The Response Pathway And Mechanism Of North Pacific Subtropical Counter Current To The Freshwater Forcing

The simulated response pathway and mechanism of North Pacific Subtropical Counter Current (STCC) and other associated changes in North Pacific Ocean to idealized freshwater perturbations (water-hosing experiment). The changes and its regional differences of sea surface temperature, wind speed and ocean circulation in North Pacific Ocean induced by freshwater input in North Atlantic Ocean are described by comparing simulated results between The Community Climate System Model version 3 (CCSM 3) and Geophysical Fluid Dynamics Laboratory coupled model (CM2.1). Further analysis for STCC, mode water, mixed layer depth (MLD), etc. are examined by its seasonal changes. Finally the changing of current velocity and transport anomaly of STCC on different sections with increasing freshwater flux in boreal winter are investigated.The results demonstrate as follow: the distribution of sea surface temperature (SST) anomaly in water-hosing experiment is similar in two models. But North Pacific is more cooler in CCSM3 than in CM2.1. Warming in particular areas on North Pacific Ocean although cooling is dominated in CCSM3; but not shows in CM2.1. There are a lot of differences between two models in the responds of wind stress. The responds even express an opposite situation in some particular area. The responds of upper ocean currents, especially STCC and equatorial current, is weaker in CM2.1 than in CCSM3.Not only current velocity and transport of STCC are changed under freshwater forcing, but also its position and depth on 180°section (international dateline). It is confirmed that mode water intrusion is the cause of STCC by comparing the position and depth of STCC on this section with the shoaling of upper pycnocline and the appearance of mode water. This mode water is North Pacific Central Mode Water (NPCMW) because of its particular potential density characteristic. In the mean time , a pathway between sudden changes in climate induced by freshwater input in North Atlantic and STCC is presented here. The strengthening and weakening of the STCC due to variations in mode-water ventilation can be further traced upstream to variability in MLD and wind tress in the Kurshio extension. The seasonal changes of current velocity of STCC on 180°section is concentrate between 20-24°N. It is highest during July and August, lowest during January and February. The difference between water-hosing experience and control run is more obvious. The average transport on this section is higher in water-hosing experience than control run for all year. But they are both reach maximum during July and August, minimum during March and April. The variability in water-hosing experience is one month ahead to control run. In water-hosing experience, the location of NPCMW is moving westward; the area that MLD is higher than 200m is smaller; the MLD gradient is increased on 40°N.The responds of current velocity and transport on 150°E section, which represents the header part of STCC, to freshwater forcing is the most evident; and on 170°W section, which the end part, is the least evident. The responds is basically increasing with the amount of freshwater input. Also it present different tendency on different sections. It changes the most on 150°E section, then 170°W. It's decreasing from 160°E to 180°, which represent the middle part. The increasing process is not linear. The most rapid change is from 0.1 Sv to 0.2 Sv. Some oscillations occur between 0.2 Sv to 0.6 Sv and varies in all sections. 150°E section is the most monotone increasing section. Other sections reach the highest on 0.6 Sv.