Decadal Variability of Climate and Winter Phytoplankton Bloom in the Mid-Atlantic Bight Yi Xu, Robert Chant, and Oscar Schofiled Coastal Ocean Observation Lab Institute of Marine & Coastal Sciences, Rutgers University, New Brunswick, NJ USA Introduction The Middle Atlantic Bight (MAB) has undergone significant decadal changes in the temperature and salinity (Mountain, 2001). Although the biological to these basin-scale physical processes have been hypothesized, the mechanisms by which process dominate remain unclear. Here, we seek to identify how local meteorological changes influence on biological productivity during the fall-winter bloom period in the Mid-Atlantic Bight. We examine the inter-annual variability of winter phytoplankton bloom in the Mid-Atlantic Bight (MAB) using a combination of satellite data and a biogeochemical ROMS model for the time period of 1983-1986 and 2004-2008 (which correspond to a negative and positive Atlantic multidecadal oscillation (AMO) periods). Factors that influence water column stability were analyzed since stability is the most important factor associated with the timing and magnitude of phytoplankton bloom. Given the counteracting effects of wind, temperature and river discharge we conducted a stability analysis to assess the balance between mixing (tide and wind) and buoyancy (heat and freshwater) processes. 3. River There is enhanced PEA in January for the 2004-2008 period. the same time in case 3; however we kept the river input as temperature and salinity sources/sinks and mass sources/sinks terms, but turned off the input of nutrients from river. The four experiments results are compared with normal model condition. The river discharge brings fresh water to the coastal of MAB, which acts to increase stability. The climatology analyses of USGS river discharge data indicate that there are higher river discharges in the 2004-2008 year especially in January. Temperature increase Salinity decrease PEA increase River discharges and temperature are increasing which will increase the MAB stability. Conversely wind and storm increases will decrease the MAB stability Discharge(m3/s) Month PEA Any increase of water column stability in winter time could decrease the light limitation so as to enhance phytoplankton growth in winter. So there is a positive relationship between the change of PEA and the change of chlorophyll. Δ Chl Δ PEA Chl For our simulated case, comparing with year 1983-1986, the 2004-2008 winter experienced increase of PEA (positive) and increased chlorophyll in winter. Decadal variability of Chlorophyll 2. Decadal variability of buoying and mixing terms The concentration of chlorophyll didn’t change much when nutrient input form rivers was turned off. However, when you turn off the fresh water discharge, there is an obvious decrease of in chlorophyll, suggesting the primary role of the rivers in winter MAB is to stabilize the water column. The interannual for chlorophyll is shown for both satellite data, MARMAP observation data and the output of the biophysical model based on ROMS (Haidvogel and Beckmann, 1999; Wilkin, et al., 2005; Fennel, et al., 2006) for the time period of 1983-1986 and 2004-2008 individually. which represents the change of water column stability due to surface neat heat at a rate Q and salt flux due to evaporation (E) and precipitation(P), stirring by tides ub and speed of wind W and the freshwater input by river. To determine whether the water column remains stratified or mixes as a result of the changes in the physical foracing, we calculated the change of PEA with time: No heat flux, resulted in decreased mixing, and PEA decreased to zero almost at the same time with the normal condition. The timing of decrease of PEA is associated with the initial of fall bloom so the timing of fall bloom is not influenced by net heat flux. Without the wind mixing, the timing of destratification is delayed associated with gradually increase of chlorophyll in fall compared with normal condition. The magnitude of bloom in mid-winter is also increased without wind mixing. The breakdown of the seasonal thermocline in fall was primarily due to wind mixing. The MARMAP in situ near-surface chlorophyll a + pheopigments concentrations in MAB inner-middle shelf show that maximum concentrations occur during fall-winter (i.e. after year day 300 and before year day 100) which is consistent with the fall-winter high chlorophyll trend captured by CZCS data. Days of year Chlorophyll()mg/m3 Decadal variability of Meteorological Forcing (wind, temperature, river) The net heat flux terms is negative in winter indicating heat loss to the air and decrease of water column stability. The river flux term is positive when the freshwater discharge is transported offshore. 1. Wind The time-series analysis of wind data from NDBC Moored Buoys shows that there is an increase trend of stormy frequency during winter for the period 1983 to 2010 . Neat heat flux Wind mixing River flux All terms(neat heat flux + river – wind) Month The climatology trends in model output for both time periods are similar to the chlorophyll observations. There is a fall-spring high of chlorophyll concentration for the inner-middle shelf of MAB. Compare with the simulation for the Month Chlorophyll()mg/m3) year 2004-2008, the climatology chlorophyll concentration show a little bit higher during winter time, especially in January. Conclusion In summary, the MAB has been experiencing significant changes over the last 30 years, with warmer atmospheric and water temperatures, increased river discharge, and increased wind forcing. Additionally there are enhanced phytoplankton blooms despite the counteracting effects of wind, net heat flux and river discharge. Model simulations suggest that the timing of phytoplankton blooms is primarily associated with wind mixing, however, the magnitude of mid-winter bloom is more sensitive to water column stability which is influenced the changes in fresh water run off , ocean cooling/heating and wind mixing. The climatology of storm frequency of 44008 and 44009 buoy show there are more storms during winter time especially in January in the years of 2004-2008. 44008 44009 Month Stormy frequency For the recent years (2004-2008), water column mixing has declined as a result of increased river flux and lower heat loss, even there is more wind mixing in that month. Diagnostic analysis of buoying and mixing terms 2. Temperature 1. PEA Sensitivity study Acknowledgements We would acknowledge the Rutgers University Coastal Ocean Observation Laboratory (RU COOL) and the Ocean Modeling group. We also thank the Northeast Fisheries Science Center who provide the MARMAP data. This work was supported by a grant from ONR MURI Espresso program (N000140610739) and NSF LaTTE program (OCE-0238957; OCE-0238745). The climatology of water temperature for the station 44009 shows that there is higher water temperature during winter which is associated with higher air temperature during this time period. The competition between buoyancy inputs through heat and fresh water and the mixing produced through winds and tides influences determines the existence or absence of stratification which in turn influence the phytoplankton bloom. The potential energy anomaly (PEA) of the water column is used to determine the water column stability. The PEA is defined as : Temperature(°C) The former study identified river run off, wind mixing and neat heat flux as the significant roles in regulate water column stability so as to influence phytoplankton bloom. Five sensitivity experiments are conducted to study the role of each factor s as the major influence of phytoplankton growth. In Experiment 1, no river input is applied; In Experiment 2, we turn off the neat heat flux; In Experiment 3, we turn off the neat heat flux since at the day the neat heat flux change from positive to negative; In Experiment 4, we turn off the wind at Month Please contact: xuyi@marine.rutgers.edu, oscar@marine.rutgers.edu, chant@marine.rutgers.edu