Mean Phase Speed Research Articles

Observed internal solitary waves in the northern Bali waters, Indonesia

Internal solitary waves (ISWs) in the northern Bali waters are frequently observed via satellite imagery, but in situ observations remain scarce. Here, we report the northward propagating Lombok solitary waves from shipboard acoustic current measurements. The four leading waves have amplitude of 45, 35, 30, and 25 m and typical mean phase speed of ∼1.8 m s−1. The waves generated strong northward currents of ∼0.8 m s−1 in the upper layer and triggered vertical velocity that reaches ∼15 cm s−1. Those observations suggest larger ISWs amplitude than recent numerical estimates at the same location. The shape of the four leading waves is consistent with the Korteweg–de Vries (KdV) model. The total energy density within the packet reached ∼147 MJ m−1, and the total mean energy flux per tidal cycle into ISWs is in the range [0.6 – 1.0] GW, representing a strong fraction of the total northward baroclinic tidal energy flux estimated from models.

Eastward-propagating planetary waves in the polar middle atmosphere

Abstract. According to Modern-Era Retrospective Research Analysis for Research and Applications (MERRA-2) temperature and wind datasets in 2019, this study presents the global variations in the eastward-propagating wavenumber 1 (E1), 2 (E2), 3 (E3) and 4 (E4) planetary waves (PWs) and their diagnostic results in the polar middle atmosphere. We clearly demonstrate the eastward wave modes exist during winter periods with westward background wind in both hemispheres. The maximum wave amplitudes in the Southern Hemisphere (SH) are slightly larger and lie lower than those in the Northern Hemisphere (NH). Moreover, the wave perturbations peak at lower latitudes with smaller amplitudes as the wavenumber increases. The period of the E1 mode varies between 3–5 d in both hemispheres, while the period of the E2 mode is slightly longer in the NH (∼ 48 h) than in the SH (∼ 40 h). The periods of the E3 are ∼ 30 h in both the SH and the NH, and the period of E4 is ∼ 24 h. Despite the shortening of wave periods with the increase in wavenumber, their mean phase speeds are relatively stable, ∼ 53, ∼ 58, ∼ 55 and ∼ 52 m/s at 70∘ latitudes for E1, E2, E3 and E4, respectively. The eastward PWs occur earlier with increasing zonal wavenumber, which agrees well with the seasonal variations in the critical layers generated by the background wind. Our diagnostic analysis also indicates that the mean flow instability in the upper stratosphere and upper mesosphere might contribute to the amplification of the eastward PWs.

Torsional Alfvénic Oscillations Discovered in the Magnetic Free Energy during Solar Flares

Abstract We report the discovery of torsional Alfvénic oscillations in solar flares, which modulate the time evolution of the magnetic free energy E f (t), while the magnetic potential energy E p (t) is uncorrelated, and the nonpotential energy varies as E np (t) = E p + E f (t). The mean observed time period of the torsional oscillations is P obs = 15.1 ± 3.9 minutes, the mean field line length is L = 135 ± 35 Mm, and the mean phase speed is v phase = 315 ± 120 km s−1, which we interpret as torsional Alfvénic waves in flare loops with enhanced electron densities. Most of the torsional oscillations are found to be decay-less, but exhibit a positive or negative trend in the evolution of the free energy, indicating new emerging flux (if positive), magnetic cancellation, or flare energy dissipation (if negative). The time evolution of the free energy has been calculated in this study with the Vertical-current Approximation (Version 4) Non-linear Force-free Field code, which incorporates automatically detected coronal loops in the solution and bypasses the non-force-freeness of the photospheric boundary condition, in contrast to traditional NLFFF codes.

Dissimilarity between turbulent heat and momentum transfer induced by a streamwise travelling wave of wall blowing and suction

