Seasonal changes in SSTs are very small in the western tropical North Pacific, in spite of the fact that significantly more solar radiation per unit time and per unit area is absorbed by the surface layer in summer than in winter and mainly at lower latitudes. Therefore, an efficient heat transport mechanism must be operating to keep up with the solar input. Sea surface temperatures from a world atlas are analyzed where it is found that the areas between the 80 and 82.5 F contours and the equator have marked seasonal variations: increasing in spring and summer as the contours move north, then decreasing in fall and winter as the contours return south. Between winter and summer, the area under the 80 F contour doubles in size. The increase of surface areas is likely due to the increase of absorption of solar radiation. When the areas decrease with time, the inferred heat in the surface layer must go somewhere. Arguments consistent with the available data suggest that the accumulated heat in spring and summer is advected north out of the tropics. The surface temperature gradient, related to the driving force for the flow, computed between the contours and the Aleutian Islands, increases slowly from January to June but then increases much faster till September, after which it rapidly decreases again. It is conjectured that cold surface water from the subarctic surges south beginning in June causing a warm tropical water pulse simultaneously to go north. The large volume of high temperature water of the surface layer in the western tropical North Pacific is thought to be the source of the permanent northward wide warm current off California as well as the additional northward warm surge in summer, both of which have been established previously on the basis of very many observations.
An important oceanographic concept is the non-seasonality of the SSTs in the western tropical North Pacific. In this large region of the ocean, the sea surface temperatures are at most only 1 C higher in summer than in winter, whereas the seasonal temperature range at mid-latitudes is typically 10 C [
Some of the earliest evidence of the non-seasonality property is contained in the classical world atlas of sea surface temperatures [
In a recent study of an extensive SST data set, covering the North Pacific above 20 N, coast to coast, and over a 30 year period, the suggestion was made that a separate wide body of warm surface water headed north crossing mid-latitudes in the central and eastern ocean every summer [
Earlier the proposal is put forward that there is a permanent wide warm surface current flowing northeast off California [
Support for the origin of the wide warm current off California, which becomes even wider in summer due to a separate warm surge, being the warmest western topical surface layer waters year around is offered below by taking into account SST data that include observations below 20 N, and putting together a few reasonable interpretations of these observations coupled with some stated assumptions.
Where the non-seasonal sea surface temperatures occur in the western tropics of the North Pacific is also the place where the warmest temperatures in the whole ocean are to be found all year long. This striking fact has important consequences, although there is very little discussion of it in the recent text books. In the 1970s I was made acquainted with that unusual feature by a professor of oceanography at Scripps, who was senior to me (I was an assistant research oceanographer). Since then I have learned that the idea was not entirely new with him because a similar feature was described in print long ago to take place in the western tropical North Atlantic by M. F. Maury [
Since the most extensive SST data set, the Namias-Scripps ship-injection temperatures, does not extend south of 20 N in the North Pacific, another source of observations is needed for exploring the surface and surface layer of that part of the ocean: the classical World Sea Surface Temperature Atlas, H. O. 225 [
In
one degree squares, provided as a background grid in the charts, was carried out by starting at the equator for a given longitude, and going north to the 82.5 F contour, beginning at 120 E east, or where the nearest land/ocean boundaries east of it are, and proceeding to the longitude where that temperature contour crosses the equator and never comes back. For the 80 F contour the counting of squares for the area was carried out in the same way but stopped at the arbitrarily chosen longitude of 160 W, since the real concern here is with the western tropics and also the 80 F contour converges toward but does not cross the equator in most months. This procedure gives surface areas in terms of the total number of one degree latitude/longitude squares contained below the 82.5 F contour and above the equator in the western North Pacific and below the 80 F contour and the equator. By interpretation, for purposes of discussing surface layer heat content, the total areas of
As can be seen in the figure, the total areas have distinct seasonal changes and vary significantly from one month to the next, in contrast to the sea surface temperatures inside the areas which do not change from month to month, or only change a little. Presumably in the months from February to August, where the areas in
When the areas in
Consider briefly the northward gradient of sea surface temperature in the North Pacific, which is set up and maintained by the sun. An estimate of the mean northward SST gradient is given in
since equatorial temperatures are always higher than subarctic temperatures, but the magnitude of the gradient has a seasonal variation.
