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Liguang Wu and Bin Wang

Abstract

A new approach is proposed to assess the possible impacts of the global climate change on tropical cyclone (TC) tracks in the western North Pacific (WNP) basin. The idea is based on the premise that the future change of TC track characteristics is primarily determined by changes in large-scale environmental steering flows and in formation locations.

It is demonstrated that the main characteristics of the current climatology of TC tracks can be derived from the climatological mean velocity field of TC motion by using a trajectory model. The climatological mean velocity of TC motion, composed of the large-scale steering and beta drift, is determined on each grid of the basin. The mean large-scale steering flow is computed from the NCEP–NCAR reanalysis for the current climate state. The mean beta drift is estimated from the best-track data by removing the steering flow. The derived mean beta drift agrees well with the results of previous observational and numerical studies in terms of its direction and magnitude.

The approach is applied to assessing the potential impacts of global warming on TC tracks in the WNP. The possible changes in the large-scale steering flows are taken from the output wind fields of two Geophysical Fluid Dynamics Laboratory (GFDL) global warming experiments and possible changes in the TC formation locations are considered by shifting the formation locations as a whole. The GFDL experiments suggested that the changes in the future large-scale steering flows are dominated by the easterly anomalies in the Tropics and westerly anomalies in the midlatitudes with the enhanced northward component during the period of 2030–59. Based on the assessments using two different ways to reduce climate model biases, the prevailing TC tracks shift slightly southwestward during the period of 2000–29, but northeastward during the period of 2030–59. More TCs will take a recurving track and move northeastward during the period of 2030–59. The El Niño–like climate change predicted in many climate models can significantly enhance the track changes if the TC formation locations in the WNP shift eastward as a whole.

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Bin Wang and Tianming Li

Abstract

The tropical atmosphere model presented here is suitable for modeling both the annual cycle and short-term (monthly to decadal time scale) climate fluctuations in sole response to the thermal forcing from the underlying surface, especially the ocean surface. The present model consists of a well-mixed planetary boundary layer and a free troposphere represented by the gravest baroclinic mode. The model dynamics involves active interactions between the boundary-layer flow driven by the momentum forcing associated with sea surface temperature (SST) gradient and the free tropospheric flow stimulated by diabatic heating that is controlled by the thermal effects of SST. This process is demonstrated to be essential for modeling Pacific basinwide low-level circulations. The convective heating is parameterized by a SST-dependent conditional heating scheme based upon the proposition that the potential convective instability increases with SST in a nonlinear fashion.

The present model integrates the virtue of a Gill-type model with that of a Lindzen–Nigam model and is capable of reproducing both the shallow intertropical convergence zone (ITCZ) in the boundary layer and the deep South Pacific convergence zone (SPCZ) and monsoon troughs in the lower troposphere. The precipitation pattern and intensity, the trade winds and associated subtropical highs, and the near-equatorial trough can also be simulated reasonably well.

The thermal contrast between oceans and continents is shown to have a profound influence on the circulation near landmasses. Changes in land surface temperature, however, do not exert significant influence on remote oceanic regions. Both the ITCZ and SPCZ primarily originate from the inhomogeneity of ocean surface thermal conditions. The continents of South and North America contribute to the formation of these oceanic convergence zones through indirect boundary effects that support coastal upwelling changing the SST distribution. The diagnosis of observed surface wind and pressure fields indicates that the nonlinear advection of momentum is generally negligible, even near the equator, in the boundary-layer momentum balance. The large SST gradients in the subtropics play an important role in forcing rotational and cross-isobaric winds.

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Xiouhua Fu and Bin Wang

Abstract

An air–sea–land coupled model of intermediate complexity was used to reveal the important roles of air–sea coupling and adjacent continental monsoons (i.e., American monsoons and Asian–Australian monsoons) on the annual cycle and mean state of the equatorial Pacific.

Excluding the effects of adjacent continental monsoons, the simulated mean SST in the western Pacific displays a warm bias; the SST seasonal cycle exhibits an erroneous, dominant annual component in the western Pacific, and insufficient strength and a 2-month phase delay in the equatorial eastern Pacific. The air–sea coupling alone cannot sustain the full strength of the annual marches of the ITCZ/cold tongue complex. This is because the diabatic heating associated with the ITCZ rainfall generates both a southerly and a westerly component to its equatorward side; while the southerly cools the cold tongue establishing a positive feedback to enhance the ITCZ, and the equatorial westerly favors cold tongue warming inducing a negative feedback that offsets the effect of the southerly component.

Including the influences from the adjacent continental monsoons significantly improves the simulations of the mean state and annual cycle of the equatorial Pacific. The Asian–Australian monsoons are found to improve the mean SST through enhancing the strength of the trades and to yield a correct semiannual cycle of surface wind speed and SST in the equatorial western Pacific. However, they have little influence on the annual cycle in the eastern Pacific SST. In contrast, the South American monsoon exerts profound impacts on the annual variations of the southeast trades and SST in the eastern Pacific, but not the mean SST. The Colombian and North American continental monsoons have little impact on the annual cycle of SST in the cold tongue.

