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John Molinari and David Vollaro

Abstract

It is frequently stated that 70%–80% of western North Pacific tropical cyclones form “within the monsoon trough,” but without an objective definition of the term. Several definitions are tested here. When the monsoon trough (MT) is defined as the contiguous region where long-term (1988–2010) mean July–November 850-hPa relative vorticity is positive, 73% of all July–November tropical cyclones form within the MT. This percentage varies interannually, however, from as low as 50% to nearly 100%. The percentage correlates with the Niño-3.4 index, with more storms forming within the MT during warm periods. When the MT is defined instead using long-term monthly mean ζ 850, more than 80% of tropical cyclones form within the MT in all months except July and August, when more than 30% of storms form poleward of the MT. It is hypothesized that the known peak in the frequency of upper-tropospheric midlatitude wave breaking in July and August is responsible. It is argued that any long-term mean provides a suitable definition of the MT. Defining it on less than seasonal time scales, however, creates a lack of conceptual separation between the MT and other tropical disturbances such as the MJO, equatorial waves, and easterly waves. The term monsoon trough should represent a climatological feature that provides an asymmetric background state within which other disturbances evolve.

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John Molinari and David Vollaro

Abstract

This paper describes a large cyclonic gyre that lasted several days in the northwest Pacific during July 1988. Cyclonic winds at 850 hPa extended beyond the 2000-km radius with a radius of maximum winds of 700–800 km. The gyre exhibited clear skies within and north of its center. Active convection extended 4000 km in longitude to its south.

The Madden–Julian oscillation (MJO) was in its active phase in the Indian Ocean prior to gyre formation. Consistent with earlier studies, diabatic heating in the MJO was associated with an anomalous upper-tropospheric westerly jet over the northeast Asian coast and a jet exit region over the northwest Pacific. Repeated equatorward wave-breaking events developed downwind of the jet exit region. One such event left behind a region of lower-tropospheric cyclonic vorticity and convection in the subtropics that played a key role in the gyre formation. A second wave-breaking event produced strong subsidence north of the mature gyre that contributed to its convective asymmetry.

Gyres from 1985 and 1989 were compared to the 1988 case. All three gyres developed during an active MJO in the Indian Ocean. Each gyre displayed the same strong convective asymmetry. Each developed in July or August during the climatological peak in breaking Rossby waves in the northwest Pacific. Finally, all of the gyres developed during La Niña at nearly the same location. This location and the convective structure of the gyres closely matched composite La Niña anomalies during boreal summer.

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John Molinari and David Vollaro

Abstract

Outflow layer winds were objectively analyzed every 12 h for 6 days during the life cycle of Hurricane Elena (1985). A high correlation was found between angular momentum fluxes by azimuthal eddies at large radii and central pressure changes in the storm 27–33 h later. Momentum flux by eddies exceeded that by the azimuthal mean outside the 800 km radius, while vortex spinup by the eddies reached instantaneous magnitudes as large as 25 m s−1/day. Outflow maxima and minima repeatedly appeared more than 1000 km from the hurricane center and tracked inward with time. The results provide evidence of significant environmental control on the behavior of the storm.

After reaching hurricane strength, Elena experienced a major secondary intensification associated with a large inward cyclonic eddy momentum flux produced by the passage of a middle latitude trough north of the hurricane. An outflow maximum appeared radially inside of the eddy momentum source, consistent with balanced vortex theory, and tracked inward with the eddy momentum source during the following 24 h. When the outflow maximum reached the storm core, an extended period of rapid pressure fails followed. It is speculated that these pressure falls represented a response to midlevel spinup forced by the outflow layer momentum sourcers.

Although environmental forcing dominated the later stages of Elena, the rapid initial intensification of the storm as it moved from land to water appeared to be a precursor to subsequent environmental interactions. The enhanced anticyclonic outflow from this initial deepening reduced the outflow-layer inertial stability, allowing a more radially extended region for external forcing. The secondary intensification of Elena is thus viewed as a cooperative interaction between mesoscale events at the hurricane core and synoptic-scale features in the upper tropospheric environment.

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John Molinari and David Vollaro

Abstract

The vertical structure of the interaction of Hurricane Elena (1985) with a baroclinic wave was evaluated using analyses from the European Centre for Medium Range Weather Forecasting. During the period of interaction, azimuthal eddies produced a localized flux convergence of cyclonic angular momentum in the upper troposphere which shifted to progressively smaller radii prior to major secondary deepening of the storm. These momentum fluxes decayed above and below the outflow layer. Eddy heat fluxes showed maximum cooling in the middle and upper troposphere and warming in the lower stratosphere, reflecting the temperature structure of the baroclinic wave as it moved into the hurricane volume.

