Revisiting the Quasi Biennial Oscillation as Seen in ERA5. Part II: Evaluation of Waves and Wave Forcing

This paper describes stratospheric waves in ERA5 reanalysis and evaluates the contributions of different types of waves to the driving of the quasi-biennial oscillation (QBO). Because of its higher spatial resolution compared to its predecessors, ERA5 is capable of resolving a broader spectrum of waves. It is shown that the resolved waves contribute to both eastward and westward accelerations near the equator, mainly by the way of the vertical flux of zonal momentum. The eastward accelerations by the resolved waves are mainly due to Kelvin waves and small-scale gravity (SSG) waves with zonal wavelengths smaller than 2000 km, whereas the westward accelerations are forced mainly by SSG waves, with smaller contributions from inertio-gravity and mixed-Rossby-gravity waves. Extratropical Rossby waves disperse upward and equatorward into the tropical region and impart a westward acceleration to the zonal flow. They appear to be responsible for at least some of the irregularities in the QBO cycle.

Interaction between the MJO and High-Frequency Waves over the Maritime Continent in Boreal Winter

The two-way interaction between Madden–Julian oscillation (MJO) and higher-frequency waves (HFW) over the Maritime Continent (MC) during boreal winter of 1984–2005 is investigated. It is noted from observational analysis that strengthened (weakened) HFW activity appears to the west (east) of and under MJO convection during the MJO active phase and the opposite is seen during the MJO suppressed phase. Sensitivity model experiments indicate that the control of HFW activity by MJO is through change of the background vertical wind shear and specific humidity. The upscale feedbacks from HFW to MJO through nonlinear rectification of condensational heating and eddy momentum transport are also investigated with observational data. A significantly large amount (25%–40%) of positive heating anomaly () at low levels to the east of MJO convection is contributed by nonlinear rectification of HFW. This nonlinear rectification is primarily attributed to eddy meridional moisture advection. A momentum budget diagnosis reveals that 60% of MJO zonal wind tendency at 850 hPa is attributed to the nonlinear interaction of HFW with other scale flows. Among them, the largest contribution arises from eddy zonal momentum flux divergence . Easterly (westerly) vertical shear to the west (east) of MJO convection during the MJO active phase causes the strengthening (weakening) of the HFW zonal wind anomaly. This leads to the increase (decrease) of eddy momentum flux activity to the east (west) of the MJO convection, which causes a positive (negative) eddy zonal momentum flux divergence in the zonal wind transitional region during the MJO active (suppressed) phase, favoring the eastward propagation of the MJO.

Variability of the Great Whirl and Its Impacts on Atmospheric Processes

Using satellite remote sensing data and re-analysis products for the period of 1993–2015, the variation of a large anticyclonic eddy, the so-called Great Whirl (GW), located in the Northwest Indian Ocean off the coast of Somali, and its impacts on atmosphere were investigated. The GW is generated in early summer and decays in late fall every year. The center of the GW is located at 7.73°N, 53.20°E. The mean lifetime, sea level anomaly (SLA) difference, sea surface temperature anomaly (SSTA), radius, normalized vorticity, eddy kinetic energy (EKE), and deformation rate are 169 days, 0.07 m, 0.83°C, 116.86 km, −0.53, 0.08 m2∙s-2, and 0.58, respectively. All these variables exhibit interannual variations. Composite analyses show that the maximum values of sea surface temperature (SST), wind, and water vapor anomalies occur in the northwest of the GW center. The fitting coefficient between the SST and wind speed anomaly is 1.1, indicating that, corresponding to 1 °C increases of the SST, the wind speed increases by about 1.1m∙s-1, and the fitting coefficient between the SST and water vapor anomaly is 0.45, indicating that water vapor increases by about 0.45 mm in response to 1 °C increases in the SST. In the vertical direction, the maximum and minimum values of vertical velocity anomalies and vertical transport of transient zonal momentum occur over the GW at about 900 hPa, and wind speed anomalies occur at about 950 hPa. Both the positive transport anomalies of transient zonal momentum and the positive vertical velocity anomalies on the west side of the GW can accelerate the wind speed in the lower level.

ITCZ Width Controls on Hadley Cell Extent and Eddy-Driven Jet Position and Their Response to Warming

The impact of global warming–induced intertropical convergence zone (ITCZ) narrowing onto the higher-latitude circulation is examined in the GFDL Atmospheric Model, version 2.1 (AM2.1), run over zonally symmetric aquaplanet boundary conditions. A striking reconfiguration of the deep tropical precipitation from double-peaked, off-equatorial ascent to a single peak at the equator occurs under a globally uniform +4 K sea surface temperature (SST) perturbation. This response is found to be highly sensitive to the SST profile used to force the model. By making small (≤1 K) perturbations to the surface temperature in the deep tropics, varying control simulation precipitation patterns with both single and double ITCZs are generated. Across the climatologies, narrower regions of ascent correspond to more equatorward Hadley cell edges and eddy-driven jets. Under the global warming perturbation, the experiments in which there is narrowing of the ITCZ show significantly less expansion of the Hadley cell and somewhat less poleward shift of the eddy-driven jet than those without ITCZ narrowing. With a narrower ITCZ, the ascending air has larger zonal momentum, causing more westerly upper-tropospheric subtropical wind. In turn, this implies 1) the subtropical jet will become baroclinically unstable at a lower latitude and 2) the critical (zero wind) line will shift equatorward, allowing midlatitude eddies to propagate farther equatorward. Both of these mechanisms modify the Hadley cell edge position, and the latter affects the jet position.

