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Yongyun Hu
and
Ka Kit Tung

Abstract

Using NCEP–NCAR reanalysis data, decadal trends in planetary wave activity in Northern Hemisphere high latitudes (50°–90°N) in late winter and early spring (January–February–March) were studied. Results show that wave activity in both the stratosphere and the troposphere has been largely reduced and exhibits statistically significant downward trends since the 1980s. In the stratosphere, the wave activity is decreased by about 30%, which is mainly due to less Eliassen–Palm (E–P) flux from the troposphere into the stratosphere. In the troposphere, the vertical E–P flux is reduced by about 30%, while equatorward horizontal E–P flux is increased by 130%. This suggests a significant refraction of planetary waves away from the high latitudes. The significant negative trends in wave activity in late winter are in contrast to the authors' previous finding of no significant changes in planetary wave activity in early winter.

The timing of the significant decline in wave activity, which starts from the early 1980s and exists only in late winter and springtime, suggests that such a decrease of wave activity is possibly a result of stratospheric ozone depletion in the Arctic. Therefore, a mechanism is proposed whereby Arctic ozone depletion leads to an enhanced meridional temperature gradient near the subpolar stratosphere, strengthening westerly winds. The strengthened winds refract planetary waves toward low latitudes and cause the reduction in wave activity in high latitudes.

Decreasing vertical E–P fluxes are found to extend to near the surface. At 850 mb, vertical E–P fluxes have been reduced by about 10% since 1979. Such a reduction in wave activity might be responsible for the observed late-winter and springtime warming over Northern Hemisphere high-latitude continents during the last two decades.

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Priscilla Cehelsky
and
Ka Kit Tung

Abstract

Previous results based on low- and intermediate-order truncations of the two-layer model suggest the existence of multiple equilibria and/or multiple weather regimes for the extratropical large-scale flow. The importance of the transient waves in the synoptic scales in organizing the large-scale flow and in the maintenance of weather regimes was emphasized. Our result shows that multiple equilibria/weather regimes that are present in lower order models examined disappear when a sufficient number of modes are kept in the spectral expansion of the solution to the governing partial differential equations. Much of the chaotic behavior of the large-scale flow that is present in intermediate order models is now found to be spurious. Physical reasons for the drastic modification are offered.

We further note a peculiarity in the formulation of most existing two-layer models that also tends to exaggerate the importance of baroclinic processes and increase the degree of unpredictability of the large-scale flow.

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Priscilla Cehelsky
and
Ka Kit Tung

Abstract

No abstract available

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Priscilla Cehelsky
and
Ka Kit Tung

Abstract

The concept of baroclinic adjustment is reexamined in the context of a fully nonlinear two-layer model on a β-plane. Based on our results we propose a single, conceptually very simple mechanism of the nonlinear equilibration of waves and the mean flow, which we term “nonlinear baroclinic adjustment.” The new concept appears applicable to cases that currently require different explanations, varying from case to case, to account for the equilibration of the mean temperature gradient in the presence of external driving.

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Wendell T. Welch
and
Ka-Kit Tung

Abstract

The spectrum of transient heat flux in the midlatitude troposphere has a maximum at a synoptic scale. The same is true of the transient energy. The wavenumber of these maxima can be explained by the theory of nonlinear baroclinic adjustment, which is also shown to predict the shape of the spectra.

According to this theory, each zonal wave has a nonlinear threshold that bounds its growth, and the bounds are larger for longer waves. The most unstable wave grows and transports heat until it reaches its threshold, at which point it breaks and saturates, passing off excess energy to the next longer wave. The process repeats, with energy cascading upscale, until the total heat transport is sufficient to reduce the meridional temperature gradient down to a relatively constant equilibrium level, independent of forcing. Thus at higher forcings more heat must be transported and the cascade extends to longer scales. At equilibrium, the longest heat-transporting wave has not saturated but rather has been rendered linearly neutral by the reduction in the temperature gradient.

Observations from the real atmosphere, and computations with a quasigeostrophic two-level model in a beta-plane channel, corroborate the theory presented.

