Cold event during summer of 2003: anomalous hydrographic and atmospheric conditions

1. The South Atlantic Bight extends from Cape Canaveral, FL to Cape Hatteras, NC. The main inputs of water to the SAB shelf are: • river discharge • interaction with Gulf Stream through meanders and filaments • exchange with the Mid Atlantic Bight through the Cape Hatteras region. • This last mechanism is generally small. SST anomaly: The cool water anomaly in July 2003 extended from the eastern coast of Florida to New Jersey. Wind anomaly spatial variability: When we compared large scale wind patterns observed during 2003 by QuickScat with COADS climatological winds (Blanton et al., 2003), differences were present in the whole SAB region, predominantly during the summer months. The upwelling favorable winds during this period were anomalously strong. By September the winds reversed and the opposite effect was found. The excess of fresh water provided by the anomalous river discharge generated a surface low salinity cell by late spring (May). When the salinity structure observed in the SAB was compared with climatological values (Blanton et al., 2003), the main difference was a significant salinity stratification in 2003. By early summer (June), the salinity stratification was replaced by a thermal stratification. This was caused by the combined effect of surface heat flux, reduced mixing and upwelling of Gulf Stream water. At this point the cold water was concentrated in the outer shelf. By late summer (August), the cold water reached all the way to the mid-shelf and was affecting the coastal waters. While the surface waters remained at climatological values, the deep part of the water column registered temperatures up to eight (8) degrees C colder. The comparable increase in stratification was obvious in the density structure. This level of stratification is not common for this region and had significant effects on the exchange processes in the shelf. Wind anomaly temporal variability: Upwelling favorable winds are common during the summer in the SAB, but during 2003, the wind was significantly stronger than in previous years. The data for NDBC Buoy 41008 is presented, but a similar pattern can be found for all the buoys and towers in the SAB region. In 2003 the upwelling favorable period extended from May until August, while in previous years was mainly concentrated in July Sea Level Data: When the 2003 sea level record for coastal water level stations in the SAB and MAB was compared with climatological values, a significant decrease in sea level was observed during the summer months. This effect is consistent with the upwelling favorable wind being anomalously strong. Another factor that could have affected sea level is the Gulf Stream position and transport. The effect of this factor is still to be determined. Mean along-shelf wind statistical test comparison for Buoy 41008 (Gray’s Reef) River Discharge: The spring of 2003 was anomalously wet for most of the South East producing a significant increase in the riverine input of fresh water into the shelf. 2003 Prev. years Signif. greater Apr -0.01 0.42 No May 1.42 0.66 Yes Jun 3.47 1.20 Yes Jul 3.83 2.51 Yes Aug 2.31 0.92 Yes Sep -3.29 -1.52 No Temporal persistence of the wind was present as well. The upwelling favorable wind was not only stronger but more persistent in direction as can be seen in the wind direction histogram. The vertical blue line indicates the upwelling favorable direction. The cold water present in the shelf during July and August had predominantly the same T-S characteristics as Gulf Stream water. To quantify this effect, a T-S distance to Gulf Stream water is estimated. The T-S characteristics of the water on the SAB shelf during this period corresponded with the process of mixing between Gulf Stream water and riverine input. The less saline water was predominant during spring and was mixed and replaced by early summer. When the whole Gulf Stream water was considered, the T-S distance values for 2003 were similar to climatological values. The anomalous cold water of 2003 was only affected by the deeper part of the Gulf Stream. where α is the thermal expansion, β is the saline contraction and TGS is the temperature of Gulf Stream and Tj is the observed temperature • Conclusions • Anomalous upwelling favorable winds (strength and persistence) was a principal driver of the event • Increased river discharge could have preconditioned the shelf through late spring salinity stratification that by early summer became thermal stratification. • The cold water upwelled onto the shelf and the deep Gulf Stream water presented similar characteristics suggesting that the anomalous water came from the deep part of the Stream. • Further research is needed to quantify the effect of the Gulf Stream meanders and transport. • References • Atkinson, L., T. Lee, J. Blanton and G. A. Paffenhofer (1987). Summer upwelling on the southeastern continental shelf of the USA during 1981. Prog. Ocean., 19, 231-266. • Blanton, B. O., A. Aretxabaleta, F. E. Werner and H. Seim (2003 ). Monthly climatology of the continental shelf waters of the South Atlantic Bight. J. Geophys. Res., 108(C8), 3264, doi:10.1029/2002JC001609. • Blanton, J. O., L. Atkinson, L. Pietrafesa and T. Lee (1981). The intrusion of Gulf Stream Water across the continental shelf due to topographically-induced upwelling. Deep-Sea Res., 28A, 393-405. • Sun, D., Z. Liu, L. Chiu, R. Yang, R. P. Singh and M. Kafatos (2004). Anomalous cold water detected along Mid Atlantic Coast. EOS, 85, 152. Cold event during summer of 2003: anomalous hydrographic and atmospheric conditions Alfredo Aretxabaleta (1), James Nelson (2), Jackson Blanton (2), Robert Weisberg (3), Harvey Seim (1), Cisco Werner (1), Brian Blanton (1),  Yonggang Liu (3) (1) University of North Carolina, Chapel Hill, NC, USA(2) Skidaway Institute of Oceanography, Savannah, GA, USA(3) University of South Florida, St. Petersburg, FL, USA Abstract Unusually cool sea-water temperatures were observed along much of the U.S. eastern seaboard during the summer of 2003. Large scale wind patterns were upwelling favorable during this period. In the South Atlantic Bight, the presence of salinity stratification in late spring, due to larger than average river discharge, could have preconditioned the shelf waters to favor shoreward penetration of upwelled cold water. The characteristics of the upwelled water corresponded with water coming from the deeper part of the Gulf Stream. •  Gulf Stream effects • Was the Gulf Stream transport for 2003 anomalous? How did the transport affect the coastal sea level?  Look at Florida Straits cable data • Was the Gulf Stream position different? Was the meander activity anomalous?  Look at SST data to estimate mean and variability of G-S position •  Modeling activities • Assess the relative importance of the different forcings. • Could the event be caused by the anomalous wind alone? • What was the effect of river discharge and the associated increased stratification in spring? • What was the effect of the Gulf Stream? • Estimate the volume of anomalous water on the shelf. • What were the effects on circulation and its possible biological implications? Remaining questions