Surface Expressions of Atmospheric Thermal Tides in the Tropical Atlantic and Their Impact on Open‐Ocean Precipitation. Issue 22 (16th November 2020)

• Record Type: Journal Article
• Authors: Christophersen, J. A.; Foltz, G. R.; Perez, R. C.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 22(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-16
• Subjects: atmospheric thermal tides -- tropical Atlantic Ocean -- ocean observing systems; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1029/2019JD031997
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Diurnal and semidiurnal variations of atmospheric pressure and surface winds are fundamental to the Earth‐Sun system. Past research in the tropical Pacific shows semidiurnal and diurnal patterns in the zonal and meridional wind anomalies, respectively. While the semidiurnal zonal wind pattern is consistent with atmospheric thermal tidal forcing, it is not yet certain what drives diurnal meridional wind variability. This study examines the diurnal cycle of meridional winds in the tropical Atlantic Ocean across four different seasons and the extent to which they impact the diurnal evolution of open‐ocean precipitation in boreal summer. Comparisons of direct observations from long‐term moored buoys to an atmospheric reanalysis (MERRA‐2) show that MERRA‐2 reproduces diurnal and semidiurnal pressure and wind variations. The MERRA‐2 atmospheric thermal tides are decomposed into migrating and nonmigrating tidal components, and the linear terms in the zonal and meridional momentum equations are calculated. There is an approximate balance between the acceleration and pressure gradient terms for the zonal migrating momentum budget in the open ocean, with larger residuals or imbalances between those terms for the other budgets. The meridional nonmigrating budget shows largest residuals near the African coastline, indicating that nonlinear processes like the evening initiation of convection are important over land. This evening convection drives the meridional component of surface convergence which is translated offshore by the nonmigrating tide in the form of gravity waves, potentially modified by the African Easterly Jet. During boreal summer, this convergence helps to induce a morning peak of precipitation over the open ocean. Plain Language Summary: The Sun's radiation is absorbed by different chemicals (ozone and water vapor) in the atmosphere, to which the surface atmosphere response can be observed as a wave‐like pattern that migrates westward in pressure data. East‐west wind patterns are explained as atmospheric thermal tides forced by the solar radiation in the tropical Atlantic that migrate westward around globe twice per day. We examine the cause of the diurnal north‐south wind velocity variations, which had yet to be fully explained, and how they influence rainfall over the open‐ocean. Nonmigrating (no westward movement) atmospheric tides drive the north‐south wind pattern, carrying with them a signal of low‐level convergence of the wind from western Africa to the middle of the tropical Atlantic Ocean, which helps to initiate early morning rainfall during the boreal summer. Climate models are deficient at simulating diurnal rainfall, and these results may aid in improving model precipitation. Key Points: MERRA‐2 represents well the diurnal cycles of surface pressure and winds, and hence atmospheric thermal tides in the tropical Atlantic Nonmigrating atmospheric thermal tides are largely responsible for diurnal cycles of meridional wind anomalies in the tropical Atlantic Diurnal cycles in divergence/convergence and precipitation in the Atlantic Ocean may be related to diurnal cycles of convection over Africa