International Journal of Marine Science2016, Vol.6, No.56, 1-9
2
by 75 m near the coast of southern Peru. Wooster and Guillen (1974) posited that the deepened coastal thermocline
off Peru during El Nino was a consequence of the Bjerknes (1969) breakthrough explanation about the weakening of
the southeasterly trade wind as the fundamental generating mechanism of El Nino. Observations along the
equatorial Pacific during the 1982-1983 El Nino confirmed that thermocline deepened when the southeasterly trade
winds weakened (Halpern, 1987). However at the coast of Peru during El Nino, τ
alongshore
increased (Enfield, 1981;
Huyer et al., 1987), which would lift the thermocline. In the present study τ
alongshore
is computed as in Halpern
(2002) showing increase during 1997-1998 El Nino, and hence emphasizing importance of Ekman pumping in
deepening of thermocline.
The definition of the mixed layer can be based on different physical parameter (e.g., temperature, density, salinity)
and may represent averages over different time interval. The theoretical and experimental foundations of the
concepts of diurnal and seasonal mixed layers are described in detail by Brainerd and Gregg, (1995). Solar warming
occurs only during the daytime, and mixing occurs primary at night time in regions of light winds (Moum, 1989).
Thus seasonal and inter-annual variations in wind mixing and surface heating can be expected to cause variations in
the SST diurnal cycle, as well as seasonal and inter-annual variations in MLD. Cronin, (2002)studied modulation of
mixed layer variability at 0
˚
, 110
˚
W in the eastern equatorial Pacific from Tropical Atmosphere Ocean (TAO)
mooring during warm and cold phases of El Nino-Southern oscillation (ENSO).In the present study MLD is also
analyzed during El Nino/La Nina events. The years 1997-1998, 1982-1983 (El Nino) and 1988-1989 (La Nina) have
been considered. Similarly the positive IOD years 1994, 1997 and negative IOD year 1992 are taken into account to
examine variability of Ekman pumping and MLD in the Indo-Pacific Ocean.
2. Data Sources
Monthly mean wind data sets are utilized from the European Center for Medium Range Weather Forecasts
(ECMWF) Re-Analysis (ERA-Interim). The data are available with a spatial resolution of 0.5˚ on both longitude
and latitude. Monthly data are used to compute wind stress and curl of wind stress. The wind stress curl is
computed with constant air density (ρ
a
=1.225 kg.mˉ³) and drag coefficient dependent on wind speed at 10-m
height (Trenberth et al., 1990). Ekman pumping is calculated from equation (2) using this data. The alongshore
wind stress (τ
alongshore
) is calculated which equals wind stress magnitude multiplied by cos
,
being the difference
in the directions of the wind stress and the 45˚ alignment of the coastline in the south east Pacific region as in
Halpern, (2002). Monthly mean Ekman pumping and anomalies of these fields are calculated using 1980-2011
base period.
The Sea surface temperature (SST) data used here is the improved Extended Reconstructed Sea surface
temperature version 4 (ERSST V4) (Smith and Reynolds, 2004). This data analyses use monthly and 2˚ spatial
resolution. Note that the main conclusion of the present study is not critical to the data period because most El
Nino, La Nina and IOD events occur since 1990.
Monthly mean Mixed Layer Depth (MLD) data available at 1˚
1˚ from the European Center for Medium Range
Weather Forecasts (ECMWF) is used. MLD analyses are from ocean mixed layer model which uses a non-local K
profile parameterization (Large et al., 1994).
Throughout the paper all of the datasets have been monthly averaged. Seasonal cycles are determined over the
period 1980–2011, which includes two strong El Nino events, 1982-83 and 1997-98 as well as a strong La Nina
event in 1988-89 and two positive IOD events in 1994, 1997 as well as a negative IOD event in 1992. The
seasonal cycle is defined as the monthly mean over these 32 years.
3. Results and discussion
3.1 El Nino and La Nina events
Figure 1 displays evolution of SST anomalies during 1997-1998 El Nino events. Positive SST anomalies are seen
to develop in the eastern tropical Pacific Ocean from the month of May 1997, which extend to the central Pacific
and intensify during subsequent months till January 1998. Afterward anomalies weaken. The Western tropical
