Oceans

Oceans

Interest of the oceanic forcing

The WAM is a coupled ocean-continent-atmosphere system that is very sensitive to Sea Surface Temperature (SST) anomalies in the different ocean basins on seasonal, interannual and decadal scales. Even if the relationships between these SST anomalies and rainfall over West Africa are not direct and completely established, because they depend on a multitude of other factors, it is essential to analyze the structure and intensity of these anomalies that will partly condition the intensity and penetration of the WAM. It is necessary to consider 2 types of SST anomaly variability modes: (i) those close to West Africa such as the Atlantic Dipole (Lamb 1978) which has a more direct and rapid effect on WAM variability, and (ii) those further away such as the El Niño-Southern Oscillation (ENSO) which has a more complex and slow effect on WAM via teleconnections.

Relationship between the annual cycles of the WAM and the SST of nearby basins

As the WAM has a strong zonal character due to the zonal orientation of the Guinean coast, it is interesting to analyze its meridional distribution in the [10°W-10°E] band (Figure opposite). The annual cycles of SST in the Gulf of Guinea (GG) and in the eastern Mediterranean appear to be opposite (lower figure). The GG is marked in spring by a strong cooling leading to the formation of the cold water tongue (CT later) in the equatorial band, while the eastern Mediterranean is very cold during the winter and warms up during the summer. At the same time, the continent, which has a lower thermal inertia, warms up very quickly. These thermal contrasts favor both the strengthening of the monsoon flow and a weakening of the Harmattan in the lower layers (blue arrows) allowing the penetration of the monsoon and precipitation on the continent (top figure).

In the absence of other factors such as the relief (low for West Africa), this (phenomenon of “regional breeze”) largely explains the mean annual cycle of the WAM. However, from one year to another, these annual cycles of the SST can be disturbed and thus modify the intensity and penetration of the monsoon. It is therefore necessary to analyze the SST anomalies to predict the quality of the monsoon.

Annual climatological cycle in the [10°W-10°E] band as a function of latitude: (top) precipitation and (bottom) SST in the tropical Atlantic (south of the coast at 5°N) and in the Mediterranean Sea (north of the coast at about 30°N). Source: Thesis of P. Peyrillé (2005).

Teleconnections with the different ocean basins

To illustrate the teleconnections with the SSTs of the different basins we use the results below of a maximum covariance analysis (MCA) between the precipitation over West Africa (dashed rectangle) and the global SSTs for the 72-year period (1916-1987) (Joly et al. 2007).

Pacific

For the highest frequencies, the main mode (HF Mode 1) suggests that the El Niño phase of ENSO disfavors rainfall over the Sahel (West and Central).

Color scale of the correlations
Maximum covariance analysis (MCA) between precipitation (left) and SST (right) for the highest frequency (HF, below 10 years) part of the data and for the 1st mode of variability (mode 1). Source: Modified figure 3 from Joly et al (2007).

Atlantic

The 2nd mode (HF Mode 2) highlights an “Atlantic El Niño” mode that favors rainfall on the Guinean coast but is slightly unfavorable over the Sahel. In other words, a cold tongue in the GG that is less cold than the climatology slows down the penetration of the monsoon over the continent and increases rainfall in the Guinean area.

Maximum covariance analysis (MCA) between precipitation (left) and SST (right) for the highest frequency (HF, below 10 years) part of the data and for the 2nd mode of variability (mode 2). Source: Modified figure 3 from Joly et al (2007).

Inter-hemispheric thermal contrast

For the low frequency (above 10 years, decadal scales) the main mode highlights an inter-hemispheric thermal contrast of the oceans (Folland et al., 1999), unfavourable for rainfall over the Sahel when the southern hemisphere is warmer, and primarily for the Indian Ocean. For example, between the 1950s and 1980s, the southern hemisphere oceans warmed more than those of the northern hemisphere, leading to a transition from abundant rainfall to drought conditions over the Sahel

Maximum covariance analysis (MCA) between precipitation (left) and SST (right) for the lowest frequency (LF, above 10 years) part of the data and for the 1st mode of variability (mode 1). Source: Figure 10 from Joly et al (2007).

Mediterranean

The role of SSTs in the Mediterranean is more delicate to establish because, given the small size of the Mediterranean basin and its lower inertia, it is difficult to separate the causes and consequences between the atmosphere and the Mediterranean sea. Thus, a strengthening of the surface winds (Harmattan) cool the SST very quickly while increasing the ventilation, both leading to the retreat of the monsoon. However, on intra-seasonal scales, it is interesting to analyze the SST anomalies over the Mediterranean that are correlated with the penetration of the monsoon.

Conceptual scheme

To summarize, given the variety of teleconnections with the SST, the Sahel is among the most complex regions of the globe, being influenced by the three tropical oceans, as well as the Mediterranean. This is illustrated in the diagram below. In addition, a significant part of the seasonal rainfall anomalies over the Sahel often

The Sahel often persists from one year to the next, resulting in prolonged periods of drought or abundance. This is also due to the influences of the SSTs on the Sahel.

Schematic illustration of teleconnections with SSTs that often result in below-average rainfall totals over the Sahel and the ‘Guinean coast’ for the period July to September. The red areas over the oceans show areas where warmer than average SSTs are related to drier than average seasons, and the blue areas over the oceans show areas where colder than average SSTs are related to drier seasons. Note that the hatched area over the Atlantic indicates an overlap of areas that affect both the Sahel and the ‘Guinean coast’. Source: Figure 8.3 in the Handbook.

Adapted products

Site MISVA :

  • Weekly SST maps (near Atlantic and Mediterranean) raw and anomalies with respect to the climatology of the week, month and year.
    • encore sous Migration OLD > NEW

Other sources :

Références

Handbook

Section 7.1.2.2        Sea Surface Temperature (SST)          pages 435-436

Section 8.1.2           Teleconnections with SSTs                                pages 491-495

Articles

Folland CK, Parker DE, Colman AW, Washington R. 1999. Large scale modes of ocean surface temperature since the late nineteenth century. In: Beyond El Niño: Decadal and Interdecadal Climate Variability, A Navarra (ed.). Springer-Verlag: Berlin; pp. 73-102.

Joly M., A. Voldoire, H. Douville, P Terray and JP Royer 2007: African monsoon teleconnections with tropical SSTs: validation and evolution in a set of IPCC4 simulations. Clim Dyn (2007) 29:1–20 DOI 10.1007/s00382-006-0215-8

Lamb PJ. 1978. Case studies of tropical Atlantic surface circulation patterns during recent sub-Saharan weather anomalies: 1967 and 1968. Mon. Weather Rev. 106: 482-491.

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