On The Effects Of Subtropical Phytoplankton On Ocean Circulation And Climate

Phytoplankton amount in upper ocean is not only limited by ocean environment,but also can affect ocean environment reversely. The main effects of phytoplankton onocean environment are to change seawater temperature, modify oceanthermodynamical status, ultimately affect ocean circulation and climate. Ifphytoplankton blooms in upper ocean, then more solar radiation will be used byphotosynthesis in upper layers, thus less solar radiation will penetrate to deeper layers.So phytoplankton amount determines the vertical distribution of solar radiation inupper ocean, this vertical pattern plays a critical role in ocean thermodynamical status.Phytoplankton amount displays large variances in different regions and seasons.Recent advances in satellite measurements allow for real-time global observations ofphytoplankton concentration, provide a broad stage for the researchs of phytoplanktonroles in climate. Previous studies highlighted the important roles of phytoplankton inimproving the tropical climatology, while the effects of subtropical phytoplankton onocean circulation and climate are relatively less considered. In this study, the effectsof subtropical phytoplankton on ocean circulation and climate are investigated basedon Fast Ocean Atmosphere Model(FOAM) and Sea-viewing Wide Field-of-viewSensor(SeaWIFS) chlorophyll concentration data set. By comparing two simulationswith different solar radiation parameterizations, on multi-timescales we analyzed thedetailed feedbacks of North Hemispheric ocean and atmosphere, as well as theinvolving mechanisms.In the North Pacific, on seasonal timescale, phytoplankton can significantlyreduce the amplitude of SST seasonal cycle. In summer, phytoplankton can partlyreduce the SST by diluting the solar radiation to the seasonal thermocline, while inwinter phytoplankton can increase SST through indirect effect of entraining the warmthermocline water into the mixed layer. Phytoplankton forces significant changes inthe midlatitudinal atmospheric circulation. The changes in the atmospheric circulation are characterized by a baroclinic structure with ridge/trough in the lower/uppertroposphere in summer, and an equivalent barotropic trough in winter. Phytoplanktonalso changes midlatitudinal low cloud amount, convective precipitation andwind-driven circulation. In summer low cloud increases and convective precipitationdecreases, while in winter low cloud decreases and convective precipitation increases.The oceanic subtropical gyre is intensified in whole year. On decadal timescale,phytoplankton can significantly reduce the amplitude and periods of Pacific DecadalOscillation(PDO). Based on Empirical Orthogonal Function(EOF) and Singular ValueDecomposition(SVD), both the amplitude and periods of SST,500mb geopotentialheight, upper400m heat content and Aleutian Low are reduced. Phytoplanktongenerates stronger upper ocean stratification and enhances baroclinic Rossby wavevelocity, thus the adjustment timescale of thermocline is shortened, which leads theincreases of PDO frequence.In the North Atlantic, on seasonal timescale, phytoplankton can significantlyreduce the amplitude of SST seasonal cycle. On annual timescale, both thewind-driven and thermohaline circulations are intensified. Phytoplankton generatesbasin-wide cooling/warming in summer/winter, respectively. Associated with SSTchanges, the annual mean surface wind stress is intensified in both subtropical andsubpolar North Atlantic, which lead to acceleration of both subtropical and subpolargyres. Due to warming in the subtropics and enhanced evaporation in the subpolarregion, the potential density decreases/increases in subtropical/subpolar NorthAtlantic, and thus the north-south meridional density gradient is enlarged, leading toan accelerating of the Atlantic meridional overturning circulation(AMOC). In addition,phytoplankton reduces stratification in the upper ocean and favors stronger verticalconvection, which also contribute to the accelerating of the AMOC. On multi-decadaltimescale, phytoplankton can significantly reduce the amplitude and periods ofAtlantic Multi-decadal Oscillation(AMO). Based on SVD, both the multi-decadaloscillation of SST,500mb geopotential height, precipitation are coincidence withAMOC. Phytoplankton will reduce the multi-decadal variance of upper oceantemperature and subpolar freshwater flux, as well as the north-south density gradient. Thus the amplitude and periods of AMOC are reduced, which lead to the changes ofAMO.