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Fei Liu and Bin Wang

Abstract

During late boreal summer (July–October), the intraseasonal oscillation (ISO) exhibits maximum variability over the western North Pacific (WNP) centered in the South China Sea and Philippine Sea, but many numerical models have difficulty in simulating this essential feature of the ISO. To understand why this maximum variability center exists, the authors advance a simple box model to elaborate the potential contribution of the mean-state-dependent atmosphere–ocean interaction. The model results suggest that the WNP seasonal mean monsoon trough plays an essential role in sustaining a strong stationary ISO, contributing to the existence of the maximum intraseasonal variability center. First, the monsoon trough provides abundant moisture supply for the growing ISO disturbances through the frictional boundary layer moisture convergence. Second, the cyclonic winds associated with the monsoon trough provide a favorable basic state to support a negative atmosphere–ocean thermodynamic feedback that sustains a prominent stationary ISO. In an active phase of the ISO, anomalous cyclonic winds enhance the monsoon trough and precipitation, which reduce shortwave radiation flux and increase evaporation; both processes cool the sea surface and lead to an ensuing high pressure anomaly and a break phase of the ISO. In the wintertime, however, the wind–evaporation feedback is positive and sustains the Philippine Sea anticyclone. The result here suggests that accurate simulation of the boreal summer climatological mean state is critical for capturing a realistic ISO over the WNP region.

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Haiyan Teng and Bin Wang

Abstract

A finite-domain wavenumber–frequency analysis was proposed to objectively measure the interannual variability of the boreal summer intraseasonal oscillation (ISO) in the Asian–Pacific region. The strongest interannual variations of the ISO are found in the off-equatorial western North Pacific (WNP). In summers when El Niño is developing, both the westward- and northward-propagating waves with periods of 15–40 and 8–10 days are enhanced in July–October. The northward-propagating ISO in the Indian summer monsoon region, however, has little linkage with El Niño–Southern Oscillation (ENSO).

ENSO affects the northwestward-propagating ISO mode in the WNP through changing the mean circulation. During July–October in the El Niño developing year, the easterly vertical shears over the tropical western Pacific are considerably increased, which in turn promote development and northwestward emanation of Rossby waves away from the equatorial western-central Pacific, reinforcing the WNP ISO. In the Indian summer monsoon region, the ENSO-induced circulation changes are too weak to significantly modify the strong easterly sheared monsoon mean circulation. Therefore, the northward-propagating ISO is insensitive to ENSO.

Unlike the wintertime Madden–Julian oscillation (MJO), which is uncorrelated with ENSO, the May–July MJO is strengthened during El Niño developing years. The questions of why there is a seasonal dependence of the MJO–ENSO relationship and how ENSO directly affects the May–July MJO require further investigations.

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Tianming Li and Bin Wang

Abstract

The development and movement of the tropical intraseasonal system (TIS) exhibit remarkable annual variations. It was hypothesized that spatial and temporal variation in sea surface temperature (SST) is one of the primary climatic factors that are responsible for the annual variation of TISs. This paper examines possible influences of SST on the TIS through numerical experiments with a 2.5-layer atmospheric model on an equatorial β plane, in which SST affects atmospheric heating via control of the horizontal distribution of moist static energy and the degree of convective instability.

The gradient of the antisymmetric (with respect to the equator) component of SST causes a southward propagation of the model TIS toward northern Australia in boreal winter and a northward propagation over the Indian and western Pacific Oceans in boreal summer. The phase speed of the meridional propagation increases with the magnitude, of antisymmetric SST gradients. The poleward propagation of the equatorial disturbance takes the form of moist antisymmetric Rossby modes and influences the summer monsoon.

During May when SST is most symmetric in the western Pacific, a disturbance approaching the date line may evolve into westward-moving, double cyclonelike, symmetric Rossby modes due to the suppression of the moist Kelvin mode by the cold ocean surface cast of the date line. The disturbance over the equatorial Indian Ocean, however, may evolve into an eastward-moving, moist Kelvin–Rossby wave packet; meanwhile, a cyclonic circulation may be induced over the Gulf of Thailand and Malaysia, drifting slowly westward into the Indian subcontinent.

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Bin Wang and Xiaofan Li

Abstract

Tropical cyclone propagation (the beta drift) is driven by a secondary circulation associated with axially asymmetric gyres (beta gyres) in the vicinity of the cyclone center. In the presence of the beta effect, the environmental flow may interact with the symmetric circulation and beta gyres of the cyclone, affecting the development of the gyres and thereby the cyclone propagation. An energetics analysis is carried out to elucidate the development mechanism of the beta gyres and to explain variations in propagation speed of a barotropic cyclone embedded in a meridionally varying zonal flow on a beta plane. Two types of zonal flows are considered: one with a constant meridional shear resembling those in the vicinity of a subtropical ridge or a monsoon trough, and the other with a constant relative vorticity gradient as in the vicinity of an easterly (westerly) jet.

