International Journal of Marine Science2016, Vol.6, No.56, 1-9
1
Research Article Open Access
Ekman Pumping and Mixed Layer Depth Variability over the Indo-Pacific
Oceans during the El Nino and IOD Events
Veeranjaneyulu Ch.
1,
, A. A. Deo
2
, G. Bharathi
1
, D. W. Ganer
2
, Prasad K. V. S. R.
1
1. Department of Meteorology and Oceanography, Andhra University, Visakhapatnam-530003, India
2. Indian Institute of Tropical Meteorology, Pune, India-411008
Corresponding author email
International Journal of Marine Science
2016, Vol.6, No.56 do
Received: 02 Nov., 2016
Accepted: 19 Dec., 2016
Published: 20 Dec., 2016
This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly cited.
Preferred citation for this article
:
Veeranjaneyulu Ch., Deo A.A, Bharathi G., Ganer D.W., and Prasad K.V.S.R., 2016, Ekman Pumping and Mixed Layer Depth Variability over the Indo-Pacific
Oceans during the El Nino and IOD Events, International Journal of Marine Science, 6(56):1-9 (doi
Abstract
The following study addresses the variability of Mixed Layer Depth (MLD) and Ekman pumping (W
EK
) during the
extreme El Nino and Indian Ocean Dipole (IOD) over the Indo-Pacific regions. Monthly anomalies considering the climatology of
the period of 1980 through 2011 show that during the El Nino years, Ekman suction (positive Ekman pumping) is replaced with
Ekman pumping (negative Ekman pumping) in the tropical eastern Pacific Ocean (88˚W-90˚W and 13˚S-15˚S) resulting in positive
MLD anomalies, and the strong Ekman pumping may be the source for the deepened thermocline during El Nino. In the La Nina
events shallow MLD exits in the tropical eastern Pacific Ocean, due to positive Ekman pumping. During the positive IOD (PIOD)
events in the south eastern Indian Ocean (97˚E-100˚E and 2˚S-5˚S) MLD becomes shallow and positive Ekman pumping anomalies
occur. During the negative IOD (NIOD) years opposite signs take place. Composite events of El Nino are compared with those of
IOD, showing more Ekman pumping anomalies during IOD events as against less deviation in SST.
Keywords
Ekman pumping; Mixed layer depth; El Nino and La Nina; IOD
1Introduction
Interactions between the atmosphere and the ocean are very important in the climate system. The oceans play an
important role in the climate system owing in part to their large heat-storage Capacity. Vertical water movement in
the ocean changes sea surface temperatures (SST) and vice versa and thus causing changes in MLD. In this paper
the main focus is to understand the variability of Ekman pumping representing vertical mixing and mixed layer
depth during strong El Nino/La Nina and IOD events. Ekman pumping is represented by τ/(ρ
f
), where τ is the
surface wind stress, ρ is seawater density (1025 kg mˉ
3
), and
f
is the Coriolis parameter (=2Ωsinθ, with Ω and θ
equal to Earth’s angular velocity and latitude, respectively). The surface wind stress τ is calculated using the bulk
formulation,
τ = [
τ
x
,
τ
y
] =
ρ
a
C
D
V [
u
,
v
],
(1)
Where, τ
x
and τ
y
are east-west and north-south components, respectively. The surface wind (nominally at 10 m) is
assumed to be parallel to the stress vector, with components [u, v] and magnitude V (= wind speed). ρ
a
is the density
of surface air, C
D
is the drag coefficient. Vertical velocity at the bottom of the Ekman layer (effective depth of
frictional influence) from wind stress, W
EK
, is named Ekman suction if upward and Ekman pumping if downward as
in (Stommel, 1958). Ekman transport is proportional to the wind stress and inversely proportional to the sine of the
latitude as discussed by (Sverdrup et al., 1942). Ekman pumping analysis is given as
W
EK
= (curl τ)/ (
ρ
f
),
(2)
Fennel (1999) showed theoretically that wind stress curl could have a substantial impact in coastal upwelling. Off
Oregon west coast of North America, Ekman suction was a major contributor to the total upward velocity during
coastal upwelling as discussed by (Halpern, 1976). Off the west coast of South America, El Nino conditions include
a deepening of the coastal thermocline (Strub, et al., 1998). During the 1997-98 El Nino, the thermocline deepened
