IAPSO 1997: Upper ocean reversing jets in the western equatorial Pacific
Warning: This writeup was done to help organize
my thoughts prior to the presentation.
The actual presentation may have been quite different!
IAPSO 1997 Melbourne, Australia
Presented in session JPM8 (Ocean-Atmosphere Coupling) on Wed 2 July
1997 AM (20 min slot)
Fig. 1: Title slide. (1min/1min)
Upper ocean reversing jets in the western equatorial Pacific
Meghan F. Cronin and Michael J. McPhaden
NOAA/Pacific Marine Environmental Laboratory, Seattle, WA USA
Robert H. Weisberg
University of South Florida, St. Petersburg, FL USA
Fig. 2: Tx, U(z,t), TSD(z,t) at 0,156E. (3 min./4 min.).
1) Introduction: Reversing Jet Structure
Acknowledgements: Kunio Kutsuwada for providing ADCP data at 0,154E,
and David Tang for providing an ATLAS mooring at 0,157.5E.
2) Data from COARE Enhanced Monitoring Array
3) Layered Momentum Balance at 0,156E
slide shows the zonal wind stress, upper ocean zonal currents, and
subsurface temperature, salinity, and density at 0,156E from Sep 1991
- Apr 1994. With the upper ocean temperatures greater than 28C (top
contour), this site is always within the western equatorial Pacific
Warm Pool. Consequently, as seen in the top panel, the easterly
trades (negative Tx) are weak and punctuated by westerly wind bursts
associated with the Madden Julian Oscillation.
As seen in the second panel, unlike in the Central Pacific where
westward South Equatorial Current lies above the eastward Equatorial
Undercurrent, here the Equatorial Undercurrent (red) is found at
around 200m and the vertical structure of the overlying flow is more
complex: There are periods where the current structure is eastward
(red) all the way to the surface, as well as periods of Central
Pacific like conditions, Reversing Jet structures with eastward flow
overlying westerward flow-overlying the eastward EUC, and for this
very interesting 6-week period in September 1992, there was westward
flow overlying eastward-overlying westward-overlying the eastward
Fig. 2b: Z28 & Z20 overlay for the U,T,S,D panels
Overlaying the depth of the 20C and 28C isotherms highlights the 3
layered structure in the zonal currents. Because of the active
hydrological cycle in this region, I should really be defining my
layers in terms of isopycnals. However, this is a preliminary
analysis, and to first approximation the density is controlled by
temperature. Note that the 20C isotherm is commonly used to define the
depth of the thermocline core, while the position where the Z28
outcrops defines the boundary of the Warm Pool.
As I will show in the next slide, while there is lots of data from
the TC-Enhance monitoring array, there are also lots of gaps. Thus in
the momentum balance analysis I will focus on the response to the
October 1992 WWB.
Fig. 3: EMA data map (1 min./5 min).
This slide shows the EMA data availability map. The grey mass is the
corner of Papau New Guinea. These are the sites along the equator,
along 147E, 156E, and 165E. Blue represents temperature between 1m and
500m. Salinity data is not shown, and in this preliminary analysis I
compute the dynamic height relative to 500m using only the temperature
data shown here and Levitus T-S relation. For about 5 months or so
(including the case study period), I can estimate the zonal
pressure gradient at 0,156E from 157.5E and 154E. However at other
times I use other pairs (e.g. 161E&154E, 165E&156E).
The Green lines represent periods with ADCP data at 0,147E, 154e, 156E, 157.5e, and 165E as well as at sites off the equator. In particular, Bob Weisberg
had ADCPs moored at 0.75N and 0.75S along 156E.
Fig. 4: U(z,t) along equator w/Z28C and Z20C overlays (1min/6min)
This slide shows the zonal velocity at all sites along the equator,
with the Z28 and Z20 overlayed. I've already shown you the time series
at 0,156E. Notice that these reversing jets are coherent across 10-20
degrees of longitude and are in fact part of the large-scale
circulation in this region.
Fig. 5: dPdx(z,t) at 0,156E (1 min/7 min)
As with the zonal velocity, the zonal pressure gradient at 0,156E
also shows an approximate 3-layer structure, with periods of surface
intensification as well as subsurface intensification. Clearly
there are a variety of processes (Kelvin waves, convergence of mass,...)
Fig. 6: THEORY SLIDE. (1 min/8 min).
d/dt [u_i] + beta y [v_i]
= -[P_x] - [uu_x] - [vu_y] + [(TAU_(i-1) - TAU_i)/(rho(h_(i-1) - h_i))]
where [...] = 1/(h_(i-1) - h_i) Integral From h_i To h_(i-1) ... dz
h0 = 0m.........\ SFC Layer
h1 = Z28C..../..........\ Intermediate Layer
h2 = Z20C.............. /............\ EUC layer
h3 = 240m............................/
TAU_0 = wind stress computed using the COARE bulk flux algorithm
TAU_i = turbulent and diffusive momentum flux
For the surface layer all terms except TAU_1/(rho(h_0-h_1)) are measured
so this term can be estimated as the residual of the suface balance and then
used as the top stress for the next layer down. Obviously errors
accumulate in these residual terms so caution is warrented when
interpreting these terms.