Climatology of mesopause region nocturnal temperature, zonal wind and sodium density observed by sodium lidar over Hefei, China (32° N, 117° E)

The University of Science and Technology of China narrowband sodium temperature and wind lidar, located in Hefei, China (32∘ N, 117∘ E), has made routine nighttime measurements since January 2012. A total of 154 nights (∼ 1400 h) of vertical profiles of temperature, sodium density, and zonal wind and 83 nights (∼ 800 h) of vertical flux of gravity wave (GW) zonal momentum in the mesopause region (80–105 km) were obtained during the period from 2012 to 2016. For temperature, it is most likely that the diurnal tide dominates below 100 km in spring, while the semidiurnal tide dominates above 100 km throughout the year. A clear semiannual variation in temperature is revealed near 90 km, in phase with the tropical mesospheric semiannual oscillation (MSAO). The variability in sodium density is positively correlated with temperature below 95 km, suggesting that in addition to dynamics, the chemistry also plays an important role in the formation of sodium atoms. The seasonal variability in sodium density observed by both lidar and satellite generally agrees well with a whole atmosphere model simulation using an updated meteoric input function which includes different cosmic dust sources. For zonal wind, the diurnal tide dominates in both spring and fall, while semidiurnal tide dominates in winter. The observed semiannual variation in zonal wind near 90 km is out of phase with that in temperature, consistent with the tropical MSAO. The lidar observations generally agree with satellite and meteor radar observations as well as model simulations at similar latitude. The 50–70 % of zonal momentum flux is induced by short-period (10 min–2 h) GWs. The large zonal momentum flux in summer and winter due to short-period GWs is clearly anticorrelated with eastward zonal wind maxima below 90 km, suggesting the filtering of short-period GWs by the SAO wind.

Is the Coefficient of Eddy Potential Vorticity Diffusion Positive? Part I: Barotropic Zonal Channel

The question of whether the coefficient of diffusivity of potential vorticity by mesoscale eddies is positive is studied for a zonally reentrant barotropic channel using the quasigeostrophic approach. The topography is limited to the first mode in the meridional direction but is unlimited in the zonal direction. We derive an analytic solution for the stationary (time independent) solution. New terms associated with parameterized eddy fluxes of potential vorticity appear both in the equations for the mean zonal momentum balance and in the kinetic energy balance. These terms are linked with the topographic form stress exerted by parameterized eddies. It is demonstrated that in regimes with zonal flow (analogous to the Antarctic Circumpolar Current), the coefficient of eddy potential vorticity diffusivity must be positive.

Climatology of mesopause region nocturnal temperature, zonal wind, and sodium density observed by sodium lidar over Hefei, China (32° N, 117° E)

The University of Science and Technology of China narrowband sodium temperature/wind lidar, located in Hefei, China (32° N, 117° E), has made routine nighttime measurements since January 2012. 154 nights (~ 1400 hours) of vertical profiles of temperature, sodium density, and zonal wind, and 83 nights (~ 800 hours) of vertical flux of gravity wave (GW) zonal momentum in the mesopause region (80–105 km) have been obtained during the period from 2012 to 2016. In temperature, it is most likely that the diurnal tide dominates below 100 km in spring, while the semidiurnal tide dominates above 100 km throughout the year. A clear semiannual variation in temperature is revealed near 90 km, in phase with the tropical mesospheric semiannual oscillation (MSAO). The variability of sodium density is positively correlated with temperature below 95 km, suggesting that in addition to dynamics, the chemistry also plays an important role in the formation of sodium atoms. The seasonal variability of sodium density observed by both lidar and satellite generally agrees well with a whole atmosphere model simulation using an updated meteoric input function which includes different cosmic dust sources. In zonal wind, the diurnal tide dominates in both spring and fall, while semidiurnal tide dominates in winter. The observed semiannual variation in zonal wind near 90 km is out-of-phase with that in temperature, consistent with the tropical MSAO. The GW zonal momentum flux is mostly westward in fall and winter, anti-correlated with eastward zonal wind. The annual mean flux averaged over 87–97 km is ~ −0.3 m2/s2 (westward), anti-correlated with eastward zonal wind of ~ 10 m/s. The lidar observations generally agree with satellite and meteor radar observations as well as model simulations at similar latitudes.

ITCZ Breakdown and Its Upscale Impact on the Planetary-Scale Circulation over the Eastern Pacific

The eastern Pacific (EP) intertropical convergence zone (ITCZ) is sometimes observed to break down into several vortices on the synoptic time scale. It is still a challenge for present-day numerical models to simulate the ITCZ breakdown in the baroclinic modes. Also, the upscale impact of the associated mesoscale fluctuations on the planetary-scale circulation is not well understood. Here, a simplified multiscale model for the modulation of the ITCZ is used to study these issues. A prescribed two-scale heating drives the planetary-scale circulation through both planetary-scale mean heating and eddy flux divergence of zonal momentum, where the latter represents the upscale impact of mesoscale disturbances. In an idealized scenario where the heating only varies on the mesoscale, key features of the ITCZ breakdown in the baroclinic modes are captured. The eddy flux divergence of zonal momentum is characterized by midlevel (low level) eastward (westward) momentum forcing at subtropical latitudes of the Northern Hemisphere and opposite-signed midlevel momentum forcing at low latitudes. Such upscale impact of mesoscale fluctuations tends to accelerate (decelerate) planetary-scale zonal jets in the middle (lower) troposphere. Compared with deep heating, shallow heating induces stronger vorticity anomalies on the mesoscale and more significant eddy flux divergence of zonal momentum and acceleration–deceleration effects on the planetary-scale mean flow. In a more realistic scenario where the heating also varies on the planetary scale, the most significant zonal velocity anomalies are confined in the diabatic heating region.