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Aiguo Dai
and
Junhong Wang

Abstract

Global surface pressure data from 1976 to 1997 from over 7500 land stations and the Comprehensive Ocean–Atmosphere Data Set have been analyzed using harmonic and zonal harmonic methods. It is found that the diurnal pressure oscillation (S 1) is comparable to the semidiurnal pressure oscillation (S 2) in magnitude over much of the globe except for the low-latitude open oceans, where S 2 is about twice as strong as S 1. Over many land areas, such as the western United States, the Tibetan Plateau, and eastern Africa, S 1 is even stronger than S 2. This is in contrast to the conventional notion that S 2 predominates over much of the globe. The highest amplitudes (∼1.3 mb) of S 1 are found over northern South America and eastern Africa close to the equator. Here S 1 is also strong (∼1.1 mb) over high terrain such as the Rockies and the Tibetan Plateau. The largest amplitudes of S 2 (∼1.0–1.3 mb) are in the Tropics over South America, the eastern and western Pacific, and the Indian Ocean. Here S 1 peaks around 0600–0800 LST at low latitudes and around 1000–1200 LST over most of midlatitudes, while S 2 peaks around 1000 and 2200 LST over low- and midlatitudes. Here S 1 is much stronger over the land than over the ocean and its amplitude distribution is strongly influenced by landmasses, while the land–sea differences of S 2 are small. The spatial variations of S 1 correlate significantly with spatial variations in the diurnal temperature range at the surface, suggesting that sensible heating from the ground is a major forcing for S 1. Although S 2 is much more homogeneous zonally than S 1, there are considerable zonal variations in the amplitude of S 2, which cannot be explained by zonal variations in ozone and water vapor. Other forcings such as those through clouds’ reflection and absorption of solar radiation and latent heating in convective precipitation are needed to explain the observed regional and zonal variations in S 2. The migrating tides S 1 1 and S 2 2 predominate over other zonal wave components. However, the nonmigrating tides are substantially stronger than previously reported. The amplitudes of both the migrating and nonmigrating tides decrease rapidly poleward with a slower pace at middle and high latitudes.

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Dehai Luo
,
Yao Yao
, and
Aiguo Dai

Abstract

Both the positive and negative phases of the North Atlantic Oscillation (NAO+ and NAO, respectively) and atmospheric blocking in the Euro-Atlantic sector reflect synoptic variability over the region and thus are intrinsically linked. This study examines their relationship from a decadal change perspective. Since the winter-mean NAO index is defined as a time average of instantaneous NAO indices over the whole winter, it is unclear how the activity of European blocking (EB) events can be related to the variation of the positive mean NAO index. Here, this question is examined by dividing the winter period 1978–2011 into two decadal epochs: 1978–94 (P1) with an increasing and high NAO index and 1995–2011 (P2) with a decreasing and low NAO index. Using atmospheric reanalysis data, it is shown that there are more intense and persistent EB events in eastern Europe during P1 than during P2, while the opposite is true for western Europe.

It is further shown that there are more NAO+ (NAO) events during P1 (P2). The EB events associated with NAO+ events extend more eastward and are associated with stronger Atlantic mean zonal wind and weaker western Atlantic storm track during P1 than during P2, but EB events associated with NAO events increase in western Europe under opposite Atlantic conditions during P2. Thus, the increase in the number of individual NAO+ (NAO) events results in more EB events in eastern (western) Europe during P1 (P2). The EB change is also associated with the increased frequency of NAO to NAO+ (NAO+ to NAO) transition events.

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Dehai Luo
,
Yao Yao
, and
Aiguo Dai

Abstract

In Part I of this study, it is revealed that decadal variations of European blocking, in its intensity, duration, and position, during 1978–2011 are modulated by decadal changes in the frequency of North Atlantic Oscillation (NAO) events associated with background Atlantic conditions. In Part II, reanalysis data are analyzed to first show that a T-bone-type structure of the climatological-mean blocking frequency in the Euro-Atlantic sector roughly results from a combination of the blocking frequency distributions along the southeast–northwest (SE–NW) direction associated with negative-phase NAO (NAO) events and along the southwest–northeast (SW–NE) direction associated with positive-phase NAO (NAO+) events.

A nonlinear multiscale interaction (NMI) model is then used to examine the physical processes behind the blocking frequency distributions. This model shows that the combination of eastward- and westward-displaced blocking frequency patterns along the SW–NE and SE–NW directions associated with NAO+ and NAO events leads to a T-bone-type frequency distribution, as seen in reanalysis data. Moreover, it is found that the westward migration of intense, long-lived blocking anomalies over Europe following NAO+ events is favored (suppressed) when the Atlantic mean zonal wind is relatively weak (strong). This result is held for the strong (weak) western Atlantic storm track. This helps explain the findings in Part I. In particular, long-lived blocking events with double peaks can form over Europe because of reintensification during the NAO+ decay phase, when the mean zonal wind weakens. But the double-peak structure disappears and becomes a strong single-peak structure as the mean zonal wind strengthens.

