final_program_abstracts[1]

11 IMSC Session Program
Extreme variations in the American Monsoon Systems: An
integrated overview based on a multivariate index
Thursday - Parallel Session 7
Leila M. V. Carvalho 1,2 and Charles Jones 2
1
Department of Geography, University of California Santa Barbara, USA
2 Earth Research Institute, University of California Santa Barbara, USA
The presence of a monsoonal type of circulation involving intense convective activity
and heavy precipitation is a common climatic feature in the Americas during the
respective summer seasons. The seasonal migration of the monsoons in the Americas
has been extensively investigated and important mechanisms on intraseasonal to
interannual (ISI) time scales controlling their variability have been identified. In this
study we show a unified view of the American Monsoon Systems (AMS) by
developing a multivariate index based on combined Empirical Orthogonal Function
(CEOF) Analysis. To derive this index, long-term annual means are first removed
from 5-day averages (pentads) of precipitation (P), zonal and meridional wind
components (U, V), specific humidity (Q) and temperature (T) at 850-hPa.
Precipitation from the Global Precipitation Climatology Precipitation (GPCP) and
atmospheric variables from the National Centers for Environmental
Prediction/National Center for Atmospheric Research reanalysis (NNR) are used. We
show that the first, second and third combined EOFs represent different phases of the
migration of convection over the Americas and characterize the transition of the
monsoon systems from the Southern to Northern Hemisphere and vice-versa. The first
combined EOF (henceforth Large-Scale Index for the American Monsoon – LIAM-1)
shows high correlation with opposite sign between the Mexican and South American
Monsoons. The second combined EOF (EOF-2) shows high correlation with
convection and circulation over Equatorial South America. Wavelet analysis of
LIAM-1 shows clear decadal variability of the amplitude of the AMS, with large
amplitudes from 1995-2008 compared with the previous decades. Large variability on
interannual time-scales is observed for LIAM-2, consistent with the influence of
ENSO and corresponding teleconnections over Equatorial South America. Large
variability in both indexes is observed on intraseasonal timescales. In this study we
explore extreme variations in the AMS by investigating extreme anomalies in LIAM-
1 and LIAM-2 indexes on ISI time-scales. Both indexes are band-filtered and
extremes are defined as the upper and lower quartiles of the respective anomalies. We
investigate the relationships between the extreme anomalies and global sea surface
temperature, precipitation, moisture fluxes and circulation on ISI time-scales. We
show that the enhancement convection over tropical continental South America
during December-February on intraseasonal timescales is associated with the
organization of the Pacific North American Pattern (PNA) and with the strengthening
of the Northern Hemisphere subtropical jet. Variations on interannual timescales are
examined along with variations in the South Atlantic and Pacific SST patterns.
Abstracts 246