A Regional Climate Mode Discovered in the North Atlantic: Dakar Niño/Niña - PubMed (original) (raw)
A Regional Climate Mode Discovered in the North Atlantic: Dakar Niño/Niña
Pascal Oettli et al. Sci Rep. 2016.
Abstract
The interrannual variability of coastal sea surface temperature (SST) anomalies confined off Senegal is explored from a new viewpoint of the ocean-land-atmosphere interaction. The phenomenon may be classified into "coastal Niño/Niña" in the North Atlantic as discussed recently in the Northeastern Pacific and Southeastern Indian Oceans. The interannual variability of the regional mixed-layer temperature anomaly that evolves in boreal late fall and peaks in spring is associated with the alongshore wind anomaly, mixed-layer depth anomaly and cross-shore atmospheric pressure gradient anomaly, suggesting the existence of ocean-land-atmosphere coupled processes. The coupled warm (cold) event is named Dakar Niño (Niña). The oceanic aspect of the Dakar Niño (Niña) may be basically explained by anomalous warming (cooling) of the anomalously thin (thick) mixed-layer, which absorbs shortwave surface heat flux. In the case of Dakar Niña, however, enhancement of the entrainment at the bottom of the mixed-layer is not negligible.
Figures
Figure 1. Composite of SST anomalies during January-April after the peak season of El-Niño years in the equatorial Pacific/north tropical Atlantic (period 1982–2011).
The figure was plotted by R software.
Figure 2. Monthly SST climatology (contour) and its variability (shading), with 10-m winds (vector).
Only isotherms between 20 °C and 30 °C are displayed in blue. The green rectangle in (c) is the region selected to study the local SST variability. The figure was plotted by R software.
Figure 3
(a) Monthly standard deviation of standardized coastal SST index and coastal wind index based on ERA Interim in the region [21°–17°W, 9°–14°N] for the period 1982–2011, (b) Same as (a) but with coastal wind index based on NCEP/DOE AMIP II, (c) Time series of the standardized coastal SST index (black line), standardized coastal wind index (black two-dash line) and standardized coastal SST index in March (colored bars) in the region [21°–17°W, 9°–14°N] for the period 1982–2011. Colors correspond to identified warm (red), neutral (dark grey) and cold (blue) events. Horizontal dotted lines denote the ± 0.8σ (0.57 °C). The figure was plotted by R software.
Figure 4. Lead/lag monthly correlation coefficients between DNI and itself, coastal wind (CWI), mixed layer depth (MLDI) and coastal upwelling (CUI) indices, over the 1982–2011 period.
The inverted autocorrelation of DNI is also displayed in the figure (grey solid line) for convenience. Correlation coefficients significant at the 95% confidence level (according to a random-phase test) are denoted by a filled circle. The figure was plotted by R software.
Figure 5
Time series of composite anomalies of the (a) DNI SST index, (b) CWI (coastal wind index), (c) gSLPI (cross-shore pressure gradient index), (d) EKMI (Ekman pumping index), (e) CUI (coastal upwelling index), (f) MLDI (mixed-layer depth) and (g) SWI (shortwave radiation) index for warm (solid line) and cold (dashed line) events, and neutral state (dotted line). Solid, dashed and dotted lines are the values of each index during warm events, cold events and neutral state, respectively. Values significant at the 95% confidence level (according to a two-sided Student’s _t_-test with 10,000 permutations) are denoted by a filled circle. The figure was plotted by R software.
Figure 6
Mixed-layer heat balance during (a) Dakar Niño and (c) Dakar Niña, and surface heat flux contribution during (b) Dakar Niño and (d) Dakar Niña. In (a,c), the time series of composite anomalies of the mixed-layer temperature tendency (bold solid line) and its three component: net surface heat flux (bold two-dash line), horizontal advection (solid line) and entrainment (dashed line). In (b,d), the time series of composite anomalies of the surface heat flux contribution (bold two-dash line) and its four components: shortwave radiation (bold solid line), longwave radiation (dashed line), sensible heat (two-dash line) and latent heat (solid line) fluxes. Values significant at the 90% and 80% confidence level (according to a two-sided Student’s _t_-test with 10,000 permutations) are denoted by a filled circle and an open circle, respectively. The figure was plotted by R software.
Figure 7
Composite analysis of SST (contour and shading) and 10-m winds (vector) anomalies from February to April, and for the period 1982–2011, during (a) Dakar Niño and (b) Dakar Niña. Only wind values significant at the 95% confidence level according to a two-sided Hotelling’s T2-test are displayed. Significant SST values at the 95% confidence level (according to a two-sided Student’s _t_-test) are shaded. The figure was plotted by R software.
Figure 8. Vertical cross-section of lead-lag correlation coefficients between DNI in March and anomalies in air temperature (shading, values below 0.4 omitted), geopotential height (contour, 0 omitted) and vertical velocity (arrow).
Atmospheric variables are averaged in latitude between 7.5°N and 15°N. The location of the Dakar Niño/Niña is symbolized by a black rectangle. The location of the land is symbolized by a grey rectangle. The figure was plotted by R software.
References
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