Vol.2, No.8, 911-914 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.28112
North pacific cool-down: 1940s-1960s
Kern E. Kenyon
4632 North Lane, Del Mar, USA;
Received 14 March 2010; revised 16 May 2010; accepted 25 May 2010.
ABSTRACT
Between the 1940s and the 1960s there was a
significant lowering of the surface temperatures
of the central North Pacific. This cool-down is
discussed on the basis of analyses of a very
large surface temperature data base, covering
most of the North Pacific, which began in 1947
and continued for at least 30 years afterwards. A
surface area more than 20 degrees of latitude by
approximately 70 degrees of longitude, centered
on 40°N, cooled down within about a ten year
period by typically 0.5℃ and by as much as
1.0℃. Previously a permanent surface and near
surface circulation was proposed in which a
shallow very broad warm surface layer flows
northeastward at mid-latitudes on the eastern
side of the North Pacific while colder water re-
turns southward to the east, west and under-
neath the warm surface current. It is suggested
that variations in this hypothesized circulation,
due to natural causes not yet completely un-
derstood, potentially provide a mechanism for
producing a cooling down (or warming up) of a
large region of the central North Pacific at mid-
latitudes in a relatively short period of time (ten
years or less).
Keywords: Climate Change; Cool-Down; North
Pacific; Sea Surface Temperatures
1. INTRODUCTION
“The earth’s average surface temperature has increased
by about 1.2°F in the past century, with most of the in-
crease occurring from about 1920 to 1950, and again
beginning around 1975” [1]. A curious reader will want
to know what happened between 1950 and 1975. One
might infer from this quote that the earth’s average sur-
face temperature also increased from 1950 to 1975, only
at a slower rate. But since 1982 it has been established
that a “confusing” cooling trend occurred from around
1940 to the 1960s as illustrated by the average surface
temperatures of the Northern Hemisphere [2]. More spe-
cifically, though not explained either, is the fact that the
annual mean temperature of the United States as a whole
lowered between about 1953 and 1968 by roughly 0.7℃
[3]. It is shown below, through analyzing an extensive
set of sea surface temperature data, involving millions of
ship injection temperatures [4], that the temperature of
most of the North Pacific Ocean’s surface waters
dropped, in the mean, by around 0.5℃ within a ten year
span between about 1949 and 1959.
From data analyses published earlier [5,Figure 1]
comes the educated guess that a large slice of the North
Pacific was cooling down simultaneously with the US
within about a 30 year period starting in 1947. The fig-
ure shows sea surface temperatures, averaged over a
month and 5 degree latitude/longitude squares, plotted
against longitude, between California and Japan, along
35°N for six consecutive months. For each month and
longitude three means are compared: 10, 20, and 29
years, all beginning with 1947. Over most of the interior
of the ocean the three curves are nested with the 29 year
mean lying on the bottom, the 10 year mean on top and
the 20 year mean in between these two. Temperature
gaps separating the curves are small, in the 0.1℃ range,
but systematic over a large band of longitudes (about 70
degrees), implying a consistent mean cooling from 1947-
1975 (This cool-down was noticed by the author at the
time the figure was made but not mentioned explicitly in
that paper because it was not relevant to the purpose at
hand).
Here an examination of the cool-down is presented
which is expanded from a single latitude line (35°N) to a
band of latitudes (30-50°N) but at the same time more
narrowly focused in time: the most intense period of
cooling is condensed from 30 to 10 years. It is not that
the author is purposefully avoiding the currently popular
subject of global warming; it is only that the data that are
readily available to him happened to coincide with the
cool-down of the North Pacific, which is sandwiched
between two perhaps better known warming periods.
A characteristic of recent books and position papers

K. E. Kenyon / Natural Science 2 (2010) 911-914
912
Figure 1. Temperature difference map for the surface of the mid-latitude central North Pa
cific. Monthly five degree temperatures from the Namias-Scripps data base were averaged
over two separate five year periods: 1947-1951 and 1957-1961, after which the annual
mean was taken over the twelve months for both periods. Then the 57-61 averages were
subtracted from the 47-51 averages for each five degree latitude/longitude square and
contours (℃) of surface temperature difference were drawn on the map. A cool-down
occurred in a large region in the middle of the map. Hatch marks indicate smaller areas
that warmed up between 47-51 and 57-61. Latitude on the vertical axis, longitude on the
horizontal axis.
on global warming and climate change is that they are
long on discussion but short on data presentation. In this
brief journal article an effort is made to have a more
balanced account. Also it appears that nobody has yet
specifically studied the particular North Pacific SST data
base in terms of short period climate variations in the
way I have chosen to look at these observations here.
