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

    Outline:
    Acknowledgements: Kunio Kutsuwada for providing ADCP data at 0,154E, and David Tang for providing an ATLAS mooring at 0,157.5E.

  • Fig. 2: Tx, U(z,t), TSD(z,t) at 0,156E. (3 min./4 min.).
    This 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 EUC!!

  • 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,...) occurring here.

  • Fig. 6: THEORY SLIDE. (1 min/8 min).

    d/dt [u_i] + beta y [v_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 be important.
    (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.

  • Fig. 11: 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.

  • Fig. 12: 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)

    Summary

    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 wind driven.

    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 Z20C.

    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)

    Further Work:

    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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    Pacific Marine Environmental Laboratory
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