Effect of seasonal forcing on global general circulation in a world ocean general circulation model

Effects of the seasonal variation in thermohaline and wind forcing on the a byssal circulation are investigated by using an ocean general circulation m odel. To isolate effects of the seasonality in the thermohaline forcing fro m those in the wind forcing, we carry out three experiments with (1) annual -mean wind forcing and perpetual-winter thermohaline forcing, (2) annual-me an wind forcing and seasonal thermohaline forcing, and (3) seasonal wind fo rcing and seasonal thermohaline forcing. The deep water under the seasonal thermohaline forcing becomes warmer than under the perpetual-winter thermoh aline forcing. Although the perpetual-winter thermohaline forcing is widely used and believed to reproduce the deep water better than the annual-mean forcing, the difference between the results of the perpetual-winter and the seasonal thermohaline forcing is significant. The seasonal variation of th e Ekman convergence and divergence produces meridional overturning cells ex tending to the bottom because the period of seasonal cycle is shorter than the adjustment timescale by baroclinic Rossby waves. The heat transport owi ng to those Ekman flows and temperature anomalies makes the upper water (0- 200 m) colder at low to mid-latitudes (40S-40N) and warmer at high latitude s. Also the deep water becomes warmer owing to the warming of the northern North Atlantic, the main source region of North Atlantic Deep Water. The mo del is also synchronously (i.e., without acceleration) integrated with seas onal forcing for 5400 y. A past study suggested that under seasonal forcing , a sufficient equilibrium state can be achieved after only decades of sync hronous integration following more than 10 000 y of accelerated integration . Here, the result so obtained is compared with that of the 5400-y synchron ous integration. The difference in the global average temperature is as sma ll as 0.12 degrees C, and most of the difference is confined to the Souther n Ocean.