Quasi-biweekly Oscillation Research Articles

Prediction of global patterns of dominant quasi-biweekly oscillation by the NCEP Climate Forecast System version 2

Daily output from the hindcasts by the National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) is analyzed to understand the skill of forecasting atmospheric variability on quasi-biweekly (QBW) time scale. Eight dominant quasi-biweekly oscillation (QBWO) modes identified by the extended empirical orthogonal function analysis are focused. In the CFSv2, QBW variability exhibits a significant weakening tendency with lead time for all seasons. For most QBWO modes, the variance drops to only 50 % of the initial value at lead time of 11–15 days. QBW variability has better prediction skill in the winter hemisphere than in the summer hemisphere. Skillful forecast can reach about 10–15 days for most modes but those in the winter hemisphere have better forecast skills. Among the eight QBWO modes, the North Pacific mode and the South Pacific (SP) mode have the highest forecast skills while the Asia–Pacific mode and the Central American mode have the lowest skills. For the Asia–Pacific and Central American modes, the forecasted QBWO phase shows an obvious eastward shift with increase in lead time compared to observations, indicating a smaller propagating speed. However, the predicted feature for the SP mode is more realistic. Air–sea coupling on the QBW time scale is perhaps responsible for the different prediction skills for different QBWO modes. In addition, most QBWO modes have better forecasting skills in El Nino years than in La Nina years. Different dynamical mechanisms for various QBWO modes may be partially responsible for the differences in prediction skill among different QBWO modes.

Sudden Tropical Cyclone Track Changes over the Western North Pacific: A Composite Study

Abstract Tropical cyclones (TCs) over the western North Pacific (WNP) are usually embedded in the multitime-scale summer monsoon circulation and occasionally experience sudden track changes, which are currently a challenge in TC forecasting. A composite analysis of 15 sudden north-turning cases and 14 west-turning cases that occurred during the period 2000–10 was conducted with a focus on influences of low-frequency monsoon circulations. It is found that TCs in the two specific categories of track changes are embedded in a monsoon gyre of about 2500 km in diameter on the quasi-biweekly oscillation (QBW) time scale, which is also embedded in a larger-scale cyclonic gyre or monsoon trough on the Madden–Julian oscillation (MJO) time scale. The two types of track changes are closely associated with interaction between low-frequency and synoptic flows. Two different types of asymmetric flow patterns are identified on the synoptic time scale in the vicinity of these TCs. In the north-turning case, enhanced winds lie mainly on the southeast side of TCs due to strong ridging associated with interactions between low-frequency and synoptic flows. In the west-turning case, the westward extension of the subtropical high leads to ridging on the northwest side of TCs and the enhanced winds can largely offset the steering of enhanced southwesterly winds on the synoptic time scale. Thus the north-turning (west turning) sudden track changes are affected primarily by the synoptic-scale (low frequency) steering. This may be one of the reasons for the larger forecasting errors in the north-turning case than in the west-turning case.

Impact of the quasi‐biweekly oscillation over the western North Pacific on East Asian subtropical monsoon during early summer

The impact of quasi‐biweekly oscillation (QBWO) over the western North Pacific on East Asian summer monsoon (EASM) is investigated. The life cycle of QBWO is divided into eight phases defined by the two leading principal components (PC1 and PC2) of an empirical orthogonal function analysis. Subtropical rainfall shows significant changes, with a northwestward propagation of convection from equatorial regions to the South China Sea (SCS). The most significant variations occur in QBWO phases 3 and 4 (enhanced convection over SCS) and phases 7 and 8 (reduced convection over SCS). The East Asia Mei‐yu significantly decreases in QBWO phases 3 and 4 but increases in phases 7 and 8. The QBWO influences EASM through modulating the subtropical monsoon flow and extratropical circulation. The response of lower tropospheric atmosphere to QBWO shows a northwestward propagation and a downstream wave train that extends northward into the western North Pacific, modulating the SCS monsoon trough and the EASM flow associated with moisture transportation. The mid‐tropospheric extratropical circulation and the western Pacific subtropical high also show obvious changes accompanying QBWO evolution, resulting in circulation patterns associated with cold air activity. Moreover, changes with QBWO are found in the upper tropospheric East Asian westerly jet stream and the South Asian high, and these changes contribute to upper level divergence over subtropical East Asia.

