Linking African Easterly Wave Activity with Equatorial Waves and the Influence of Rossby Waves from the Southern Hemisphere
Linking African Easterly Wave Activity with Equatorial Waves and the Influence of Rossby Waves from the Southern Hemisphere
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DOI：
10.1175/jas-d-17-0184.1
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发表时间：
2018-06-01
影响因子：
3.1
通讯作者：
Hoskins, Brian
中科院分区：
文献类型：
--
作者：
Yang, Gui-Ying;Methven, John;Hoskins, Brian
非洲东风波（AEW）、赤道向西移动的混合罗斯贝重力波（WMRG）和来自南半球（SH）的等效正压罗斯贝波（RW）之间存在联系。赤道波的振幅和相位是通过将宽带滤波的 ERA-Interim 数据投影到从赤道波理论获得的水平结构基础上来计算的。确定了波类型之间相互作用的机制。 AEW 以涡旋波为主，涡旋波在非洲东风急流下方向东倾斜，上方向西倾斜：斜压波增长所需的倾斜。然而，AEW 内放大涡度中心与赤道 WMRG 波之间存在密切关系。虽然波不锁相，但只要 WMRG 波的跨赤道运动位于对流层上层（南向流）和对流层下层以东（北向流）的同一经度，正涡度中心就会放大。有两种机制可以解释涡度放大：涡旋在对流层上层辐散度下方延伸，以及与对流层下层潮湿向北流中对流潜热相关的上升。在AEW活动强烈的年份，SH和赤道对流层上层纬向风更加偏东。较强的东风有两个影响：(i) 它们多普勒频移 WMRG 波，使其周期随波数变化很小（3-4 天）；(ii) 它们使向西移动的 RW 能够从 SH 传播到热带波导。 RW 相速度可以与 WMRG 波的相速度相匹配，从而实现 WMRG 的持续激发。 WMRG 波具有向东的群速度，波活动在非洲上空累积，并通过涡度放大机制以相似的频率激发预警波。
A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Rossby-gravity (WMRG) waves, and equivalent barotropic Rossby waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of equatorial waves is calculated by projection of broadband-filtered ERA-Interim data onto a horizontal structure basis obtained from equatorial wave theory. Mechanisms enabling interaction between the wave types are identified. AEWs are dominated by a vorticity wave that tilts eastward below the African easterly jet and westward above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centers within AEWs and equatorial WMRG waves. Although the waves do not phase lock, positive vorticity centers amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southward flow) and east of this in the lower troposphere (northward flow). Two mechanisms could explain the vorticity amplification: vortex stretching below the upper-tropospheric divergence and ascent associated with latent heating in convection in the lower-tropospheric moist northward flow.In years of strong AEW activity, SH and equatorial upper-tropospheric zonal winds are more easterly. Stronger easterlies have two effects: (i) they Doppler shift WMRG waves so that their period varies little with wavenumber (3-4 days) and (ii) they enable westward-moving RWs to propagate into the tropical waveguide from the SH. The RW phase speeds can match those of WMRG waves, enabling sustained excitation of WMRG. The WMRG waves have an eastward group velocity with wave activity accumulating over Africa and invigorating AEWs at similar frequencies through the vorticity amplification mechanism.