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Julien Crétat
and
Benjamin Pohl

Abstract

The authors analyze to what extent the internal variability simulated by a regional climate model is sensitive to its physical parameterizations. The influence of two convection schemes is quantified over southern Africa, where convective rainfall predominates. Internal variability is much larger with the Kain–Fritsch scheme than for the Grell–Dévényi scheme at the seasonal, intraseasonal, and daily time scales, and from the regional to the local (grid point) spatial scales. Phenomenological analyses reveal that the core (periphery) of the rain-bearing systems tends to be highly (weakly) reproducible, showing that it is their morphological features that induce the largest internal variability in the model. In addition to the domain settings and the lateral forcing conditions extensively analyzed in the literature, the physical package appears thus as a key factor that modulates the reproducible and irreproducible components of regional climate variability.

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Benjamin Pohl
and
Nicolas Fauchereau

Abstract

This article investigates the prominent features of the Southern Hemisphere (south of 20°S) atmospheric circulation when extracted using EOF analysis and a k-means clustering algorithm. The focus is on the southern annular mode (SAM), the nature of its recent trend, and the zonal symmetry of associated spatial patterns. The study uses the NCEP–Department of Energy Atmospheric Model Intercomparison Project II Reanalysis (NCEP-2) (period 1979–2009) to obtain robust patterns over the recent years and the Twentieth Century Reanalysis Project (period 1871–2008) to document decadal changes. Also presented is a comparison of these signals against a station-based reconstruction of the SAM index and a gridded interpolated dataset [Hadley Centre Sea Level Pressure dataset version 2 (HadSLP2)].

Over their common period, both reanalyses are in fair agreement, both in terms of spatial patterns and temporal variability. In particular, both datasets show weather regimes that can be interpreted as the opposite phases of the SAM. At the decadal time scale, the study shows that the trend toward the positive SAM phase (as inferred from the usual EOF-based index) is related more to an increase in the frequency of clusters corresponding to the positive phase, with little changes in the frequency of the negative SAM events. Similarly, the long-term tropospheric warming trend already discussed in the literature is shown to be related more to a decrease in the number of abnormally cold days, with little changes in the number of abnormally warm days. The cluster analysis therefore allows for complement descriptions based on simple indexes or EOF decompositions, highlighting the nonlinear nature of the decadal changes in the Southern Hemisphere atmospheric circulation and temperature.

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Benjamin Pohl
and
Adrian J. Matthews

Abstract

The Madden–Julian oscillation (MJO) is analyzed using the reanalysis zonal wind– and satellite outgoing longwave radiation–based indices of Wheeler and Hendon for the 1974–2005 period. The average lifetime of the MJO events varies with season (36 days for events whose central date occurs in December, and 48 days for events in September). The lifetime of the MJO in the equinoctial seasons (March–May and October–December) is also dependent on the state of El Niño–Southern Oscillation (ENSO). During October–December it is only 32 days under El Niño conditions, increasing to 48 days under La Niña conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Niño, consistent with theoretical arguments concerning equatorial wave speeds.

The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is found in 1975, which is at the same time as the well-known rupture in the ENSO time series that has been associated with the Pacific decadal oscillation. The mean amplitude of the MJO is 16% larger in the postrupture (1976–2005) compared to the prerupture (1950–75) period. Before the 1975 rupture, the amplitude of the MJO is maximum (minimum) under El Niño (La Niña) conditions during northern winter, and minimum (maximum) under El Niño (La Niña) conditions during northern summer. After the rupture, this relationship disappears. When the MJO–ENSO relationship is analyzed using all-year-round data, or a shorter dataset (as in some previous studies), no relationship is found.

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Béatrice Morel
,
Benjamin Pohl
,
Yves Richard
,
Benjamin Bois
, and
Miloud Bessafi

Abstract

Regional climate models (RCMs) should be evaluated with respect to their ability to downscale large-scale climate information to the local scales, which are sometimes strongly modulated by surface conditions. This is the case for La Réunion (southwest Indian Ocean) because of its island context and its complex topography. Large-scale atmospheric configurations such as tropical cyclones (TCs) may have an amplifying effect on local rainfall patterns that only a very high-resolution RCM, forced by the large scales and resolving finescale processes, may simulate properly.