Modulation of Western North Pacific Tropical Cyclone Activity by the ISO. Part I: Genesis and Intensity

Abstract This study investigates the intraseasonal variability of tropical cyclones (TCs) by systematically examining the two major components of the intraseasonal oscillation (ISO), the 30–60-day Madden–Julian oscillation (MJO) and the 10–20-day quasi-biweekly oscillation (QBWO). Results suggest that these two ISO modes exhibit different origins, spatial scales, and propagation characteristics, which result in distinctive TC modulation in the western North Pacific Ocean (WNP). The northeastward-propagating MJO predominantly controls the basinwide TC frequency. The significant increase (reduction) in cyclogenesis in the convective (nonconvective) phase is found to be associated with the concomitant strengthening (weakening) of the monsoon trough. In addition, the large contrast in TC frequency also results in a significant difference in daily accumulated cyclone energy (ACE) between the convective and nonconvective MJO phases. The northwestward-propagating QBWO, in contrast, is characterized by alternating signals of positive and negative convection. It leads to the opposite TC modulation in the WNP1 (0°–30°N, 120°–150°E) and WNP2 (0°–30°N, 150°E–180°) regions and results in a northwestward shift in TC genesis locations, which in turn causes substantial differences in intensity distribution and daily ACE for different QBWO phases. Finally, a brief examination of the dual mode situation suggests that the QBWO generally exerts modulation upon the background MJO, and the modulation seems to vary under different MJO conditions.

Modulation of Western North Pacific Tropical Cyclone Activity by the ISO. Part II: Tracks and Landfalls

Abstract This study investigates how tropical cyclone (TC) tracks and landfalls are modulated by the two major components of the intraseasonal oscillation (ISO), the 30–60-day Madden–Julian oscillation (MJO) and the 10–20-day quasi-biweekly oscillation (QBWO). In the convective phases of the MJO (phases 7 + 8 and 1 + 2), the western North Pacific Ocean (WNP) is mainly clustered with westward- and northwestward-moving TCs. The strong easterlies (southeasterlies) in the southern flank of the subtropical high lead to an increase in TC activity and landfalls in the Philippines and Vietnam (China and Japan) in phase 7 + 8 (phase 1 + 2). In the nonconvective phases (phases 3 + 4 and 5 + 6), TCs change from the original straight-moving type to the recurving type, such that the tendency for landfalls is significantly reduced. The QBWO, on the other hand, has a significant influence on TC landfalls in the Philippines and Japan. The strengthening of the subtropical high in phase 1 + 2 favors the development of westward-moving TCs and results in an increase in landfalls in the Philippines, while in phase 3 + 4 (phase 5 + 6), there is an increase (decrease) in TC activity and landfalls in Japan because of changes in genesis locations and large-scale circulations. The results herein suggest that both the MJO and QBWO exert distinctive impacts on TCs in the WNP.

Estimating the Predictability of the Quasi-Biweekly Oscillation Using the Nonlinear Local Lyapunov Exponent Approach

AbstractThe quasi-biweekly oscillation (QBWO) is a major intraseasonal variability (ISV) in the tropics. Based on bandpass-filtered outgoing longwave radiation (OLR) and wind field data, the predictability limits of the QBWO in boreal summer and boreal winter are investigated using the nonlinear local Lyapunov exponent (NLLE) approach. The analysis shows that the evolution of the mean error growth of the QBWO in boreal summer and the evolution of the mean error growth in boreal winter are comparable. Both curves exhibit rapid growth in the initial stage followed by a slowly fluctuating, ascending trend before saturation is reached. As a result, the potential predictability limits for the boreal summer QBWO are very close to those for the boreal winter QBWO, with a lead time of approximately three weeks. Given the current limitations in the simulation and prediction of ISV, including the QBWO, the results of this study provide a useful reference for assessing the predictability of the QBWO using model simu...

