariri (7.4°S, 36.5°W), in the ionospheric minimum virtual height h'F and the maximum critical frequency foF2 at Fortaleza (3.9°S, 38.4°W), and in the TIMED/SABER satellite temperature data in the stratosphere‐mesosphere. Wavelet analyses of these time series reveal that the 3–4‐day oscillation was observed for all of these data during the period from March 1 to 11, 2005. From the characteristics of the downward phase propagation (wavelength of ∼40 km), longitudinal and latitudinal extension, we conclude that this oscillation must be a 3.5–day Ultra Fast Kelvin (UFK) wave. This is the first report of clear evidence of propagation of a UFK wave from the stratosphere to the ionosphere. The UFK wave could have an important role in the day‐to‐day variability of the equatorial ionosphere evening uplift.

Signatures of 3–6 day planetary waves in the equatorial mesosphere and ionosphere

Abstract. Common periodic oscillations have been observed in meteor radar measurements of the MLT winds at Cariri (7.4° S, 36.5° W) and Ascension Island (7.9° S, 14.4° W) and in the minimum ionospheric virtual height, h'F, measured at Fortaleza (3.9° S, 38.4° W) in 2004, all located in the near equatorial region. Wavelet analysis of these time series reveals that there are 3–4-day, 6–8-day and 12–16-day oscillations in the zonal winds and h'F. The 3–4 day oscillation appeared as a form of a wave packet from 7–17 August 2004. From the wave characteristics analyzed this might be a 3.5-day Ultra Fast Kelvin wave. The 6-day oscillation in the mesosphere was prominent during the period of August to November. In the ionosphere, however, it was apparent only in November. Spectral analysis suggests that this might be a 6.5-day wave previously identified. The 3.5-day and 6.5-day waves in the ionosphere could have important roles in the initiation of equatorial spread F (plasma bubble). These waves might modulate the post-sunset E×B uplifting of the base of the F-layer via the induced lower thermosphere zonal wind and/or the E-region conductivity.

Rayleigh lidar observations of planetary waves in the middle atmosphere over Gadanki (13.5°N, 79.2°E)

Rayleigh lidar temperature data collected at Gadanki (13.5°N, 79.2°E) during two campaign periods in 1999 and 2000 have been analyzed to study the planetary wave activities in the stratosphere and lower mesosphere. Attention has been given to the Rossby-gravity (RG) wave, fast Kelvin wave, and slow Kelvin wave with remarkable periodicity of 3.6, 6.5, and 16 days, respectively. The well-known Lomb–Scargle periodogram (L–S) analysis was performed on the temperature profile to extract the wave characteristics. The analysis indicated the dominance of these waves in the stratosphere and mesosphere. The results obtained are consistent with other measurements based on radar, rocketsonde or satellite.

Kelvin waves and ozone Kelvin waves in the quasi‐biennial oscillation and semiannual oscillation: A simulation by a high‐resolution chemistry‐coupled general circulation model

Equatorial Kelvin waves and ozone Kelvin waves were simulated by a T63L250 chemistry‐coupled general circulation model with a high vertical resolution (300 m). The model produces a realistic quasi‐biennial oscillation (QBO) and a semiannual oscillation (SAO) in the equatorial stratosphere. The QBO has a period slightly longer than 2 years, and the SAO shows rapid reversals from westerly to easterly regimes and gradual descents of westerlies. Results for the zonal wave number 1 slow and fast Kelvin waves are discussed. Structure of the waves and phase relationships between temperature and ozone perturbations coincide well with satellite observations made by LIMS, CLAES, and MLS. They are generally in phase (antiphase) in the lower (upper) stratosphere as theoretically expected. The fast Kelvin waves in the temperature and ozone are dominant in the upper stratosphere because the slow Kelvin waves are effectively filtered by the QBO westerly. In this simulation, the fast Kelvin waves encounter their critical levels in the upper stratosphere when zonal asymmetry of the SAO westerly is enhanced by an intrusion of the extratropical planetary waves. In addition to the critical level filtering effect, modulations of wave properties by background winds are evident near easterly and westerly shears associated with the QBO and SAO. Enhancement of wave amplitude in the QBO westerly shear is well coincident with radiosonde observations. Increase/decrease of vertical wavelength in the QBO easterly/westerly is obvious in this simulation, which is consistent with the linear wave theory. Shortening of wave period due to the descending QBO westerly shear zone is demonstrated for the first time. Moreover, dominant periods during the QBO westerly phase are longer than those during the QBO easterly phase for both the slow and fast Kelvin waves.

