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  • Author or Editor: Benjamin Pohl x
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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
,
Thomas Saucède
,
Vincent Favier
,
Julien Pergaud
,
Deborah Verfaillie
,
Jean-Pierre Féral
,
Ylber Krasniqi
, and
Yves Richard

Abstract

Daily weather regimes are defined around the Kerguelen Islands (Southern Ocean) on the basis of daily 500-hPa geopotential height anomalies derived from the ERA5 ensemble reanalysis over the period 1979–2018. Ten regimes are retained as significant. Their occurrences are highly consistent across reanalysis ensemble members. Regimes show weak seasonality and nonsignificant long-term trends in their occurrences. Their sequences are usually short (1–3 days), with extreme persistence values above 10 days. Seasonal regime frequency is mostly driven by the phase of the southern annular mode over Antarctica, midlatitude dynamics over the Southern Ocean such as the Pacific–South American mode, and, to a lesser extent, tropical variability, with significant but weaker relationships with El Niño–Southern Oscillation. At the local scale over the Kerguelen Islands, regimes have a strong influence on measured atmospheric and oceanic variables, including minimum and maximum air temperature, mostly driven by horizontal advections, seawater temperature recorded 5 m below the surface, wind speed, and sea level pressure. Relationships are weaker for precipitation amounts. Regimes also modify regional contrasts between observational sites in Kerguelen, highlighting strong exposure contrasts. The regimes allow us to improve our understanding of weather and climate variability and interactions in this region; they will be used in future work to assess past and projected long-term circulation changes in the southern midlatitudes.

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