Diabatic Flux Research Articles

Circulation conservation in the outflow of warm conveyor belts and consequences for Rossby wave evolution

AbstractRossby waves on the jet stream are associated with meridional motions, displacing air and the strong potential vorticity (PV) gradient on isentropic surfaces. Poleward motion along sloping isentropic surfaces typically results in ascent and a ridge of air with low PV values. Latent heating in the ascending warm conveyor belt (WCB) enables air to cross isentropic surfaces so that the WCB outflow into a ridge occurs in a higher isentropic layer than the inflow. However, the PV impermeability theorem states that there can be no PV flux across isentropic surfaces, so how can heating alter the PV pattern of a Rossby wave? Here, the ways in which heating in WCBs can influence Rossby wave evolution at tropopause level are explained in the context of the PV impermeability theorem. First, a WCB outflow volume is defined by the upper tropospheric air in a ridge that has experienced net heating over the last few days, using a tracer within short global model forecasts. Second, the boundary of this outflow volume is tracked backwards using isentropic trajectories allowing quantification of the degree to which circulation is conserved, as predicted by theory, even though the WCB transports mass into the volume from lower isentropic layers. This diabatic flux of mass into the outflow volume results in an increase in density and expansion in the outflow area, the partition being determined approximately by PV inversion. The area expansion, combined with conservation of circulation, implies stronger anticyclonic vorticity. The relative vorticity change from divergent outflow can be as large as the decrease relative to the background planetary vorticity associated with poleward displacement of the circuit. The additional anticyclonic relative motion results in enhanced anticyclonic overturning of PV contours on the eastern flank of the ridge, altering qualitatively the nonlinear evolution of the Rossby wave.

Key Role of Diabatic Processes in Regulating Warm Water Volume Variability over ENSO Events

AbstractThe equatorial Pacific warm water volume (WWV), defined as the volume of water warmer than 20°C near the equator, is a key predictor for El Niño–Southern Oscillation (ENSO), and yet much about the individual processes that influence it remains unknown. In this study, we conduct idealized ENSO simulations forced with symmetric El Niño– and La Niña–associated atmospheric anomalies as well as a historical 1979–2016 hindcast simulation. We use the water mass transformation framework to examine the individual contributions of diabatic and adiabatic processes to changes in WWV. We find that in both sets of simulations, El Niño’s discharge and La Niña’s recharge periods are initiated by diabatic fluxes of volume across the 20°C isotherm associated with changes in surface forcing and vertical mixing. Changes in adiabatic horizontal volume transport above 20°C between the equator and subtropical latitudes dominate at a later stage. While surface forcing and vertical mixing deplete WWV during El Niño, surface forcing during La Niña drives a large increase partially compensated for by a decrease driven by vertical mixing. On average, the ratio of diabatic to adiabatic contributions to changes in WWV during El Niño is about 40% to 60%; during La Niña this ratio changes to 75% to 25%. The increased importance of the diabatic processes during La Niña, especially the surface heat fluxes, is linked to the shoaling of the 20°C isotherm in the eastern equatorial Pacific and is a major source of asymmetry between the two ENSO phases, even in the idealized simulations where the wind forcing and adiabatic fluxes are symmetric.

Available Potential Energy and the General Circulation: Partitioning Wind, Buoyancy Forcing, and Diapycnal Mixing

AbstractThe ocean energy cycle is calculated using a new available potential energy (APE) decomposition, which partitions adiabatic buoyancy fluxes from diapycnal mixing, applied to results from the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2), eddy-permitting ocean state estimate and observed surface buoyancy fluxes from the WHOI OAFlux project. Compared with the traditional Lorenz energy cycle, this framework provides a more accurate estimate of the background potential energy (PE) of the global oceans and the surface generation and interior fluxes of APE. Calculations of the global energy budget using 16 yr of ECCO2 output suggest that the adiabatic portion of the general circulation is maintained by a balance between the mean wind-driven upwelling that increases APE (+0.27 TW) and time-fluctuating processes, including mesoscale eddies, which release APE (−0.27 TW). The APE generated by surface buoyancy fluxes (0.46 TW) is comparable to the generation by the mean winds. The global rate of irreversible mixing (0.46 TW), which balances surface APE generation, is consistent with previous estimates of the diapycnal fluxes associated with maintaining deep stratification (see Munk and Wunsch) and a global diapycnal diffusivity of O(1 × 10−4) m2 s−1. However, the net contribution of diapycnal mixing to the total potential energy is negligible, which suggests that mixing, contrary to one current paradigm, does not place a global demand on kinetic energy dissipation. However, there are regions where mixing is significant, for example, between 3000 and 5000 m (in ECCO2), where mixing increases PE by 0.1 TW. The work provides a new framework for separating adiabatic–diabatic fluxes and for monitoring the global rate of diapycnal mixing rate using measurable surface properties such as SST and heat flux.

