Posted by Dongxiao Zhang on November 08, 2000 at 19:01:56:
In Reply to: seasonal variation of heat trasport in the East China Sea posted by Hiroshi Ichikawa on November 08, 2000 at 02:58:44:
Dear Hiroshi,
Thanks for your interest and comments.
First, let me make it clear that the Q_K in our poster includes the
contribution of the Kuroshio in the East Taiwan Channel (ETC, between
Taiwan and southern tip of Ryukyus) where the PCM-1 moored array was
deployed, and the flows in the Taiwan Strait west of Taiwan.
: Dear Dr. Zhang and other co-authors,
: I have estimated the annual and seasonal means of total transport of volume, heat and fresh water through a fixed section across the Kuroshio in the central East China Sea using the quarterly hydrographic and sea surface current data in 1981-1992. Therefore, I have enjoyed very much to look your good poster.
: Comparing your results with mine, I have following comments.
: 1) In the estimation of Q_k, what values of flow mean temperature in the Kuroshio region you have used? I got annual
: mean of flow mean (volume-transport-averaged) temp of 18.60 C with maximum of 19.47 C in winter and miminum of 17.64 C. Winter value has been affected largely by the return flow in the section.
We did not calculate the flow averaged temperature theta_K (note the
flow averaged temperature is defined as theta_K = temperature transport
/ volume transport) for the combined flow in the ETC and Taiwan Strait.
The flow averaged temperature we calculated from the moored data in the
ETC is 19.79+/-0.65 degC. This value has little change with time just as
the case in the Florida Current (Molinari et al., 1990), and this is the
reason why a very tight linear relationship exist between the
temperature transport and volume transport in the ETC. But the flow mean
temperature in the shallow Taiwan Strait should have seasonal variation.
: 2) In my estimation, temp transport in the ECS ranges from 1.13 PW to 2.23 PW about annual mean of 1.76 PW. This variation range seems much larger than your estimates even if I take into account the air-sea heat exchange process in the ECS.
Your estimation should be compared to the sum of the temperature
transport (1.76 PW and the variation shown by the respective figure) and the
temperature transport in the Taiwan Strait (not shown). This is probably
the reason why your estimate has larger seasonal variation.
One concern I have for the use of synoptic surveys to calculate
seasonal mean transport is the alias by the strong 100-day variability
of the Kuroshio in the ETC (the energetic transport variabilities can be
seen in one of our plots, and this is caused by the Kuroshio meandering
to the south of Suao ridge. Zhang et al., 2000) and likely in the East China
Sea. This alias is especially worrisome when using surface current as
reference since the additional tidal alias is introduced as pointed out
by Ichikawa and Beardsley (1993). I have compared the results of
Ichikawa and Beardsley with the JMA published geostrophic calculation
and PCM-1 moored time series, the use of surface current as reference
seems give larger transport variabilities. But the mean transport in
Ichikawa and Beardsley is really a GOOD and SOLID estimate because more
than 20 sections were used to derive the mean.
: 3) I have used a constant value of 15.11 C as mean temp of southward return flow region in the estimation of Q_k. Would you show me its seasonal variation. It may very important value for estimating the seasonal variation of Q_net only from the volume transport of the Kuroshio in the ECS and west of Taiwan.
The beginning to use the single flow averaged temperature in Bryden
et al. in our community to calculate the Q_K is bothering me.
Since that is a flow averaged temperature, any transport change
could result in its change. (What can we use? Bryden et al. only gave us
one number. :-( ).
I hate to show you the seasonal variation of the flow averaged
temperature in the interior ocean in POP model (a very nice high
resolution ocean model run by Los Alomas National Laboratory group),
because it is very large (can be from 10 to 20 deg C seasonally).
The reason is because the ocean at this latitude response to the wind
forcing by barotropic flow on seasonal time scales or high frequency
(see the figure of the transport variation in temperature classes and
Boning et al., 1994, Wilkin et al., 1995, as well as the thesis of two
WHOI students as recommended by Alison's comment.)
: 4) You have disccused the global meridional heat transport along 24 N using Q_Ek estimated by various wind data. In my opinion, the estmation error has mainly come from Q_Ek and we need to make further efforts for getting good Q_Ek value before disccsing Q_net.
Yes, we definitely need high quality wind measurements over the
interior ocean. With the new satellite measurements, things will get
better. One of the purpose of this study is to show the
variability and possible error sources for different
components and to understand what the total error could be for the
method and data we use.
Another concern about the heat transport estimate is the East Ryukyu Current.
We still know too little about its mean state and variability. Considering the
importance of the heat transport by the western boundary current and the
Kuroshio recirculation to the climate variability (see Bill Johns's
comment on Bo Qiu's poster), a more extensive study about the ERC is
necessary. Have Jananese scientists considered to have moored
measurements on this current? If you or other Japanese scientists have any
plans, we (my advisors, Tom Lee and Bill Johns) would be very interested in any
type of collaboration.
Thanks again for your interest and comment.
P.S. You paper Ichikawa and Beardsley (1993) is really one of my
favorate. Much of my initial knowledge about the Kuroshio in the East
China Sea was gained from reading it.
Best Regards,
Dongxiao