A series of direct numerical simulations of a fully developed turbulent channel flow is conducted in order to clarify the effects of travelling wave-like wall blowing and suction on dissimilar heat transfer enhancement. While the wave form is kept sinusoidal and its amplitude is set to be 5 % of the bulk mean velocity, the wavelength and phase speed of the travelling wave are systematically changed in a wide parameter space. As a result, the global optimum of the parameter set for maximizing the analogy factor, which is defined as the ratio between the Stanton number and the skin-friction coefficient, is identified. Interestingly, the obtained globally optimal mode agrees well with that predicted from the optimal control theory taking into account the future dynamics within a limited time horizon by Yamamoto et al. (J. Fluid Mech., vol. 733, 2013, pp. 189–220). The instantaneous velocity and thermal fields are decomposed into coherent and random components in order to evaluate the contribution from each component to dissimilar heat transfer enhancement. The detailed mechanisms of dissimilarity are explained by the budget analyses of the coherent and random contributions. Also, their relationships with the near-wall turbulent structures modified by the applied control are discussed through flow visualization. It is found that the random component makes a dominant contribution to dissimilarity, and this can be explained by an indirect effect through the modification of the coherent field by the applied control. Based on the above mechanisms, we propose a simple unsteady Reynolds-averaged Navier–Stokes (URANS) approach, where the phase-averaged velocity and thermal fields are solved directly whereas the effects of the random component are modelled by the Boussinesq eddy viscosity and diffusivity hypothesis. It is shown that the present URANS can capture the overall trend of dissimilar heat transfer enhancement in a wide parameter range. The present results also explain why the optimal control theory with a limited time horizon succeeds in predicting the globally optimal control mode.

Near 5-Day Nonisostatic Response to Atmospheric Surface Pressure and Coastal-Trapped Waves Observed in the Northern South China Sea

AbstractOceanic, nonisostatic responses to near 5-day Rossby–Haurwitz atmospheric pressure waves have been observed in open oceans; however, such responses based on observations in marginal seas such as the South China Sea have not been reported, owing to the limited ocean bottom pressure Pbot records. The Pbot measurements from pressure recording inverted echo sounders (PIESs) at sites in the northern South China Sea revealed a nonisostatic-like response near 5 days, although the coastal-trapped waves (CTWs) appeared to obscure it because their broadband periods include the near 5-day band. Cross-spectral analysis revealed that the PIES Pbot records and the sea level (SL) records of Hong Kong all correlate strongly with the atmospheric pressure and winds over the East China Sea. This is indicative of remotely forced CTWs. The PIES Pbot records showed higher coherence near 5 days with the zonal low-pass wavelength filters applied to the atmospheric pressure, and the phase analysis results strongly suggest nonisostatic oceanic responses to the westward-propagating Rossby–Haurwitz waves. Effective separation of CTWs and the nonisostatic responses from the Pbot records at the near 5-day period was achieved. The oceanic responses to the Rossby–Haurwitz waves in the northern South China Sea were nonisostatic; a 1-mbar change in air pressure resulted in a 1.58-mbar change in Pbot with a phase lag of 14.8°. The mean phase speed of CTWs from Hong Kong to station P3 was estimated to be 9.9 m s−1.

Equatorial Kelvin waves generated in the western tropical Pacific Ocean trigger mass and heat transport within the Middle America Trench off Costa Rica

AbstractSequences of correlated seafloor temperature, current velocity, and acoustic backscatter events recorded at Ocean Drilling Program (ODP) sites at 4300 m depth in the Middle America Trench have been inferred to result from tidally induced turbidity currents generated in the vicinity of the 3300 m deep sill at the southern end of the trench. New data from the borehole observatories extend the temperature records to 11 years (November 2002 to December 2013) and confirm the highly episodic nature of the events. We present satellite altimetry data and ocean circulation model results to show that event timing is correlated with intraseasonal Kelvin wave motions in the equatorial Pacific. The observed temperature events had a mean (±1 standard deviation) occurrence interval of 61 (±24) days, which spans the periods of the first two baroclinic modes. Lag times between peak bottom water temperatures at the ODP sites and the passage of eastward‐propagating Kelvin wave crests at locations in the eastern equatorial Pacific are consistent with the time for mode‐1 waves to propagate to the southern end of the trench at a mean phase speed of 2.0 m s−1. Findings indicate that Kelvin wave currents augment tidal motions in the vicinity of the sill, triggering turbidity currents that travel northwestward along the trench axis at mean speeds of ∼0.1 m s−1. We conclude that mode‐1 (or, possibly, mixed mode‐1 and mode‐2) baroclinic Kelvin waves generated by large‐scale atmospheric processes in the western tropical Pacific lead to heat and mass transport deep within Middle America Trench in the eastern tropical Pacific.

