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Ping Huang, Chia Chou, and Ronghui Huang

Abstract

The seasonal modulation of tropical intraseasonal oscillation (TISO) on tropical cyclone (TC) geneses over the western North Pacific Ocean (WNP) is investigated in three periods of the WNP TC season: May–June (MJ), July–September (JAS), and October–December (OND). The modulation of the TISO–TC geneses over the WNP is strong in MJ, while it appears weaker in JAS and OND. In MJ, TISO propagates northward via two routes, the west route through the South China Sea and the east route through the WNP monsoon trough region, which are two clustering locations of TC geneses. TISO can synchronously influence most TC geneses over these two regions. In JAS, however, the modulation is out of phase between the monsoon trough region and the East Asian summer monsoon region, as well as the WNP subtropical high region, as a result of further northward propagation of TISO and scattered TC geneses. The TISO–TC genesis modulation in each individual region is comparable to that in MJ, although the modulation over the entire WNP in JAS appears weaker. In OND, TISO has a stronger influence on TC geneses west than east of 150°E because TISO decays and its convection center located at the equator is out of the TC genesis region when propagating eastward into east of 150°E. Midlevel relative humidity is the primary contribution to the modulations of TISO on the genesis environment, while vorticity could contribute to the modulation over the subtropics in JAS.

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Houk Paek and Huei-Ping Huang

Abstract

The climatology and trend of atmospheric angular momentum from the phase 3 and the phase 5 Climate Model Intercomparison Project (CMIP3 and CMIP5, respectively) simulations are diagnosed and validated with the Twentieth Century Reanalysis (20CR). It is found that CMIP5 models produced a significantly smaller bias in the twentieth-century climatology of the relative MR and omega M Ω angular momentum compared to CMIP3. The CMIP5 models also produced a narrower ensemble spread of the climatology and trend of MR and M Ω. Both CMIP3 and CMIP5 simulations consistently produced a positive trend in MR and M Ω for the twentieth and twenty-first centuries. The trend for the twenty-first century is much greater, reflecting the role of greenhouse gas (GHG) forcing in inducing the trend. The simulated increase in MR for the twentieth century is consistent with reanalysis. Both CMIP3 and CMIP5 models produced a wide range of magnitudes of decadal and interdecadal variability of MR compared to 20CR. The ratio of the simulated standard deviation of decadal or interdecadal variability to its observed counterpart ranges from 0.5 to over 2.0 for individual models. Nevertheless, the bias is largely random and ensemble averaging brings the ratio to within 18% of the reanalysis for decadal and interdecadal variability for both CMIP3 and CMIP5. The twenty-first-century simulations from both CMIP3 and CMIP5 produced only a small trend in the amplitude of decadal or interdecadal variability, which is not statistically significant. Thus, while GHG forcing induces a significant increase in the climatological mean of angular momentum, it does not significantly affect its decadal-to-interdecadal variability in the twenty-first century.

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Houk Paek and Huei-Ping Huang

Abstract

An intercomparison of the global relative angular momentum MR in five reanalysis datasets, including the Twentieth Century Reanalysis (20CR), is performed for the second half of the twentieth century. The intercomparison forms a stringent test for 20CR because the variability of MR is known to be strongly influenced by the variability of upper-tropospheric zonal wind whereas 20CR assimilated only surface observations. The analysis reveals good agreement for decadal-to-multidecadal variability among all of the datasets, including 20CR, for the second half of the twentieth century. The discrepancies among different datasets are mainly in the slowest component, the long-term trend, of MR. Once the data are detrended, the resulting decadal-to-multidecadal variability shows even better agreement among all of the datasets. This result indicates that 20CR can be reliably used for the analysis of decadal-to-interdecadal variability in the pre-1950 era, provided that the data are properly detrended. As a quick application, it is found that the increase in MR during the 1976/77 climate-shift event remains the sharpest over the entire period from 1871 to 2008 covered by 20CR. The nontrivial difference in the long-term trend between 20CR and the other reanalysis datasets found in this study provides a caution against using 20CR to determine the trend on the centennial time scale that is relevant to climate change. These conclusions are restricted to the quantities that depend strongly on the upper-tropospheric zonal wind, but the approach adopted in this work will be useful for future intercomparisons of the low-frequency behavior of other climate indices in the reanalysis datasets.