Observational Analysis of Sudden Tropical Cyclone Track Changes in the Vicinity of the East China Sea

Abstract An observational analysis of observed sudden typhoon track changes is conducted with a focus on the underlying mechanism and the possible role of slowly varying low-frequency flows. Four typhoons that took a generally northwestward track prior to sharply turning northeastward in the vicinity of the East China Sea are investigated. It is found that the sudden track changes occurred near the center of the Madden–Julian oscillation (MJO)-scale cyclonic circulation or at the bifurcation point of the steering flows at 700 hPa, and they were all associated with a well-developed quasi-biweekly oscillation (QBW)-scale gyre. Calculation of vorticity advection suggests that the peripheral ridging resulting from the interaction between the typhoons and the flows on the MJO and QBW scales can compress the typhoon circulation, leading to an area of high winds to the east or south of the typhoon center. The enhanced synoptic-scale winds shifted the typhoons northward and placed them in a northeastward orbit under the steering of the flows associated with the Pacific subtropical high. The sudden track change can be likened to the maneuvering of satellite orbit change in that the enhanced synoptic-scale winds act as a booster rocket to shift the typhoons northward to the southwesterly steering flows.

Propagation characteristic of atmospheric responses to abnormal warm SST in the Kuroshio Extension in winter

The propagation characteristic of atmospheric responses to the abnormal warm sea-surface temperature (SST) in the Kuroshio Extension in winter was investigated using National Centre for Atmospheric Research CAM3.0. The results show that geopotential height perturbations at 500 hPa occupy much of the mid- and high-latitude areas north of 20°N and are stronger in winter and spring than in summer. Power spectrum analysis reveals that the perturbations contain both quasi-biweekly and intraseasonal oscillations. In the latitude band with maximum perturbation amplitude, the oscillations propagate mainly eastward. The centers of dominant oscillations are situated in the mid- and higher-latitude areas north to 40°N. The perturbations in the Arctic mainly propagate meridionally, whereas those south of the Arctic propagate zonally, at a steady-phase velocity basically. The propagation characteristics of wind perturbations and temperature perturbations are similar to those of geopotential height perturbations.

Monsoonal Influence on Typhoon Morakot (2009). Part II: Numerical Study

Abstract In the second part of this study, numerical experiments are conducted to investigate the influences of multi-time-scale monsoonal flows on the track change of Typhoon Morakot (2009). While the control simulation captures the slowing and northward deflections in the vicinity of Taiwan Island, the highly asymmetric rainfall structure, and the associated rainfall pattern, sensitivity experiments suggest that the westward movement prior to the landfall on Taiwan and the subsequent northward shifts in the vicinity of Taiwan were closely associated with the interaction between Morakot and multi-time-scale monsoonal flows. Prior to the landfall on Taiwan, Morakot moved westward directly toward Taiwan because of a synoptic wave train–like pattern, which consisted of Goni over mainland China, Morakot, and a cyclone over the western North Pacific with an anticyclone to the west of Morakot. Numerical simulation suggests that strong northerly winds between Morakot and the anticyclone reduced the northward steering component associated with the low-frequency flow prior to the landfall. Numerical experiments confirm that the northward track shifts that occurred in the vicinity of Taiwan Island were a result of the coalescences of Morakot with a quasi-biweekly oscillation (QBW)-scale gyre prior to the landfall on Taiwan and a Madden–Julian oscillation (MJO)-scale gyre in the Taiwan Strait. The simulation of Morakot and the associated sensitivity experiments agree with the previous barotropic study that the interaction between tropical cyclones and low-frequency monsoon gyres can cause sudden changes in tropical cyclone tracks.

Monsoonal Influence on Typhoon Morakot (2009). Part I: Observational Analysis

AbstractTyphoon Morakot made landfall on Taiwan with a record rainfall of 3031.5 mm during 6–13 August 2009. While previous studies have emphasized the influence of southwesterly winds associated with intraseasonal oscillations and monsoon surges on moisture supply, the interaction between Morakot and low-frequency monsoon flows and the resulting influence on the slow movement and asymmetric precipitation structure of the typhoon were examined observationally.Embedded in multi-time-scale monsoonal flows, Morakot generally moved westward prior to its landfall on Taiwan and underwent a coalescence process first with a cyclonic gyre on the quasi-biweekly oscillation time scale and then with a cyclonic gyre on the Madden–Julian oscillation time scale. The coalescence enhanced the synoptic-scale southwesterly winds of Morakot and thus decreased its westward movement and turned the track northward, leading to an unusually long residence time in the vicinity of Taiwan. The resulting slow movement and collocation with the low-frequency gyres also maintained the major rainfall in southern Taiwan because the low-frequency flows played an important role in enhancing the winds on the southern side, especially during 6–9 August 2009. In addition to the lifting effect of the Taiwan terrain and the moisture supply associated with monsoon flows, the study suggests that the monsoonal influence maintained the major rainfall area in southern Taiwan through reducing the translation speed, shifting Morakot northward, and enhancing the low-frequency flows on the southern side of the typhoon. Since the enhanced low-frequency flows did not shift northward with the movement of Morakot, its primary rainfall expanded outward with time as the typhoon center moved northwestward after its landfall on Taiwan.