Evidence for coherent spicule oscillations from correcting Hinode/SOT Ca ii H in the south-east limb of the Sun

Wave theories of heating the chromosphere, corona, and solar wind due to photospheric fluctuations are strengthened by the existence of observed wave coherency up to the transition region (TR). The coherency of solar spicules' intensity oscillations was explored using the Solar Optical Telescope (SOT) on the Hinode spacecraft with a height increase above the solar limb in active region (AR). We used time sequences near the southeast region from the Hinode/SOT in Ca II H line obtained on April 3, 2015 and applied the de-convolution procedure to the spicule in order to illustrate how effectively our restoration method works on fine structures such as spicules. Moreover, the intensity oscillations at different heights above the solar limb were analysed through wavelet transforms. Afterwards, the phase difference was measured among oscillations at two certain heights in search of evidence for coherent oscillations. The results of wavelet transformations revealed dominant period peaks in 2, 4, 5.5, and 6.5 min at four separate heights. The dominant frequencies for coherency level higher than 75 percent was found to be around 5.5 and 8.5 mHz. Mean phase speeds of 155-360 km s-1 were measured. We found that the mean phase speeds increased with height. The results suggest that the energy flux carried by coherent waves into the corona and heliosphere may be several times larger than previous estimates that were based solely on constant velocities. We provide compelling evidence for the existence of upwardly propagating coherent waves.

Eddy Phase Speeds in a Two-Layer Model of Quasigeostrophic Baroclinic Turbulence with Applications to Ocean Observations

AbstractThe phase speed spectrum of ocean mesoscale eddies is fundamental to understanding turbulent baroclinic flows. Since eddy phase propagation has been shown to modulate eddy fluxes, an understanding of eddy phase speeds is also of practical importance for the development of improved eddy parameterizations for coarse resolution ocean models. However, it is not totally clear whether and how linear Rossby wave theory can be used to explain the phase speed spectra in various weakly turbulent flow regimes. Using linear analysis, theoretical constraints are identified that control the eddy phase speed in a two-layer quasigeostrophic (QG) model. These constraints are then verified in a series of nonlinear two-layer QG simulations, spanning a range of parameters with potential relevance to the ocean. In the two-layer QG model, the strength of the inverse cascade exerts an important control on the eddy phase speed. If the inverse cascade is weak, the phase speed spectrum is reasonably well approximated by the phase speed of the linearly most unstable mode. A significant inverse cascade instead leads to barotropization, which in turn leads to mean phase speeds closer to those of barotropic-mode Rossby waves. The two-layer QG results are qualitatively consistent with the observed eddy phase speed spectra in the Antarctic Circumpolar Current and may also shed light on the interpretation of phase speed spectra observed in other regions.

Evidence of Energy Supply by Active-Region Spicules to the Solar Atmosphere

We investigate the role of active region spicules in the mass balance of the solar wind and energy supply for heating the solar atmosphere. We use high cadence observations from the Solar Optical Telescope (SOT) onboard the Hinode satellite in the Ca II H line filter obtained on 26 January 2007. The observational technique provides the high spatio-temporal resolution required to detect fine structures such as spicules. We apply Fourier power spectrum and wavelet analysis to SOT/Hinode time series of an active region data to explore the existence of coherent intensity oscillations. The presence of coherent waves could be an evidence for energy transport to heat the solar atmosphere. Using time series, we measure the phase difference between two intensity profiles obtained at two different heights, which gives information about the phase difference between oscillations at those heights as a function of frequency. The results of a fast Fourier transform (FFT) show peaks in the power spectrum at frequencies in the range from 2 to 8 mHz at four different heights (above the limb), while the wavelet analysis indicate dominant frequencies similar to those of the Fourier power spectrum results. A coherency study indicates the presence of coherent oscillations at about 5.5 mHz (3 min). We measure mean phase speeds in the range 250 to 425 km/s increasing with height.

INTRA-SEASONAL VARIABILITY OF NEAR-BOTTOM CURRENT IN THE HALMAHERA SEA

The secondary entry portal of the Indonesian Throughflow (ITF) from the Pacific to Indian Oceans is considered to be via the Halmahera Sea (HS). However, few ITF studies have been done within the passage. This motivated the Internal Tides and Mixing in the Indonesian Througflow (INDOMIX) program to conduct direct measurements of currents and its variability across the eastern path of the ITF. This study focused on the intra-seasonal variability of near-bottom current in HS (129°E, 0°S), its origin and correlation with surface zonal winds and sea surface height over the equatorial Pacific Ocean. The result showed a strong northwestward mean flow with velocity exceeding 40 cm/s, which represented the current-following topography with the northwest orientation. Meridional current component was much stronger than the zonal component. The energy of power spectral density (PSD) of the current peaked on 14-days and 27-days periods. The first period was presumably related to the tidal oscillation, but the latter may be associated with surface winds perturbation. Furthermore, cross-PSD revealed a significant coherency between the observed currents and the surface zonal winds in the central equatorial Pacific zonal winds (180°E-160°W), which corroborates westward propagation of intra-seasonal sea surface height signals along the 5°S with its mean phase speeds of 50 cm/s, depicting the low-latitude westward Rossby waves on intra-seasonal band. Keywords: current, equatorial Pacific Ocean, zonal winds, sea surface height, Halmahera Sea

