Takatoshi Sakazaki

Three-dimensional structures of tropical nonmigrating tides in a high-vertical-resolution general circulation model

This paper investigates nonmigrating tides from the ground to the lower mesosphere using data from a high-resolution general circulation model (KANTO GCM), as well as observational data from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on board the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite and from GPS radio occultation measurements obtained with the COSMIC/FORMOSAT-3 mission. We extract nonmigrating tides using a composite as a function of universal time in physical space, without performing a zonal wave number decomposition. The KANTO GCM clearly demonstrates that tropical nonmigrating tides are regarded as gravity waves excited by diabatic heating enhanced over two major continents, specifically Africa and South America. They propagate zonally, in a direction away from their sources; that is, west and eastward propagating waves are dominant on the western and eastern sides of the continents, respectively. These characteristics are observed in two satellite data sets as well, except that the amplitudes in the KANTO GCM are larger than those in the observations. Seasonal variations of nonmigrating tides are also investigated. It is suggested that filtering owing to the stratopause semiannual oscillation, as well as diabatic heating in the troposphere, is important for the seasonal variations of nonmigrating tides in the stratosphere and the lower mesosphere.

Physical Processes Controlling the Tide in the Tropical Lower Atmosphere Investigated Using a Comprehensive Numerical Model

AbstractThe lower-atmospheric circulation in the tropics is strongly influenced by large-scale daily variations referred to as atmospheric solar tides. Most earlier studies have used simplified linear theory to explain daily variations in the tropics. The present study employs a comprehensive limited-area atmospheric model and revisits some longstanding issues related to atmospheric tidal dynamics. The tides in the tropical lower atmosphere are realistically simulated in the control experiment with a near-global (75°S–75°N) version of the model. Sensitivity experiments with different aspects of the solar heating suppressed showed that the semidiurnal (S2) tide near the surface can be attributed roughly equally to stratospheric and tropospheric direct solar heating and that the diurnal (S1) tide is excited almost entirely by tropospheric direct solar heating as well as solar heating of Earth’s surface. Linear theory with forcing only by direct radiative heating predicts the phase of the S2 barometric oscil...

Is there a stratospheric pacemaker controlling the daily cycle of tropical rainfall

Rainfall in the tropics exhibits a large, 12-hour sun-synchronous variation with coherent phase around the globe. A long-standing, but unproved, hypothesis for this phenomenon is excitation by the prominent 12-hour atmospheric tide, which itself is significantly forced remotely by solar heating of the stratospheric ozone layer. We investigated the relative roles of large-scale tidal forcing and more local effects in accounting for the 12-hour variation of tropical rainfall. A model of the atmosphere run with the diurnal cycle of solar heating artificially suppressed below the stratosphere still simulated a strong coherent 12-hour rainfall variation (~50% of control run), demonstrating that stratospherically-forced atmospheric tide propagates downward to the troposphere and contributes to the organization of large-scale convection. The results have implications for theories of excitation of tropical atmospheric waves by moist convection, for the evaluation of climate models, and for explaining the recently-discovered lunar tidal rainfall cycle.

Zonally uniform tidal oscillations in the tropical stratosphere

By analyzing data from satellite measurements, a reanalysis, and a chemistry-climate model, we found clear zonally uniform tidal signals in the tropical stratosphere, particularly during the Northern Hemisphere summer. Antisymmetric components with respect to the equator are dominant and are characterized by a vertical wavelength of ~15 km and a diurnal frequency. The temperature and vertical wind diurnal amplitudes in the stratosphere are 0.2–1 K and 1–7 mm s–1, respectively. The latter is generally larger than the climatological ascent motion in the stratosphere. The observed latitudinal and vertical structures can be explained by the second, propagating, antisymmetric Hough mode. These tidal oscillations should be carefully considered in analyses of zonal mean fields at a particular universal time.

Discovery of a lunar air temperature tide over the ocean: a diagnostic of air-sea coupling

The lunar semidiurnal (L2) tide in the Earth’s atmosphere is unique as a purely mechanically forced periodic signal and it has been detected in upper atmosphere winds and temperature and in surface barometric pressure. L2 signals in surface air temperature, L2(T), have only been detected at a single land station (results published almost a century ago). We report observational determinations of L2(T) over the ocean by using data from 38 moored buoys across the tropical Pacific and Atlantic. In contrast to published speculation that L2(T) should be negligible over ocean, we find that the observed L2(T) is fairly close to that consistent with an adiabatic L2 pressure variation. Any deviations from purely adiabatic behavior are a measure of diabatic effects on the surface air—expected to be dominated by damping processes, notably heat exchange with the ocean surface. With the aid of climate model simulations that include L2-tide-like variations, we demonstrate that our observations of L2(T) provide a unique diagnosis for the strength of air-sea coupling and a useful constraint on climate model formulations of this coupling.Atmospheric Dynamics: Moon excites global surface air temperature waveDetection of air temperature variations forced by the lunar tide enables a novel diagnosis of the strength of air–sea coupling. The gravitational tidal force of the moon drives a roughly twice daily cycle in the atmosphere just as in the ocean. Earlier investigators had used long time series of barometric observations to quantify the surface air pressure variations associated with this lunar tide at many locations around the world. Takatoshi Sakazaki and Kevin Hamilton at the University of Hawai’i have now discovered comparable signals in the surface air temperature using two decades of observations at 38 moored buoys across the tropical Pacific and Atlantic. The relative amplitudes and phases of the lunar pressure and temperature signals are shown to provide valuable information about the exchange of heat between the atmosphere and underlying ocean surface.