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

npj Climate and Atmospheric Science, Oct 2018

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.

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

Abstract 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. Introduction The gravitational pull of the moon excites tidal variations in atmospheric circulation with L2 frequency (period ~12.42 h). These variations are small—typically an order of magnitude less than solar diurnal and semidiurnal variations (which are forced primarily by the daily cycle of solar heating).1,2 However the lunar tide is singular as a purely mechanically (i.e., adiabatically) forced monochromatic (i.e., single frequency) variation and so observations of the L2 tide can provide uniquely valuable diagnostics of atmospheric behavior that enhance understanding of the atmospheric circulation. A historically important example is provided by long-standing observations of L2 variations in surface geomagnetic field that have informed our understanding of the ionospheric dynamo and upper atmospheric winds.3 Very recently it has been demonstrated that the L2 circulation variations in the troposphere modulate tropical rainfall,4 and the measurements of the L2 rainfall variation provide a valuable constraint on the representation of moist convective processes in climate models.5 In this study, we focus on measurements of the L2 tide in surface air and show that the results provide important information on the thermal coupling of the atmosphere with the underlying surface. The L2 variation in surface air pressure, L2(p), has been reliably determined at many stations where very long records of hourly measurements are available and its geographical and seasonal dependence have been established1,6,7,8 (Supplementary discussion for a brief review). The basic features of the observed L2(p) have been reproduced by simplified linear theory calculations of the response of the global atmosphere to the lunar gravitational forcing.2,9,10 The L2 surface air temperature cycle, L2(T), is expected to have an amplitude ~0.01 K making it quite hard to detect among all the other atmospheric disturbances present. The only direct observational report so far was obtained at a single location, Batavia (6°S, 106.5°E), by Chapman11 using hourly data during 1866–1928 and this determination had large uncertainty due to sampling noise (Note that a recent study4 estimated the size of the troposphere-average L2(T) based on global reanalysis model data). In this study, we analyzed long data records at many ocean buoys to determine accurate values of L2(T) over the tropical ocean. The L2 forcing will excite a linear wave response in the atmosphere whose air temperature perturbations near the surface (\(\hat T\): complex amplitude) are governed by $$\hat T = \frac{{i\omega }}{{i\omega + \beta }}\hat T_{{\rm ad}},$$ (1) where ω is he frequency (=2π/12.42 h−1), and \(\hat T_{{\rm ad}}\) is the adiabatic component defined as, $$\hat T_{ad} \equiv \frac{{R\bar T}}{{C_p\bar p}}\hat p,$$ (2) where \(\hat p\) is the complex amplitude of L2(p) (see Methods section for details). Because there is no “external” thermal forcing for L2, the diabatic heating is expressed here as an effective Newtonian cooling with rate, β. Chapman12 suggested that (over land at least) L2(T) should to first order be in adiabatic relationship with L2(p), with deviations from this purely adiabatic behavior serving as a diagnostic of damping of the temperature signal in the air near ground (which he suggested would be dominated by heat exchange with the underlying surface). Chapman’s single-station determination11 indeed showed a first order adiabatic relation between L2(T) and L2(p) but was not sufficiently precise to reliably determine the strength of any thermodynamic damping. Curiously Chapman12 also suggested that air-sea coupling would be so strong that “over the oceans the lunar atmospheric tide will appear to be isothermal at sea level”. Our results will contradict this last assertion but will show that Chap (...truncated)


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T. Sakazaki, K. Hamilton. Discovery of a lunar air temperature tide over the ocean: a diagnostic of air-sea coupling, npj Climate and Atmospheric Science, 2018, Issue: 1, DOI: 10.1038/s41612-018-0033-9