A multi-scale study of the dynamical processes of the tropical Pacific Ocean
Date
2016
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
In recent years, it has been observed that there are different types of El Niño
events. The warm events can be divided into two categories: those centered in the
central Pacific (CP) and those centered in the eastern Pacific (EP). We examined
the variability of western Pacific warm pool (WP) horizontal migration and size from
January 1982 to December 2011 by applying Ensemble Empirical Mode Decomposition
(EEMD) and Hilbert Huang Spectrum (HHS) to the optimally interpolated sea surface
temperature (OISST) data set. The analysis shows that the long-term residual trend of
the zonal centroid movement is migrating to the west by 3.78° from the mean location
during the past 30 years. The size of the warm pool has also increased 18% during this
period. These analysis techniques isolated two separate time series for the migration of
the zonal component of the WPWP for both CP and EP events and showed that these
two types of El Niño generally operate at different time-scales. The EP time-series
shows the strong traditional EP El Niño and the transition between strong El Niño
conditions and La Niña conditions. The CP time-series shows that CP El Niños occur
more often than EP El Niños. The changes of the El Niño type in conjunction with
westward drift and increasing warm pool size shows an interesting multidecadal change
in the warm pool.
To extend the centroid study, we also examine the role of the Western Pacific
Warm Pool (WP) in the past thirty years of mixed interannual variability, decadal
variability, and climate change. Our analysis method involves a multi-dimensional
study of the vertical centroid migration, volume, and heat content of the WP and an
application of the advanced time-series analysis technique known as Multidimensional
Ensemble Empirical Mode Decomposition (MEEMD). We show the 30-year evolution
of the warm pool and how it interacts with the PDO, ENSO, and anthropogenic climate
forcing mechanisms. Our results show that the WP increasing in size and increasing
in heat content anomaly in response to global warming forcing.
The variability of the South Pacific Convergence Zone (SPCZ) is evaluated using
ocean surface wind products derived from the ERA-Interim atmospheric reanalysis for
the period of 1981-2014 and QuickSCAT satellite scatterometer for the period of 1999-
2009. From these products, indices were developed to represent the SPCZ strength,
area, and centroid location. Excellent agreement is found between the indices derived
from the two wind products during the QuikSCAT period in terms of the spatiotemporal
structures of the SPCZ. The longer ERA-Interim product is used to study the
variations of SPCZ properties on intraseasonal, seasonal, interannual, and decadal time
scales. The SPCZ strength, area, and centroid latitude have a dominant seasonal cycle.
In contrast, the SPCZ centroid longitude is dominated by intraseasonal variability
due to MJO influence. The SPCZ indices are all correlated with El Niño-Southern
Oscillation indices. Interannual and intraseasonal variations of SPCZ strength during
strong El Niño are approximately twice as large as the respective seasonal variations.
SPCZ strength depends more on the intensity of El Niño rather than the central vs.
eastern-Pacific type. The change from positive to negative PDO around 1999
results in a westward shift of the SPCZ centroid longitude, much smaller interannual
swing in centroid latitude, and a decrease in SPCZ area. This study improves the
understanding of the variations of the SPCZ on multiple time scales and reveals the
variations of SPCZ strength not reported previously. The diagnostics analyses can be
used to evaluate climate models to gauge their fidelity.
We have analyzed multiple data sets (wind, SST, heat content) in an effort to
better understand the the WP and the SPCZ. As a major component of the tropical
Pacific Ocean dynamics and global climate dynamics, these two intertwined components
are critical to study. This is especially true in the context of global climate
change.