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Jian-Ping Huang and Gerald R. North

Abstract

Due to the variety of periodic or quasi-periodic deterministic forcings (e.g., diurnal cycle, seasonal cycle, Milankovitch cycles, etc.), most climate fluctuations may be modeled as cyclostationary processes since their properties are modulated by these cycles. Difficulties in using conventional spectral analysis to explore the seasonal variation of climate fluctuations have indicated the need for some new statistical techniques. It is suggested here that the cyclic spectral analysis he used for interpreting such fluctuations. The technique is adapted from cyclostationarity theory in signal processing. To demonstrate the usefulness of this technique, a very simple cyclostationarity stochastic climate model is constructed. The results show that the seasonal cycle strongly modulates the amplitude of the covariance and spectrum. The seasonal variation of intraseasonal oscillations in the Tropics has also been studied on a zonally symmetric all-land planet in the absence of external forcing. The idealized planet has no ocean no topography. A 15-year length seasonal run of the atmosphere is analyzed with the NCAR Community Climate Model (CCM2, R15). Analysis of the simulation data indicates the presence of intraseaonal oscillations in the Tropics, which are also localized in the time of year.

Both examples suggest that these techniques might be useful for analysis of fluctuations that exhibit locality in both frequency and season.

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Huei-Ping Huang and Prashant D. Sardeshmukh

Abstract

The annual variation of global atmospheric angular momentum (AAM) is dominated by its first and second harmonic components. The first harmonic is associated with maximum global AAM in winter (December– January–February) and minimum in summer, but the second harmonic is important enough to produce a distinct secondary midwinter minimum. Locally, the second harmonic has largest amplitude in the Tropics and subtropics of the upper troposphere. At present, little is known concerning the fundamental cause of this semiannual variation. The problem is investigated here by focusing on the upper-tropospheric winds, whose angular momentum is an excellent proxy of global AAM. The annual variation of the rotational part of these winds (the part that contributes to the global AAM) is diagnosed in a nonlinear upper-tropospheric vorticity-equation model with specified horizontal wind divergence and transient-eddy forcing. The divergence forcing is the more important of the two, especially in the Tropics and subtropics, where it is associated with tropical heating and cooling. Given the harmonics of the forcing, the model predicts the harmonics of the response, that is, the vorticity, from which the harmonics of angular momentum can then be calculated. The surprising but clear conclusion from this diagnosis is that the second harmonic of AAM arises more as a nonlinear response to the first harmonic of the divergence forcing than as a linear response to the second harmonic of the divergence forcing. This result has implications for general circulation model simulations of semiannual variations, not only of global AAM but also of other quantities.

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Huei-Ping Huang and Klaus M. Weickmann

Abstract

This note evaluates the numerical schemes used for computing the axial component of the mountain torque from gridded global surface pressure and topography datasets. It is shown that the two formulas of the mountain torque based on (i) an integral of the product of the surface pressure and the gradient of topography, and (ii) an integral of the product of the topography and the surface pressure gradient, should produce identical results if a centered even-ordered finite-difference scheme or the spectral method is used to evaluate the integrand. Noncentered finite-difference schemes are not recommended not only because they produce extremely large errors but also because they produce different results for the two formulas. When compared with the benchmark calculation using the spectral method, it is found that the centered fourth-order finite-difference scheme is an efficient and generally accurate approximation for practical applications. Using the data from NCEP–NCAR reanalysis, the finite-difference schemes generally underestimate the global mountain torque compared to the benchmark. This negative error is interpreted as due to the asymmetry in the distribution of surface pressure and in the steepness of the topography between the western and eastern slopes of the mountains.

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Huei-Ping Huang and Walter A. Robinson

Abstract

Branstator–Kushnir-type large-scale westward propagating waves are investigated using linear and nonlinear global barotropic models with an idealized zonally asymmetric basic state. Retrograde waves are found in the most unstable normal mode of the zonally asymmetric basic state with a jet in the Northern Hemisphere. West-ward propagating waves also exist in nonlinear equilibrium states under a wide range of supercriticality and in both periodic and chaotic regimes. The frequency of the most unstable mode remains as a peak in the frequency spectrum through the nonlinear equilibration process. That frequency matches the frequency of the westward propagating waves in the nonlinear equilibrium states. Local energetics analyses of the linear and nonlinear cases show that the barotropic energy conversion concentrated in the jet exit supplies the perturbation energy of the disturbances all over the globe. Under a traditional spherical-harmonic decomposition, the westward propagating waves consist of several spherical-harmonic components. In the weakly chaotic nonlinear equilibrium states, these components show higher regularity in time than the others and may possess higher predictability.