INTRA-SEASONAL VARIABILITY OF NEAR-BOTTOM CURRENT IN THE HALMAHERA SEA

<p><em>The secondary entry portal of the Indonesian Throughflow (ITF) from the Pacific to Indian Oceans is considered to be via the Halmahera Sea (HS). However, few ITF studies have been done within the passage. This motivated the Internal Tides and Mixing in the Indonesian Througflow (INDOMIX) program to conduct direct measurements of currents and its variability across the eastern path of the ITF. This study focused on the intra-seasonal variability of near-bottom current in HS (129°E, 0°S), its origin and correlation with surface zonal winds and sea surface height over the equatorial Pacific Ocean. The result showed a strong northwestward mean flow with velocity exceeding 40 cm/s, which represented the current-following topography with the northwest orientation. Meridional current component was much stronger than the zonal component. The energy of power spectral density (PSD) of the current peaked on 14-days and 27-days periods. The first period was presumably related to the tidal oscillation, but the latter may be associated with surface winds perturbation. Furthermore, cross-PSD revealed a significant coherency between the observed currents and the surface zonal winds in the central equatorial Pacific zonal winds (180°E-160°W), which corroborates westward propagation of intra-seasonal sea surface height signals along the 5°S with its mean phase speeds of 50 cm/s, depicting the low-latitude westward Rossby waves on intra-seasonal band.</em></p> <p><em> </em></p> <strong><em>Keywords</em></strong><em>: current, equatorial Pacific Ocean, zonal winds, sea surface height, Halmahera Sea</em>

Gravity waves generated by deep tropical convection: Estimates from balloon observations and mesoscale simulations

Convective gravity waves in the Tropics are studied by analyzing in situ measurements from long‐duration stratospheric balloons launched during the PreConcordiasi campaign (2010) and mesoscale simulations. An improved temporal resolution of the observations as well as the balloon quasi‐Lagrangian behavior allow an unprecedented investigation of the whole gravity wave spectrum. First, a case study of gravity waves generated by a developing cyclone, Tropical Storm Gelane (February 2010), is carried out using observations complemented by numerical simulations with the Weather Research and Forecast model, with a resolution down to 1 km. Distributions of momentum fluxes obtained from both data show reasonable agreement and emphasize waves with short wavelengths (<15 km) and short periods (<20 min). Still, some differences are also found, which can likely be related to errors of the modeled background flow. Second, observations from the whole PreConcordiasi flights are analyzed with an emphasis on gravity wave momentum fluxes. Their phase speed distribution has a robust shape, with maximum fluxes with near‐zero ground‐based phase speeds. Yet, significant momentum fluxes are also found for larger values, yielding a mean phase speed of about 27 ms−1. The momentum fluxes are concentrated in short episodes with intense values, and their intermittency is quantified using probability distribution functions and the Gini coefficient (0.5–0.6). The relative importance of convective and topographic sources are investigated, suggesting comparable intensities, but a greater occurrence frequency of convective events. Waves emitted by Tropical Storm Gelane do not stand out relative to other convective events.

On the Speed of the Eddy-Driven Jet and the Width of the Hadley Cell in the Southern Hemisphere

Abstract A strong correlation between the speed of the eddy-driven jet and the width of the Hadley cell is found to exist in the Southern Hemisphere, both in reanalysis data and in twenty-first-century integrations from the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report multimodel archive. Analysis of the space–time spectra of eddy momentum flux reveals that variations in eddy-driven jet speed are related to changes in the mean phase speed of midlatitude eddies. An increase in eddy phase speeds induces a poleward shift of the critical latitudes and a poleward expansion of the region of subtropical wave breaking. The associated changes in eddy momentum flux convergence are balanced by anomalous meridional winds consistent with a wider Hadley cell. At the same time, faster eddies are also associated with a strengthened poleward eddy momentum flux, sustaining a stronger westerly jet in midlatitudes. The proposed mechanism is consistent with the seasonal dependence of the interannual variability of the Hadley cell width and appears to explain at least part of the projected twenty-first-century trends.

