EDDY CHARACTERISTICS IN THE LEE OF HAWAIIAN ISLANDS
S. Yoshida, B. Qiu, P. Hacker
Universith of Hawaii, Honolulu,USA
Abstract
Strong sea surface height variabilities in the lee of Hawaiian Island are investigated by weekly altimetic
satellite data. Northeasterly trade winds throughout most of the year are obstructed by high mountains in
each islands and generate positive and negative wind stress curls dipole structures in the west side of the
islands. This vorticity input of the wind stress curl is thought to be a major cause to force Hawaiian Lee
Counter Current (HLCC). The HLCC region has a strong north-south horizontal shear centered on the HLCC
at 19°N, with the north-westward flowing Hawaiian Lee Current (HLC) to the north and westward flowing
North Equatorial Current (NEC) to the south. Eddies are dominant features in the lee of the Big Islands of
Hawaii, and are caused mostly by local wind stress curl and intrinsic oceanic instability processes. The Navy
Coastal Ocean Model (NCOM), Hybrid Coordinate
Ocean Model (HYCOM) and OFES output are also
applied to understand the formation of eddies
which have different temporal and spatial
resolutions.
(a)
As shown in Figure.1(b), relatively high eddy
variability bands with RMS SSH amplitude
exceeding 10 cm are seen along 19.5°N over
HLCC. A spectral analysis of the 15 year long SSH
anomaly time series quantifies the dominant time
scale of the observed SSH anomalies, and spectral
inside lee of Hawaii region has two peaks. One is
around 60-day which locates right behind the Big
Island, and the other is around 90-day which
centered at 164°W with continuously extension to
farther west. Michum (1995) reports the existence
of 90-day variability from tide gauge records at
Wake Islands, more than 4000km west of Hawaii.
In the past studies, these two signals had not been
distinguished, especially the one closer to the Big
Island had not been known well, even though they
have different characteristics. We examine the
formation mechanisms of these two different eddy
variabilities individually. Hereafter, we call these
east and west high variability areas as region E
and W to make an easy recognition.
Numerical models seem to have some difficulties
to reproduce region E variability, but have good
agreement with region W. The mean EKE
calculated from altimetric data has a maximum to
the west of the Big Island. This pattern weakens to
the west. The pattern is quantitatively different in
the NCOM EKE where the high values are seen
(cm)
(b)
Figure 1. (a) Schematic diagram showing the main surface
current system around the Hawaiian Island. (Lumpkin 1998)
(b) RMS sea surface height anomaly around Hawaii from
satellite data.
further to the west in the region of the HLCC. The estimation of the horizontal scale of eddies indicates that
the eddy over the region E have smaller horizontal scale with higher frequency signals than that of region W.
Figure.2 shows times series of eddy kinetic energy of two locations along 19.5°N. EKE over region E
changes with larger amplitude than that of region W and this result is consistent with mean EKE spatial
distribution.
Wind stress curl has a dipole structure in the wake of the island of Maui and Hawaii (Calil et al, 2008), and
the wind stress curl itself doesn’t have specific energy peak around 60-day which matches dominant eddy
variability period, but has more energy in high frequency band. In the real ocean, dissipation tends to remove
higher frequency signals preferentially, we apply a simple Ekman pumping model over the strong wind
stress curl region in the wake of islands to estimate SSH anomalies resulting from the
convergence/divergence of the anomalous surface Ekman pumping over the region E using,
∂h' EK
g'∇ ×τ
=−
ρ 0 gf
∂t
Where g ' is the reduced gravity (~0.03m/s2), ρ 0 is the reference density and ∇ × τ is QSCAT wind stress
curl. The time integrated h' EK gives the importance of 60-day signal as the ocean response to overlying
atmospheric forcing. The SSH anomaly calculated from the Ekman pumping model explains about 10 to 15
percent of the altimeter or NCOM SSH variability with no clear correlation pattern.
Since the current system in the lee of island is horizontally sheared, it possesses the mean kinetic energy
necessary for barotropic instability. In order to clarify the energy sources from mean to eddy variability,
energetic analysis with the convergence of the Reynolds stress in the momentum equations is conducted.
References
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Lumpkin, C.F., 1998. Eddies and currents in the Hawaiian Islands. Ph.D. Thesis, SOEST-School of Ocean
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Figure 2. Time series of eddy kinetic energy at two locations along 19.5°N calculated from satellite SSH anomaly
data. Blue and red curves represent region W and E variability maximums which locate 164°W and 157°W,
respectively.