Bi-daily variation of meteorological properties at sea level across the Pacific along 35°N

The bi-daily (or two-day) variation of meteorological properties is presented for an oceanographic cruise that sailed along 35°N from California to Japan during March and April, 1976. The data were sampled every two hours for 35 days. All the weather variables recorded quantitatively show a two-day periodicity including air temperature (dry- and wet-bulb), sea level pressure, wind velocity (both north and east components as well as speed and direction), total cloud amount and visibility. For pressure and velocity the amplitude of the bi-daily variation is larger than that of the diurnal signal in the same data set analyzed previously; for temperature and cloud amount the bi-daily and diurnal signals have comparable amplitudes. In addition one computed meteorological quantity, relative humidity, exhibits the two-day variation. Also all the oceanographic quantities, sampled once per two hours or oftener, show a two-day cycle: sea-surface temperature, swell height and direction, and sea direction. The bi-daily periodicity can be seen in the raw data but it is more clearly visible after the higher and lower frequencies are reduced by a simple filtering procedure. At 50°N, 145°W (Weather Ship Papa) air temperature, sea level pressure and wind velocity also show a two-day variation during the time of the cruise. Apparently there is no previous documentation of a bi-daily periodicity in meteorological or oceanographic quantities at sea level with which to compare these results. It is hypothesized that the atmosphere is partially adjusting its heat balance horizontally on a time-scale of two days, based on the correlation between air temperature and the north-wind velocity component, which is consistent with a net poleward heat flux all across the ocean. This suggests that the ultimate cause of the bi-daily variation is related to the heating and cooling set up by the sun.