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Dehai Luo
,
Wenqi Zhang
,
Linhao Zhong
, and
Aiguo Dai

Abstract

In this paper, an extended nonlinear multiscale interaction model of blocking events in the equivalent barotropic atmosphere is used to investigate the effect of a slowly varying zonal wind in the meridional direction on dipole blocking that is regarded as a nonlinear Rossby wave packet. It is shown that the meridional gradient of potential vorticity ( PV y = PV / y ) prior to the blocking onset, which is related to the background zonal wind and its nonuniform meridional shear, can significantly affect the lifetime, intensity, and north–south asymmetry of dipole blocking, while the blocking dipole itself is driven by preexisting incident synoptic-scale eddies. The magnitude of the background PV y determines the energy dispersion and nonlinearity of blocking. It is revealed that a small background PV y is a prerequisite for strong and long-lived eddy-driven blocking that behaves as a persistent meandering westerly jet stream, while the blocking establishment further reduces the PV y within the blocking region, resulting in a positive feedback between blocking and PV y . When the core of the background westerly jet shifts from higher to lower latitudes, the blocking shows a northwest–southeast-oriented dipole with a strong anticyclonic anomaly to the northwest and a weak cyclonic anomaly to the southeast as its northern pole moves westward more rapidly and has weaker energy dispersion and stronger nonlinearity than its southern pole because of the smaller PV y in higher latitudes. The opposite is true when the background jet shifts toward higher latitudes. The asymmetry of dipole blocking vanishes when the background jet shows a symmetric double-peak structure. Thus, a small prior PV y is a favorable precursor for the occurrence of long-lived and large-amplitude blocking.

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Dehai Luo
,
Yao Ge
,
Wenqi Zhang
, and
Aiguo Dai

Abstract

In this paper, reanalysis data are first analyzed to reveal that the individual negative (positive)-phase Pacific–North American pattern (PNA) or PNA (PNA+) has a lifetime of 10–20 days, is characterized by strong (weak) westerly jet stream meanders, and exhibits clear wave train structures, whereas the PNA with rapid retrogression tends to have longer lifetime and larger amplitude than the PNA+ with slow retrogression. In contrast, the wave train structure of the North Atlantic Oscillation (NAO) is less distinct, and the positive (negative)-phase NAO shows eastward (westward) movement around a higher latitude than the PNA. Moreover, it is found that the PNA wave train occurs under a larger background meridional potential vorticity gradient (PV y ) over the North Pacific than that over the North Atlantic for the NAO. A unified nonlinear multiscale interaction (UNMI) model is then developed to explain why the PNA as a nonlinear wave packet has such characteristics and its large difference from the NAO. The model results reveal that the larger background PV y for the PNA (due to its location at lower latitudes) leads to its larger energy dispersion and weaker nonlinearity than the NAO, thus explaining why the PNA (NAO) is largely a linear (nonlinear) process with a strong (weak) wave train structure, though it is regarded as a nonlinear initial-value problem. The smaller PV y for the PNA than for the PNA+ leads to lower energy dispersion and stronger nonlinearity for PNA, which allows it to maintain larger amplitude and have a longer lifetime than the PNA+. Thus, the difference in the background PV y is responsible for the asymmetry between the two phases of PNA and the difference between the PNA and NAO.

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Curt Covey
,
Aiguo Dai
,
Dan Marsh
, and
Richard S. Lindzen

Abstract

Although atmospheric tides driven by solar heating are readily detectable at the earth’s surface as variations in air pressure, their simulations in current coupled global climate models have not been fully examined. This work examines near-surface-pressure tides in climate models that contributed to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC); it compares them with tides both from observations and from the Whole Atmosphere Community Climate Model (WACCM), which extends from the earth’s surface to the thermosphere. Surprising consistency is found among observations and all model simulations, despite variation of the altitudes of model upper boundaries from 32 to 76 km in the IPCC models and at 135 km for WACCM. These results are consistent with previous suggestions that placing a model’s upper boundary at low altitude leads to partly compensating errors—such as reducing the forcing of the tides by ozone heating, but also introducing spurious waves at the upper boundary, which propagate to the surface.

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Curt Covey
,
Aiguo Dai
,
Richard S. Lindzen
, and
Daniel R. Marsh

Abstract

For atmospheric tides driven by solar heating, the database of climate model output used in the most recent assessment report of the Intergovernmental Panel on Climate Change (IPCC) confirms and extends the authors’ earlier results based on the previous generation of models. Both the present study and the earlier one examine the surface pressure signature of the tides, but the new database removes a shortcoming of the earlier study in which model simulations were not strictly comparable to observations. The present study confirms an approximate consistency among observations and all model simulations, despite variation of model tops from 31 to 144 km. On its face, this result is surprising because the dominant (semidiurnal) component of the tides is forced mostly by ozone heating around 30–70-km altitude. Classical linear tide calculations and occasional numerical experimentation have long suggested that models with low tops achieve some consistency with observations by means of compensating errors, with wave reflection from the model top making up for reduced ozone forcing. Future work with the new database may confirm this hypothesis by additional classical calculations and analyses of the ozone heating profiles and wave reflection in Coupled Model Intercomparison Project (CMIP) models. The new generation of models also extends CMIP's purview to free-atmosphere fields including the middle atmosphere and above.

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