Zonal flow with a constant meridional shear changes the generation rate of the gyre kinetic energy through an exchange of energy directly with the gyres. The momentum flux associated with gyres acting on the meridional shear of zonal flow accounts for this energy conversion process. Zonal flow with an anticyclonic (cyclonic) shear feeds (extracts) kinetic energy to (from) the gyres. The magnitude of this energy conversion is proportional to the strength of the meridional shear and the gyre intensity. As a result, the gyres are stronger and the beta drift is faster near a subtropical ridge (anticyclonic shear) than within a monsoon trough (cyclonic shear).

Zonal flow with a constant relative vorticity gradient affects gyre intensity via two processes that have opposing effects. A southward vorticity gradient, on the one hand, weakens the gyres by reducing the energy conversion rate from symmetric circulation to gyres; on the other hand, it enhances the gyres by indirectly feeding energy to the symmetric circulation, whose strengthening in turn accelerates the energy conversion from symmetric circulation to gyres. The effect of the second process tends to eventually become dominant.

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Guosen Chen and Bin Wang

Abstract

Current theoretical studies have a debate on whether the Madden–Julian oscillation (MJO) has a zero or westward group velocity. A recent analysis of the observed Hovmöller diagram of MJO signals suggested that the MJO has a significant westward group velocity. Here it is shown that the observed MJO has a negligibly small group velocity, which is manifested in two aspects. First, on the wavenumber–frequency spectra diagram the precipitation spectra indicate quasi independence of the MJO frequency on wavenumber, suggesting a nearly vanishing group velocity. Second, on the Hovmöller diagram of the regressed intraseasonal daily precipitation, the MJO group velocity is defined by the propagation of the wave envelopes of the precipitation and is shown to be negligibly small for the eastward propagating signals. The causes of the discrepancy between this study and the recent study mentioned above are the calculating method and the data filtering process. The group velocity in the recent study is calculated by the propagation of local convection extrema, which does not necessarily indicate the propagation of the wave envelopes. More importantly, the westward propagation of the local convection extrema is an artifact of the data filtering. The Hovmöller diagram in the recent study was constructed by using only the eastward propagating wavenumber-1–5 signals. This truncation of data onto the planetary scales of the eastward wavenumber domain fails to resolve the Maritime Continent “barrier effect,” causing significant artificial westward propagation of local convection extrema.

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Xiouhua Fu and Bin Wang

Abstract

This study assesses the impact of stratiform rainfall (i.e., large-scale rainfall) in the development and maintenance of the Madden–Julian oscillation (MJO) in a contemporary general circulation model: ECHAM4 AGCM and its coupled version. To examine how the model MJO would change as the stratiform proportion (the ratio of the stratiform versus total rainfall) varies, a suite of sensitivity experiments has been carried out under a weather forecast setting and with three 20-yr free integrations. In these experiments, the detrainment rates of deep/shallow convections that function as a water supply to stratiform clouds were modified, which results in significant changes of stratiform rainfall.

Both the forecast experiments and long-term free integrations indicate that only when the model produces a significant proportion (≥30%) of stratiform rainfall can a robust MJO be sustained. When the stratiform rainfall proportion becomes small, the tropical rainfall in the model is dominated by drizzle-like regimes with neither eastward-propagating nor northward-propagating MJO being sustained.

It is found that the latent heat release of stratiform rainfall significantly warms up the upper troposphere. The covariability between the heating and positive temperature anomaly produces eddy available potential energy that sustains the MJO against dissipation and also allows the direct interaction between the precipitation heating and large-scale low-frequency circulations, which is critical to the development and maintenance of the MJO. This finding calls for better representations of stratiform rainfall and its connections with the convective component in GCMs in order to improve their simulations of the MJO.

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Tianming Li and Bin Wang

Abstract

Diagnosis of the dynamic and thermodynamic balances using observed climatological monthly mean data reveals that 1) anisotropic, latitude-dependent Rayleigh friction coefficients lead to much improved modeling of the monthly mean surface wind field for a given monthly mean sea level pressure field, and 2) the annual variation of the vertically averaged lapse rate is important for modeling sea level pressure.

Based on the aforementioned observations, a thermodynamic equilibrium climate model for the tropical Pacific is proposed. In this model, the sea level pressure is thermodynamically determined from sea surface temperature (SST) through a vertically integrated hydrostatic equation in which the vertical mean lapse rate is a function of SST plus a time-independent correction. The surface winds are then computed from sea level pressure gradients through a linear surface momentum balance with anisotropic, latitude-dependent Rayleigh friction coefficients. The precipitation is finally obtained from a moisture budget by taking into account the effects of SST on convective instability.

Despite its simplicity, the model is capable of simulating realistic annual cycles as well as interannual variations of the surface wind, sea level pressure, and precipitation over the tropical Pacific. The success of the model suggests that the tropical atmosphere on a monthly mean time scale is, to the lowest-order approximation, in a thermodynamic equilibrium state in which sea level pressure is primarily controlled by SST and the effects of dynamic feedback on sea level pressure may be parameterized by an empirical SST-lapse rate relationship. Further studies are needed to establish a firm physical basis for the proposed parameterization.

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