Ka Kit Tung

Varying planetary heat sink led to global-warming slowdown and acceleration

Deep-sea warming slows down global warming Global warming seems to have paused over the past 15 years while the deep ocean takes the heat instead. The thermal capacity of the oceans far exceeds that of the atmosphere, so the oceans can store up to 90% of the heat buildup caused by increased concentrations of greenhouse gases such as carbon dioxide. Chen and Tung used observational data to trace the pathways of recent ocean heating. They conclude that the deep Atlantic and Southern Oceans, but not the Pacific, have absorbed the excess heat that would otherwise have fueled continued warming. Science, this issue p. 897 The slowdown in global warming over the beginning of the 21st century has resulted from heat transport into the deep ocean. A vacillating global heat sink at intermediate ocean depths is associated with different climate regimes of surface warming under anthropogenic forcing: The latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake. In addition to the shallow La Niña–like patterns in the Pacific that were the previous focus, we found that the slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic. Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.

A Theory of Stationary Long Waves. Part I: A Simple Theory of Blocking

Abstract A theory is presented that attempts to explain the tropospheric blocking phenomenon caused by the resonant amplification of large-scale planetary waves forced by topography and surface heating. It is shown that a wave becomes resonant with the stationary forcings when the wind condition in the lower atmosphere is such that the phase speed of the wave is reduced to zero. The resonant behavior of the wave in the presence of Ekman pumping and other damping mechanisms is used to account for the time amplification of the pressure ridges that is an essential part of the blocking phenomenon. This same time behavior also allows the waves to interact with the mean flow in the stratosphere, possibly initiating major sudden warmings. Such a situation was, in fact, assumed by Matsuno (1971) in the lower boundary of his stratospheric model of sudden warming. The basis for reviving the classical normal-mode theory (when faced with the difficulties associated with the zero-wind line) is presented in Part II. Th...

The k−3 and k−5/3 Energy Spectrum of Atmospheric Turbulence: Quasigeostrophic Two-Level Model Simulation

The Nastrom‐Gage energy spectrum of atmospheric turbulence as a function of wavelength is simulated here with a two-level quasigeostrophic (QG) model. This simple model has no topography, no direct wave forcing, and no small-scale forcing, nor any kind of gravity wave generation. The two-level model does, however, allow for the simple mechanism of baroclinic energy injection at the large (synoptic) scales as the model atmosphere relaxes to a specified north‐south ‘‘radiative equilibrium’’ temperature gradient. It also has a small sink of energy at the small scales due to subgrid hyperdiffusion; this attempts to model the small-scale sink not resolved by the two-level QG model, in particular, enhanced viscous dissipation in atmospheric fronts. The magnitude and shape of the observed energy spectrum, with its characteristic k23 power-law behavior in the synoptic and subsynoptic scales (from several thousand to about eight hundred kilometers) and the characteristic k25/3 behavior in the mesoscales (less than about six hundred kilometers), are reproduced convincingly in the model. The picture that emerges for the energy spectrum of atmospheric turbulence from a few kilometers to tens of thousands of kilometers is actually quite simple. The potential energy of the mean flow, which is derived from solar heating with no scale dependence, is transferred selectively to the long synoptic scales of motion via the mechanism of (nonlinear) baroclinic instability. The injected energy moves both upscale, to the planetary waves where it is damped by Ekman damping, and also downscale, through the short synoptic waves, through the mesoscales, to the short mesoscales, where it can be damped by viscous dissipation. There is no need for dynamics other than QG to produce the spectrum. (However, the present work cannot be used to rule out other explanations, such as gravity wave generation, or a separate energy source at the small scales.)

Interannual and Decadal Variations of Planetary Wave Activity, Stratospheric Cooling, and Northern Hemisphere Annular Mode

Using NCEP‐NCAR 51-yr reanalysis data, the interannual and decadal variations of planetary wave activity and its relationship to stratospheric cooling, and the Northern Hemisphere Annular mode (NAM), are studied. It is found that winter stratospheric polar temperature is highly correlated on a year-to-year basis with the Eliassen‐Palm (E‐P) wave flux from the troposphere, implying a dynamical control of the former by the latter, as often suggested. Greater (lower) wave activity from the troposphere implies larger (smaller) poleward heat flux into the polar region, which leads to warmer (colder) polar temperature. A similar highly correlated antiphase relationship holds for E‐P flux divergence and the strength of the polar vortex in the stratosphere. It is tempting to extrapolate these relationships found for interannual timescales to explain the recent stratospheric polar cooling trend in the past few decades as caused by decreased wave activity in the polar region. This speculation is not supported by the data. On timescales of decades the cooling trend is not correlated with the trend in planetary wave activity. In fact, it is found that planetary wave amplitude, E‐P flux, and E‐P flux convergence all show little statistical evidence of decrease in the past 51 yr, while the stratosphere is experiencing a cooling trend and the NAM index has a positive trend during the past 30 yr. This suggests that the trends in the winter polar temperature and the NAM index can reasonably be attributed to the radiative cooling of the stratosphere, due possibly to increasing greenhouse gases and ozone depletion. It is further shown that the positive trend of the NAM index in the past few decades is not through the inhibition of upward planetary wave propagation from the troposphere to the stratosphere, as previously suggested.