At this point an assumption is inserted in order to help make further progress in the argument: no north or south currents exist along the equator that could transport heat from one hemisphere to the other. Behind the assumption is the reasonable hypothesis that each ocean, North and South Pacific, can effectively and individually accomplish its heat balance requirements on its own. One reason is that in the North and South Pacific Oceans there are wide open water regions that connect the equatorial zones with the subarctic (or subantarctic) zones in
which the possibility exists that continuous streams of flow can connect the low and high latitudes and thereby partially, at least, satisfy the necessary heat balances, by means of cold currents coming south and warm currents going north, on a permanent basis.
Thus, warm North Pacific surface layer water does not cross the equator in the western Pacific by assumption. Consistent with this picture is the cold surface water in the eastern Pacific that flows west along the equator, as suggested by the sea surface temperatures of the H. O. 225 Atlas. Undoubtedly originating near Antarctica, since penguins exist on the Galapagos Islands right on the equator, and moving north along the coast of South America in the Humboldt Current, when it reaches the equator the cold water turns left and goes straight west, apparently not crossing the equator. In addition, the submerged undercurrent is symmetric about the equator and swiftly flows due east.
In the far west of the basin (around 120 E), and between about 20 N and the equator, there is not a continuous continental boundary in the usual sense. Nevertheless, a westward flow should be mostly stopped by land from moving into the northern Indian Ocean, at least according to the way the land is depicted in the temperature atlas (
Returning to the primary question sparked by
Neither is it anticipated that any significant amount of heat from the surface layer would be transported vertically upward into the atmosphere by conduction or evaporation. Cloud bands parallel to and just north of the equator [
Plausible conjectures for negligible vertical heat transfer from the sea surface, up or down, for the western tropical North Pacific need to be substantiated with observations when they become available. Also the assumed zero cross-equatorial flow of heat in the surface layer should be verified. Hopefully, once that work is successfully completed, then there is only one option left: northward exporting of the excess tropical heat.
Assuming minimal heat transfer between the ocean and atmosphere in the tropics, then a northward movement of the temperature contours in spring and summer (
When a large volume of warm surface water heads north and leaves the tropics, starting around August or shortly before on the average, the net effect will be to cause the latitude of the temperature contours to drop south. That will qualitatively account for the negative slope of the curve on the right side of
Within the surface layer the temperature decreases from the tropics to the subarctic (
As cold water goes south due to a horizontal pressure force created by the north/south density difference in the surface layer, there is no force to oppose that motion except for internal friction. The Coriolis force acts to the right of the flow in the northern hemisphere. Thus a warm flow that may start out heading due north in the tropics will become a northeastward flow, and a cold flow that begins going due south in the subarctic will turn into a southwestern flow.
What can be inferred from
Earlier studies of ocean observations have revealed the existence of a wide warm current flowing northeast off California that is a real and permanent part of the surface and near surface circulation at mid-latitudes of the eastern North Pacific. Also each summer, a separate surge of warm surface water heads north to the west of the permanent flow. These findings have been based on extensive ship-injection sea surface temperatures as well as more accurate and closely spaced surface and subsurface hydrographic measurements. To understand the source of the wide warm flows, it is necessary to analyze surface temperature data below 20 N, which has been done here using a classical world atlas. Results are that in the western tropical North Pacific there is a large body of the warmest sea surface temperatures which have a very small seasonal variation, because at all times of the year, and particularly in summer, excess heat from absorbed solar radiation in the top 100 m is advected north out of the tropics. Then, the Coriolis force bends these northward currents to the east to join up with those flows already established at mid-latitudes. Cold surface waters from the subarctic moving south are the cause of the warm flows going north above them. After some assumptions have been made, these conclusions are consistent with the data which are available at this time.
Kern E.Kenyon, (2015) Non-Seasonal SSTs of the Western Tropical North Pacific. Natural Science,07,605-612. doi: 10.4236/ns.2015.713060