NAMIAS: SPACE SCALES OF SEA-SURFACE TEMPERATURE PATTERNS 



was chosen for the atmospheric variable because 

 mid-tropospheric wind patterns often give a bet- 

 ter specification of contemporaneous SST pat- 

 terns than do sea-level maps. In general terms 

 the physical reasons underlying this better spec- 

 ification from 700-mb rather than sea-level pat- 

 terns lie in the fact that the 700-mb anomalies 

 reflect not only the character of the anomalous 

 sea-level pressure and wind distribution but they 

 also contain implicitly the vertical stability of the 

 lower troposphere. This association results 

 from the high positive correlation between 700- 

 mb height and mid-tropospheric temperature, 

 and with cyclonic or anticyclonic contour cur- 

 vature (Namias, 1947). The stability of the 

 1,000-700-mb layer to a large extent determines 

 the vertical heat, moisture, and momentum flux 

 from the surface — fluxes which are naturally im- 

 portant factors entering into the heat budget of 

 the sea. Furthermore, since 700-mb height usu- 

 ally implies 700-mb temperature over ocean 

 areas, some rough measure of back radiation is 

 implied by the 700-mb heights. 



Returning now to Figures 1-3 several points 

 clearly emerge: 



1. The space scale of coherent SST anomaly 

 patterns is large — perhaps of the order of one- 

 fourth to one-third of the North Pacific. 



2. The SST scales are not far different from 

 the 700-mb height scales and indeed display a 

 remarkably similar form. 



3. Strong teleconnections exist between SST 

 anomaly fields of opposite sign over areas span- 

 ning almost one-half of the North Pacific; these 

 teleconnections appear to be related to similar 

 700-mb teleconnections. 



To aid the explanation of the above three em- 

 pirical findings it should be remembered that 

 time-averaged atmospheric patterns over large 

 areas in temperate latitudes always reveal the 

 presence of long or planetary waves whose hori- 

 zontal dimensions are about 4,000 miles from 

 crest to crest, but whose geographical position- 

 ing varies from month to month and also be- 

 tween the same month of diflferent years. Thus, 

 the atmospheric anomalies (and resultant pre- 

 vailing winds and air masses) also vary. A 

 quasi-stationary anomalous wind pattern gener- 

 ally creates anomalous SST patterns of a certain 



form. Most often cold water is advected and 

 appreciable heat extracted in the cold dry north- 

 erly air currents behind mid-tropospheric 

 troughs, while warmer water masses are ad- 

 vected and less heat extracted in the warm moist 

 air currents ahead of troughs. Therefore, with 

 atmospheric long-wave dimensions such as 4,000 

 miles (crest-to-crest) it is not surprising to find 

 in Figure 1 that both 700-mb heights and SST 

 anomalies are negatively correlated between the 

 Central Pacific and the West Coast of the United 

 States (one-half wave length) while both have 

 similar coherence fields around the diamond. 



Further clarification of the physical mech- 

 anism for this coupling can be obtained from the 

 cross-correlations of SST with 700-mb height 

 (Figure 1, lower). With anomalously warm 

 water in the Central Pacific there are apt to be 

 high 700-mb heights (anticyclonic ridge) in the 

 north but low heights (cyclonic trough) in the 

 south. Since correlation fields of the type shown 

 in the upper and lower portions of Figures 1 and 

 3 may be interpreted in terms of flow (Stidd, 

 1954) , we can approximate the resultant wind by 

 remembering that it flows parallel to the con- 

 tours with lower heights to the left. The anom- 

 alous wind component at the diamond is thus 

 southeasterly and leads to less loss of heat by 

 evaporation and sensible heat and greater ad- 

 vection of warm water from the south. Note 

 also the contemporary reaction to northerly 

 anomalous wind components in the eastern 

 North Pacific — accounting for the colder than 

 normal waters usually found there when the 

 diamond area is warm. Of course, the above 

 reasoning may also be applied (in reverse sense) 

 when anomalously cold water exists at the dia- 

 mond. 



Figure 2, relating surrounding values to the 

 diamond at lat 40°N, long 130°W, also shows the 

 large-scale coherence and area of negative cor- 

 relation one-half wave length upstream. The 

 isopleths in the bottom figure indicate warm 

 southerly air flow and warm sea transport ac- 

 companying positive SST values at the diamond 

 and negative 700-mb height departures from nor- 

 mal in the Central Pacific. Of course, abnor- 

 mally low SST values at the diamond imply 



615 