Using data to attribute episodes of warming and cooling in instrumental records

The observed global-warming rate has been nonuniform, and the cause of each episode of slowing in the expected warming rate is the subject of intense debate. To explain this, nonrecurrent events have commonly been invoked for each episode separately. After reviewing evidence in both the latest global data (HadCRUT4) and the longest instrumental record, Central England Temperature, a revised picture is emerging that gives a consistent attribution for each multidecadal episode of warming and cooling in recent history, and suggests that the anthropogenic global warming trends might have been overestimated by a factor of two in the second half of the 20th century. A recurrent multidecadal oscillation is found to extend to the preindustrial era in the 353-y Central England Temperature and is likely an internal variability related to the Atlantic Multidecadal Oscillation (AMO), possibly caused by the thermohaline circulation variability. The perspective of a long record helps in quantifying the contribution from internal variability, especially one with a period so long that it is often confused with secular trends in shorter records. Solar contribution is found to be minimal for the second half of the 20th century and less than 10% for the first half. The underlying net anthropogenic warming rate in the industrial era is found to have been steady since 1910 at 0.07–0.08 °C/decade, with superimposed AMO-related ups and downs that included the early 20th century warming, the cooling of the 1960s and 1970s, the accelerated warming of the 1980s and 1990s, and the recent slowing of the warming rates. Quantitatively, the recurrent multidecadal internal variability, often underestimated in attribution studies, accounts for 40% of the observed recent 50-y warming trend.

On the Two-Dimensional Transport of Stratospheric Trace Gases in Isentropic Coordinates

Abstract A zonally averaged model of stratospheric tracer transport is formulated in isentropic coordinated There are some conceptual and computational advantages, as well as some disadvantages in adopting the potential temperature, instead of pressure, as the vertical coordinate. The main disadvantage is that the “density” (mass per unit coordinate volume) in isentropic coordinates is no longer a constant as in the pressure coordinate system under the hydrostatic approximation. However, it can be shown that this density effect is almost negligible in the calculation of the mean diabatic circulation and the eddy advective transports. What is gained by adopting the new formulation is a conceptually simpler picture of the interplay of diabatic and adiabatic process in the transport of tracers. Mean diabatic heating (cooling) forces a direct rising (descending) mean mass flow. Along the streamlines of this mean mass circulation, tracers are advected in the mean. These surfaces slope downward and poleward in ...

The Influence of the Solar Cycle and QBO on the Late-Winter Stratospheric Polar Vortex

Abstract A statistical analysis of 51 years of NCEP–NCAR reanalysis data is conducted to isolate the separate effects of the 11-yr solar cycle (SC) and the equatorial quasi-biennial oscillation (QBO) on the Northern Hemisphere (NH) stratosphere in late winter (February–March). In a four-group [SC maximum (SC-max) versus minimum (SC-min) and east-phase versus west-phase QBO] linear discriminant analysis, the state of the westerly phase QBO (wQBO) during SC-min emerges as a distinct least-perturbed (and coldest) state of the stratospheric polar vortex, statistically well separated from the other perturbed states. Relative to this least-perturbed state, the SC-max and easterly QBO (eQBO) each independently provides perturbation and warming as does the combined perturbation of the SC-max–eQBO. All of these results (except the eQBO perturbation) are significant at the 95% confidence level as confirmed by Monte Carlo tests; the eQBO perturbation is marginally significant at the 90% level. This observational res...

A Theory of Stationary Long Waves. Part II: Resonant Rossby Waves in the Presence of Realistic Vertical Shears

Abstract In Part I a simple theory of resonant Rossby waves in a uniform zonal flow was developed. The present paper extends the previous results to the case of an atmosphere with winds varying with height. The wave responses to a large number of physically possible wind configurations are studied to help determine whether the observed wind fields in the winter atmosphere permit resonance of the large-scale waves and, in cases where resonance is possible, to search for the most favorable conditions for resonance. It is found that an increase in stratospheric jet strength and the descent of the stratospheric jet are both capable of exciting the resonant waves of zonal wavenumbers 1 and 2, with the latter (the descent of the stratospheric jet) being most effective in resonating the large-scale waves. The shorter waves (with zonal wavenumbers 3, 4 and up) are found to be insensitive to changes in wind conditions in the stratosphere as they are mostly trapped in the troposphere. These waves are easier to exci...

Wave Overreflection and Shear Instability

Abstract It is shown that the necessary conditions for the instability of unstratified plane-parallel shear flow, rotating barotropic flows and rotating baroclinic flows are also sufficient conditions for the existence of propagating waves (essentially Rossby waves) and their overreflection (reflection coefficient exceeds 1 in magnitude) from critical levels (where flow speed and phase speed are equal). The identification of the unstable modes with overreflected waves is strongly suggested and allows greater insight into the meaning of various theorems such as Rayleigh’s inflection point theorem. The present results also suggest an important distinction between instabilities associated with, redistribution such as Benard convective instability and instabilities, such as those we are concerned with, associated with the self-excitation of waves.

A reexamination of the radiative balance of the stratosphere

Abstract Previous diagnostic calculations of the stratospheric radiation budget using observed temperature and absorber distributions produce net heating rates that, although qualitatively similar in their overall patterns, differ quantitatively from each other. Furthermore, when horizontally averaged over the globe, most heating rates reveal significant departures from radiative equilibrium. It is shown that globally averaged infrared cooling and solar heating should theoretically be in balance to within 0.03 K day−1 throughout the stratosphere over monthly means and to within smaller ranges over longer time periods. Such accuracies cannot be attained with current methods and available data. Since it is shown here that distributions of important chemical tracers are sensitive to diabatic transport differences larger than 0.1 K day−1 in the lower stratosphere, global radiative imbalances should at least be kept to within 0.1 K day−1. This last, less ambitious goal appears to be almost achievable using cur...