Abstract
On seasonal and inter-annual time scales, vertically integrated moisture divergence provides a useful measure of the tropical atmospheric hydrological cycle. It reflects the combined dynamical and thermodynamical effects, and is not subject to the limitations that afflict observations of evaporation minus precipitation. An empirical orthogonal function (EOF) analysis of the tropical Pacific moisture divergence fields calculated from the ERA-Interim reanalysis reveals the dominant effects of the El Nino-Southern Oscillation (ENSO) on inter-annual time scales. Two EOFs are necessary to capture the ENSO signature, and regression relationships between their Principal Components and indices of equatorial Pacific sea surface temperature (SST) demonstrate that the transition from strong La Nina through to extreme El Nino events is not a linear one. The largest deviation from linearity is for the strongest El Ninos, and we interpret that this arises at least partly because the EOF analysis cannot easily separate different patterns of responses that are not orthogonal to each other. To overcome the orthogonality constraints, a self-organizing map (SOM) analysis of the same moisture divergence fields was performed. The SOM analysis captures the range of responses to ENSO, including the distinction between the moderate and strong El Ninos identified by the EOF analysis. The work demonstrates the potential for the application of SOM to large scale climatic analysis, by virtue of its easier interpretation, relaxation of orthogonality constraints and its versatility for serving as an alternative classification method. Both the EOF and SOM analyses suggest a classification of “moderate” and “extreme” El Ninos by their differences in the magnitudes of the hydrological cycle responses, spatial patterns and evolutionary paths. Classification from the moisture divergence point of view shows consistency with results based on other physical variables such as SST.
Highlights
Around 60 % of the terrestrial precipitation directly originates from moisture transported from the ocean (Trenberth et al 2007; Gimeno et al 2012)
We have used empirical orthogonal function (EOF) and self-organizing map (SOM) analyses to characterize the spatial patterns of inter-annual variability in the atmospheric moisture divergence over the tropical Pacific, a key component of the hydrological cycle that is linked directly to anomalies in the surface water balance (E−P)
This variability is dominated by El Niño-Southern Oscillation (ENSO) influences, with the moisture divergence shifting eastwards to follow the eastward shift of the warmest equatorial sea surface temperature (SST) during moderate El Niños, accompanied by an equatorward rotation of the South Pacific Convergence Zone (SPCZ)
Summary
Around 60 % of the terrestrial precipitation directly originates from moisture transported from the ocean (Trenberth et al 2007; Gimeno et al 2012). Most of the major oceanic source regions of atmospheric moisture are confined to the tropics and subtropics, where the high sea surface temperature (SST) and anticyclonic circulations provide favorable conditions for evaporation to occur under clear sky conditions. The surplus evaporation (E) over precipitation (P) provides a useful estimate of the net water input to the atmosphere (E−P). Evaporation from reanalysis is not constrained by precipitation and radiation (Hartmann et al 2013), spurious trends and biases can be introduced by changing satellite observations E and P computed oceanic freshwater fluxes show poorer performance in closing the water budget, compared with atmospheric moisture fluxes derived values (Rodríguez et al 2010)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
