TITLE:
MJO Modulation of Station Rainfall in the Semiarid Seridó, Northeast Brazil
AUTHORS:
Cati E. A. Valadão, Paulo S. Lucio, Rosane R. Chaves, Leila M. V. Carvalho
KEYWORDS:
Madden-Julian Oscillation, Intraseasonal, Raingauge
JOURNAL NAME:
Atmospheric and Climate Sciences,
Vol.5 No.4,
October
9,
2015
ABSTRACT: The
influence of the Madden Julian Oscillation (MJO) on station rainfall over the
Seridó region of Rio Grande do Norte state, Northeast Brazil is examined based
on 17 raingauge daily data over 30-year period (1 January-31 December,
1981-2010). The Seridó is one of the driest regions in Northeast Brazil and is
recognized as particularly vulnerable to desertification by the United Nations
Convention to Combat Desertification. Firstly, daily anomalies were calculated
by removing the 30-year daily climatology. Then, to distinguish the MJO signal
from other patterns of climate variability, the daily anomalies were band pass
filtered in the frequency domain (20 - 90 days) by applying Fast Fourier
Transform (FFT). Composites of rainfall anomalies were computed for each of the
eight phases of the MJO during February-May (FMAM) rainy season, based on the
Jones-Carvalho MJO index. Only days classified as MJO events were considered in
the composites. For each phase composite, statistical significance tests were
computed independently at each individual station by applying a two-tailed
Student’s t-test at 5% significance level. Preliminary results showed that the
rainfall anomalies have a spatial coherence throughout the MJO cycle. Extreme
positive (negative) anomalies occurred in phase 2 (phase 5), where 13 (12) out
of the 17 stations showed statistically significant anomalies in the range of
0.9 - 1.9 mm/day (0.8 - 1.7 mm/day). The typical difference between the wet MJO
phase 2 and dry phase 5 represented at least 50% modulation of the daily mean
rainfall.