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Noel C. Baker and Huei-Ping Huang

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The twentieth-century climatology and twenty-first-century trend in precipitation P, evaporation E, and P − E for selected semiarid U.S. Southwest and Mediterranean regions are compared between ensembles from phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). The twentieth-century simulations are validated with precipitation from observation and evaporation from reanalysis. It is found that the Special Report on Emissions Scenarios (SRES) A1B simulations in CMIP3 and the simulations with representative concentration pathways (RCPs) 4.5 and 8.5 in CMIP5 produce qualitatively similar seasonal cycles of the twenty-first-century trend in P − E for both semiarid regions. For the southwestern United States, it is characterized by a strong drying trend in spring, a weak moistening trend in summer, a weak drying trend in winter, and an overall drying trend for the annual mean. For the Mediterranean region, a drying trend is simulated for all seasons with an October maximum and July minimum. The consistency between CMIP3 and CMIP5 scenarios indicates that the simulated trend is robust; however, while the trend in P − E is negative in spring for the southwestern United States for all CMIP ensembles, CMIP3 predicts a strongly negative trend in P and minor negative trend in E whereas both CMIP5 scenarios predict a nearly zero trend in P and positive trend in E. For the twentieth-century simulations, the P, E, and P − E of the two model ensembles are statistically indistinguishable for most seasons. This “stagnation” of the simulated climatology from CMIP3 to CMIP5 implies that the hydroclimatic variable biases have not decreased in the newer generation of models. Notably, over the southwestern United States the CMIP3 models produce too much precipitation in the cold season. This bias remains almost unchanged in CMIP5.

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Huei-Ping Huang and Walter A. Robinson

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The dynamics of two-dimensional turbulence on a rotating sphere are examined. The anisotropic Rhines scale is derived and verified in decaying turbulence simulations. Due to the anisotropic nature of the Rossby waves, the Rhines barrier is displaced toward small total wavenumber n with decreasing zonal wavenumber m. Up-scale energy transfer along the zonal axis (m = 0) is not directly arrested by beta. A forced dissipative model with high-wavenumber forcing is used to investigate the dynamics of persistent zonal jets. Persistent jets form in the low energy (strong rotation) cases with the root-mean-square velocity V*rmsaΩ. Under a fixed rotation rate, the jet scale decreases with the energy. The equilibrated jets generally stay at fixed latitudes. The zonal bands are nearly uniformly distributed in latitude, except that bands in the high latitudes tend to be wider and weaker, as clearly affected by a decreasing beta with latitude. The time-mean zonal winds in the forced simulations appear to be stable, with their absolute vorticity gradient dominated by beta. The increase of the jet scale with energy as required by stability is consistent with the simulated results.

Diagnostic analysis shows that the persistent jets are primarily maintained by the shear-straining mechanism involving small-scale eddies and large-scale zonal jets, with a clear scale separation between them. Although large-scale eddies, those at scales near the Rhines scale, possess most of the eddy energy, in the time mean they contribute little to the maintenance of the zonal jets. Thus, despite the similarity between the Rhines scale and the jet scale, their dynamical link is not obvious in the time-mean statistics. The presence of persistent zonal jets modifies the normal modes of the system. Pure Rossby–Haurwitz modes at small and medium scales are severely modified and fall into the continuum. Large-scale modes, however, may remain discrete. The discreteness of the large-scale modes limits their ability to exchange energy with the zonal jets in the time mean.

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Ping Huang, Dong Chen, and Jun Ying

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In the tropics, the atmospheric circulation response to sea surface temperature (SST) anomalies is a crucial part of the tropical air–sea interaction—the primary process of tropical climate. How it will change under global warming is of great importance to tropical climate change. Here, it is shown that the atmospheric vertical circulation response to local SST anomalies will likely be weakened under global warming using 28 selected models from phase 5 of the Coupled Model Intercomparison Project. The weakening of the circulation response to SST anomalies is closely tied to the increased atmospheric stability under global warming, which increases at the same rate as the circulation response decreases—around 8% for 1 K of tropical-mean SST warming. The spatial pattern of background warming can modify—especially in the equatorial central-eastern Pacific—the spatial distribution of the changes in the circulation response. The atmospheric response to SST anomalies may increase where the local background warming is pronouncedly greater than the tropical mean. The general weakening of the atmospheric circulation response to SST anomalies leads to a decreased circulation response to the structured variability of tropical SST anomalies, such as the El Niño–Southern Oscillation and the Indian Ocean dipole. The decreased circulation response will offset some of the enhancement of the tropical rainfall response to these SST modes as a result of global-warming-induced moisture increase and also implies a decreased amplitude of the tropical air–sea interaction modes.

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