Abstract
The El Nino Southern Oscillation plays a key role in modulating interannual rainfall variability in Mexico. While El Nino events are linked to drought in Mexico, uncertainty exists about the spatial pattern and causal mechanisms behind El Nino-induced drought. We use lead/lag correlation analysis of rainfall station data to identify the spatial pattern of drought associated with the summer before, and the spring following, the peak of warm SST anomalies in the eastern equatorial Pacific. We also use atmospheric fields from the North American Regional Reanalysis to calculate the anomalous moisture budget and diagnose the mechanisms associated with El Nino-induced drought in Mexico. We find that reduced rainfall occurs in Mexico in both the summer before and the spring after a peak El Nino event, especially in regions of climatologically strong convection. The teleconnection in the developing phase of El Nino is primarily driven by changes in subsidence resulting from anomalous convection in the equatorial Pacific. The causes of drought during the decaying phase of El Nino events are varied: in some years, descent anomalies dominate other moisture budget terms, while in other years, drying of the boundary layer on the Mexican plateau is important. We suggest that the latter may result from the interaction of weakened southeasterly winds in the Intra-Americas Sea with high topography along the Atlantic coast of Mexico. Weakened winds are likely driven by a reduced sea level pressure gradient between the Atlantic and the Pacific. Changes in easterly wave activity may contribute to drought in the developing phase of El Nino, but may be less important in the decaying phase of El Nino.
Highlights
The role of El Nino events in causing summer drought across Mexico is well documented (Magana et al 2003; Cavazos and Hastenrath 1990; Seager et al 2009; Englehart and Douglas 2002; Gochis et al 2007)
We clarified the intraseasonal evolution of the ENSO teleconnection to Mexican rainfall, and explored the atmospheric changes associated with El Nino-induced drought in Mexico
Our major findings resulted in an explanation for the changing intraseasonal spatial pattern of ENSO-induced negative rainfall anomalies, an evaluation of the anomalous subsidence versus land-sea thermal contrast mechanisms proposed for the developing phase of an El Nino event, as well as an understanding of the diverse mechanisms that may result in drought in the spring following a warm ENSO event
Summary
The role of El Nino events in causing summer drought across Mexico is well documented (Magana et al 2003; Cavazos and Hastenrath 1990; Seager et al 2009; Englehart and Douglas 2002; Gochis et al 2007). Giannini et al (2001) documented this phenomenon in the Caribbean during the summer before a peak El Nino event, and by Seager et al (2009) in a modeling study for Mexico This subsidence may be driven by the ability of anomalous convection in the eastern equatorial Pacific to warm the tropical troposphere, suppressing convection in remote tropical regions, other mechanisms may drive subsidence (Chiang and Sobel 2002). We assess the intraseasonal evolution of the spatial pattern of ENSO-induced drought using lead and lag correlation analysis This analysis helps us distinguish between the changes in the teleconnection in the developing versus the decaying phase of anomalously warm Nino 3.4 region SSTs. Second, we use a diagnostic approach to shed light on the mechanisms underlying drought by analyzing the anomalous moisture budget associated with the 1997–1998 El Nino event. We analyze the composite moisture budget anomalies for El Nino events from 1979 to 2007, in order to see how the dynamical mechanisms associated with rainfall anomalies in Mexico may differ across El Nino events
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