Deep water warming and steric height change in the Pacific Ocean
Kawano, T; Doi, T; Kouketsu, S; Uchida, H; Fukasawa, M; Katsumata, K; Kawai, Y
JAMSTEC, 2-15, Natsushima, Yokosuka, Japan
Changes in the heat content in the Pacific Ocean were studied by comparing
results from ship-based basin-scale repeat hydrographic surveys mainly conducted in
the 2000s in CLIVAR/IOCCP and previous surveys conducted in World Ocean
Circulation Experiment (Kawano et al., 2009). The area for heat content calculation was
broken into 23 boxes delimited by the intersections of the WHP observation lines in the
Pacific Ocean (Figure 1). We calculated the horizontal area of the ocean in each box
from the surface to the bottom in 100-m increments. Then, the volume of each 100-m
layer in each box was obtained from the product of the area and the thickness of the
layer. The spacing of stations and sampling depths is important for the analysis. We
made adjustments so that the data grids from the first and second surveys were identical.
For station locations, the data from surveys with a smaller number of stations were
interpolated to match the station locations of surveys with a greater number of stations.
The observation depth of each station was set to the shallower of the depths from the
first and second surveys.
We estimated the change in heat content for each 100-m thick layer in each
Box. First, the heat content change was averaged over a layer k with 100-m thickness on
face i:
x2
1 1 100k
HCR (i, k , BOX  n)    
HeatContent (i, x, z )dxdz
t A 100( k 1) x1
(1)
HeatContent  2 (i, x, z)Cp2 (i, x, z)2 (i, x, z)  1 (i, x, z)Cp1 (i, x, z)1 (i, x, z)
100k
A　 
100( k 1)
( x2  x1 )　dz
(2)
(3)
where subscripts 1 and 2 denote the first and second visits, x and z represents horizontal
and vertical direction, respectively. ΔHCR(i, k, BOX-n) is the heat content change rate
(W/m3) of layer k between x1 and x2 on face i in BOX-n. The averaged heat content
change rate of layer k in BOX-n [ΔHC(k, BOX-n)] is calculated as follows:
HC (k , BOX  n)  V1 (k , BOX  n) 
I
1
I
D
i 1
 HCR(i, k , BOX  n)  D
i 1
i
(4)
i
where I is the number of WHP lines that enclose BOX-n, Di is the width of face i, and
V1(k, BOX-n) is the volume of layer k in BOX-n. Equation (3) was used to estimate the
heat content change for layers with a thickness of 1000 m in each Box:
k 10l
1
HCL(l , BOX  n)    HC (k , BOX  n)
V2 k 10( l 1) 1
(5)
where ΔHCL(l, BOX-n) and V2(l, BOX-n) are the heat content change rate (W/m3) and
volume (m3) of layer l in BOX-n, respectively. The first layer, from the surface to a
depth of 1000 m, accounted for 90% of the increased heat content. In this layer, heat
content increased in the western Pacific, with large increases in the western Pacific
warm pool and near New Zealand, and decreased in the eastern Pacific. The horizontal
distributions of heat content change in the third and fourth layers are similar. These
layers correspond to the layers with the smallest change in heat content (not shown). In
the fifth and sixth (bottom) layers, the heat content increased in almost all boxes. There
is a notable heat content increase along the pathway of LCDW and the trend toward the
northwest can be seen (Figure 2). These results are consistent with those of Kawano et
al. (2006) and Johnson et al. (2007). In particular, the contribution from the deep ocean
below 2000 dbar is estimated as 5% of the total.
Change in the thickness of layer k of each box is estimated by the similar
method by Antonov et al. (2002) as follows;
1
1
hk  (
 )   k  dz
(6)
 k   k  k
where dz is thickness (100 m) of layer k and ρk is density averaged in each box. Density
change Δρk is calculated as a function of thermal expansion coefficient (α) and saline
contraction coefficient (β) as follows (McDougall, 1987);
 k    , S , P   d   k    , S , P   dS   k
(7)
where θ, S and P are potential temperature, salinity and pressure averaged in each box
and dθ and dS represent change in averaged potential temperature and salinity,
respectively. The steric height change rate was obtained by Δhk divided by the interval
of observations. The steric height change rate below 700m averaged over the entire
Pacific was calculated to be 0.3 mm/year. Willis et al. (2004) estimated that trend of
thermo-steric height in upper 750m was 1.6 +/- 0.3 mm/year and Lombard et al. (2007)
estimated that trend of global steric height increase was 1.9 +/- 0.2 mm/year by
combining sea level data from satellite altimeters with time-variable gravity data from
the Gravity Recovery and Climate Experiment (GRACE) space mission.. When we
consider the difference between those trends, that is 0.3 mm/year, to be a steric height
increase below 750m, this is consistent with our estimation of steric height change rate
below 700m. The steric height change below 2000 dbar averaged over the Pacific Ocean
was calculated to be 0.1 mm/year. The contribution of thermal expansion was 0.16
mm/year and that of saline contraction was -0.07 mm/year. The steric height below
2000 dbar increased in the western Pacific and decreased in the eastern Pacific. The
largest increase was seen in the Southern Oceans and as well as the western boundary
region off the coast of Japan.
Reference
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Figure 1. The 23 boxes delineated for the calculation of heat content change. Boxes
connect the intersection points of the WHP observation lines in the Pacific Ocean. The
Solid lines indicate the WHP observational lines used for delineating BOXes. Broken
lines are additional lines added for BOX division
Figure 2. Horizontal distribution of heat content change in the Pacific Ocean between
4000-500 m. Red and blue indicate increasing and decreasing heat content, respectively.
The value within each box represents the heat input per volume required (×10–3 W/m3)
to produce the changes in each box.