Climate of the Seasonal Cycle in the North Pacific and the North Atlantic Oceans

Abstract

Time series of monthly sea-surface temperature (SST), air temperature (AT) and sea level pressure (SLP) were constructed from merged releases of the Comprehensive Ocean-Atmosphere Data Set (COADS). The time series were decomposed into seasonal and non-seasonal (short and long-term) components. The contribution of the seasonal cycle to the total variance of SST and AT exceeds 80% in the mid and in some high latitude locations and reaches its peak (> 95%) in the centres of subtropical gyres. In most cases, a combination of annual and semiannual harmonics accounts for more than 95% of the seasonal variability. Amplitudes of SST and AT annual cycles are highest near the western boundaries of the oceans; annual phases of SST and AT increase toward the eastern tropical oceans, revealing a southeastern propagation of the annual cycle over the Northern Hemisphere oceans. The annual cycle of AT leads that of SST by 1-3 weeks. The largest phase differences are observed in the regions of western boundary currents in the North Pacific and the North Atlantic oceans. This is consistent with spatial patterns of integral air-sea heat fluxes. Annual phases of SST increase along the Gulf Stream and the Kuroshio Current. This points to the importance of signal transport by the major ocean currents. The lowest annual amplitudes of SLP are observed along the equator (0°- 10°N) in both oceans. There are three distinct areas of high annual amplitudes of SLP in the North Pacific Ocean: Asian, Aleutian and Californian. Unlike the North Pacific, only one such area exists in the North Atlantic centred to the west of Iceland. A remarkable feature in the climate of the North Pacific is a maximum of semiannual SLP amplitudes, centred near 40°N and 170°W. It is also an absolute maximum in the entire Northern Hemisphere. Analysis of phases of harmonics of SLP seasonal cycle has revealed the trajectories of propagation of the annual and semiannual cycles. Analysis of semiannual to annual amplitudes ratio has revealed the regions of semiannual cycle dominance. Copyright © 2001 Royal Meteorological Society.