9:40 AM - 9:55 AM
[AAS06-03] Tropical coastal bidirectional gravity waves robustizing QBO
Keywords:diurnal-cycle sea-land breeze circulations, internal gravity waves, wave-mean flow interaction, quasi-biennial oscillation
The equatorial lower-stratospheric quasi-biennial oscillation (QBO) is explained basically by the wave-mean flow interaction theory, but equi-amplitude east-/west-ward waves suggested from the robust periodicity and meridional-vertical extension have not been clarified. Recent observations have revealed that the coastal diurnal cycle (CDC) is the most dominant mode of cloud-rainfall generation in the equatorial troposphere, in particular along the world’s longest coastlines surrounding major islands of the Indonesian maritime continent (IMC) [1]. The CDC (sea-land breeze) circulation is a sum of internal gravity waves forced by the sea-land temperature contrast dependent on sea-surface temperature variations and land-surface modifications. Therefore, the CDC gravity waves may be a key candidate to explain the robustness of QBO and its interactions with El Niño, global warming etc.
The CDC cloud (updraft) area migrations observed in the IMC [1] start from mountains in the evening with destination to seas in the morning, or from seas before sunrise to the mountainous destination in the evening. The horizontal phase velocity is 102 km/half a day ≒ 10 km/h ≒ 3 m/s on land, and faster off the coast. Individual cloud movements and cloud-bottom convergent winds reach about 20 m/s. Under the linear theory for |latitude| < 30° (|Coriolis factor| < 2π/1 day) the CDC circulation is given by a superposition of internal inertio-gravity waves of which the dispersion relation is a cubic equation for vertical wavenumber squared m2. At the surface both the up-/down-ward components exist to satisfy zero vertical velocity condition, and large eddy viscosity/diffusivity (m6 term) balancing the buoyancy (m0 term) makes the velocity field almost similar to a steady horizontal-convection cell, of which the intensity and horizontal scale (inverse horizontal wavenumber k–1 ≈ 102 km) are governed by the land-sea temperature contrast kΔT. Above this cell, the acceleration (m2 term) balances the buoyancy, and the radiation condition selects upward propagating waves. kΔT is reversed between day/night (with frequency ω = 2π/1 day), and the horizontal phase velocity c = ω/k ≃ 100 m/s directs land-/sea-ward, respectively.
The CDC bidirectional (east-/west-ward) gravity-wave generation resembles the bottom condition in the Plumb’s QBO analog [3] (Fig. 1). In this case the meridional range of QBO is narrower than the internal region of inertio-gravity waves, and is given by the region (|latitude| < about 10°) where the solar diurnal cycle is stronger than annual, which is close to the meridional range of the IMC. The zonal-wind amplitude of QBO is dependent on migration velocities of individual clouds, somewhat faster than c for CDC. The period of QBO decreases with an increase of the CDC amplitude, i.e., kΔT, and becomes biperiodic for too strong CDC. The QBO-period becomes longer for a weaker CDC, and decayed if too weak.
The CDCs in the low-latitudes, as well as mountain waves in the mid-latitudes [4], are mechanisms of the rotating earth controlling the atmosphere. Longitudinal locations of Africa (–90°) and South America (+180°) and rapid rotation of the earth homogenize the local solar heating zonally (Fig. 2). If the sea surface around the IMC becomes cooler by El Niño and/or Indian-Ocean dipole mode, CDCs become weaker and the QBO period becomes longer. The CDCs do not generate a "moving frame" mechanism [5] considered for global diurnal cycles (tides) in the Venusian atmosphere. The CDCs appear only on an sea-land coexistent planet such as the earth. The CDCs interact with meridional circulations, water cycles (cold trap), vegetation and human activity (heat island), and hence the QBO also may be affected by them.
References: [1] Yamanaka et al., 2018: Prog. Earth Planet. Sci., 5, 21. [2] Rotunno, 1983: J. Atmos. Sci., 40, 1999. [3] Plumb, 1977: J. Atmos. Sci., 34, 1847. [4] Tanaka & Yamanaka, 1985: J. Meteor. Soc. Japan, 63, 1047. [5] Schubert & Whitehead, 1969: Science, 163(3862), 71.
The CDC cloud (updraft) area migrations observed in the IMC [1] start from mountains in the evening with destination to seas in the morning, or from seas before sunrise to the mountainous destination in the evening. The horizontal phase velocity is 102 km/half a day ≒ 10 km/h ≒ 3 m/s on land, and faster off the coast. Individual cloud movements and cloud-bottom convergent winds reach about 20 m/s. Under the linear theory for |latitude| < 30° (|Coriolis factor| < 2π/1 day) the CDC circulation is given by a superposition of internal inertio-gravity waves of which the dispersion relation is a cubic equation for vertical wavenumber squared m2. At the surface both the up-/down-ward components exist to satisfy zero vertical velocity condition, and large eddy viscosity/diffusivity (m6 term) balancing the buoyancy (m0 term) makes the velocity field almost similar to a steady horizontal-convection cell, of which the intensity and horizontal scale (inverse horizontal wavenumber k–1 ≈ 102 km) are governed by the land-sea temperature contrast kΔT. Above this cell, the acceleration (m2 term) balances the buoyancy, and the radiation condition selects upward propagating waves. kΔT is reversed between day/night (with frequency ω = 2π/1 day), and the horizontal phase velocity c = ω/k ≃ 100 m/s directs land-/sea-ward, respectively.
The CDC bidirectional (east-/west-ward) gravity-wave generation resembles the bottom condition in the Plumb’s QBO analog [3] (Fig. 1). In this case the meridional range of QBO is narrower than the internal region of inertio-gravity waves, and is given by the region (|latitude| < about 10°) where the solar diurnal cycle is stronger than annual, which is close to the meridional range of the IMC. The zonal-wind amplitude of QBO is dependent on migration velocities of individual clouds, somewhat faster than c for CDC. The period of QBO decreases with an increase of the CDC amplitude, i.e., kΔT, and becomes biperiodic for too strong CDC. The QBO-period becomes longer for a weaker CDC, and decayed if too weak.
The CDCs in the low-latitudes, as well as mountain waves in the mid-latitudes [4], are mechanisms of the rotating earth controlling the atmosphere. Longitudinal locations of Africa (–90°) and South America (+180°) and rapid rotation of the earth homogenize the local solar heating zonally (Fig. 2). If the sea surface around the IMC becomes cooler by El Niño and/or Indian-Ocean dipole mode, CDCs become weaker and the QBO period becomes longer. The CDCs do not generate a "moving frame" mechanism [5] considered for global diurnal cycles (tides) in the Venusian atmosphere. The CDCs appear only on an sea-land coexistent planet such as the earth. The CDCs interact with meridional circulations, water cycles (cold trap), vegetation and human activity (heat island), and hence the QBO also may be affected by them.
References: [1] Yamanaka et al., 2018: Prog. Earth Planet. Sci., 5, 21. [2] Rotunno, 1983: J. Atmos. Sci., 40, 1999. [3] Plumb, 1977: J. Atmos. Sci., 34, 1847. [4] Tanaka & Yamanaka, 1985: J. Meteor. Soc. Japan, 63, 1047. [5] Schubert & Whitehead, 1969: Science, 163(3862), 71.