5:15 PM - 7:15 PM
[AAS06-P08] Long-term variations in surface pressure tides
Keywords:Atmospheric tide, Trend, Reanalysis, climate model
Atmospheric tides are periodic daily fluctuations in the atmosphere that are primarily excited by diurnal, diabatic heating. While their climatological characteristics have been studied by both theoretical and observational approaches, there has been little research on long-term trends over decades or centuries. The purpose of this study is to investigate long-term variations in atmospheric tides, using long-term datasets including observation data, reanalysis data, and output data from the climate models.
Surface pressure data from observations (ISPD, period: 1950-2023), a reanalysis (ERA5, period: 1950-2017), and output data from climate models (MRI-ESM v2.0 (period: 1850-2014) and MIROC6 (period: 1850-2020)) were used for the analysis. For MRI-ESM v2.0, two types of data were used: an experiment that reproduced the real Earth (hereafter referred to as “Historical”) and an experiment in which only solar radiation was varied and the atmospheric CO2 concentration was fixed from before the Industrial Revolution (hereafter referred to as “pictl”). For MIROC6 also, two types of data were used: the “Historical” experiment and an experiment in which atmospheric CO2 concentrations were continuously increased at a rate of 1% per year (hereafter referred to as “1pctCO2”).
For each surface pressure data, monthly mean diurnal (24 hr) and semidiurnal (12 hr) tidal components were retrieved. Of these, this presentation focuses on the so-called migrating component that moves westward with the Sun (westward wavenumber 1 for the diurnal tide and westward wavenumber 2 for the semidiurnal tide). Since both diurnal and semidiurnal tides have large amplitudes in the tropics, the long-term trend of the components averaged over the tropics (10°S to 10°N) is examined below. We find that the observation and ERA5 results are relatively similar in the semidiurnal tides, showing a decreasing trend from the 1950s to the 1980s and an increasing trend after the 1980s, although there are some differences in places. The model results showed an increasing trend throughout the analysis period except for MRI-ESM v2.0-pictl, while the magnitude of the positive trend was smaller than that in observations and reanalysis. For the diurnal tide, a slight increasing trend was observed only for MRI-ESM v2.0-Historical, but no significant trend was observed for the other datasets (either of observation or model).
Possible factors for the trend in semidiurnal tide are changes in the non-adiabatic heating of the atmosphere and changes in the atmospheric background field (especially the temperature structure). To quantify these effects, we performed numerical calculations using classical tidal theory with diabatic heating and temperature data from ERA5. Specifically, the tidal component of monthly diabatic heating and the background temperature field (averaged at 30°S to 30°N) were given for 40 years from 1980 to 2019, and the surface pressure response was calculated. The results show that the amplitude of the semidiurnal tide shows a decreasing trend unlike the observed data (including reanalysis), which does not explain the observed results well.
In the future, we would like to investigate the robustness in the observed trend by comparing and analyzing other reanalysis, model data, and observational data, and at the same time, we would like to investigate the causes of the trend in detail e.g. by performing the classical tidal theory calculation again using the climate model output data.
Surface pressure data from observations (ISPD, period: 1950-2023), a reanalysis (ERA5, period: 1950-2017), and output data from climate models (MRI-ESM v2.0 (period: 1850-2014) and MIROC6 (period: 1850-2020)) were used for the analysis. For MRI-ESM v2.0, two types of data were used: an experiment that reproduced the real Earth (hereafter referred to as “Historical”) and an experiment in which only solar radiation was varied and the atmospheric CO2 concentration was fixed from before the Industrial Revolution (hereafter referred to as “pictl”). For MIROC6 also, two types of data were used: the “Historical” experiment and an experiment in which atmospheric CO2 concentrations were continuously increased at a rate of 1% per year (hereafter referred to as “1pctCO2”).
For each surface pressure data, monthly mean diurnal (24 hr) and semidiurnal (12 hr) tidal components were retrieved. Of these, this presentation focuses on the so-called migrating component that moves westward with the Sun (westward wavenumber 1 for the diurnal tide and westward wavenumber 2 for the semidiurnal tide). Since both diurnal and semidiurnal tides have large amplitudes in the tropics, the long-term trend of the components averaged over the tropics (10°S to 10°N) is examined below. We find that the observation and ERA5 results are relatively similar in the semidiurnal tides, showing a decreasing trend from the 1950s to the 1980s and an increasing trend after the 1980s, although there are some differences in places. The model results showed an increasing trend throughout the analysis period except for MRI-ESM v2.0-pictl, while the magnitude of the positive trend was smaller than that in observations and reanalysis. For the diurnal tide, a slight increasing trend was observed only for MRI-ESM v2.0-Historical, but no significant trend was observed for the other datasets (either of observation or model).
Possible factors for the trend in semidiurnal tide are changes in the non-adiabatic heating of the atmosphere and changes in the atmospheric background field (especially the temperature structure). To quantify these effects, we performed numerical calculations using classical tidal theory with diabatic heating and temperature data from ERA5. Specifically, the tidal component of monthly diabatic heating and the background temperature field (averaged at 30°S to 30°N) were given for 40 years from 1980 to 2019, and the surface pressure response was calculated. The results show that the amplitude of the semidiurnal tide shows a decreasing trend unlike the observed data (including reanalysis), which does not explain the observed results well.
In the future, we would like to investigate the robustness in the observed trend by comparing and analyzing other reanalysis, model data, and observational data, and at the same time, we would like to investigate the causes of the trend in detail e.g. by performing the classical tidal theory calculation again using the climate model output data.