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Yongyun Hu
,
Ka Kit Tung
, and
Jiping Liu

Abstract

Decadal trends are compared in various fields between Northern Hemisphere early winter, November–December (ND), and late-winter, February–March (FM), months using reanalysis data. It is found that in the extratropics and polar region the decadal trends display nearly opposite tendencies between ND and FM during the period from 1979 to 2003. Dynamical trends in late winter (FM) reveal that the polar vortex has become stronger and much colder and wave fluxes from the troposphere to the stratosphere are weaker, consistent with the positive trend of the Arctic Oscillation (AO) as found in earlier studies, while trends in ND appear to resemble a trend toward the low-index polarity of the AO. In the Tropics, the Hadley circulation shows significant intensification in both ND and FM, with stronger intensification in FM. Unlike the Hadley cell, the Ferrel cell shows opposite trends between ND and FM, with weakening in ND and strengthening in FM. Comparison of the observational results with general circulation model simulations is also discussed.

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Li Yi
,
King-Fai Li
,
Xianyao Chen
, and
Ka-Kit Tung

Abstract

The rapid increase in open-water surface area in the Arctic, resulting from sea ice melting during the summer likely as a result of global warming, may lead to an increase in fog [defined as a cloud with a base height below 1000 ft (~304 m)], which may imperil ships and small aircraft transportation in the region. There is a need for monitoring fog formation over the Arctic. Given that ground-based observations of fog over Arctic open water are very sparse, satellite observations may become the most effective way for Arctic fog monitoring. We developed a fog detection algorithm using the temperature difference between the cloud top and the surface, called ∂T in this work. A fog event is said to be detected if ∂T is greater than a threshold, which is typically between −6 and −12 K, depending on the time of the day (day or night) and the surface types (open water or sea ice). We applied this method to the coastal regions of Chukchi Sea and Beaufort Sea near Barrow, Alaska (now known as Utqiaġvik), during the months of March–October. Training with satellite observations between 2007 and 2014 over this region, the ∂T method can detect Arctic fog with an optimal probability of detection (POD) between 74% and 90% and false alarm rate (FAR) between 5% and 17%. These statistics are validated with data between 2015 and 2016 and are shown to be robust from one subperiod to another.

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Richard S. Lindzen
,
Brian Farrell
, and
Ka-Kit Tung

Abstract

The concept of wave overreflection is reviewed. The physical origin of this effect is discussed as is its relation to hydrodynamic instability. It is noted that the instabilities associated with overreflection are likely to be identical to what are commonly called critical layer instabilities.

The bulk of this paper examines baroclinic instability in terms of the overreflection of vertically propagating Rossby waves. This approach leads to rapid estimates of growth rates and phase speeds of unstable modes for arbitrary distributions of zonal velocities in models with and without lids; it also leads to efficient algorithms for calculating unstable modes “exactly.”

Among our findings are the following: (i) Charney and Green modes are both essentially critical-layer instabilities. (ii) When tropospheric shear is brought to zero above some height (one scale height, for example) so that long waves may radiate to infinity (ignoring for a moment the growth rate), the growth rates are reduced somewhat, but the modes remain unstable. (iii) Baroclinic instability can be eliminated by stretching the transition region from zero shear at the ground to the interior shear sufficiently without altering the shear above this region. Explicit calculations show this depth to be about a quarter of a scale height.

Consistent with item (iii) above, we show that the potential vorticity flux of baroclinically unstable modes (a measure of their interaction with the mean flow) is confined primarily to a layer between the ground and the neighborhood of the steering level—even when the unstable eigenmodes extend to much greater heights.

