On the seasonal changes in the depth of the mixed layer in the north Pacific Ocean

Analysis was made of approximately 135,000 bathythermograph observations taken in the Pacific Ocean from the equator to 70°N to determine the seasonal changes in the depth of the mixed layer. The results show that the mixed-layer depth undergoes a strong seasonal change with increasing latitude in response to summer heating. This change begins during the spring in the central western North Pacific, expands across midlatitudes by early summer, and extends to high latitudes by midsummer. In contrast, the development of a shallow summer mixed layer is not pronounced at low latitudes. The over-all annual average mixed-layer depth throughout the North Pacific is 75 meters. This annual average varies from 15 meters off the coast of Panama to 122 meters in the Bering Sea. In general, the average mixed-layer depth decreases northward from the equator to approximately 20°–40°N and then increases again at higher latitudes. The seasonal range was found to be greatest (188 meters) in the Bering Sea and smallest (10 meters) in the eastern tropical Pacific north of the Galapagos Islands.