Three-dimensional characterization of boreal spring-summer climate variability over West Africa

The main scientific interest is to characterize the three-dimensional boreal spring-summer climate anomaly patterns from 1948-2006 using monthly University of Delaware terrestrial precipitation, ERSST, and NCEP/NCAR reanalysis data for two major rainfall seasons, March-April-May-June(MAMJ) and June-July-August-September (JJAS) over tropical West Africa. The study is organized into two parts, which entail the use of: (i) unfiltered data for general characterization, and (ii) a 10-year low-pass-filtered data to focus on decadal/low frequency (LF) structures.

Analysis techniques involve application of EOF analysis, simultaneous heterogeneous grid point correlations, linear/partial correlations, composite and simple linear regression analyses. EOF analysis is performed on West African precipitation and global SSTs from which the leading and most relevant PCs are retained. Grid point correlations between the total precipitation time coefficients (TPTCs) and global SST anomalies (SSTAs) are computed to highlight spatial SST mode(s) associated with the precipitations. Linear/partial correlations between TPTCs and 19 well-known climatic indices are computed to highlight the most important indices associated with the temporal character of the precipitations. Composite analysis is computed based on the positive (negative) phases of the decomposed TPTCs, using vertical motion, horizontal wind, and geopotential height (GH) anomaly fields in the monsoon and midtropospheric layers, represented by 1000 hPa, 850 hPa, and 500 hPa isobaric surfaces. Velocity potential (divergence) and streamfunction (rotational) are computed from the horizontal wind fields. The linear regression model is used to evaluate the functional relationship between the net precipitation and net vertically integrated moisture budget (NVIMB) anomalies for the two seasons over West Africa, Sahel, and Gulf of Guinea Coast (GOGC), and also, test the sensitivity of the divergent circulations over these climatic zones.

The composite analyses show that the unfiltered circulations at the surface and 850 hPa levels for each of the seasons are similar and are closely related to contrasting SST patterns over the Mediterranean Sea, Atlantic and Indian Oceans and the associated horizontal winds, as well as their divergent and rotational flows. At 500 hPa, the winds are more intense and well organized. Within and between the seasons, the commonalities and disparities are highlighted on the basis of SSTAs, nature of the horizontal winds, rotational wind fields- number, intensity, location, and size of their cyclonic/anticyclonic flows, as well as centers of action of their divergent circulations. These features, which generally coincide with the GH and omega fields, together, reflect the precipitation patterns over the region. However, the MAMJ divergent circulations generally do not synchronize with the omega and precipitation fields, thus, appear to be closely related to subtropical Saharan high and Saharan thermal low. The regression model reveals that while the other two zones are insignificant, 70% unfiltered JJAS Sahel net precipitation anomalies (NPAs) can be explained by NVIMB anomalies (r= +0.84), implicitly, suggesting their compatibility with the divergent circulations. The LF circulation anomalies are similar to the generalized events, suggesting that the latter are dominated by at least the first two LF modes. However, the features, which serve as the main dividers between these two and also, within and between the LF events of the two seasons relate to intensely contrasting SST patterns, which may be coupled to intense LF atmospheric circulations, manifesting themselves especially in the rotational and divergent fields. The regression analysis also shows that 73% LF JJAS Sahel NPAs can be explained by LF NVIMB anomalies (r= -0.85), linking them to divergent circulations. The most outstanding conclusion is that the two dominant and competing mechanisms that drive precipitation over Sahel and GOGC are momentum convergence (divergence) and horizontal advection, respectively. These two terms will be investigated further using a combination of observation and modeling techniques to provide valuable insights into their properties.