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Ming Fang
and
Ka Kit Tung

Abstract

The classical problem of viscosity-driven, axially symmetric meridional circulation, partly solved only for the midlatitudes by Charney, is solved here analytically in the whole globe and for any value of viscosity coefficient ν. The solution satisfies Hide's theorem for any Ekman number when Ro < 80E 2, where Ro is the Rossby number and E is the Ekman number. For Ro > 80E 2, the linear solution ceases to be asymptotically valid. The nonlinear, nearly inviscid regime of Held and Hou presumably is a subset of the second regime (for E → 0+ and Ro fixed).

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Ming Fang
and
Ka Kit Tung

Abstract

Simple analytic solutions are constructed for an axially symmetric, nonlinear, slightly viscous circulation in a Boussinesq atmosphere in the presence of intense convection at an intertropical convergence zone. The latitude–height extent of the Hadley circulation is obtained, as well as its streamfunction, zonal wind, and temperature distribution. Numerical solutions of the viscous primitive equations are also obtained to verify the analytic solutions. The strength of the circulation is stronger than previous results based on dry models and is now close to the observed value. The extent of the Hadley region is also quite realistic.

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Ming Fang
and
Ka Kit Tung

Abstract

Analytic and numerical solutions are found for the nonlinear Hadley circulation problem with respect to the dependence of the strength and the extent of the Hadley circulation on the thermal relaxation time. The dependence on the thermal relaxation time is a crucial parameter to investigate since the simplifications used in previous studies assumed a large thermal relaxation time, to justify the geostrophic assumption, but in the presence of moist convection, thermal relaxation may be fast in the convection regions. In this study, a primitive equation model is used to investigate the effect of different latitudinal distribution of thermal relaxation time on the extent of the circulation cells, the zonal wind, the temperature distribution, and the strength of the meridional circulations. It is found that the extent of the Hadley circulation is insensitive to the value of the thermal relaxation time τ, while the strength of the circulation is very sensitive to τ (but in a way that is predictable based on the 1/τ scaling).

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Jiansong Zhou
and
Ka-Kit Tung

Abstract

The purpose of the present work is to demonstrate that a solar cycle response exists in surface temperature using the longest global dataset available, which is in the form of 1854–2007 sea surface temperature (SST), with an emphasis on methods and procedures, data quality, and statistical tests and the removal of deterministic signals, such as volcano aerosol forcing and greenhouse gas warming. Using the method of composite-mean difference (CMD) projection, signals of warming during solar maximum and cooling during solar minimum years are found in the global SST over the 14 cycles, dispelling previous claims that the solar cycle response before 1920 is opposite to that of the modern era. The magnitude of the solar cycle response averaged over the oceans between 60°S and 60°N is about 0.1°C of warming for each W m−2 variation of the solar constant (but is slightly lower, at ~0.085°C, when periods of suspected bad data are averaged in, which is consistent with previous work). The signal is robust provided that the years near the Second World War are excluded, during which transitions from British ships to U.S. ships introduced warm bias in the SST, as recently pointed out by D. Thompson and his colleagues. Monte Carlo tests show that the extracted signal has less than 0.02% chance of being a random occurrence. This establishes the existence of a solar cycle response at the earth’s surface at high statistical confidence. Contamination of the signal by volcano aerosols is estimated using the multiple CMD inversion method and found to be small over this long record, although ENSO contamination varies depending on the period chosen but is also small.

The multidecadal trend of response to solar forcing is found to account for no more than a quarter of the observed warming in SST during the past 150 yr, under a reasonable but unproven assumption that the climate response to secular solar forcing and to solar cycle forcing has the same spatial pattern.

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Maurizio Fantini
and
Ka-Kit Tung

Abstract

Results from barotropic stability analyses of strong westerly jets generally tend to conclude that those waves that can extract energy from the shear of the jet are largely trapped by the jet. Therefore, the available shear energy of the flow cannot be transmitted by propagating waves away from the strong zone. We show here that the situation is different if the jet is in the meridional instead of the zonal direction. In this case, unstable waves are generated that are able to propagate energy eastward even in the presence of a realistic dissipation. It is then possible for the shear energy of a western boundary current to be transported large distances into the interior of the ocean.

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