Characteristics of the Dominant Modes of Atmospheric Quasi-Biweekly Oscillation over Tropical–Subtropical Americas

Abstract During the boreal summer (June–August), vigorous convection appears over the eastern Pacific, southern Mexico, and northern South America, and oscillates on a distinct time scale of 10–20 days. Extended empirical orthogonal function (EEOF) analysis shows that the quasi-biweekly oscillation (QBWO) of the convection has two major modes: a west–east-orientated mode (WEM) and a north–south-orientated mode (NSM). The WEM, which is explained by the first two EEOF modes, originates over the eastern Atlantic, propagates westward along 15°N, and enhances over the Caribbean Sea before disappearing over the central Pacific. The NSM, explained by the third and fourth EEOF modes, originates over the western Pacific, moves eastward, and strengthens over the eastern Pacific. It shifts northward after arriving over the Caribbean Sea. Both modes have notable seasonal dependence, with the WEM more active in July and August and the NSM more active in June or earlier. The two distinct QBWO modes are linked to different rainfall patterns over the United States and Mexico. When the WEM is active in July and August, wet conditions occur over the southern central United States and dry conditions appear to the north. When the NSM is active in June, northern Mexico, the southwestern United States, the Missouri basin, and the northern Great Lakes are drier than normal, while southern Mexico and the eastern United States are wetter than normal. Significant variations in atmospheric circulation are found to be associated with the interannual variability of the NSM activity in June. However, these variations may not necessarily result from QBWO but, rather, provide a background for QBWO activity instead. In July and August, the association of QBWO with the precipitation pattern over North America may sometimes be related to hurricane activity.

Characteristics and origin of quasi‐biweekly oscillation over the western North Pacific during boreal summer

This study investigates the structure, energetic, and origin of quasi‐biweekly oscillation (QBWO) over the western North Pacific (WNP), using NCEP reanalyses for the years 2000–2007. In the context of vorticity there appears to be a significant QBWO mode over the WNP during the summer. QBWO emerges from the equatorial region and propagates northwestward. Its horizontal structure exhibits a slight southwest‐northeast tilt but mainly longitudinal elongation. In the vertical the QBWO has a northwest tilt with height that gives rise to a structure of the first baroclinic mode. The centers of vorticity and vertical motion near the equator show a phase lag of about one‐quarter wavelength, consistent with the characteristics of equatorial waves, whereas the cyclonic circulation is tightly coupled with anomalous convection as the wave moves away from the equator. Energetic analysis of the QBWO reveals that diabatic heating in the tropics and baroclinic processes in the subtropics play important roles in the generation of eddy available potential energy (EAPE). In turn, the conversion from EAPE to eddy kinetic energy (EKE) and the barotropic conversion are major sources for EKE to compensate the loss by EKE redistribution and dissipation. Tracing the QBWO to equatorial disturbances, our results show some features of equatorially trapped n = 1 Rossby mode, such as phase speed and group velocity. This mode is generally characterized by a zonal planetary wave number of about 6 and nearly symmetric circulation about the equator. A typical case from 2002 is chosen to illustrate that the origin of the QBWO is closely associated with the theoretical equatorial Rossby wave.

