Westward Winds Research Articles

An association between mesospheric winds and electron densities

Mesospheric winds and electron densities at 44S have been measured by partial reflection of 2.4 MHz radio waves. Observations suggest that a decreased zonal wind is frequently accompanied by increased meridional wind, D-region electron densities and fmin. In this paper, wind is considered as a vector quantity. Thus, an increased zonal wind means an increase in the magnitude of the eastward wind, a decrease in the magnitude of the westward wind, or a change in direction from westward to eastward.

A pacific equatorial temperature section from 172°E to 110°W during winter and spring 1979

A composite zonal-depth temperature section was occupied along the equator between 172°E and 110°W, nearly 8.6 × 10 3 km, by combining temperature profiles recorded at about 1- degree intervals on 28 February 1979 between 172°E and 158°W with profiles recorded at 1 4 - degree interval from 23 April to 2 May 1979 between 153°W and 110°W. Coincident with the zonal temperature section, which combined with salinity data produced dynamic height anomalies, were moored current measurements in the upper ocean at three equator sites (166°E, 152°W, and 110°W). The thickness of the mixed layer decreased from about 100 m near 180° to 10 m near 110°W. The depth of the thermocline was about 125 m near 180° and approximately 50 m east of 120°W. At 166°E the velocity core of the Cromwell Current was near the bottom of the thermocline; at 152°W and 110°W it was within the central region of the thermocline. East of about 140°W a 100-m-thick 13°C thermostad occurred below about 175 m. At 110°W the bulk Richardson number encompassing the 150- to 200-m interval was virtually always less than unity from 20 January to 2 May, suggesting that the thermostad was formed by shear-induced dynamic instabilities below the velocity core of the Cromwell Current. The surface dynamic height anomaly relative to 27 dbar tilted upward toward the west between 172°E and 120°W, with the maximum inclination from 153°W to 133°W. East of 120°W the zonal gradient of the 0/270 dbar steric sea level was indistinguishable from a horizontal surface. East of 132°W the directions of the 270/400 dbar pressure force and of the zonal wind field were both westward, indicating that the influence of the prevailing westward wind stress was confined to the region above the thermocline.

The ionospheric disturbance dynamo

A numerical simulation study of the thermospheric winds produced by auroral heating during magnetic storms, and of their global dynamo effects, establishes the main features of the ionospheric disturbance dynamo. Driven by auroral heating, a Hadley cell is created with equatorward winds blowing above about 120 km at mid‐latitudes. The transport of angular momentum by these winds produces a subrotation of the mid‐latitude thermosphere or westward motion with respect to the earth. The westward winds in turn drive equatorward Pedersen currents which accumulate charge toward the equator, resulting in the generation of a poleward electric field, a westward E × B drift, and an eastward current. When realistic local time conductivity variations are simulated, the eastward mid‐latitude current is found to close partly via lower latitudes, resulting in an ‘anti‐Sq’ type of current vortex. Both electric field and current at low latitudes thus vary in opposition to their normal quiet‐day behavior. This total pattern of disturbance winds, electric fields, and currents is superimposed upon the background quiet‐day pattern. When the neutral winds are artificially confined on the nightside, the basic pattern of predominantly westward E × B plasma drifts still prevails on the nightside but no longer extends into the dayside. Considerable observational evidence exists, suggesting that the ionospheric disturbance dynamo has an appreciable influence on storm‐time ionospheric electric fields at middle and low latitudes.