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Eduardo P. Olaguer
,
Hu Yang
, and
Ka Kit Tung

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 current technology.A comprehensive radiative transfer algorithm has been constructed based on accurate and efficient methods for use in coupled stratospheric models of chemistry, dynamics, and radiative transfer. The individual components of our code are validated here against available line-by-line calculations. Compared to line-by-line calculations, the total IR heating has an accuracy of 10% in the stratosphere and the errors are less than 0.07 K day−1 in the lower stratosphere. Our result happens to have radiative heating and cooling rates that are globally balanced to about 0.1 K day−1 in the lower stratosphere. Furthermore, the net heating rate in the tropical lower stratosphere, which controls the strength of the important Brewer–Dobson circulation, is found to be weaker than previously thought, with important implications for the global distribution of chemical species. In an attempt to explain the differences among existing models, a set of nine case runs is performed with different input datasets of temperature and ozone, with various degrees of degradation of accurate methods and physical parameterizations, and with different numerical implementations. Although it is difficult to perform a true intercomparison of existing models based solely on published material, one finds, based on experiments conducted using our model, that some commonly adopted approximations in IR schemes and physical parameterizations tend generally to increase the tropical net heating in the lower stratosphere by over a factor of 2 and also to significantly increase the global radiative imbalance at other heights.

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Xianyao Chen
,
John M. Wallace
, and
Ka-Kit Tung

Abstract

Empirical orthogonal function (EOF) analysis of global sea surface temperature yields modes in which interannual variability associated with ENSO and the lower-frequency variability associated with the Pacific decadal oscillation (PDO) and the Atlantic multidecadal oscillation (AMO) are confounded with one another and with the signature of global warming. The confounded EOFs exhibit overlapping centers of action with polarities of the perturbations juxtaposed such that the respective modes are mutually orthogonal in the global domain. When physical modes with different time scales appear in the same pair of EOFs, the principal component (PC) time series tend to be positively correlated in one frequency band and negatively correlated in another. Mode mixing may be a reflection of sampling variability or it may reflect the lack of spatial orthogonality of the physical modes themselves. Using sequences of pairwise orthogonal rotations of selected PCs, it is possible, without recourse to filtering, to recover a global warming mode with a bland spatial pattern and a nearly linear upward trend, along with dynamical modes, each with its own characteristic time scale, that resemble ENSO, the PDO, and the AMO. Novel elements of this analysis include a rationale for choosing the optimal angle for pairwise rotation and a simple algorithm for eliminating mode mixing between the dynamical modes and the global warming mode by transferring the linear trends from the former to the latter.

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Mao-Chang Liang
,
Li-Ching Lin
,
Ka-Kit Tung
,
Yuk L. Yung
, and
Shan Sun

Abstract

The equilibrium climate sensitivity (ECS) has a large uncertainty range among models participating in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) and has recently been presented as “inherently unpredictable.” One way to circumvent this problem is to consider the transient climate response (TCR). However, the TCR among AR4 models also differs by more than a factor of 2. The authors argue that the situation may not necessarily be so pessimistic, because much of the intermodel difference may be due to the fact that the models were run with their oceans at various stages of flux adjustment with their atmosphere. This is shown by comparing multimillennium-long runs of the Goddard Institute for Space Studies model, version E, coupled with the Hybrid Coordinate Ocean Model (GISS-EH) and the Community Climate System Model, version 4 (CCSM4) with what were reported to AR4. The long model runs here reveal the range of variability (~30%) in their TCR within the same model with the same ECS. The commonly adopted remedy of subtracting the “climate drift” is ineffective and adds to the variability. The culprit is the natural variability of the control runs, which exists even at quasi equilibration. Fortunately, for simulations with multidecadal time horizon, robust solutions can be obtained by branching off thousand-year-long control runs that reach “quasi equilibration” using a new protocol, which takes advantage of the fact that forced solutions to radiative forcing forget their initial condition after 30–40 yr and instead depend mostly on the trajectory of the radiative forcing.

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Mao-Chang Liang
,
Li-Ching Lin
,
Ka-Kit Tung
,
Yuk L. Yung
, and
Shan Sun

Abstract

Reducing climate drift in coupled atmosphere–ocean general circulation models (AOGCMs) usually requires 1000–2000 years of spinup, which has not been practical for every modeling group to do. For the purpose of evaluating the impact of climate drift, the authors have performed a multimillennium-long control run of the Goddard Institute for Space Studies model (GISS-EH) AOGCM and produced different twentieth-century historical simulations and subsequent twenty-first-century projections by branching off the control run at various stages of equilibration. The control run for this model is considered at quasi equilibration after a 1200-yr spinup from a cold start. The simulations that branched off different points after 1200 years are robust, in the sense that their ensemble means all produce the same future projection of warming, both in the global mean and in spatial detail. These robust projections differ from the one that was originally submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), which branched off a not-yet-equilibrated control run. The authors test various common postprocessing schemes in removing climate drift caused by a not-yet-equilibrated ocean initial state and find them to be ineffective, judging by the fact that they differ from each other and from the robust results that branched off an equilibrated control. The authors' results suggest that robust twenty-first-century projections of the forced response can be achieved by running climate simulations from an equilibrated ocean state, because memory of the different initial ocean state is lost in about 40 years if the forced run is started from a quasi-equilibrated state.

