Analysis Of The Characteristics And The Mechanisms Of Physical And Biological Processes In The California Current System

In-situ observations, satellite data and a three dimensional (3-D) physical and biological model (ROMS-CoSiNE) are used to study the dynamics of physical and biological processes in California Current System (CCS) from 1993 to 2012, especially in the central CCS which is a typical upwelling region in the Eastern Boundary Current Systems, and investigate how climate change influences CCS ecosystem. Conclusions are shown as follows.(1) Scarce observations and physical-biological models with only one phytoplankton lead to little understanding of plankton dynamics on seasonal cycle in the central CCS. Thus, ROMS-CoSiNE is adopted in our study, which includes two phytoplankton. According to the model, diatoms (s2) and mesozooplankton (zz2) are dominant in the high-nutrient waters, while picoplankton (s1) and microplankton (zz1) are prevailing in the low-nutrient waters. Data comparisons show that ROMS-CoSiNE can capture the seasonal cycle of key variables (such as currents, temperature, nutrients, chlorophyll and so on) in CCS. Besides, model shows that s2 reaches annual maximum in May-June, zz2 peaks in July, s1 peaks in August and zzl reaches annual maximum in September. Simulated plankton also show significant seasonal evolutions along the CalCOFI Line 67. In summer, maximal nutrients occur at the coast, s2 peaks in the coastal (0-100km) region and maximal zz2 is about 150 km offshore. This spatial pattern agrees well with the offshore transport of coastal upwelled water. In fall, all plankton decrease in the offshore region. However, in the coastal upwelling region, s2 and zz2 dropped dramatically, while s1 and zzl increase and their maxima move onshore. The model shows that the components of coastal ecosystem change from summer to fall. From summer to fall, the model indicates that the drop of s2 in the coastal upwelling region is mainly caused by zz2 grazing, not by nutrient limitation, while the decrease of s2 in the offshore region is caused by low nitrate which induces low growth rate.(2) The dynamics of low-frequency variability of physical and biological processes in the central CCS is still unclear, especially in the offshore region, because in-situ observations and most studies are limited to the coastal region. ROMS-CoSiNE can simulate well the low-frequency variability of physical and biological processes in the central CCS from 1993 to 2012, although there are differences in the amplitudes of primary production (PP) between observations and model results, which may result from the ecosystem parameters in model or from the system errors of observations. The model could capture the 1998-99 regime shift of Pacific in CCS. For example, after 1999, temperature becomes lower and subsurface nutrients turns higher. Besides, the coastal upwelling system shows significant interannual and decadal variability in physical, chemical and biological variables, which is mainly driven by the alongshore wind stress. In the offshore California Current region, the low-frequency variability is also strong and is mainly modulated by vertical mixing and stratification in the upper layer. In the transition zone, things become more complicated. Not only coastal offshore transport work on the low-frequency variations of physical and biological processes, but also vertical mixing and water stratification in upper layer influence the ecosystem.(3) Physical and biological models are used to investigate how regional ecosystem responds to climate change, because of scarce observations. However, most of the models only contain one phytoplankton, or are run by monthly data, neglecting the sensitivity of plankton to meso-scale processes (such as eddies). Thus, a model containing two or more phytoplankton with high resolutions and outputs close to observations is needed.ROMS-CoSiNE has two phytoplankton and two zooplankton, with spatial resolution of 12.5 km and temporal resolution of 3 days, which can capture mesoscale processes, such as eddies. Data comparisons show that low-frequency variations of ROMS-CoSiNE output is close to observations from 1993-2012. Besides, ROMS-CoSiNE simulates well the evolutions of physical and biological processes iin the CCS during the strong 1997-99 ENSO event. The model shows that the first El Nino signal reached CCS in May 1997 in the form of the first baroclinic Kelvin wave or coastal trapped wave, the second El Nino signal was strongest in 1997-98 winter and lasted till October 1998, and the CCS went into the La Nina phase in 1998-99 winter. Meanwhile, the model indicates the evolutions of different phytoplankton in the central CCS during the 1997～1999 ENSO. In the coastal upwelling region, when El Nino occured in the summer of 1997, s2 biomass decreased and became lower than the climatological mean, but s1 increased and peaked in 1997, thus s1 anomalies became positive. In 1998 summer, conditions were similar. In 1999 summer, CCS was in La Nina phase with nutrients lifted, so s2 increased and became positive, while s1 dropped, became lower than the climatological mean and was significantly negative.ROMS-CoSiNE also indicates how physical and biological variables in CCS respond to Pacific Decadal Oscillation (PDO) and North Pacific Gyre Oscillation (NPGO). When PDO is positive, the anomalies of wind stress in CCS are cyclonic, centered in the offshore oceanic region, and the alongshore wind stress anomalies are poleward along the whole eastern coast. According to the Ekman theories, anomalous subsurface waters are brought to surface offshore, while surface waters are anomalously accumulated and downwelled near shore. Thus stratification becomes stronger offshore and turns weaker in the coastal upwelling regions. Correspondingly, nutrients are raised offshore and are depleted near shore. As a result, PP is increased offshore and is decreased at the coast. When NPGO is positive, the anomalies of wind stress are cyclonic north of 40 °N, and are anticyclonic south of 40 °N. Accordingly, in the offshore region waters are pumped north of 40 °N and downwelled south of 40 °N, while coastal waters are accumulated north of 40°N but upwelled south of 40 °N. Thus, north of 40 °N, stratification becomes weaker offshore but turns stronger near coast; south of 40 °N, stratification is stronger offshore but turns weaker near coast. As a result, north of 40 °N, nutrient anomalies are positive offshore but negative near coast, while south of 40°N, nutrient anomalies are negative offshore but positive near coast. Consequently, PP is raised offshore but reduced at the coast north of 40 °N, but south of 40 °N is quite the reverse.