This paper documents the capability of the Weather Research and Forecasting Model (WRF) RCM to regionalize rainfall variability at very high resolution (680 m) over La Réunion island for daily to seasonal time scales and year-to-year differences. Two contrasted wet seasons (November–April) are selected: 2000–01 (abnormally dry) and 2004–05 (abnormally wet). WRF rainfall is compared to a dense network of rain gauge records interpolated onto the WRF grid through the regression-kriging (RK) technique. RK avoids the point-to-grid comparison issue, but produces imperfect estimates due to sampling, so its quality also needs to be tested.

Seasonal rainfall amounts and contrasts produced by WRF are fairly realistic. At intraseasonal and daily time scales, differences to RK are more sizable. These differences are not easy to interpret in sectors where the rain gauge network is less dense and the quality of RK more uncertain, as over the eastern slopes of Piton de la Fournaise volcano where WRF seems to simulate more realistic rainfall than RK. Finally, the heavy rainfall associated with TC Ando on 6 January 2001, is documented. WRF shows weak disagreements with RK, indicating its capability to regionalize rainfall during extreme events.

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Benjamin Pohl
,
Béatrice Morel
,
Christelle Barthe
, and
Olivier Bousquet

Abstract

Ensemble simulations of Tropical Cyclone (TC) Ando (31 December 2000–9 January 2001) are performed over the southwest Indian Ocean using the nonhydrostatic WRF Model. Nested domains centered over the island of La Réunion allow for the simulation of local rainfall amounts associated with TC Ando at very high resolution (680-m grid spacing). The model is forced by and nudged toward ERA-Interim during the first (1–6) day(s) of the TC’s life cycle. The nudging ends at various dates to constrain either the whole life cycle or only parts of it.

As expected, results show weakened member dispersion, as the relaxation lasts longer, with more members producing similar cyclone tracks and intensities. The model shows reasonable skill to simulate local rainfall amounts and distribution, as soon as the simulated TC approaches La Réunion with a realistic distance and azimuth. Strong lower-level wind associated with the TC is forced to ascend over the slopes of the island. The model is able to successfully simulate the extreme daily precipitation amounts (>1200 mm) and their distribution over the highest parts of La Réunion. Nevertheless, smaller-scale features of the rainfall field are less realistic in the simulations. The wind speed and direction upstream of the island are the main drivers of such local uncertainties and errors, and they appear as an important issue to assess the local impacts of the TC over such a complex terrain.

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Benjamin Pohl
,
Serge Janicot
,
Bernard Fontaine
, and
Romain Marteau

Abstract

Madden–Julian oscillations (MJOs) are extracted over the Indo-Pacific basin using a local mode analysis. The convective perturbations are then projected over a larger domain to evaluate their remote consequences over the West African monsoon (WAM) intraseasonal variability. Rather weak (4–6 W m−2) convective fluctuations occurring in phase with those over the southern Indian basin are found over Africa, confirming the results of Matthews. In reverse, 40-day fluctuations in the WAM, similarly detected and projected over a widened area, demonstrate that a large majority of these events are embedded in the larger-scale patterns of the MJO. The regional amplitude of intraseasonal perturbations of the West African convection is not statistically associated with the amplitude of the MJO over the Indian basin but is instead closely related to background vertical velocity anomalies over Africa, possibly embedded in changes in the regional Walker-type circulation. Subsiding motion over Africa is recorded during the most energetic convective perturbations in the WAM.

Composites analyses over the MJO life cycle, as depicted by the real-time daily indices developed by Wheeler and Hendon, show that positive outgoing longwave radiation (OLR) anomalies during the dry phase are of larger amplitude and spatially more coherent than negative anomalies during the wet phase, especially over the Sahel region. Over West Africa, the phase of suppressed convection is thus of greater importance for the region than the phase of enhanced convection. Rain gauge records fully confirm these results. The MJO appears to be significantly involved in the occurrences of dry spells during the monsoon over the Sahel, whereas large-scale convective clusters are only restricted to the equatorial latitudes and thus affect the Guinean belt, which experiences its short dry season at this time of the year.