The response of the hurricane vortex to the fluxes of heat and angular momentum was determined by solution of Eliassen's balanced vortex equation. The balanced solutions showed a band of upward motion, with deep inflow and narrow outflow, which shifted inward from the 500 km radius to the hurricane core in the 24 hours prior to the secondary deepening. The position and timing of this feature corresponded to the contracting outflow maximum found in Part I. Eddy heat fluxes contributed to the induced circulation in the same manner as momentum fluxes near the core, but with smaller magnitude and areal coverage. The contracting outflow maximum thus appeared to represent the upper branch of a secondary circulation excited primarily by the eddy momentum fluxes.

The reintensification of hurricanes is often directly associated with formation of a wind maximum at inner radii which replaces or reinforces the original eye wall as it contracts. Such a feature was seen in reconnaissance data in Elena at the time the secondary circulation reached inner radii. It is speculated that the relatively weak secondary circulation evolved into a local wind maximum through the actions of diabatic heat sources. The approaching trough is thus viewed not as a direct cause of deepening, but as a catalyst which organized the diabatic sources in such a way as to excite internal instabilities of the system.

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John Molinari and David Vollaro

Abstract

A set of 327 dropsondes from the NOAA G-IV aircraft was used to create a composite analysis of the azimuthally averaged absolute angular momentum in the outflow layer of major Hurricane Ivan (2004). Inertial instability existed over a narrow layer in the upper troposphere between the 350- and 450-km radii. Isolines of potential and equivalent potential temperature showed that the conditions for both dry and moist symmetric instability were satisfied in the same region, but over a deeper layer from 9 to 12 km. The radial flow maximized at the outer edge of the unstable region. The symmetrically unstable state existed above a region of outward decrease of temperature between the cirrus overcast of the storm and clear air outside. It is hypothesized that the temperature gradient was created as a result of longwave heating within the cirrus overcast and longwave cooling outside the cloudy region. This produced isentropes that sloped upward with radius in the same region that absolute momentum surfaces were flat or sloping downward, thus creating symmetric instability. Although this instability typically follows rather than precedes intensification, limited numerical evidence suggests that the reestablishment of a symmetrically neutral state might influence the length of the intensification period.

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John Molinari and David Vollaro

Abstract

An objective definition of monsoon gyres in the northwest Pacific was developed in order to construct a gyre climatology. Over a 31-yr period, 53 gyres were identified with a median formation location at 16.5°N, 135°E. More than 80% formed during July–September. More than half of gyres developed during El Niño periods at a median location 1200 km farther to the east-southeast than during La Niña. Cyclonic winds at 850 hPa extended across a diameter of more than 4000 km, with maximum tangential wind near the 1000-km radius. A precipitation maximum extended westward for several thousand kilometers south of the gyre. Typhoons were most common north and east of the gyre centers. More than 70% of gyres developed during large-amplitude MJO events, with a strong preference for Real-time Multivariate MJO (RMM) phases 5–7. In boreal summer these phases contain circulation and convective anomalies that coincide most closely with those of the climatological monsoon trough. Gyres are most likely to form when an active, large-amplitude MJO event superposes with the monsoon trough in the presence of high sea surface temperature. Gyres exhibited 850-hPa wind, height, and vorticity anomalies and surface latent heat flux anomalies that closely resembled the active Pacific–Japan pattern (PJP). This was especially true during La Niña, even though no attempt was made to isolate the PJP. It is hypothesized that an active MJO modulates gyre formation, and the gyres project onto the active phase of the PJP as they move westward.

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John Molinari and David Vollaro

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The structure and evolution of lowpass-filtered background flow and synoptic-scale easterly waves were examined during the 1991 eastern Pacific hurricane season. Active and inactive cyclogenesis periods conformed well to the sign of the near-equatorial, lowpass-filtered, 850-mb zonal wind anomaly, consistent with the recent results of Maloney and Hartmann. This behavior emphasizes the importance of westerly wind bursts associated with the Madden–Julian oscillation (MJO) in creating an environment favorable for eastern Pacific tropical cyclogenesis.

Synoptic-scale easterly waves reached the western Caribbean and eastern Pacific regularly from upstream, usually from Africa. The amplitude of waves leaving Africa had little correlation with the likelihood of a wave producing an eastern Pacific storm. Rather, easterly waves intensified, and tropical depressions formed, during the convectively active phase of the MJO in the western Caribbean and eastern Pacific. Wave growth, measured by strengthening of convection within the waves, occurred in the regions of sign reversal of the meridional potential vorticity gradient found previously. For the 1991 season cyclogenesis occurs when westward-moving synoptic-scale waves amplify within the superclusters that represent the favorable MJO envelope. Analogously, waves existed but failed to grow during the unfavorable part of the MJO.

During each active period of the MJO, the region of active convection moved eastward and northward with time in the eastern Pacific, with strongest convection reaching as far as the southwestern Gulf of Mexico by the end of such periods. The locations of tropical depression formation followed a similar path, shifting eastward with time following the MJO, and northward following the eastern Pacific intertropical convergence zone. The latter was defined by the locations of low-pass-filtered background vorticity maxima at 1000 mb.