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Le Kuai
,
Run-Lie Shia
,
Xun Jiang
,
Ka Kit Tung
, and
Yuk L. Yung

Abstract

The authors examine the mechanism of solar cycle modulation of the Quasi-Biennial Oscillation (QBO) period using the Two-and-a-Half-Dimensional Interactive Isentropic Research (THINAIR) model. Previous model results (using 2D and 3D models of varying complexity) have not convincingly established the proposed link of longer QBO periods during solar minima. Observational evidence for such a modulation is also controversial because it is only found during the period from the 1960s to the early 1990s, which is contaminated by volcanic aerosols. In the model, 200- and 400-yr runs without volcano influence can be obtained, long enough to establish some statistical robustness. Both in model and observed data, there is a strong synchronization of the QBO period with integer multiples of the semiannual oscillation (SAO) in the upper stratosphere. Under the current level of wave forcing, the period of the QBO jumps from one multiple of SAO to another and back so that it averages to 28 months, never settling down to a constant period. The “decadal” variability in the QBO period takes the form of “quantum” jumps; these, however, do not appear to follow the level of the solar flux in either the observation or the model using realistic quasi-periodic solar cycle (SC) forcing. To understand the solar modulation of the QBO period, the authors perform model runs with a range of perpetual solar forcing, either lower or higher than the current level. At the current level of solar forcing, the model QBO period consists of a distribution of four and five SAO periods, similar to the observed distribution. This distribution changes as solar forcing changes. For lower (higher) solar forcing, the distribution shifts to more (less) four SAO periods than five SAO periods. The record-averaged QBO period increases with the solar forcing. However, because this effect is rather weak and is detectable only with exaggerated forcing, the authors suggest that the previous result of the anticorrelation of the QBO period with the SC seen in short observational records reflects only a chance behavior of the QBO period, which naturally jumps in a nonstationary manner even if the solar forcing is held constant, and the correlation can change as the record gets longer.

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Le Kuai
,
Run-Lie Shia
,
Xun Jiang
,
Ka-Kit Tung
, and
Yuk L. Yung

Abstract

It has often been suggested that the period of the quasi-biennial oscillation (QBO) has a tendency to synchronize with the semiannual oscillation (SAO). Apparently the synchronization is better the higher up the observation extends. Using 45 yr of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data of the equatorial stratosphere up to the stratopause, the authors confirm that this synchronization is not just a tendency but a robust phenomenon in the upper stratosphere. A QBO period starts when a westerly SAO (w-SAO) descends from the stratopause to 7 hPa and initiates the westerly phase of the QBO (w-QBO) below. It ends when another w-SAO, a few SAO periods later, descends again to 7 hPa to initiate the next w-QBO. The fact that it is the westerly but not the easterly SAO (e-SAO) that initiates the QBO is also explained by the general easterly bias of the angular momentum in the equatorial stratosphere so that the e-SAO does not create a zero-wind line, unlike the w-SAO. The currently observed average QBO period of 28 months, which is not an integer multiple of SAO periods, is a result of intermittent jumps of the QBO period from four SAO to five SAO periods. The same behavior is also found in the Two and a Half Dimensional Interactive Isentropic Research (THINAIR) model. It is found that the nonstationary behavior in both the observation and model is caused not by the 11-yr solar-cycle forcing but by the incompatibility of the QBO’s natural period (determined by its wave forcing) and the “quantized” period determined by the SAO. The wave forcing parameter for the QBO period in the current climate probably lies between four SAO and five SAO periods. If the wave forcing for the QBO is tuned so that its natural period is compatible with the SAO period above (e.g., at 24 or 30 months), nonstationary behavior disappears.

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