Respondence to the paper Atmospheric Response to North Pacific SST: The
Role of Ocean-Atmosphere Coupling

The two-way interaction between the atmosphere and ocean is one of the
hot topics in the atmospheric and oceanic sciences. Generally, most
people believe that ocean drives the atmosphere in the tropics while the
atmosphere drives the ocean in the midlatitude. Recently, most
scientists in the author's climate group in Wiscosin are focusing on
showing evidences of the atmospheric forcing on ocean in midlatitude. So
it motivates me to read his earlier paper on the ocean-atmosphere
coupling issue.

His argument is very elegant. And also his style of presenting the
conclusion before the detailed the explanation is quite unique. So
reading his paper is much more like reading a story. The main conclusion
of the paper is that SST forcing have a warm-ridge response on the
downstream area, while the heat flux forcing have a warm-low response on
the downstream area. Due to this basic difference, he explained why in
the fully coupled ensemble(FCE), the warm-ridge response is stronger
than that in the thermodynamically coupled ensemble (TCE) and
Atmospheric Model Intercomparsion Project. (We first have to know that
fully coupled experiment include both the dynamically and
thermodynamically coupling. The former is mainly the advection by the
Ekman flow.) The heat flux in TCE and AMIP are stronger than that in the
FCE. and Assume that the TCE is the sum of the FCE and the additional
heat flux forcing, then the warm-ridge and warm-low response by the FCE
and additional heat flux forcing tends to cancel with each other, and it
follows a weak warm-ridge response in the TCE. If the above logics are
true, then there two questions arise. One is why there is additional
heat flux or excessive heat flux in the TCE and AMIP. The other is why
the stronger heat flux forcing is associated with warm-low response.

The answer to the first question which is shown in the Figure 9 is very
clear. With the dynamic coupling, the warm SST induces warm ridge which
is associated with easterly winds. It follows the northward Ekman flow
advecting warm SST to the north. Then there is positive feedback on the
SSTA, so that the SST forcing is stronger in the FCE. However, without
the Ekman flow in the TCE, there is strong surface cooling which will
destabilize the surface of ocean and induces a stronger convective
warming. The warming causes much stronger surface heat loss and enhance
the damping of SSTA, as well as enhance the forcing (upward) heat flux
to the atmosphere.

The answer to the second question is through the additional experiment
which is flux forced ensemble. And results in Figure 10 shows a warm low
in the downstream region.

I also want to mention the application of a new way to estimate the
atmospheric response to midlatitude SST. First of all, we need to know
that atmospheric variability in the midlatitude are caused by midlattude
SST as well as the atmospheric internal variability. To filter out the
noise associated with the atmospheric internal variability, the author
used SST-lead variance rather than the instantaneous covariance (See
equation (1) ).

However, there remain two questions which are not addressed in the
paper. One is due to the limitation of the model they used, which have
only the monthly data, so that the transient eddy heat and vorticity
flux cannot be calculated without the daily outputs. The other is due to
their assumption of the separation of the SST forcing and heat forcing.
Whether they are independent or dependent are unknown. But physically,
SST forces the atmosphere through the changes in heat flux, and
therefore is not independent from heat flux forcing.