4.0 



27N.114W 



27N.115W 



c5 o!i 04 3s 



05 0.0 0.1 



FREQUENCY (CYCLES /MONTH) 



^ - / 









95% 





o!i 



d.2 



da 



o'a 



OS 



Figure 21.— Power and coherence spectra of monthly mean net heat exchange estimates for 1950-72. Spectra are plotted for time series at A) lat. 32°N, long. 117°W; lat. 32°N, 

 long. 1 18°\V; lat. 32°N. long. 119°\V and B) lat. 27°N, long. 114°W; lat. 27°N, long. 11S°W. Spectra of the monthly anomalies from the long-term monthly means at lat. 27°N, 

 long. U4°W and lat. 27°N, long. 115°\V are displayed in Figure C. 



ern California and Oregon. On an annual time scale the entire 

 region experiences net heat gain to the ocean. Previous summaries 

 have shown that the North Pacific Ocean gains heat primarily on 

 the eastern side (Wyrtki 1965), and we have shown that the net heat 

 gain reaches a maximum in the coastal upwelling region off north- 

 ern California as a consequence of relatively low cloudiness and a 

 change in sign of the turbulent heat fluxes at the sea surface due to 

 the presence of cold water during summer. In the absence of a cli- 

 matic increase in the mean sea surface temperature along the coast 

 (i.e., in a steady state balance), this net annual heat gain must be 

 balanced by mixing and cold advection, i.e., the influx of cold 

 water into the region, either by horizontal or vertical motions. 



In the region of maximum offshore Ekman transport between 

 Cape Mendocino and Cape Blanco, the amplitude of the annual 

 cycle of sea surface temperature is suppressed (Bakun et al. 1974). 

 Near the coast, the difference between minimum and maximum 

 temperatures is <3°C. From May to September, f, increases at a 

 mean rate of 0.5°C/mo. In the offshore areas, the seasonal range of 

 sea surface temperature exceeds 6°C and T, increases at a rate 

 > 1.5°C/mo during summer. In the California Current, maximum 

 sea surface temperatures occur in September, well after the period 

 of maximum net oceanic heat gain in June and July. However, the 

 maximum in the annual cycle of Q N corresponds directly to the 

 maximum rate of change in surface temperature. 



The equation of conservation of heat for an incompressible fluid , 

 expressed in terms of mean temperature, T , is given by: 



8T_ Qn 



dt p„CpZ 



-V„.v„T. 



r.dT 



d-T 



OZ dz 



(ll) 



where dldt is the local time rate of change with time scales large 

 compared with the averaging time; Q N is the mean net heat 

 exchange across the sea surface which heats a column of water extend- 

 ing from the surface to a depth z; p is density; c p is the spe- 

 cific heat of water at constant pressure; V= ui +vj is the mean 

 horizontal velocity; w is the mean vertical velocity component 



(positive upward); v H and v 2 H are the horizontal gradient and Laplac- 

 ian, respectively; and A H and A z are the horizontal and vertical coeffi- 

 cients of eddy thermal conductivity. Assuming a constant mixed layer 

 depth, the terms in Equation (11) express a balance between the local 

 change of temperature (heat content) and those changes resulting from 

 radiative and turbulent heat fluxes across the air-sea interface and hori- 

 zontal and vertical advection and diffusion (i.e., mixing). 



To test the general validity of our heat exchange calculations we 

 appl ied Equation ( 1 1 ) to the upper 30 m of the water column in a 3 ° 

 square adjacent to Cape Mendocino and centered at lat. 40°N. 

 long. 125°W. The terms were evaluated by month and averaged 

 over the "upwelling season" from May to September to obtain a 

 mean seasonal heat budget appropriate for this region. We used cli- 

 matological surface and subsurface temperature data (Robinson 

 1976) to estimate the local change, the horizontal and vertical gra- 

 dients, and the second derivatives of temperature. Mean east and 

 north components of velocity were obtained from seasonal distribu- 

 tions of surface currents based on ship drift observations (Stidd 

 1974). 



During summer, the mean net heat flux, Q N , across the air-sea 

 interface is 180 W/m : . To calculate the equivalent rate of change in 

 temperature, we used the constant values p„=1.0xl0 3 kg/ 

 m\c,, = 4. 180x 10" J/kg per °C, and z = 30 m. This mean surface 

 flux equals a temperature increase at the rate of 3.7°C/mo. How- 

 ever, the local change of temperature in the upper 30 m is only 

 0.6°C/mo. The excess heat input at the surface must be balanced by 

 horizontal and vertical advection and diffusion. 



We estimated mean zonal and meridional temperature gradients of 

 -7.4 x lf> 3 and -3.6 x 10"' °C/km, respectively. The corresponding sec- 

 ond derivatives were 4.7 x lO -5 °C/km 2 in the east-west direction and 

 2.0 x 10 5 °C/km : in the north-south direction. Combining these values 

 with mean east and north velocity components of -2.0 and -10.0 cm/s 

 and assuming a horizontal eddy diffusion coefficient of 100 mr/s 

 (Bathen 1971) gives a mean horizontal advection (-K H » v H f) of 

 - 1 .3°C/mo and mean horizontal diffusion (A H v 2 H T) of 0.02 °C/mo. 

 The sign of the advection term implies cold advection which is consist- 



29 