The generation of equatorial currents

In response to the sudden onset of zonal winds the surface layers of the ocean accelerate in the direction of the wind. Motion is most intense near the equator where a jet forms within a week. The next stage in the evolution of equilibrium conditions is associated with wave fronts, excited initially at the coasts, that propagate across the ocean basin and establish zonal density gradients. Wave modes trapped in and above the strong shallow tropical thermocline because of internal reflection there are responsible for the adjustment of the upper ocean in low latitudes. These thermocline‐trapped modes extend over a depth greater than that of the wind‐driven surface currents and hence give rise to an undercurrent in the thermocline. This undercurrent is zonal and particularly intense near the equator, where it appears in the wake of an eastward traveling Kelvin or westward traveling Rossby wave after about 1 month. In the case of eastward winds, nonlinearities intensify the eastward equatorial surface jet and weaken the westward undercurrent. In the case of westward winds a different nonlinear mechanisms intensifies the eastward Equatorial Undercurrent and weakens the westward surface flow. In a 5000‐km wide basin, equilibrium equatorial currents are established about 150 days after the onset of the winds. The response time of the ocean below a depth of a few hundred meters is much longer. Winds with no spatial and a simple temporal structure generate currents with a complex vertical structure in the deep ocean. Closed current systems are possible in a confined forced region of an unbounded ocean; meridional coasts are not essential for their maintenance. The intensity of equatorial current is sensitive to dissipation parameters.

Seasonal and solar cycle variations in the thermospheric circulation observed over Millstone Hill

Incoherent scatter measurements of the ionospheric F region electron density, electron and ion temperature, and vertical ion drift made at Millstone Hill have been used to derive estimates of the exospheric neutral temperature T∞ and the horizontal component of the neutral wind along the magnetic meridian VHn. These data then were employed in a semiempirical model for the local thermosphere (Emery, 1978a) to calculate the diurnal variation of the zonal and meridional winds for the days of the measurements. A total of 64 geomagnetically quiet days from the years 1970, near solar maximum, through 1975 were analyzed. The mean meridional and zonal winds were found to exhibit a marked seasonal variation of about ±50 and ±30 m/s, respectively. The amplitude of this oscillation did not appear to decrease with decreasing sunspot activity; however, the annual mean meridional wind was found to decrease from 25 m/s equatorward to about 0 m/s over the 6‐year period. It is believed that this is indicative of a decrease in the auroral forcing relative to the solar forcing at solar minimum. The meridional winds near equinox shifted from being generally equatorward at solar maximum to being poleward at solar minimum, also suggesting decreased auroral forcing. The diurnally averaged zonal winds maintained a fairly consistent pattern of eastward winds in winter and westward winds in summer over the 6‐year interval.

Equatorial oceanography

The equatorial ocean has always captured the attention of oceanographers. Almost all medium‐to‐large scale oceanic motions are dominated by the effect of the earth's rotation, the Coriolis acceleration. At the equator, this apparent acceleration vanishes and very special physics apply. The most striking phenomena is upwelling that occurs in a narrow latitude band a few degrees wide along the equator due to the prevailing easterlies (westward winds).

The Seasonal Upwelling in the Gulf of Guinea Due to Remote Forcing

A linear model on an equatorial β plane is integrated over a 120-day period in a basin that approximates the tropical Atlantic Ocean. An increase in the westward wind stress of 0.025 N m−2 in the western Atlantic excites an equatorially trapped Kelvin wave that propagates eastward along the equator, moves poleward at the eastern boundary, and produces upwelling throughout the Gulf of Guinea. Cases that study the effects of nonlinearities and the inclusion of a northward wind stress are included. Nonlinearities are shown to have the effect of amplifying the effects of the Kelvin wave and prolonging the upwelling event. The inclusion of a southerly wind stress in the eastern basin provides a secondary mechanism for upwelling south of the equator along the eastern basin. Local winds. cannot account for the seasonal upwelling in the Gulf of Guinea. The simple baroclinic ocean model is integrated from rest. The effects of mean currents and bottom topography are not considered in detail.

Equatorial adjustment in the eastern Atlantic

Observations suggest that the annual upwelling event in the Gulf of Guinea is not associated with changes in the local winds. A possible explanation is that a strong upwelling signal, generated by increased westward wind stress in the western Atlantic, can travel to the eastern Atlantic as an equatorially trapped Kelvin wave. This explanation is analogous to current theories of El Niño in the Pacific Ocean.

