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Tropical Convection on 7–21-Day Timescales over the Western Pacific

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  • 1 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana
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Abstract

Evidence is presented to demonstrate that the oscillations of convection on 7–21-day timescales are an important component of the intraseasonal variability over the region spanning the equatorial western Pacific to the subtropical South Pacific. In that area of the world, these oscillations are largely confined to regions with high sea surface temperatures (SSTS) or SST gradients. Consequently, the patterns of 7–21-day variability of convection undergo significant changes, as the El Nin˜o/Southern Oscillation reconfigures the distributions of SST.

A test is developed that detects episodes in which the 7–21-day oscillation of outgoing longwave radiation (OLR) is particularly well defined for several cycles. Applying this test, 29 episodes of high 7–21-day variability were defined. Based on this information, the annual and longitudinal distribution of 7–21-day variability is discussed.

The 7–21-day oscillations of convection found at subtropical southern latitudes tend to have stronger wind shear in the vertical column than oscillations detected in the equatorial Tropics. Vertical motion maxima were generally found at lower levels of the atmosphere in the subtropical episodes than in those found along the equator. As predicted by other studies, the subtropical latitude cases appear to be caused by the passage of a series of baroclinic waves.

Two of the 29 episodes are described in detail. The atmospheric state is composited with respect to the active and inactive phases of the 7–21-day oscillation of OLR. Contrasting events when the OLR values are low and high reveals patterns of circulation features both upstream and downstream from the convection. Composite profiles of vertical velocity and horizontal divergence, as well as maps of divergence and geopotential height anomalies at 200 hPa, were consistent with an atmosphere that had alternately enhanced and suppressed convective activity.

Abstract

Evidence is presented to demonstrate that the oscillations of convection on 7–21-day timescales are an important component of the intraseasonal variability over the region spanning the equatorial western Pacific to the subtropical South Pacific. In that area of the world, these oscillations are largely confined to regions with high sea surface temperatures (SSTS) or SST gradients. Consequently, the patterns of 7–21-day variability of convection undergo significant changes, as the El Nin˜o/Southern Oscillation reconfigures the distributions of SST.

A test is developed that detects episodes in which the 7–21-day oscillation of outgoing longwave radiation (OLR) is particularly well defined for several cycles. Applying this test, 29 episodes of high 7–21-day variability were defined. Based on this information, the annual and longitudinal distribution of 7–21-day variability is discussed.

The 7–21-day oscillations of convection found at subtropical southern latitudes tend to have stronger wind shear in the vertical column than oscillations detected in the equatorial Tropics. Vertical motion maxima were generally found at lower levels of the atmosphere in the subtropical episodes than in those found along the equator. As predicted by other studies, the subtropical latitude cases appear to be caused by the passage of a series of baroclinic waves.

Two of the 29 episodes are described in detail. The atmospheric state is composited with respect to the active and inactive phases of the 7–21-day oscillation of OLR. Contrasting events when the OLR values are low and high reveals patterns of circulation features both upstream and downstream from the convection. Composite profiles of vertical velocity and horizontal divergence, as well as maps of divergence and geopotential height anomalies at 200 hPa, were consistent with an atmosphere that had alternately enhanced and suppressed convective activity.

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