Structure and Origin of the Quasi-Biweekly Oscillation over the Tropical Indian Ocean in Boreal Spring

Abstract The structure and evolution features of the quasi-biweekly (10–20 day) oscillation (QBWO) in boreal spring over the tropical Indian Ocean (IO) are investigated using 27-yr daily outgoing longwave radiation (OLR) and the National Centers for Environment Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis data. It is found that a convective disturbance is initiated over the western IO and moves slowly eastward. After passing the central IO, it abruptly jumps into the eastern IO. Meanwhile, the preexisting suppressed convective anomaly in the eastern IO moves poleward in the form of double-cell Rossby gyres. The analysis of vertical circulation shows that a few days prior to the onset of local convection in the eastern equatorial IO an ascending motion appears in the boundary layer. Based on the diagnosis of the zonal momentum equation, a possible boundary layer–triggering mechanism over the eastern equatorial IO is proposed. The cause of the boundary layer convergence and vertical motion is attributed to the free-atmospheric divergence in association with the development of the barotropic wind. It is the downward transport of the background mean easterly momentum by perturbation vertical motion during the suppressed convective phase of the QBWO that leads to the generation of a barotropic easterly—the latter of which further causes the free-atmospheric divergence and, thus, the boundary layer convergence. The result suggests that the local process, rather than the eastward propagation of the disturbance from the western IO, is essential for the phase transition of the QBWO convection over the eastern equatorial IO.

Global Perspective of the Quasi-Biweekly Oscillation*

Abstract The quasi-biweekly oscillation (QBW: here defined as a 12–20-day oscillation) is one of the major systems that affect tropical and subtropical weather and seasonal mean climate. However, knowledge is limited concerning its temporal and spatial structures and dynamics, particularly in a global perspective. To advance understanding of the QBW, its life cycle is documented using a tracking method and extended EOF analysis. Both methods yield consistent results. The analyses reveal a wide variety of QBW activity in terms of initiation, movement, development, and dissipation. The convective anomalies associated with the QBW are predominant in the latitude bands between 10° and 30° in both hemispheres. The QBW modes tend to occur regionally and be associated with monsoons. Three boreal summer modes are identified in the Asia–Pacific, Central America, and subtropical South Pacific regions. Five austral summer modes are identified in the Australia–southwest Pacific, South Africa–Indian Ocean, South America–Atlantic, subtropical North Pacific, and North Atlantic–North Africa regions. The QBW modes are classified into two categories: westward- and eastward-propagating modes. The westward mode is found in the Asia–Pacific and Central America regions during boreal summer; it originates in the tropics and dissipates in the subtropics. The behavior of the westward-propagating mode can be understood in terms of equatorial Rossby waves in the presence of monsoon mean flow and convective coupling. The eastward-propagating mode, on the other hand, connects with upstream extratropical Rossby wave trains and propagates primarily eastward and equatorward. Barotropic Rossby wave trains play an essential role in controlling initiation, development, and propagation of the eastward QBW mode in the subtropics. The results therefore suggest that not only tropical but also extratropical dynamics are required for fully understanding the behavior of the QBW systems worldwide. The new conceptual picture of QBW obtained here based on long-term observation provides valuable information on the behavior of QBW systems in a global perspective, which is important for a thorough understanding of tropical variability on a time scale between day-to-day weather and the Madden–Julian oscillation.

Quasi-Biweekly Oscillation of the Convection around Sumatra and Low-Level Tropical Circulation in Boreal Spring

Abstract The quasi-biweekly oscillation (QBWO) of the tropical convection around Sumatra and its relation to the low-level circulation over the tropical Indian Ocean in boreal spring is investigated. From March to May, the convection over northern Sumatra increases continuously and oscillates with a pronounced period of 10–20 days. Time-lag cross correlations among the QBWOs of the convection, the apparent heat source, and winds in the lower troposphere reveal a possible mechanism of QBWO maintenance. In the strongest phase of the QBWO of the convection around Sumatra, there is an anomalous convective heating symmetric about the equator. The atmospheric Rossby wave response to the heating produces twin cyclones straddling the equator in the west of the convection area. The development of the twin cyclones induces an anomalous southerly north of the equator and a northerly south of the equator at 850 hPa, giving rise to the divergence of the low-level wind field, which weakens the convection around Sumatra. The weakening of the convection leads to the negative phase of convection. In the weakest phase, the Rossby wave response to the anomalous convective cooling produces twin anticyclones symmetric about the equator, resulting in the convergence of the low-level winds and, in turn, enhancing the convection around Sumatra. Consequently, the feedbacks among convection, the Rossby wave response, and the associated wind field at the lower troposphere may be important maintenance mechanisms of the tropical QBWO. The appearance of a tropical westerly is a crucial index of the Asian summer monsoon onset. In the northern equatorial region, the westerly first occurs just to the west of Sumatra, and then extends westward in boreal spring. The westerly around the equator associated with the Rossby wave response to the convective heating of the QBWO of the convection around Sumatra displays a notable intraseasonal feature, which may play an important role in modulating the process of the Asian summer monsoon onset.