Study of equatorial Kelvin waves using the MST radar and radiosonde observations

Abstract. In this paper an attempt has been made to study equatorial Kelvin waves using a high power coherent VHF radar located at Gadanki (13.5° N, 79.2° E), a tropical station in the Indian sub-continent. Simultaneous radiosonde observations taken from a nearby meteorological station located in Chennai (13.04° N, 80.17° E) were also used to see the coherence in the observed structures. These data sets were analyzed to study the mean winds and equatorial waves in the troposphere and lower stratosphere. Equatorial waves with different periodicities were identified. In the present study, particular attention has been given to the fast Kelvin wave (6.5-day) and slow Kelvin wave (16-day). Mean zonal wind structures were similar at both locations. The fast Kelvin wave amplitudes were somewhat similar in both observations and the maximum amplitude is about 8m/s. The phase profiles indicated a slow downward progression. The slow Kelvin wave (16-day) amplitudes shown by the radiosonde measurements are a little larger than the radar derived amplitudes. The phase profiles showed downward phase progression and it translates into a vertical wavelength of ~10-12km. The radar and radiosonde derived amplitudes of fast and slow Kelvin waves are larger at altitudes near the tropopause (15-17km), where the mean wind attains westward maximum.

Evidence on 2–4 day oscillations of the equatorial ionosphere h′F and mesospheric airglow emissions

Equatorial ionospheric sounding has been carried out at São Luís (2.6°S, 44.2°W). The upper mesosphere‐lower thermosphere (MLT) airglow OI5577, O2b(0,1) and OH(6,2) emissions and OH rotational temperature have been observed at Cariri airglow observatory (7.4°S, 36.5°W) within a distance of approximately 1000 km to the east of São Luís. Both observation sites are located in the equatorial region of South America. Spectral analyses of the ionospheric F‐layer bottom height (h′F) and airglow emission intensity reveal that there are quasi 2‐ and 4‐day period oscillations in their temporal variations. This might indicate that planetary scale oscillations, Rossby‐gravity waves (or inertial‐gravity waves) and Ultra Fast Kelvin waves, are present in the ionosphere. This is the first time that the planetary scale waves in the MLT region and in the ionosphere are discussed by airglow and ionospheric observations, respectively.

A study of equatorial wave characteristics using rockets, balloons, lidar and radar

A co-ordmated experimental campaign was conducted for 40 consecutive days from 21 February to 01 April 2000 using RH-200 rockets, balloons, Rayleigh lidar and MST radar, with the objective of delineating the equatorial waves and estimating momentum fluxes associated with them. Winds and temperatures in the troposphere, stratosphere and mesosphere over two low latitude stations Gadanki (13.5°N, 79.2°E) and SHAR (13.7°N, 80.2°E) were measured and were used for the study of equatorial waves and their interactions with the background mean flow in various atmospheric regions. The study shows the occurrence of a strong stratospheric cooling (∼25 K) anomaly along with a zonal wind anomaly and this low-latitude event appears to be linked to high-latitude stratospheric warming event and followed by subsequent generation of short period (∼5 days) oscillations lasting for a few cycles in the stratosphere. Slow and fast Kelvin waves and RG wave (∼-17-day and ∼7.2-day and ∼4.2-day periods respectively) have been identified. The mean flow acceleration produced by the divergence of the momentum flux due to the observed Kelvin waves in the 35–60 km height region were compared with the zonal flow accelerations computed from the observed zonal winds. Contribution by the slow and fast Kelvin waves was found to be only ∼25 % of the observed acceleration during the evolution of the westerly phase of the semi-annual oscillation.

