Role of the Convection Scheme in Modeling Initiation and Intensification of Tropical Depressions over the North Atlantic

The authors analyze how modifications of the convective scheme modify the initiation of tropical depression vortices (TDVs) and their intensification into stronger warm-cored tropical cyclone–like vortices (TCs) in global climate model (GCM) simulations. The model’s original convection scheme has entrainment and cloud-base mass flux closures based on moisture convergence. Two modifications are considered: one in which entrainment is dependent on relative humidity and another in which the closure is based on the convective available potential energy (CAPE). Compared to reanalysis, TDVs are more numerous and intense in all three simulations, probably as a result of excessive parameterized deep convection at the expense of convection detraining at midlevel. The relative humidity–dependent entrainment rate increases both TDV initiation and intensification relative to the control. This is because this entrainment rate is reduced in the moist center of the TDVs, giving more intense convective precipitation, and also because it generates a moister environment that may favor the development of early stage TDVs. The CAPE closure inhibits the parameterized convection in strong TDVs, thus limiting their development despite a slight increase in the resolved convection. However, the maximum intensity reached by TC-like TDVs is similar in the three simulations, showing the statistical character of these tendencies. The simulated TCs develop from TDVs with different dynamical origins than those observed. For instance, too many TDVs and TCs initiate near or over southern West Africa in the GCM, collocated with the maximum in easterly wave activity, whose characteristics are also dependent on the convection scheme considered

On Vortices Initiated over West Africa and Their Impact on North Atlantic Tropical Cyclones

International audienceAbstract Using 38 years of the ERA-Interim dataset, an objective tracking approach is used to analyze the origin, characteristics, and cyclogenesis efficiency (CE) of synoptic-scale vortices initiated over West Africa and the Atlantic Ocean. Vortices initiated over the ocean at a given pressure level often result from a vertical expansion of a “primary” vortex track initiated earlier over West Africa. Low-level (850 hPa) primary vortices are initiated mainly in July near the Hoggar Mountains (24°N, 5°E), while midlevel (700 hPa) primary vortices are initiated mainly in August–September near the Guinea Highlands (10°N, 10°W). The CE of all these vortices is about 10% in July and 30% in August. The average CE is, however, smaller for low-level “Hoggar” vortices because they peak in July when the cyclogenesis potential index of the Atlantic Ocean is weak. Seasonal and interannual modulations of the cyclogenesis is related more to this index than to the number of vortices crossing the West African coast. Cyclogenesis is nearly equally distributed between the coast and 60°W, but the part of the cyclogenesis due to vortices initiated over West Africa decreases from 80% near the coast to about 30% at 60°W. The most probable delay between the vortex vertical expansion and cyclogenesis is 2 days, but it can be up to 10 days. This analysis also confirms previous results, such as the larger CE for vortices extending at low levels over the continent at 10°N, or the delayed and therefore west-shifted cyclogenesis of low-level “Hoggar” vortices

Origin and Evolution of Synoptic-Scale Vortices Initiated at Low Level Downwind of the Hoggar Mountains

International audienceAbstract Numerous low-level vortices are initiated downwind of the Hoggar Mountains and progress toward the Atlantic coast on the northern path of African easterly waves (AEWs). These vortices occur mostly in July and August and more specifically when the northern position of the Saharan heat low (SHL) generates stronger and vertically expanded easterly winds over the Hoggar Mountains. At synoptic time scales, a composite analysis reveals that vortex initiation and westward motion are also statistically triggered by a reinforcement of these easterly winds by a wide and persistent high pressure anomaly developing around the Strait of Gibraltar and by a weak wave trough approaching from the east. The vortices are generated in the lee of the Hoggar, about 1000 km west of this approaching trough, and intensify rapidly. The evolution of the vortex perturbation is afterward comparable with the known evolution of the AEWs of the northern path and suggest a growth due to dry barotropic and baroclinic processes induced in particular by the strong cyclonic shear between the reinforced easterly winds and the monsoon flow. These results show that vortex genesis promoted by changes in orographic forcing due to the strengthening of easterly winds over the Hoggar Mountains is a source of intensification of the northern path of AEWs in July and August. These results also provide a possible mechanism to explain the role of the SHL and of particular midlatitude intraseasonal disturbances on the intensity of these waves

Rôle de l'interaction océan-atmosphère dans la variabilité intrasaisonnière de la convection tropicale