Relationship between equatorial pressure oscillations and tropical cyclones landing over China

With daily reanalysis data by NCEP/NCAR and data of tropical cyclones landing over China from 1949 to 2005, the variation of low-frequency oscillations of equatorial pressure and their relationship with tropical cyclones landing over China in the summer half of the years (June through October) are studied for the 57 years, using spectral analysis and correlation analysis. The results show that the summertime equatorial pressure is mainly of periodic oscillations of 5–7 days and 10–30 days and the interannual variation of the intensity of its quasi-biweekly oscillation is significantly positive correlation with the number of tropical cyclones landing over China. The quasi-biweekly oscillation is filtered from daily equatorial pressure in May–November over the 57 years with inverse wavelet transform and the probability for tropical cyclones landing on coastal China within four days before and after the oscillatory valleys of quasi-biweekly pressure at the equator is 59.7% and 73.0% for June to October and July to September respectively. The model of atmospheric circulation for quasi-biweekly oscillatory valleys of equatorial pressure in association with or without tropical cyclones landing over China in July–September is set up with the composite analysis method. When the valleys are associated with (without) landfall, zonal (meridional) circulation prevails in the mid and high latitudes of the Eastern Hemisphere, the high pressure ridge is weak (strong) near the Sea of Okhotsk, the westerly zone is northward (southward), the subtropical high is westward (eastward) in location and strong (weak) in intensity, the cross-equatorial flow is strong (weak) in southeast Asia, Southwest Monsoon is strong (weak) and stronger (weaker) while in the valleys of pressure, being favorable (unfavorable) for tropical cyclones landing over China. The atmospheric circulation model for oscillatory valleys of biweekly equatorial pressure in association with (without) tropical cyclones landing over China, which can reflect the difference of atmospheric circulation between them, is beneficial to medium-term forecasts of tropical cyclones landing over China.

Variability and Singularity of Seoul, South Korea, Rainy Season (1778–2004)

Abstract The 227-yr daily precipitation record gathered for Seoul, South Korea, represents one of the longest instrumental measurements, which provides an exceptional opportunity for detecting climate singularity (a property of phase locking to annual cycle) of extreme weather events and multidecadal–centennial variability of the rainy season structure. From late June to early September, the occurrence of heavy rain events shows a climatological quasi-biweekly oscillation. The rainy season characteristics, including the dates of onset, retreat, summit, and the duration, all show significant centennial variations. The rainy season summit shows a tendency toward delayed occurrence, which changed from the 37th pentad (P37; 30 June–4 July) during the 1778–1807 period to P44 (4–8 August) during the 1975–2004 period. The amplitude of the interannual (2–6 yr) variation of summer precipitation shows a prominent fluctuation with a 50-yr rhythm. A notable climatological break (around 9–13 August) divides the rainy season into a changma (Korean for continuous rain period) and a post-changma period. The major modes of subseasonal variability of the rainy season are characterized by an advanced changma and an enhanced post-changma, respectively. The former is dominated by biennial variation, whereas the latter has a major 5-yr spectral peak, suggesting that the processes leading to their variability are different. The occurrence of severe drought events exhibits a 4-yr spectral peak along with large power on a centennial time scale, while the severe flood events have a spectral peak at 3 and 19 yr, respectively. The remarkable climate variability in Seoul rainfall suggests that trends detected by using a 50-yr-or-shorter precipitation record likely reflect natural variability.

