The Seasonal Cycle in Coupled Ocean-Atmosphere Model

A coupled ocean-atmosphere model is used to investigate the seasonal cycle of sea surface temperature and wind stress in the Tropics. A control run is presented that gives a realistic annual cycle with a cold tongue in the eastern Pacific and Atlantic Oceans. In an attempt to isolate the mechanisms responsible for the particular annual cycle that is observed, the authors conducted a series of numerical experiments in which they alter the solar forcing. These experiments include changing the longitude of perihelion, increasing the heat capacity of land, and changing the length of the solar year. The results demonstrate that the date of perihelion and land heating do not, by themselves control the annual cycle. However, there is a natural timescale for the development of the annual cycle. When the solar year is shortened to just 6 months, the seasonal variations of climate remain similar in timing to the control run except that they are weaker. When the solar year is lengthened to 18 months, surface temperature in the eastern Pacific develops a prominent semiannual cycle. The semiannual cycle results from the ITCZ crossing the equator into the Southern Hemisphere and the development of a Northern Hemisphere cold tongue during northern winter. The meridional winds maintain an annual cycle, while the zonal winds have a semiannual component. The Atlantic maintains an annual cycle in all variables regardless of changes in the length of the solar year. A final experiment addresses the factors determining the season in which upwelling occurs. In this experiment the sun is maintained perpetually over the equator (simulating March or September conditions). In this case the atmosphere and ocean move toward September conditions, with a Southern Hemisphere cold tongue and convection north of the equator.