Heat Low

Heat Low

Definition and interest of the HL

The penetration and retreat of the monsoon over the African continent is largely driven by the maximum Saharan thermal low at the surface. In general, thermal lows (HL) are limited to the lowest layers of the atmosphere, but during the boreal summer due to strong warming over West Africa, the HL has a very large vertical extension, sometimes up to 600 hPa, and a high intensity. The potential temperature field at 850 hPa is a convenient and efficient product to identify the HL (Ɵ850> 38°C).

Annual cycle of the HL

The thermal low identified by its potential temperature at 850 hPa, oscillates during its annual cycle between two positions illustrated by the figure below (Lavaysse et al. 2009). In winter it extends zonally along 10°N with a maximum over the Democratic Republic of the Congo (DRC), then migrates northwest to settle during the northern summer over the Sahara between the Hoggar, Tibesti and Atlas mountains. In autumn it returns southward following the annual cycle of the sun with a delay of about 2 months.

Probability of HL occurrence in summer (right JJAS) and winter (left November to April). Relief areas above 925 hPa are shown in gray. Personal communication from C. Lavaysse.

The animation opposite illustrates the annual climatological cycle of the HL (1979-2001). The surface flow allows to identify: the ventilation zones coming from the Atlantic and the Mediterranean, also channeled by the reliefs (in grey), as well as the monsoon flow coming from the Gulf of Guinea. These low-level flows are partly driven by the HL acting as a vacuum cleaner. The convergence between these northern and southern flows (blue isoline) determines the areas favorable to convection.

This corresponds to a mean position of the LH which presents a strong spatio-temporal variability of its location and its intensity, especially on diurnal and intraseasonal scales.

Animation of the annual climatological cycle of the HL position (probability in color) and the wind at 925 hPa (vectors, in m/s). Personal communication from C. Lavaysse.

Conceptual model of the LH intra-seasonal mode

In addition to a strong diurnal cycle, the HL presents a mode of intra-seasonal variability with a period of about 15 days, marked by two opposite phases illustrated by the figure below.

  • During the Eastern phase (HLE) the ventilation by the northern wind at 850 hPa is strengthened over the Mauritanian-Moroccan coast and on the contrary weakened over the eastern Mediterranean. The result is a strengthening of the HL in the east and conversely its weakening in the west with a shift of its position to the east away from the Atlantic coast. This is coherent with the strengthening of the trade winds and the Azores high, which is more intense and narrower.
  • The situation reverses during the Western phase (HLW) with an increased ventilation over the eastern Sahel. This phase is characterized by an extension of the Azores high over Europe and its weakening over the Atlantic.
Composite structure of the HLE (left) and HLW (right) phases. Top for temperature (shaded above 38°C to identify HL) and wind at 850 hPa. Bottom the MSLP reduced pressure field (color) and wind at 925 hPa.

The HL mode modulates the convective activity over the Sahel as illustrated by the composite below.

  • During the HLE phase, the monsoon system strengthens in the East with a more powerful HL, resulting in an increase of the convective activity.
  • Then, the HL weakens due to the strengthening of the ventilation in the East (corresponding to the arrival of a “Cold surge” event as described by Vizy and Cook, 2009), making the ITD retreat and reducing the convective activity in the East.
  • Convection then propagates westward (~7 ms-1) to reach the western Sahel during the HLW phase one week after the HLE phase, enabling a strengthening of the WAM in the west and of the convection.
Lead-lag composite evolution of the OLR anomaly (averaged between 12.5 and 17.5N) between HLW and HLE phases. The x and axis represent the longitude and the lag in days respectively. Shading and contour intervals are every 2 W m-2. The thick line outlines areas with a confidence level greater than 95%. Source: Adapted from Chauvin et al. (2010).

Main characteristics

Diurnal cycle

  • The HL reaches its maximum (thus minimum surface pressure) in late afternoon, in response to the intense diurnal heating.
  • The wind response is not instantaneous, so the wind maximum occurs late at night.
  • In general, the ITD follows the position of the HL pressure minimum, but there is a northward extension of the HL in the afternoon.
  • The surface convergence on the ITD reaches its maximum between 0000 and 0600 UTC, it is weaker during the day.

Intra-seasonal mode

  • Period ~15 days. Opposition between the East and the West of the Sahel
  • HLE phase : Weak ventilation in the East with a HL strengthening of the HL, the monsoon flow and the convective activity.
  • HLW phase : Strong cooling over the East Mediterranean spreading southward towards the East Sahel (~4 ms-1) which it reaches 4 days later, corresponding to a “Cold surge” event that reduces convective activity. On the other hand, over the western Sahel, the LH strengthens and extends as it approaches the Atlantic coast and convective activity intensifies.
  • Between the 2 phases, the strengthening of convection moves from East to West.
  • The switch between HLE and HLW phases is favorable to the monsoon onset over the western Sahel.
  • This HL intra-seasonal mode is forced by the Rossby waves of the mid-latitudes that precede the HL mode by 4 days (see the object “Interactions with the mid-latitudes” for more details).
  • It also interacts with equatorial waves (Rossby waves and Sahelian mode) – (see object “Equatorial waves” for more details).

The adapted products

Références

Section 2.1.2.1 Saharan Heat Low pages 41-43

Handbook

Section 11.3 WASA-F: Tracé HL pages 707-711

Articles

Chauvin F, Roehrig R, Lafore JP. 2010. Intraseasonal variability of the Saharan heat low and its link with midlatitudes. J. Climate. 23: 2544-2561.

Lavaysse, C., C. Flamant, S. Janicot, D. J. Parker, J.-P. Lafore, B. Sultan, and J. Pelon, 2009: Seasonal evolution of the West
African heat low: A climatological perspective. Climate Dyn., 33, 313–330

Roehrig R, Chauvin F, Lafore J-P. 2011. 10-25 day intraseasonal variability of convection over the Sahel: a role of the Saharan heat low and midlatitudes. J. Climate 24: 5863-5878.

Vizy EK, Cook KH. 2009. A mechanism for African monsoon breaks: Mediterranean cold air surges. J. Geophys. Res. 114 : D01104. doi :10.1029/2008JD010654.

Vizy EK, Cook KH. 2014. Impact of cold air surges on rainfall variability in the Sahel and wet African tropics: a multi-scale analysis. Clim. Dyn. 43: 1057-1081. doi: 10.1007/s00382-013-1953-z.

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