Impact of Tibetan Orography and Heating on the Summer Flow over Asia

The influence on the summer flow over Asia of both the orographic and thermal forcing of the Tibetan Plateau is investigated using a sequence of idealised experiments with a global primitive equation model. The zonally averaged flow is prescribed and both realistic and idealised orography and heating are used. There is some similarity between the responses to the two forcings when applied separately. The upper tropospheric Tibetan anticyclone is predominantly forced by the heating but also weakly by the orography. Below this, both forcings tend to give air descending in an equatorward anticyclonic circulation down the isentropes to the west and rising in a similar poleward circulation to the east. However the heating-only response has a strong ascending southwesterly flow that is guided around the south and south-east of the orography when it is included. On the northern side, the westerly flow over the orography gives ascent on the upslope and descent on the downslope. It is found that heating over the Plateau leads to a potential vorticity (PV) minimum and that if it is sufficiently strong the flow is unstable, producing a quasi-biweekly oscillation. During this oscillation the Tibetan anticyclone changes between a single centre over the southwestern side of the Plateau and a split/double structure with centres over China and the Middle East. These characteristics are similar to observed variability in the region. Associated with this quasi-biweekly oscillation are significant variations in the strength of the ascent over the Plateau and the Rossby wave pattern over the North Pacific. The origin of the variability is instability associated with the zonally extended potential vorticity PV minimum on a θ-surface, as proposed by Hsu and Plumb (2000). This minimum is due to the tendency to reduce the PV above the heating over the Plateau and to advection by the consequent anticyclone of high PV around from the east and low PV to the west. The deep convection to the south and southeast of the Plateau tends to suppress the quasi-biweekly oscillation because the low PV produced above it acts to reduce the meridional PV gradient reversal. The occurrence of the oscillation depends on the relative magnitude of the heating in the two regions.

Role of the quasi‐biweekly oscillation in the onset of convection over the Indochina Peninsula

AbstractThe characteristics of convection onset over the Indochina Peninsula and its possible mechanism are investigated using the reanalysis of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and the black‐body temperature at the top of the cloud (TBB). Results show that the convection onset occurs before the systemic northward progression of the tropical convection and is earlier than the onset of the summer monsoon over this region. The prominent difference between the onset of convection and that of the summer monsoon appears in the westmost part of the western Pacific subtropical high. When the convection bursts, the Indochina Peninsula is under the control of the western Pacific subtropical high. However, the summer monsoon onset follows the splitting of the subtropical high belt and the eastward retreat of the western Pacific subtropical high out of the Indochina Peninsula.The climatological features of TBB over the Indochina Peninsula indicate that the convective activities have a close relationship with the quasi‐biweekly oscillation (QBWO). Based on the onset dates determined by the TBB data, case‐studies and composite analyses are implemented on exploring the possible mechanism of the convection onset. It is found that the onset of convection is mainly influenced by factors as follows. The wet phase of QBWO of the tropical convection propagates northward from Sumatra to the Indochina Peninsula. Over the southern Indochina Peninsula, anomalous southerlies (the Rossby wave response to the tropical convective QBWO) are beneficial to both the northward progression of the convective disturbances and the convergence of the low‐level moisture flux. Furthermore, the Indochina Peninsula is easily affected by any cold surge invading southwards from higher latitudes, which intensifies the convective activities when merging with the wet/warm current. Copyright © 2007 Royal Meteorological Society

A Mechanism of Scale Selection in Tropical Circulation at Observed Intraseasonal Frequencies

It is shown in this work that two prominent intraseasonal oscillations of the tropical atmosphere—namely, the 3–4-day westward propagating wave observed over the equatorial Atlantic and Pacific Oceans and the quasibiweekly (or 10–20 day) oscillation observed over the Indian Ocean summer monsoon region—can be understood as arising from selective excitation of the tropical normal modes in the presence of moist feedbacks and a moisture relaxation timescale. Unlike in earlier studies of a similar nature, the central theme of the present work is that the dynamics of the moisture variable is governed by a moisture relaxation timescale τ. In other words, the large-scale flow is not in exact quasi equilibrium with the precipitational heating. The implications of this hypothesis are investigated by using a shallow-water model of the tropical atmosphere on an equatorial β plane, with a fixed vertical structure. The two major findings of the present work are: 1) both the 3–4-day wave and the 10–20-day wave can be understood as intrinsic modes of the tropical atmosphere, excited by the same basic mechanism—namely, moist feedbacks in the presence of a moisture relaxation timescale, and 2) while the 3–4-day wave is represented by a maximally growing mixed Rossby gravity wave, driven selectively unstable by moist feedbacks, the 10–20-day wave represents a new mode of the tropical atmosphere that is excited by the moist feedbacks in the presence of mean westerlies. For both of these waves, the agreement between observed structure and theoretical predictions is excellent. Thus, the present work presents a mechanism that explains two major low-frequency tropical oscillations as intrinsic modes of the tropical atmosphere.