Fourier analysis of a virtual satellite view of the NIR atmospheric emission over Europe

Observations of wave-driven fluctuations in the near-infrared nightglow at the mesopause level were performed using a CCD camera located at Châteaurenard (44°41′ N, 6°54′E) in the French Alps. In this paper, a few examples of panoramas and top circular views of the emissive layer during summer show wave fields with wide arches and stripes extending from a point in the E-SE to the diameter opposite W-NW point. The direction of the wave field is retrieved by using specific image processing techniques which mainly consist of enhancing the wave front contrast. The emission is mainly due to the OH radical; its intensity varies during the night by as much as 50% due to photochemical and tidal effects. In addition, the intensity shows smaller fluctuations induced by the propagation of wave systems. Spectral Fourier Analysis was applied to the intensity spectra in order to retrieve the wavelengths and temporal periods of the waves travelling through the emissive layer at the altitude of 88 km. The measured wavelengths range from 25 to 69 km. The apparent periods range from 10 to 40 minutes. As a result, the mean phase speed of the waves can be calculated and is found equal to 35 m/s.

Observation of equatorially trapped waves in the Pacific using Geosat altimeter data

A spatial integration filtering method is applied to process 4.5 years of Geosat altimeter sea level anomalies(SLA) recorded from 1 April 1985 to 17 September 1989. The integral limits are chosen as ±1.5° latitude centered at the equator and ±4° longitude centered at the each central longitude. These limits are chosen in order to eliminate the random noise and to enhance the equatorially trapped wave signals. The spatial-integration-filtered SLA time series show periodical signals with different time scales. Processing the SLA time series with a Short Time Fourier Transform (STFT) yields the time-frequency spectra of the SLA which show a frequency spectral splitting-combining phenomenon in the periods during and after the 1986–1987 El Niñio-Southern Oscillation (ENSO) event. Analysis of non-linear wave-wave interaction indicates that this phenomenon results from amplitude modulation of the short period oscillations in the 60–100 day band by a long period oscillation with a period of about 500 days. The wave components of the short period oscillations are further analysed using the equatorial wave modes to fit the meridional distribution of the SLA. The results indicate that the downwelling Kelvin wave mode before and during the peak phase of 1986–1987 ENSO and the upwelling Kelvin wave mode during the 1988 La Niña are the dominant components. The first and second Rossby wave modes play important roles for entire time series and occasionally become the dominant components. The mean phase speed of Kelvin wave mode during the 1986–1987 ENSO is 3.0 m s −1 which is 25% higher than those in non-ENSO periods.

Analysis of remotely forced oceanic Rossby waves off California

Complex empirical orthogonal function (CEOF) analysis is used to investigate the coastal Kelvin wave driven Rossby wave response in the northeast Pacific. Using CEOF analysis, a spatial structure function is obtained from model upper layer thickness data. The model is a nonlinear, reduced gravity model of the northeast Pacific forced by coastal Kelvin waves originating in the equatorial Pacific. The spatial structure function is used to extract the interannual Rossby wave response from observed 300‐m‐depth temperature anomalies. The observed Rossby wave signal is termed the projection mode. Rossby wavelike features observed in the projection mode are order 1000 km long with most periods ranging between 2 and 4 years. The wave numbers and frequencies found are consistent with Rossby dynamics. The mean phase speed of the Rossby wavelike features within the projection mode is 1.3 cm s−1, in agreement with the theoretical Rossby wave phase speed at 40°N. Large amplitude nearshore and decreasing amplitude away from shore suggests nearshore generation of these waves. An important source of sea level variability along the east coast of North America at periods of 2–4 years was identified by Pares‐Sierra and O'Brien (1989) as poleward propagating Kelvin waves. Since the Rossby wavelike features observed in the projection mode have a majority of their periods ranging between 2 and 4 years, their forcing can be attributed to long‐period Kelvin waves. Spectral comparisons between the nearshore values in the projection mode and coastal sea level show greater than 90% coherence in the period band 3–4.4 years. The high coherence between coastal sea level variations and the projection mode shows that there is a strong correlation between the Rossby wavelike features within the projection mode and coastal Kelvin wave propagation. It is concluded that the Rossby waves within the projection mode are forced by coastal Kelvin wave propagation. The projection mode accounts for 47.5% of the variance in the 300‐m‐depth temperature anomalies. The implication of this result is that the physical mechanism of the numerical model, i.e., Rossby waves excited by coastal Kelvin wave propagation, accounts for 47.5% of the variance in the observed 300‐m depth temperature anomalies.

