@article{fdi:010046168,
  title = {{S}easonal evolution of the {W}est {A}frican heat low : a climatological perspective},
  author = {{L}avaysse, {C}. and {F}lamant, {C}. and {J}anicot, {S}erge and {P}arker, {D}. {J}. and {L}afore, {J}. {P}. and {S}ultan, {B}enjamin and {P}elon, {J}.},
  editor = {},
  language = {{ENG}},
  abstract = {{T}he {W}est {A}frican heat low ({WAHL}), a region of high surface temperatures and low surface pressures, is a key element of the {W}est {A}frican monsoon system. {I}n this study, we propose a method to detect the {WAHL} in order to monitor its climatological seasonal displacement over {W}est {A}frica during the period 1979-2001, using the {E}uropean {C}entre for {M}edium-range {W}eather {F}orecast ({ECMWF}) {ERA}-40 reanalyses. {T}he low-level atmospheric thickness ({LLAT}), a variable defined as the difference of geopotential heights at 700 and 925 h{P}a, is used to detect the dilatation of these levels generated by an increase of the temperature. {W}e define grid points with 10% highest values of the {LLAT} as the {WAHL}. {W}e show that our method reliably positions the {WAHL} over areas of high surface temperatures and low surface pressures, and that it is effective at detecting heat lows. {I}n the course of the year, the climatological {WAHL} is shown to migrate north-westward from a position south of the {D}arfur mountains in the winter ({N}ovember-{M}arch) to a location over the {S}ahara, between the {H}oggar and the {A}tlas mountains, during the summer ({J}une-{S}eptember). {T}he temperature tendency equation is used to investigate the processes controlling the displacement of the {WAHL}, and more particularly the heating at low levels. {T}he specific period of the onset of the {WAHL} in its summer location over the {S}ahara (referred to as the {S}aharan heat low -{SHL}- onset) is also analysed during the 1984-2001 period, using complementary brightness temperature data from the {E}uropean {U}nion-funded {C}loud {A}rchive {U}ser {S}ervice ({CLAUS}). {T}he climatological onset of the {SHL} occurs around 20 {J}une, i.e. just before the climatological monsoon onset date. {T}he present study suggests that the onset of the {WAHL} occurs approximately 5 days before the monsoon onset for the 1984-2001 period. {T}his is confirmed independently by comparing the {SHL} onset date and the monsoon onset date for the 1984-2001 period. {T}he seasonal evolution of the {WAHL} for the year 2006 (the year of the {A}frican {M}onsoon {M}ultidisciplinary {A}nalysis project {S}pecial {O}bservation {P}eriod) is analysed and compared with the climatological results. {T}he operational {ECMWF} analyses were used for that purpose. {E}xcept in {A}pril, the spatial distribution of the {WAHL} remains relatively unchanged and agrees with the climatology. {T}he onset of the {SHL} in 2006 occurs on 18 {J}une, which is close to the climatological date, in spite of the delay in the onset of the rainy season in {S}ahel.},
  keywords = {{W}est {A}frica ; {H}eat low ; {M}onsoon ; {C}limate ; {L}ow level circulation ; {G}eopotential height},
  booktitle = {},
  journal = {{C}limate {D}ynamics},
  volume = {33},
  numero = {2-3},
  pages = {313--330},
  ISSN = {0930-7575},
  year = {2009},
  DOI = {10.1007/s00382-009-0553-4},
  URL = {https://www.documentation.ird.fr/hor/fdi:010046168},
}