2. MAIN RESULT
Figure 1 shows the final product of the analysis: a map
of sea surface temperature differences for the central
area and mid-latitude region of the North Pacific. It is
based on the Namias-Scripps sea surface temperatures,
which extend from coast to coast and from 20-55°N. For
a given month and five degree latitude/longitude square
all reported injection temperatures from merchant ships
were combined to get a single number. In my possession
are computer printouts of the temperatures from January
1947 through December 1977. These printouts were
used to make Figure 1 by means of averaging and sub-
tracting. I do not know if the one month five degree
square average data set continued after 1977. If it did,
somebody could easily use these data to study global
warming as reflected in the surface temperatures of the
North Pacific.
Construction of Figure 1 was carried out as follows.
For each five degree latitude/longitude square and month
a straight five year average was computed starting in
1947 and ending in 1951. Then the annual mean was
made by averaging over the twelve months. Exactly the
same procedure was done for the interval 1957-1961. By
subtracting one set of five-year means from the other a
map of sea surface temperature differences was obtained.
What Figure 1 shows is that between the 1947-1951
annual mean and the 1957-1961 annual mean the whole
central mid-latitude region of the North Pacific cooled
down by typically 0.5℃ and by as much as 1.0℃.
Although the cool-down may have started as early as
about 1940, as mentioned above, there is no way I have
to investigate any warming or cooling of the North Pa-
cific before 1947, which is the start of the Namias-
Scripps sea surface temperature data set. However, a
systematic cooling was found to take place over the 29
year period 1947-1977 along 35°N [5]. Here the cooling
period has been narrowed from the upper end of this 30
year period such that the most intense cooling occurred
within the interval 1947-1961. If desired perhaps a fur-
ther narrowing could be investigated in the time domain
by taking running means instead of five year block av-
erages, which was the simplest way to begin the investi-
gation using the given temperature printouts. Also the
map in Figure 1 could be extended southward to 20°N
and northward to 55°N, but then there are occasional
gaps in the temperature record for some five degree
squares and certain seasons which are outside the normal
shipping lanes.
3. PROCEDURE
Figure 1 can be dissected in various ways, but what fol-
lows comes closest to the actual way it was put together
in the first place. One intermediate step between the raw
data and the final result is shown in Figure 2, which ex-
hibits the five year annual mean surface temperatures as a
function of longitude along 40°N: the solid curve is for
1947-1951, the dashed curve is for 1957-1961. If the
dashed curve is subtracted from the solid one in Figure 2,

K. E. Kenyon / Natural Science 2 (2010) 911-914
913
Figure 2. Five year annual mean surface
temperatures (℃, vertical axis) as a function
of longitude along 40°N (horizontal axis).
Solid curve is for 1947-1951; dashed curve
is for 1957-1961.
the temperature differences along 40°N in Figure 1 are
reproduced. It can be seen from Figure 2 that the aver-
aged surface temperatures of 1957-1961 were signifi-
cantly lower by about half a degree Centigrade than the
corresponding ones of 1947-1951 over the entire central
portion of the slice of the North Pacific Ocean at 40°N.
A striking feature is revealed in Figure 2, but due to
the subtraction involved Figure 1 contains no hint of it.
In the eastern half of the ocean there is a large-scale lon-
gitudinal maximum in surface temperature. Proceeding
west from California the surface temperature rises to a
maximum at around 1450W and then decreases again.
The maximum occurs in both curves, and it is a real and
permanent feature as documented previously [5]. Along
35°N there is an analogous maximum is surface tem-
perature. It is interesting that the eastern maximum in
Figure 2 has already survived the annual mean plus five
years of averaging. Were the averaging interval to extend
over 10, 20 or even 30 years the maximum would still be
there.
Figure 3 comes one step closer to the raw data in that
it shows five year averages of surface temperature along
40°N but just for the month of January. Again the solid
curve is for 1947-1951 and the dashed one is for
1957-1961. Here a much larger lowering of the tem-
perature by almost 2℃ between the two periods oc-
curred in the central North Pacific. Also prominent in
both the solid and dashed curves of Figure 3 is the lon-
gitudinal maximum temperature in the eastern Pacific.