Time-Dependent Adjustment in a Simple Model of the Mid-Depth Meridional Overturning Cell

Abstract A recently proposed reduced-gravity model of the warm-water branch of the middepth meridional overturning circulation in a rectangular basin with a circumpolar connection is extended to include time dependence. The model describes the balance between gain of warm water through northward Ekman advection across the circumpolar current, loss of warm water through eddy fluxes southward across the current, net gain or loss of warm water through diabatic processes north of the current, and changes in the thickness of the warm-water layer. The steady solutions are the same as those found previously, when the previous parameterization of diabatic fluxes is used. Time-dependent solutions are considered for the approach of the solution to a new equilibrium when the forcing or parameters are abruptly changed and then held fixed. An initial adjustment occurs through a combination of boundary and equatorial adjustment, followed by planetary wave propagation. The longer-term adjustment to equilibrium consists primarily of the slow change in eastern boundary thickness of the warm layer, which controls the mean depth of the entire layer. An approximate analytical solution of the time-dependent equations yields an explicit expression for the intrinsic time scale of the long-term adjustment, which depends on the eddy and diabatic flux parameters and on the equilibrium solution toward which the time-dependent solution adjusts. Numerical solutions are also considered with a second, advective–diffusive diabatic flux parameterization. These solutions differ quantitatively but not qualitatively from those with the original parameterization. For the range of parameter values considered, the adjustment time scale has dimensional values of several decades to more than a century, but the meridional flux of warm water may respond to changes in external parameters or forcing much more rapidly than this time scale for equilibration of the eastern boundary thickness and thermocline structure.

Vertical Velocities and Available Potential Energy Generated by Landscape Variability—Theory

Abstract It is shown that landscape variability decreases the temperature in the surface layer when, through mesoscale flow, cool air intrudes over warm patches, lifting warm air and weakening the static stability of the upper part of the planetary boundary layer. This mechanism generates regions of upward vertical motion and a sizable amount of available potential energy and can make the environment of the lower troposphere more favorable to cloud formation. This process is enhanced by light ambient wind through the generation of trapped propagating waves, which penetrate into the midtropospheric levels, transporting upward the thermal perturbations and weakening the static stability around the top of the boundary layer. At moderate ambient wind speeds, the presence of surface roughness changes strengthens the wave activity, further favoring the vertical transport of the thermal perturbations. When the intensity of the ambient wind is larger than 5 m s−1, the vertical velocities induced by the surface roughness changes prevail over those induced by the diabatic flux changes. The analysis is performed using a linear theory in which the mesoscale dynamics are forced by the diurnal diabatic sensible heat flux and by the surface stress. Results are shown as a function of ambient flow intensity and of the wavelength of a sinusoidal landscape variability.

Stratosphere‐troposphere exchange of mass and ozone

This study examines the relationship between the extratropical cross‐tropopause fluxes of mass and ozone. The adiabatic and diabatic components of the net fluxes are also compared. The rate of change of mass in the lowermost stratosphere and the flux across the 380 K isentropic surface are used to determine the net tropopause mass flux in the framework of a global circulation model. The diabatic mass flux is calculated from the heating rate at the tropopause, and the adiabatic component is determined by the difference of the net and diabatic fluxes. Consistent ozone fields are obtained by driving the Goddard Chemistry and Transport Model with meteorological output of the global circulation model for the same years. The ozone flux is determined by convolving the mass flux and ozone mixing ratio. The results show the following: (1) The seasonal cycle of the ozone mixing ratio is out of phase with the transport cycle leading to a temporal offset of the mass and ozone fluxes; (2) the downward net diabatic flux of mass and ozone occurs primarily at middle latitudes while the adiabatic mass flux is dominated by troposphere‐to‐stratosphere transport at higher latitudes; and (3) the Southern Hemisphere stratospheric vortex is more effective at blocking meridional transport, resulting in a phase difference of mean tropopause ozone mixing ratio in the higher Southern Hemisphere latitudes with respect to the corresponding Northern Hemisphere season and location. Finally, this study suggests that individual pathways of cross‐tropopause transport are unlikely to be the result of simultaneous adiabatic and diabatic mechanisms.

Extratropical stratosphere‐troposphere mass exchange

The net mass flux from the stratosphere to the troposphere can be computed from the heating rate along the 380 K isentropic surface and the time rate of change of the mass of the lowermost stratosphere (the region between the tropopause and the 380 K isentrope) following Appenzeller et al. [1996]. Given this net mass flux and the cross‐tropopause diabatic mass flux, the residual adiabatic mass flux across the tropopause can also be estimated. These fluxes have been computed using meteorological fields from a free‐running general circulation model (the finite volume general circulation model) and two assimilated data sets, finite volume data assimilation system (FVDAS), and UK Met Office (UKMO). All of the calculations agree that the annual average net mass flux for the Northern Hemisphere is about 1010 kg/s. There is less agreement on the Southern Hemisphere flux that might be half as large. For all three data sets the adiabatic mass flux is from the upper troposphere into the lowermost stratosphere. This adiabatic flux into the lowermost stratosphere is roughly 5 times larger than the diabatic mass flux into the stratosphere across the tropical tropopause. For the FVDAS the midlatitude tropopause diabatic flux is smaller than UKMO, apparently due to a systematically colder, higher FVDAS tropopause. Both data assimilation systems have a warmer, lower midlatitude tropopause compared to radiosondes, so the mass flux estimates can be considered upper bounds. Finally, we note that the difference in the diabatic mass fluxes between the two assimilated meteorological analyses is much larger than the interannual variability in either.