Fig. 7: SFC & INT layer momentum balances for Fall92. (2 min/10 min).
The left hand side shows the surface layer balance for Oct 1-Dec 1, 1992,
while the right hand side shows the Intermediate layer balance.
The layer tendency rate (dudt) is repeated in each panel as a dark black line.
Walking through the balance: The surface acceleration is almost
entirely forced by the wind. Although during the mature stage of the
wind burst, the pressure gradient adjusts to the wind forcing and the
layer flow reaches a new steady state balance. Prior to and during the
early stages of this particular WWB, zonal advection and residual
mixing were large, consistent with results from our heat and salt budget
analyses of this period, (and with the Richardson number timeseries which
I will show later).
Going down into the intermediate layer, we see that the flow is
accelerated WESTWARD due to the pressure gradient which was set up by the
westerly wind burst. The residual mixing term indicates that the westward
surface flow is mixed down and the eastward EUC flow is mixed up into
the layer. After the surface current has completely reversed directions,
eastward momentum is mixed down.
Fig. 8: Bulk Richardson number (g alpha dTdz)/(ddz(sqrt(u^2 + v^2)))^2 with
contours of the Z28C and Z20C depths. (OPTIONAL).
The mixing inferred from this analysis makes sense when consider the
Ri number profile time series. Only RI numbers in the range of -1 and
1 are shown. Negative RI numbers represent convectively unstable
profiles. A gradient RI number less than 0.25 indicates shear
instability. Since the bulk RI is estimated from temperature data with
approx 10-30 m resolution, and velocity resolution of 8 m bins, a RI
which is small (but larger than 0.25) can still indicate shear
instability conditions. From this we see that prior to the Oct 92 WWB,
the strong shears (low Ri) could be generating Kelvin Helmholtz
mixing. Also because the thermocline is so shallow, surface generated
turbulence can be efficient here.
Fig. 8: INT&EUC layer momentum balances for Fall92. (1min/11min)
The left hand side repeats the INT layer balance already discussed; the
right hand side shows the EUC layer momentum balance for Oct 1-Dec 1, 1992.
Notice that the pressure gradient due to the WWB adjustment in late Oct
-early Nov 1992 is weakens with depth, in contrast to the pressure
gradient signal in late November, which is of approximately the same magnitude
in all three layers. (I believe that this pressure gradient is due to the
KW propagating from the trailing edge of the WWB).
Fig. 9: Box diagram of mean zonal momentum sources/sinks into each
layer (1 min/12 min).
The mean balance in each layer during the WWB can be summarized in
a box diagram. Surface winds provide an eastward stress on the top of the
surface layer, which is balanced to a certain extent by the zonal pressure
gradient. In the intermediate layer this pressure gradient is responsible
for the mean westward acceleration. In the EUC layer, all terms appear to
(Note this would have been cleaner numbers if had started case study
later, i.e. not included big residual mixing event).
[A mystery: why does the interfacial stress increase with depth?]
Fig. 10: Surface layer (0-Z28) momentum balance for full 2-years (1min/13min)
Before ending, it's useful to look at the full time series to see how
typical the case study balances are. The black line in all panels is the
surface layer acceleration (dudt) and is correlated to 0.6 with the
zonal wind stress forcing.
Intermediate layer (Z28-Z20) momentum balance for full 2-years. (30sec/14min)
Stepping down into the Intermediate layer... The zonal acceleration is
highly correlated with pressure gradient, particularly during the
period when the pressure gradient is estimated from the nearest moorings.
For the full period, the correlation is 0.3.
EUC layer (Z20-240m) momentum balance. (30sec/15 min)
In the EUC layer the zonal acceleration is weakly correlated with the zonal
pressure gradient (0.2) as well as with the zonal advection (0.4).
Although I haven't estimated the correlation with the residual "mixing"
terms, I expect that there is some meaningful correlation there too.
Fig. 12: Summary. (1 min/16min)
Complex upper ocean reversing jet structures are observed in the
western equatorial Pacific, with zonal flow changing direction in time
and with depth.
Above the top of the thermocline (Z28C), the surface layer currents are mainly
The layer flow in the upper part of the thermocline (i.e. between Z28C and
Z20C) is mainly driven by pressure gradients setup by wind bursts.
The Equatorial Undercurrent is in the lower part of the thermocline, below
The reversing jet response to a westerly wind burst is in many ways
similar to basin scale EUC/trade wind dynamics, whereby the
easterly trades accelerate the surface currents westward setting up
a compensating pressure gradient, which then is unbalance in the thermocline
and thus accelerates the thermocline water eastward.
Fig. 15. Further Work: (1min/17min)
1) Incorporate salinity data into the analysis. What is the role of barrier
layer in reversing jet evolution?
2) Identify physics of pressure gradient. Local vs. non-local physics,
role of heat and salt advection events.
3) Identify physics of the interfacial stress profile.
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