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Lloyd J. Shapiro and Stanley B. Goldenberg

Abstract

Winds at low (near-surface) and 200-mb levels from National Hurricane Center objective analyses are used to elucidate the structure and dynamics of the tropical and subtropical intraseasonal oscillations for the North Atlantic/northeast Pacific regions, including over the continents, for the years 1980–1989. The intraseasonal oscillations are broken into three bands, with long (50–85 day), intermediate (30–55 day), and short (13–29 day) periods. Winter and summer seasons are analyzed separately. A complex empirical orthogonal function technique is used to derive the dominant modes of intraseasonal variability over the region, including their propagation characteristics. Statistically distinct modes of variability are found only during the winter and only for the long-period and short-period bands.

The dominant mode of coupled 200-mb low-level long-period variability during winter has a dipole structure. It has a substantial equivalent barotropic component in the subtropics, as well as a baroclinic structure consistent with quasigeostrophic midlatitude systems. Negative outgoing longwave radiation anomalies tend to be in phase with a low-level convergence-upper-level divergence couplet, which lies approximately one-quarter wavelength to the east of the cyclonic vorticity centers. The long-period oscillations during 1981–1988 are dominated by three events, with periods between about 60 and 70 days. There is a negative correlation, explaining about 50% of the variance, between the magnitude of the mode and an index of El Niño based on sea surface temperatures in the eastern equatorial Pacific.

The dominant modes of short-period variability during winter appear as zonally oriented wave trains similar to those found by previous investigators of global-scale fluctuations. Rotation of the modes of 200-mb variability effectively separates them into their propagating and standing components. Approximately one-half of the variance in the meridional wind near teleconnection centers of action is found in the eastward propagating component. The dominant mode of coupled 200-mb/iow-level variability propagates to the east, and has a vertical structure similar to that in the long-period band. It has a predominant period near 18 days.

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Lloyd J. Shapiro and Stanley B. Goldenberg

Abstract

It has long been accepted that interannual fluctuations in sea surface temperature (SST) in the Atlantic are associated with fluctuations in seasonal Atlantic basin tropical cyclone frequency. To isolate the physical mechanism responsible for this relationship, a singular value decomposition (SVD) is used to establish the dominant covarying modes of tropospheric wind shear and SST as well as horizontal SST gradients. The dominant SVD mode of covarying vertical shear and SST gradients, which comprises equatorially confined near-zonal vertical wind shear fluctuations across the Atlantic basin, is highly correlated with both equatorial eastern Pacific SST anomalies (associated with El Niño) and West African Sahel rainfall. While this mode is strongly related to tropical storm, hurricanes, and major hurricane frequency in the Atlantic, it is not associated with any appreciable Atlantic SST signal.

By contrast, the second SVD mode of covarying vertical shear and horizontal SST gradient variability, which is effectively uncorrelated with the dominant mode, is associated with SST fluctuations concentrated in the main tropical cyclone development region between 10° and 20°N. This mode is significantly correlated with tropical storm and hurricane frequency but not with major hurricane frequency. Statistical tests confirm the robustness of the mode, and lag correlations and physical reasoning demonstrate that the SST anomalies are not due to the developing tropical cyclones themselves. Anomalies of SST and vertical shear during years where the mode has substantial amplitude confirm the resemblance of the individual fields to the modal structure, as well as the association of hurricane development with the warmer SSTs. Although SSTs are of secondary importance to vertical shear in modulating hurricane formation, explaining only ∼10% of the interannual variability in hurricane frequency over the ∼50% explained by vertical shear, the results support the conclusion that warmer SSTs directly enhance development. The lack of correlation with major hurricanes implies that the underlying SSTs are not a significant factor in the development of these stronger systems.

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Stanley B. Goldenberg and Lloyd J. Shapiro

Abstract

Physical mechanisms responsible for the contemporaneous association, shown in earlier studies, of North Atlantic basin major hurricane (MH) activity with western Sahelian monsoon rainfall and an equatorial eastern Pacific sea surface temperature index of El Niño are examined, using correlations with 200- and 700-mb level wind data for the period 1968–92. The use of partial correlations isolates some of the relationships associated with the various parameters.

The results support previous suggestions that the upper- and lower-level winds over the region in the basin between ∼10° and 20°N where most MHs begin developing are critical determinants of the MH activity in each hurricane season. In particular, interannual fluctuations in the winds that produce changes in the magnitude of vertical shear are one of the most important factors, with reduced shear being associated with increased activity and stronger shear with decreased activity. The results show that most of these critical wind fluctuations are explained by their relationship to the SST and rainfall fluctuations. Results confirm previous findings that positive (warm) eastern Pacific SST and negative (drought) Sahelian rainfall anomalies are associated with suppressed Atlantic basin tropical cyclone activity through an equatorially confined near-zonal circulation with upper-level westerlies and lower-level easterlies that act to increase the climatological westerly vertical shear in the main development region. SST and rainfall anomalies of the opposite sense are related to MH activity through a zonal circulation with upper-level easterly and lower-level westerly wind anomalies that act to cancel out some of the climatological westerly vertical shear. The results also show that changes in vertical shear to the north of the main development region are unrelated to, or possibly even out of phase with, changes in the development region, providing a possible physical explanation for the observations from recent studies of the out-of-phase relationship of interannual fluctuations in MH activity in the region poleward of ∼25°N with fluctuations in activity to the south.

The interannual variability of MH activity explained by Sahel rainfall is almost three times that explained by the eastern Pacific SSTs. It is demonstrated that a likely reason for this result is that the SST-associated vertical shears are more equatorially confined, so that the changes in shear in the main development region have a stronger association with the rainfall than with the SSTs.

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