Equatorial electrojet and regular daily variation SR—III. Comparison of observations with a physical model

Latitudinal profiles of magnetic variations across the magnetic equator in Chad, are compared with a physical model of the equatorial electrojet which includes the effects of ionospheric winds and plasma instabilities. According to the model, east-west winds can have two types of influence on the ionospheric currents, both of which are clearly reflected in the observed magnetic profiles. Firstly, the winds can create the appearance of a secondary current ribbon, opposed to and wider than the primary electrojet ribbon due to an east-west electric field. Secondly, winds can augment (or diminish) the level of the ‘planetary’ current component in the low-latitude region, in comparison to that due to a pure electric field. We present arguments strongly supporting the existence of mean westward winds at high altitudes (125–200 km) in the daytime equatorial ionosphere. The data also suggest the possible presence of plasma instability effects, which the model indicates should tend to inhibit the electrojet enhancement current and widen the primary current ribbon. The influence of the two-stream (Type I) instability, which the model takes into account, is not entirely obvious. However, we suggest that the gradient-drift (Type II) instability, which the model does not take into account, may have an important influence on the electrojet currents.

Solar rotation: Direct evidence from prominences for a westward wind

Hα and K-line spectra of quiescent prominences, taken with the slit placed normal to the limb, commonly reveal a gas streaming (5–50 km/s) that is peculiar to the upper edge of these objects. On the average this streaming is uni-directional and consistent with a hypothetical east-west wind.

Structure of Wind-Driven Equatorial Currents in Homogeneous Oceans

The properties of wind-driven equatorial currents in homogeneous oceans are investigated for variable wind stresses and variable eddy viscosities. In the case of a uniform westward stress, the calculated cross-stream and depth variations of the undercurrent are found to be in qualitative agreement with observations for a suitable constant value of the eddy viscosity. When the stress has a cross-equatorial component, the undercurrent is displaced slightly upwind of the equator but preserves its boundary-layer character, including the vorticity discontinuity due to the juxtaposition of water particles with different histories. The currents produced by pure north-south wind stresses are in qualitative agreement with Indian Ocean observations in the southwest and northeast monsoon regimes. An eastward wind stress produces an inertial equatorial current rising to a maximum at the surface. The current becomes extremely strong for moderate viscosities and probably unstable. This may account for the absence of a steady equatorial current in eastward wind regimes. A similar explosive growth can occur for a westward wind stress but at much smaller viscosities. Thermohaline undercurrents are simulated by decoupling the pressure force from the wind stress.

The apparent rotation of the upper atmosphere

This paper computes the F-region winds expected from ion motion and neutral air pressure gradients, and reconciles these with the belief, based on satellite observations, that the upper atmosphere rotates faster than the Earth. The paper points out that the satellite observations refer to wind momentum flux rather than speed, and are weighted to low latitudes and daytime. Here indeed eastward winds are expected; maximumeastward winds are predicted in the early evening, in agreement with satellite findings. But the predicted global wind pattern is not eastward. During the day when ion densities are great, eleetrodynamic drag between the ions and neutral molecules is important. Under these conditions, a strong eastward wind is expected near the equator, where ion velocities are greatest. At night, however, when eleetrodynamic drag is weak, there is no corresponding equatorial westward wind and pressure gradients take control at all latitudes. These cause a strong equatorward wind near midnight, which could have the same effect on a satellite orbit as a weaker eastward wind.

Winds in the upper atmosphere

AbstractA systematic study of the winds in the upper atmosphere has been carried out during the 5‐yr period 1953 to 1958. At heights of 85 to 100 km regular prevailing and periodic tidal wind components have been resolved. Components of the prevailing wind of about 20 m sec–1 towards east are observed during the summer and winter months, with similar westward components during spring and weak westward winds in autumn. Large southward components of approximately 20 m sec–1 are observed during the summer. The amplitude of the semi‐diurnal periodic wind is also about 20 m sec–1, and this component can usually be represented by a vector which rotates in a clockwise direction with constant amplitude twice every 24 hours. Large departures from this simple model occur at certain times of the year. Measurements of the 24‐hr and 8‐hr periodic wind systems are also described, and an upper limit of 2 m sec–1 is determined for the lunar semi‐diurnal wind component.