On the contribution of the Gulf stream to high frequency coastal sea level variability

Long-term Atlantic Ocean and Gulf Stream (GS) variability were linked in past studies to coastal sea level (CSL) change along the U.S. East Coast – a weakening GS can lead to rise in CSL and increased coastal flooding. However, high frequency variability (HFV) in CSL is, in most cases, attributed to atmospheric weather events. This study is focused on HFV (intraseasonal variations with periods between ~ 1 week and ~ 2 months) in the GS and in CSL. First, wavelet and spectral analysis of observations of the Florida Current transport and CSL characterized the HFV in the data, and then idealized numerical simulations were conducted to study the response of CSL to imposed GS variations with known frequencies. Three experiments were conducted: a control run with constant surface and boundary forcing, and two experiments with imposed oscillations in the Florida Current transport into the model domain- a “high-frequency experiment” (HFE) and a “low-frequency experiment” (LFE), where the period of the GS oscillations were ~ 1–2 weeks and ~ 1–2 months, respectively. The observations and the model show statistically significant anticorrelation between the GS flow and the CSL, but the LFE resulted in higher GS-CSL correlations and was more like the observations than the HFE was. The results also show large spatial differences in the CSL response to GS variations - the South-Atlantic Bight (SAB) responded more strongly to the LFE while the Mid-Atlantic Bight (MAB) responded more strongly to the HFE. Power spectra of the model simulations show that even small, imposed GS oscillations at high frequency, can interact with natural variability to excite unpredictable CSL variabilities over a wide range of frequencies, including oscillations at much longer time scales than the forcing. The variability in the Florida Current flow generated a northward propagating signal along the GS path and a southward propagating sea level signal along the coast. However, the topography near Cape Hatteras seems to partly block communication between the MAB and the SAB. The study demonstrates the important contribution of high frequency GS variability to CSL variability - better understanding of the role of remote forcing on coastal sea level can help to improve prediction of coastal sea level variations and associated flooding.