Synoptic timescale linkage between midlatitude winter troughs Sahara temperature patterns and northern Congo rainfall: A building block of regional climate variability

Abstract

A coherent synoptic sequence, mostly over North Africa, is identified whereby an upper‐level midlatitude trough (in November–March) excites several days of quasi‐stationary near‐surface warming across the Sahara, leading to rainfall events over northern Congo (NC), and perturbed weather more widely. Ahead of NC rainfall events, composite sequences first identify troughs for several days near Iberia, followed by relatively quick transfer to the Central Mediterranean (CMed). Iberia and CMed daily trough‐strength indices reveal that both lead to warming and NC rainfall. Iberia trough linkages develop through West Africa and take longer to reach NC, while CMed linkages reach NC faster (2–3 days), with impact extent focused mostly south and east of CMed. Building up to the rainfall events, initial warming over the central Sahara migrates southeastward close to NC, ultimately with typical magnitude of about 1–2°C at 10–15°N. Such anomalies are statistically predictive for NC daily rainfall and associated nearby atmospheric features: anomalous low‐level southerly wind and increased moisture; anomalous low‐level westerly wind and vertical easterly shear to 600 hPa; increased mid‐level moisture (600 hPa), which along with low‐level moisture, connects northward into midlatitudes. A secondary route identified by which Iberia troughs can impact NC rainfall is through direct atmospheric teleconnection with precipitation to the west of NC, and subsequent migration of that convection eastward into NC. The eastern side of NC generally shows a small lag on western parts, and links more strongly to CMed troughs. Taken together, the lagged synoptic expression of Iberia and CMed troughs is widespread over several days, including much of North Africa (to equatorial latitudes), southwestern Asia, eastern Africa and the western Indian Ocean. Overall, these results can contribute to situational awareness for weather forecasters across the zones influenced by the troughs, while also providing a framework for climate timescale analyses.

West Africa's moist convective environment as observed by the Atmospheric InfraRed Sounder (AIRS)

“…This results in an approximate 3 ° northward surge anomaly on the thunderstorm days. The anomalous northward displacement of the ITD in DJF has been attributed to the interaction of the feature with the mid‐latitude trough (Parker and Diop‐Kane, 2017), and upper‐level trough (Knippertz and Fink, 2009; Ward et al ., 2021). It is found over West Africa that, December–March days in the upper quintile of upper‐level trough strength occur when the ITD is displaced on‐average 1.5 ° north of its climatological position (Ward et al ., 2022).…”

Mid‐level dry air intrusions over the southern Maritime Continent

“…Several researchers have associated amplification of upper level troughs with enhanced convection at the edges and to the east of descending dry air masses elsewhere, thereby steering convection (e.g., Tompkins, 2001;Pohl et al, 2009;de Vries et al, 2016). DAIs and the associated disturbances themselves are driving the flow of moist air necessary to form convection, observed in past studies such as Knippertz (2007) and Ward et al (2021) over Africa.…”

“…However, the environment near to the intrusion must be moist enough to be conducive to the formation of deep convection (de Vries et al, 2016; de Vries et al, 2018; Kumar et al, 2019). As DAIs themselves modify the lower‐level wind field, they can therefore enhance the moisture flux enabling convection downstream (Knippertz, 2007; Rodwell, 1997; Ward et al, 2021).…”

Predictability of the East Africa long rains through Congo zonal winds

“…Possible sources of model error pointed to include the weak amplitude of NAO signals (e.g., Figure 1a) and errors in tropical Atlantic variability. In addition, in terms of EA rainfall linkage to northern midlatitudes, the primary role of the NAO in MAM is likely gradually replaced with linkages to sharper upper‐level troughs and ridges over the Mediterranean region (Camberlin & Philippon, 2002; Wainwright et al, 2022; Ward et al, 2021) during months that are closer to winter (e.g., February–March), motivating assessment at monthly resolution.…”