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Clémence Macron
,
Benjamin Pohl
,
Yves Richard
, and
Miloud Bessafi
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Clémence Macron
,
Benjamin Pohl
,
Yves Richard
, and
Miloud Bessafi

Abstract

This paper aims at separating the respective influences of tropical and midlatitude variability on the development and life cycle of tropical temperate troughs (TTTs) over southern Africa in austral summer (November–February). Cluster analysis is applied to 1971–2000 40-yr ECMWF Re-Analysis (ERA-40) daily outgoing longwave radiation (OLR) anomalies to identify TTTs and monitor tropical convection. The same analysis applied to the zonal wind stretching deformation at 200 hPa (ZDEF) characterizes midlatitude transient perturbations. Results based on the comparison between these two classifications first confirm that midlatitude baroclinic waves are a necessary condition for TTT development, but they are not sufficient. Roughly 40% of those occurring in austral summer are associated with a TTT. They tend to be stronger than the baroclinic waves not associated with TTT development. In the tropics, additional conditions needed to form a TTT consist of an excess of latent energy over the Mozambique Channel, mostly because of moisture advections and convergence from the Atlantic and Indian Oceans. Taken together, these conditions are highly favorable for deep atmospheric convection over and near southern Africa and seem to explain a large fraction of TTT variability.

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Vincent Moron
,
Boutheina Oueslati
,
Benjamin Pohl
, and
Serge Janicot

Abstract

This study investigates to what extent weather types (WTs) computed over tropical North Africa and the tropical North Atlantic Ocean (40°W–40°E, 0°–30°N) are relevant for documenting intraseasonal and interannual temperature variability in tropical North Africa (west of 37°E, 2°–27°N). Nine WTs are extracted by using clustering analysis of the daily anomalies of sea level pressure and low-level 925-hPa winds from two reanalyses (NCEP–DOE and ERA-Interim) from 1979 to 2016. The analyses are carried out separately for February–March and for April–June, when temperatures reach their annual peak across most of the region. The WT patterns mix the effects of different multiscale phenomena, including the extratropical Rossby waves that travel on the northern edge of the domain (and are partly related to the North Atlantic Oscillation), the Madden–Julian oscillation, and Kelvin waves in the subequatorial zone. For each WT, warm (cold) minimum (TN) and maximum (TX) daily temperature anomalies tend to be systematically located east of cyclonic (anticyclonic) low-level circulation anomalies associated with the WT patterns. By modulating the greenhouse effect, the water vapor anomalies exert a major influence, leading to warm (cold) TX and TN anomalies associated with moister (drier) air, through advection from the tropical Atlantic or equatorial latitudes (the Sahara or northern latitudes) toward tropical North Africa. WTs are also useful for monitoring interannual variability of TX/TN anomalies mostly north of 10°N in February–March, even if they greatly underestimate the long-term warming trend. Most WTs significantly raise or lower the probability of regional-scale heat peaks, defined as the crossing of the 90th percentile of daily TX or TN.

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Benjamin Pohl
,
Bastien Dieppois
,
Julien Crétat
,
Damian Lawler
, and
Mathieu Rouault

Abstract

During the austral summer season (November–February), southern African rainfall, south of 20°S, has been shown to vary over a range of time scales, from synoptic variability (3–7 days, mostly tropical temperate troughs) to interannual variability (2–8 years, reflecting the regional effects of El Niño–Southern Oscillation). There is also evidence for variability at quasi-decadal (8–13 years) and interdecadal (15–28 years) time scales, linked to the interdecadal Pacific oscillation and the Pacific decadal oscillation, respectively. This study aims to provide an overview of these ranges of variability and their influence on regional climate and large-scale atmospheric convection and quantify uncertainties associated with each time scale. We do this by applying k-means clustering onto long-term (1901–2011) daily outgoing longwave radiation anomalies derived from the 56 individual members of the Twentieth Century Reanalysis. Eight large-scale convective regimes are identified. Results show that 1) the seasonal occurrence of the regimes significantly varies at the low-frequency time scales mentioned above; 2) these modulations account for a significant fraction of seasonal rainfall variability over the region; 3) significant associations are found between some of the regimes and the aforementioned modes of climate variability; and 4) associated uncertainties in the regime occurrence and convection anomalies strongly decrease with time, especially the phasing of transient variability. The short-lived synoptic anomalies and the low-frequency anomalies are shown to be approximately additive, but even if they combine their respective influence at both scales, the magnitude of short-lived perturbations remains much larger.

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