N. CAMPOS, MARLOS GÓES, ANDRÉA S. TASCHETTO, ILANA WAINER
Oceanográfico da Universidade de São Paulo
Tropical Atlantic Ocean variability has been the target of several studies
in the last decades. Differently from Pacific Ocean variability which
presents an interannual signal (3-5 years) dominated by El-Niño/Southern-Oscillation
(ENSO) phenomena (Wyrtki, 1975; Leetmaa et al., 1983), the Atlantic variability
shows the greatest peak in the seasonal scale (Merle et al., 1980). In
lower frequencies, the Atlantic Ocean presents two main modes of variability
: an equatorial mode similar to Pacific ENSO, and a decadal mode associated
with an inter-hemispheric sea surface temperature (SST) gradient symmetric
about Inter-tropical Convergence Zone (ITCZ). Decadal and multidecadal
scales of variability in the Tropical Atlantic were already shown to exist
in the Tropical Atlantic by Mehta and Delwoth (1995).
The Equatorial mode, which is the subject of this study, is predominantly
associated with ocean dynamics. Changes in the trade wind system in the
western Equatorial Atlantic basin causes the thermocline to adjust, which
in turn impacts on the equatorial upwelling pattern. Kelvin waves are
triggered and cross the Atlantic basin in few weeks. These waves are reflected
as Rossby waves or trapped at the African coast propagating polewards
generating a higher latitude east-west response.
The events associated with the equatorial mode have important local social-economic
consequences. For example, tuna fishing in open ocean and distribution
of several pelagic species along the African shore are considerably affected.
Adjacent coastal areas also feel the ocean temperature variation as changes
in the regional distribution and intensity of precipitation (Servain et
In the present work the equatorial mode was obtained through an empirical
orthogonal function analysis (EOF) as the dominant mode of variability
in the Tropical Atlantic, showing periods of warming or cooling along
the equatorial band. Based on it, we will build upon the study of Carton
and Huang (1994), who have identified and characterized warm events in
the region but using a shorter record.
The SST data used here is based on monthly means since January 1964 to
December 1999 on a 2°x2° grid. The study area extends from 30°N to 20°S
and from to 60oW to African coast. Merchant ships measurements that integrate
the Volunteer Observing System (VOS), data obtained from the National
Climatic Data Center (NCDC) and data from the National Meteorological
Center (NMC) compose this product (Servain and Lukas, 1990).
Figure 1 presents the two main EOF modes and the associated time series
of SST anomalies
(with respect to the annual cycle) for the Tropical Atlantic. The EOF
modes were obtained using conventional techniques (Matlab). EOF1 (figure
1a) explains 26% of total SST variability and is associated with a generalized
warming trend of the whole basin. EOF2 (figure 1b), which explains 16%
of SST variability, corresponds to the interhemispheric gradient of SST
to evaluate the occurrence of warm and cold events in a fashion consistent
with the criteria used by Zebiak (1993) and Carton e Huang (1994), the
SST anomalies time series for the equatorial area between 6oS to 2oN and
20°W to 10°E is examined (Figure 2). This region, allows an easier identification
of extremes events due to its great variability about the Equator.
criterion for extreme years used was the occurrence of SST anomalies with
absolute value greater than 1°C, lasting for more than a month. Using
this definition 1964, 1965, 1967, and 1976 were defined as cold years
and 1984, 1987, 1997, 1998, and 1999 were defined as warm years. Thus,
before 1980 four cold episodes occurred in the studied period, while after
this date 5 warm episodes occurred. This fact must be associated with
the positive trend observed in figure 2, where through the least squares
method a temperature rise of 0.0221°C/yr reaching an overall value of
0.7956°C for the whole period is observed.
It is clear that SST anomalies are predominantly negative before 1984
and become basically positive after this date. The warm episodes for the
Atlantic, according to the adopted selection criteria present a frequency
of occurrence close to that of the ENSO phenomena in the Pacific Ocean,
however, the greatest correlation between the SST anomalies time series
of figure 2 and the Southern Oscillation Index is not greater than 0.3.
The SST anomalies series presents a visible trend associated with the
equatorial variability. This way the cooling events observed earlier in
the period were more intense and frequent while after 80's the warming
events have become more intense and frequent. Trends like the one showed
here can also be observed in others data sets as DASILVA (Da Silva et
al., 1994) and GOSTA (Bottomley et al., 1990). Some authors associate
the positive SST trend to natural variability, anthropogenic effects or
even a combination of both factors (Cane et al., 1997). Other authors
consider these trends a result of changes in measurement techniques and
also by the increased number of observations available (Folland and Parker,
1995). We don't discard the biases induced by observational techniques
but the trend appears as a robust climatic signal in several data sets
and variables even after the main known biases are corrected.
Another interesting point observed and to be further explored is that
an extreme event always starts in boreal summer (on August, typically).
This seasonal character is linked to the tropical wind regime acting on
is a preliminary attempt to understand a shift in the occurrence of extreme
cold and warm events, and their characteristics, in Equatorial Atlantic
Ocean. Sea surface temperature data from merchant ship observations between
1964 through 2000 is used. Through the least squares method a sea surface
temperature rise of 0.0221°C/yr reaching an overall value of 0.7956°C for
the whole period is observed. Further more, by classifying extreme warm
and cold events as those when the sea surface temperature is above or
below 1°C, it is found that there is a predominance of extreme cold events
prior do 1980 and extreme warm events after that. Compositing the extreme
events, according to the criteria above, show that there is a phase locking
with the annual cycle with the extreme event always starting in boreal
summer (on August, typically)
Instituto Oceanográfico da Universidade de São Paulo
Praça do Oceanográfico
191 - Cidade Universitária
ZIP: 05508-900 - São Paulo - SP
Fig.1 - (a) First and (b) second EOF loadings computed for Servain's 1964-1999 SST anomalies (ºC). (click Image to Enlarge)
Fig.2 - Time series of SST anomalies for 2°N-6°S and 20°W-10°E (Click Image to Enlarge)
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