PALAISEAU-Polytechnique (914772301) / SudocSudocFranceF

The event-to-event variability of the boreal winter MJO

International audienceDuring boreal winters, perturbations of the convection by the Madden-Julian Oscillation (MJO) peak over three basins distributed in longitude south of the Equator: the eastern Indian Ocean (IO), the south of the Maritime Continent (MC) and the western Pacific Ocean (PO). We use the observed Outgoing Longwave Radiation (OLR) and low-level wind to identify and characterize all wintertime MJO events between 1979 and 2010. There is a large event-to-event variability with some MJO events organized at the planetary-scale having their amplitude well distributed over the 3 basins and some showing only basin-scale organization with a convective perturbation peaking over one or two basins. The average of the MJO amplitude for the three basins shows an intriguing decadal variability consistent for both OLR and low-level wind. The disparity between the 3 basins is dominated by an alternation between MJO amplitude peaking on either the Indian or the Pacific Ocean. This Indo-Pacific alternation, depicted by an Indo-Pacific Index (IPI), is partly related to ENSO. In El Niño conditions, there is not only an extension of the MJO perturbation further east, but also an increase of the MJO perturbation over the western Pacific and a diminution of the MJO perturbation over the eastern Indian Ocean

Indo-Pacific Sea Surface Temperature Perturbations Associated with Intraseasonal Oscillations of Tropical Convection

International audienceSince the ISV of the convection is an intermittent phenomenon, the local mode analysis (LMA) technique is used to detect only the ensemble of intraseasonal events that are well organized at large scale. The LMA technique is further developed in this paper in order to perform multivariate analysis given patterns of SST and surface wind perturbations associated specifically with these intraseasonal events. During boreal winter, the basin-scale eastward propagation of the convective perturbation is present only over the Indian Ocean Basin. The intraseasonal SST response to convective perturbations is large and recurrent over thin mixed layer regions located north of Australia and in the Indian Ocean between 5° and 10°S. By contrast, there is little SST response in the western Pacific basin and no clear eastward propagation of the convective perturbation. During boreal summer, the SST response is large over regions with thin mixed layers located north of the Bay of Bengal, in the Arabian Sea, and in the China Sea. The northeastward propagation of the convective perturbation over the Bay of Bengal is associated with a standing oscillation of the SST and the surface wind between the equator and the northern part of the bay. In fact, many intraseasonal events mostly concern a single basin, suggesting that the interbasin organization is not a necessary condition for the existence of coupled intraseasonal perturbations of the convection.The perturbation of the surface wind tends to be larger to the west of the large-scale convective perturbation (like for a Gill-type dynamical response). For eastward propagating perturbations, the cooling due to the reinforcement of the wind (i.e., surface turbulent heat flux) thus generally lags the radiative cooling due to the reduction of the surface solar flux by the convective cloudiness. This large-scale Gill-type response of the surface wind also cools the surface to the west of the basin (northwest Arabian Sea and northwest Pacific Ocean), even if the convection is locally weak. An intriguing result is a frequently occurring small delay between the maximum surface wind and the minimum SST. Different explanations are invoked, like a rapid surface cooling due to the vanishing of an ocean warm layer (diurnal surface warming due to solar radiation in low wind conditions) as soon as the wind increases

Intraseasonal Convective Perturbations Related to the Seasonal March of the Indo-Pacific Monsoons

International audienc

Analyses des perturbations synoptiques et de la modulation diurne des systèmes convectifs sur l'Afrique centrale

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Typology of intraseasonal oscillations based on a Local Mode Analysis

The Intraseasonal Oscillation (ISO; 20-120 day) is an important component of the variability of the Tropical convection that strongly perturbs the Asian and the Aus- tralian monsoons. Our knowledge of the physical origin of the ISO however remains largely incomplete, partially because of the large variability of its characteristics from one event to another. The aim is to determine how the patterns of the different ISO events may be objectively regrouped in a few types, related for example to season, ENSO or other large scale forcing. Patterns of the different ISO events are extracted using a Local Mode Analysis (LMA, Goulet and Duvel 2000). The LMA is based on complex empirical orthogonal functions computed for successive positions of a moving temporal window. When a maximum of variance percentage is detected, the pattern of the corresponding ISO event is extracted. For the present study, the ISO events are detected by an LMA applied on the 20-120-day band pass filtered OLR field for a large tropical region (0˚-200˚E; 30˚N-30˚S) and for the 1979-2005 period. The 144 events extracted are then objectively clustered into homogeneous types us- ing a Hierarchical Ascending Classification. Our classification in 8 types synthetically describes 50% of the overall diversity of the ISO events. The 8 types depict primarily the seasonality in the ISO modes, with the known north- ward (eastward) propagations over the Indian and Western Pacific basins during the northern summer (winter) seasons. While some similarities are noted from one type to another, important differences confirm however that the ISO may be classified in more than two types (i.e. the summer and winter types). In particular, some modes are very specific to particular months (bogus onset in May over the Bay of Bengal, for example). Also, different types are obtained for each season in relation to interannual variability of the sea surface temperature field related primarily to ENSO. This study also shows different equatorial propagation characteristics (i.e. Madden Julian Oscil- lation) for each type confirming that the MJO is not a phenomenon distinct from the ISO