Synoptic-Scale Disturbances near the Equator

Abstract A spectral analysis of winds analyzed and initialized at the European Centre for Medium-Range Weather Forecasts reveals an abundance of power in the 850 mb meridional wind along the equator with periods near four days. The power is mostly in the westward propagating component. Using high-pass filtered data it is shown that the waves have westward phase and eastward group propagation relative to the mean wind. The longest wavelengths are found over the Pacific Ocean, while the shortest are found over the convectively variable regions of Indonesia, South America, and Africa. Mean phase speeds at 850 mb are positively correlated with the mean wind on the equator at 500 mb and below, and negatively correlated with the mean wind above that level. The effective advecting zonal wind of the disturbances seems to be the density weighted average of the lower troposphere. The structure of the disturbances bears resemblance to the expected structure of an equatorially trapped mixed Rossby-gravity wave over t...

Accurate PE calculations for range-dependent environments based on c updates

The standard parabolic approximation to the acoustic wave equation is known to have intrinsic phase errors, which will degrade the accuracy of any PE solution for long-range propagation in the ocean. Pierce recently suggested a remedy to minimize these phase errors by simply choosing an appropriate mean phase speed (c0), eventually updated with range, representing a weighted average of all phase speeds involved in a particular propagation problem. We have now extended Pierce's formalism to include the wide-angle parabolic equation of Thomson and Chapman [J. Acoust. Soc. Am. 74, 1848 (1983)], which inherently has smaller phase errors than the standard parabolic equation. The importance of using c0 updates in PE calculations for range-dependent environments is demonstrated through numerical results for a wedge-shaped ocean. Comparison with alternative solution techniques (coupled modes, intrinsic modes) shows that accurate PE solutions for the wedge problem can be obtained only for single-mode situations, even when using c0 updates with range.

Large-Scale Aspects of Deep Convective Activity over the GATE Area

Using digital IR data obtained by SMS I satellite, the large-scale behavior of convective activity was investigated over the tropical Atlantic within the GATE area. Spectral analysis has revealed the existence of two remarkable periodicities which show good association with the large-scale atmospheric field, namely, a 4-5 day mode and a diurnal mode. The area of the enhanced convection associated with the 4-5 day mode moves westward from Africa through the tropical Atlantic. Its mean phase speed turned out to be about 7° day-1 and the mean wavelength about 30° in longitude. The vertical structure of the corresponding disturbance, was investigated by the time-composite technique applied to upper air data over the GATE A/B area. The result shows that the enhancement of convection is associated with deep upward motion throughout the troposphere and southerly (northerly) winds in the lower (upper) troposphere. It also shows that the enhancement is accompanied by moistening in the cloud layer, while drying appears in the subcloud layer. As for the diurnal mode, composited variation shows the maximum convective activity in the late afternoon over the GATE A/B area. Horizontal distributions of the composited anomalies revealed that the late afternoon maxima have a zonal distribution roughly along 10°N rather than the confined one in the vicinity of West Africa. It also revealed the coexistence of the region which shows the early morning maximum. The latter takes place to the south of the former one over the middle Atlantic. Diurnal components of the large-scale kinematic and thermal fields were also examined over the A/B area. During the daytime, the vertical motion shows upward anomalies and the temperature shows distinct warm anomalies in the upper troposphere. As for the moisture variation, the atmosphere tends to show dry anomalies near local noon and moist anomalies near midnight.

Propagation of a 14.7-Day Wave along the Northern Coast of the Guinea Gulf

Long time series of sea level and sea surface temperatures measured at different coastal stations along the northern coast of the Gulf of Guinea are analyzed statistically. The results indicate that pronounced fortnightly oscillations in sea level are composed of two waves: one is the lunar fortnightly tide Mf (13.661-day period) which has a constant phase all along the coast. The other wave has a period of 14.765 days which is the period of the luni-solar fortnightly tide Msf; this wave propagates westward along the east-west oriented coastline with a mean phase speed of 53 cm s−1 and a wavelength of 675 km. These waves have important effects on the thermal structure and give rise to strong vertical oscillations of the subsurface isotherms throughout the year. The sea surface temperature, however, has pronounced oscillations around the Msf frequency during the upwelling season (June–September) only. The 14.765-day wave is of tidal origin and is due to a nonlinear interaction of the M2 and S2 (barotropic or baroclinic) tides but the generation mechanism is obscure.