Modelling Average Residence Time Of The Waterbody In Bohai And Its Seasonal Variation

Average residence time (ART) is defined as the average time taken by waterparcels to leave the domain of interest. As an auxiliary variable reflecting theenvironmental functions of flow field, ART helps to measure the water exchangecapacity of the sea. The knowledge on the spatial and temporal distribution of ART inBohai is still limited. In this study, the spatial distribution and seasonal variation ofART in Bohai is modelled, by solving the adjoint equation of ART. And the influencesof main dynamic factors on water exchange in Bohai are analyzed.The annual mean of ART for the entire Bohai is406days, with a weak seasonalvariation (~3%of the annual mean). The seasonal variation of ART is highest inLaizhou Bay (~37%) and lowest in Liaodong Bay (~4%). The vertical difference ofART is high in spring and summer due to the stratification of sea water, while theART is vertically homogenous in autumn and winter. The spatial distribution of ARTis affected by the flow field and the distance to the Bohai strait The ART demonstratesan increasing trend in the following order: Laizhou Bay (319days), Central Basin(338days), Liaodong Bay (502days) and Bohai Bay (599days). The annualdifference of ART between the surface and bottom is~40days. The vertical variationof ART is mainly located in the upper column (<5m) of Laizhou Bay and CentralBasin.Tidal forcing plays the most important role in controlling the spatial distributionand weak seasonal variation of ART in Bohai. The exclusion of tidal forcingsignificantly changes the spatial distribution of ART, and greatly increases theseasonal variation of ART in most regions of Bohai (e.g.,~113%in Laizhou Bay).The two major hydrodynamic functions of tidal forcing are changing the flow fieldand strengthening the vertical mixing, respectively. Wind is the secondary factorcontrolling the ART in Bohai, which favors water exchange and decreases the ART in autumn and winter. Under the effects of wind, ART at south part of Bohai Strait issmaller than north, but wind does not change the spatial distribution of ART in mostregions of Bohai. Wind-induced vertical mixing makes the ART homogenous near thesea surface. Density-driven current could also promote the water exchange anddecrease the ART, especially at surface layer in spring and summer, and make theisolines of ART in Bohai Strait, Laizhou Bay and Central Basin present in SW-NEdirection. Vertically, different direction of density-driven current between the surfaceand bottom and the reduction of vertical mixing caused by thermocline and haloclinewould produce vertical variation of ART in spring and summer. The Yellow Riverdischarge mainly accelerates the water exchange in Laizhou Bay. Re-entry processdoes not change the spatial distribution of ART in Bohai but increases the value ofART in most regions more than100%. And the vertical and seasonal variations ofART in Bohai are weakened by the re-entry process.