Fast and slow Kelvin waves in the Madden‐Julian oscillation of a GCM

AbstractThe structure of the Madden‐Julian Oscillation (MJO) in an 1800‐day integration of the Hadley Centre Unified Model was analysed, and interpreted within a Kelvin‐wave framework. the model was forced with constant equinoctial (March) boundary conditions so that a ‘clean’ MJO signal could be separated from the effects of the seasonal cycle and forced interannual variability. the simulated MJO was fairly realistic in terms of its large‐scale spatial structure and propagation characteristics, although its period of 30 days (corresponding to an average phase speed of 15 m s−1) was shorter than that observed. the signal in deep convection was less coherent than in observations, and appeared to move eastwards as a sequence of discrete convective anomalies, rather than by a smooth eastward propagation. Both ‘fast’ and ‘slow’ equatorial Kelvin waves appeared to play an important role in the eastward propagation of the simulated MJO. Enhanced convection over the Indian Ocean was associated with a ‘fast’ equatorial Kelvin wave that propagated eastwards at 55 m s−1 over the Pacific. On reaching the west coast of South America, a component of this Kelvin wave propagated northwards and southwards as a trapped wave along the mountain ranges of Central America and the Andes, in agreement with observations. the anomalous surface easterlies over the tropical eastern Pacific associated with this fast Kelvin wave enhanced the climatological mean easterlies and led to positive convective anomalies over the eastern Pacific consistent with the wind‐induced surface heat exchange (WISHE) mechanism. However, WISHE was not able to account for the eastward development of the convective anomalies over the Indian Ocean/western Pacific region. By splitting the equatorial divergence anomalies of the simulated MJO into their ϱu/ϱx and ϱv/ϱy components, the role of Kelvin‐wave dynamics in the ‘slow’ (15 m s−1) average eastward propagation of the simulated MJO was examined. Although the two components were of comparable magnitude, the ϱu/ϱx component exhibited a pronounced eastward propagation which tended to be disrupted by the ϱv/ϱy component, thus supporting the paradigm of an underlying, but strongly modified, Kelvin‐wave mechanism.

Estimation of equatorial wave momentum fluxes using MST radar winds observed at Gadanki (13.5°N, 79.2°E)

A method of computing the vertical flux of zonal momentum (associated with equatorial waves) from the zonal and vertical components of the winds measured by the Indian MST radar at Gadanki (13.5°N, 79.2°E) is presented. The application of the method to the radar data gives flux values of 16×10 −3, 8.0×10 −3 and 5.5×10 −3 m 2 s −2 for slow Kelvin (12-day period), fast Kelvin (5.33-day period) and Rossby-gravity (RG) (3.43-day period) waves, respectively, in the upper troposphere. These flux values compare quite well with the values 4×10 −3 m 2 s −2 and 1×10 −3 m 2 s −2 obtained from radiosonde zonal wind and temperature data by Wallace and Kousky, 1968for slow Kelvin and RG waves, respectively. An estimate of the error in the fluxes gives a value of ∼ 1.2×10 −3 m 2 s −2.

Detection of the fast Kelvin wave teleconnection due to El Niño‐Southern Oscillation

Previous analyses of the ocean state along the western American coast have often indicated unexpectedly slow and limited propagation of coastally trapped Kelvin waves associated with the El Niño‐Southern Oscillation. In contrast, theoretical and numerical ocean models demonstrate that these Kelvin waves are a rapid and long‐range teleconnection between the low‐ and high‐latitude Pacific Ocean, strongly impacting both the surface coastal currents and nutrient upwelling. Sea level variations along the western coast of North America are reexamined under the assumption that tropically forced Kelvin waves are produced in bursts of several months duration. A cross‐correlation analysis, restricted to mid‐1982 to mid‐1983, is performed between Galapagos Island and stations along western Central and North America. A coastally trapped Kelvin wave is revealed to propagate at a speed of 2–3 m s−1 from the tropical Pacific to the Aleutian Island Chain. The observed phase speed agrees with the estimated speed of a Kelvin wave based on the average density profile of the ocean near the coast. Weaker El Niño events in 1986/1987 and 1991/1992 appear to contain a combination of this remote signal and local wind forcing. The wave propagation speed calculated from the spectral phase is shown to be sensitive to the presence of other (noise) processes in the observations. This is demonstrated through an analysis of a synthetic sea level data set that contains many of the essential features of the real sea level data. A relatively small level of red noise can give a 100% expected error in the estimated propagation speed. This suggests a new explanation for this important inconsistency within dynamical oceanography.