It is argued based on previous work in the literature that the western Pacific might behave similarly, with upstream easterly waves growing and producing depressions within the convectively active envelope of the MJO.

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John Molinari and David Vollaro

Abstract

A weak tropical storm (Gabrielle in 2001) experienced a 22-hPa pressure fall in less than 3 h in the presence of 13 m s−1 ambient vertical wind shear. A convective cell developed downshear left of the center and moved cyclonically and inward to the 17-km radius during the period of rapid intensification. This cell had one of the most intense 85-GHz scattering signatures ever observed by the Tropical Rainfall Measuring Mission (TRMM). The cell developed at the downwind end of a band in the storm core. Maximum vorticity in the cell exceeded 2.5 × 10−2 s−1. The cell structure broadly resembled that of a vortical hot tower rather than a supercell.

At the time of minimum central pressure, the storm consisted of a strong vortex adjacent to the cell with a radius of maximum winds of about 10 km that exhibited almost no tilt in the vertical. This was surrounded by a broader vortex that tilted approximately left of the ambient shear vector, in a similar direction as the broad precipitation shield. This structure is consistent with the recent results of Riemer et al.

The rapid deepening of the storm is attributed to the cell growth within a region of high efficiency of latent heating following the theories of Nolan and Vigh and Schubert. This view is supported by a rapid growth of wind speed and vorticity in the storm core during the 1-h lifetime of the cell, and by the creation of a narrow 7°C spike in 700-hPa temperature adjacent to the cell and coincident with the lowest pressure. The cell is not seen as the cause of rapid intensification. Rather, it is part of a multiscale process: (i) development of a new circulation center within the downshear precipitation shield, (ii) continued ambient shear creating a favored region for cell formation just downshear of the new center, and (iii) the development of the intense cell that enhanced diabatic heating close to the center in a region of high efficiency of kinetic energy production. This sheared, asymmetric rapid intensification of Tropical Storm Gabrielle is contrasted with the nearly symmetric composite given by Kaplan and DeMaria.

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John Molinari and David Vollaro

Abstract

The previous study of helicity, CAPE, and shear in Hurricane Bonnie (1998) was extended to all eight tropical cyclones sampled by NASA during the Convection and Moisture Experiments (CAMEX). Storms were categorized as having large or small ambient vertical wind shear, with 10 m s−1 as the dividing line. In strongly sheared storms, the downshear mean helicity exceeded the upshear mean by a factor of 4. As in the previous study, the helicity differences resulted directly from the tropical cyclone response to ambient shear, with enhanced in-up-out flow and veering of the wind with height present downshear. CAPE in strongly sheared storms was 60% larger downshear. Mean inflow near the surface and the depth of the inflow layer each were 4 times larger downshear. At more than 30% of observation points outside the 100-km radius in the downshear right quadrant, midlatitude empirical parameters indicated a strong likelihood of supercells. No such points existed upshear in highly sheared storms. Much smaller upshear–downshear differences and little likelihood of severe cells occurred in storms with ambient wind shear below 10 m s−1. In addition to these azimuthal asymmetries, highly sheared storms produced 30% larger area-averaged CAPE and double the area-averaged helicity versus relatively unsheared storms. The vortex-scale increase in these quantities lessens the negative impact of large vertical wind shear.

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John Molinari and David Vollaro

Abstract

A strong MJO event produced an upper-tropospheric jet streak in northeast Asia and repeated wave breaking in the jet exit region along 150°E during July 1988. A midlatitude low moved equatorward and intensified in the presence of bandpass-filtered (15–100 day) Q vector forcing for upward motion associated with the wave breaking. This forced ascent helped to moisten the atmosphere enough to increase the column water vapor to above 55 mm. This value was sufficiently large to support a self-sustaining low even after the upper forcing weakened. The horizontal scale of the Q vector forcing was about 1500 km, consistent with the scale of most favorable convective response to quasigeostrophic forcing in the subtropics described by Nie and Sobel. The low lasted one month as it moved southwestward, then westward, while remaining north of 20°N. Maximum precipitation along the track of the low exceeded 700 mm, with an anomaly more than 400 mm. A climatology of long-lasting lows was carried out for the monsoon gyre cases studied previously. During El Niño, long-lasting lows often began near the equator in the central Pacific, and were likely to have a mixed Rossby–gravity wave or equatorial Rossby wave structure. It is speculated that the quasi-biweekly mode, the submonthly oscillation, the 20–25-day mode, and the Pacific–Japan pattern are each variations on this kind of event. During La Niña, long-lasting lows that originated in midlatitudes were more common. It is argued that these lows from midlatitudes represent a unique disturbance type in boreal summer.

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