Finally, two examples of the basic data are given
along 40°N in Figure 4 for January of two particular
years: 1948 and 1958. A cool-down occurred between
these two Januaries, ten years apart, but it is distributed
mainly in the eastern side of the ocean. Very pronounced
is the longitudinal maximum for January of 1948.
4. DISCUSSION
Although the question of the ocean’s role in climate
change has been forcefully raised [6], so far there are no
answers coming back that can explain a significant sur-
face temperature change (0.5℃) over a large area (70
degrees of longitude by more than 20 degrees of latitude)
in a short time (10 years or less), outside of extreme
conditions like the beginnings and endings of the ice
ages, which are presumed to take place relatively
quickly.
Consider the permanent longitudinal maximum in
surface temperature on the eastern side of the ocean at
mid-latitudes, examples of which are illustrated in Fig-
ures 2-4 (and more in depth studies of which were car-
ried out before [5]). What is the most reasonable inter-
Figure 3. Five year mean temperatures
(℃, vertical axis) for January as a func-
tion of longitude along 40°N (horizontal
axis. Solid curve is for 1047-1951;
dashed curve is for 1957-1961.

K. E. Kenyon / Natural Science 2 (2010) 911-914
914
Figure 4. Monthly mean tempera-
tures (℃, vertical axis) for January as
a function of longitude along 40°N
(horizontal axis). Solid curve is for
1948; dashed curve is for 1958.
pretation of this real feature? Since the water to the east,
west and underneath the temperature maximum is colder,
and the air temperature above it is colder too (es-
tab-lished from meteorological data taken on individual
cruises), a plausible explanation is that in order to sus-
tain the permanent longitudinal temperature maximum,
warm surface water must continually be brought in to
the area of the maximum from lower latitudes. In other
words, the longitudinal maximum temperature is the
signature of a broad northward flow of warm surface
water. A comprehensive hydrographic section along
35°N [7] strongly suggests that the broad warm current is
also shallow, about 100 m at the most. Consistent with
this idea is the notion that colder water returns south,
crossing mid-latitudes, to the east and west of the longi-
tudinal temperature maximum at the surface and under
the surface maximum at about 100 m below the surface.
Such a broad surface current, hiding in plain sight un-
til recently put forward [8], is potentially capable of
changing the surface temperature of a large region of the
North Pacific by about 1℃. For example, notice the
variability displayed by the longitude and temperature
scales of the temperature maximum in Figures 2-4.
While existing in every month of every year so far stud-
ied along 35 and 40°N, there are month to month
changes within a single year and year to year changes for
a given month. Inquiries as to exactly how such changes
in sea surface temperature are brought about by changes
in the warm current are projects for the future. Also, al-
though the reality of the permanent longitudinal tem-
perature maximum feature itself at mid-latitudes on the
eastern side of the ocean is not in any doubt, there have
been no independent observational checks on the month
to month changes in this feature that are revealed in the
Namias-Scripps surface temperature data set. Nor have
year to year changes of the longitudinal maximum for a
given month been confirmed by separate means.
How quickly could a change in surface temperature of
1.0℃ be expected to occur over broad reaches of the
North Pacific? If the warm current is indeed chiefly re-
sponsible, then one way to estimate a lower bound on
the required time is the distance traveled by the warm
water divided by its mean flow rate, i.e. an advective
time-scale. Earlier analyses gave the flow rate in the ball
park of 10 cm/sec or less [8] and the travel distance is in
the range of 1,000 to 10,000 km, from which a rough
time-scale of 3 years or less is computed.
Finally, one should add that all the conceivable causes
of the North Pacific cool-down are still not known at this
point, nor is it yet possible to definitely link the observed
cool-down to the influences of man. Optimistically, if
the cool-down were better understood, it may follow that
an increase takes place in understanding the warmings
that came before and after the cooling. Natural causes
involving the proposed northward warm current and its
colder southward return flow are suggested here for con-
tributing to the cool-down, but this method could work
equally well for the warmings. On the other hand, if a
good fraction of the warmings is to be blamed on man’s
activities, then the question is: how can the cool-down in
between two warming periods be accounted for in terms
of the interference of man?
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