1. Propagation of wave signals in models and altimetry for the North Atlantic Vassil Roussenov 1, Chris Hughes 2, Ric Williams 1, David Marshall 3 and Mike Meredith 2 1.Department of Earth and Ocean Sciences, University of Liverpool, UK., 2. Proudman Oceanography Laboratory, Liverpool, U.K. 3.Meteorology Department, Reading University References Johnson and Marshall (2002) April JPO Surface layer thickness after an overturning of 10 Sv is switched on at time t = 0 on the northern boundary of the domain (Johnson and Marshall, 2002, JPO) How is localised forcing communicated over the ocean? Contact e-mail: ric@liv.ac.uk Model Studies Model depth. Resolution is 0.28º (30 km at the equator & 15km at 60N). Model initialised from Levitus and forced by annual winds for 44 years. Twin perturbation experiments for 4 years where interfaces raised 120m/10 d over northern relaxation zone. Model density interfaces along 20ºW. Altimetric Changes TEXT Model simulations carried out using a 6 layer, isopycnic model (MICOM). Data study TEXT Path and order of stations over which the altimetric SSH signals are extracted from 1993 to 2002 along the continental slope. External forcing can induce local advective changes in the circulation fast, wave-like responses propagating against the sidewalls and along the equator. Using an idealised model, Johnson and Marshall (2002) demonstrate how overturning changes are communicated through the propagation of fast Kelvin and slower Rossby waves. Is there any evidence of a similar response in more complex models or in the observations? High-pass altimetric data along sequence of coastal stations with mean annual cycle removed plus smoothing in space (100 grid points along track) and high pass filtering in time (periods shorter than annual). Correlation of high pass filtered altimetry everywhere with that averaged in the northern NE Atlantic (100 grid point section 35-36 marked in black dots). Places where correlation is not significant at the 95% level are left white. See large-scale patterns in SSH. Signals correlate on western N. Atlantic and eastern edge of the N. Atlantic The WAVE array (Western Atlantic Variability Experiment) is currently under way (Hughes, Marshall, Williams, Meredith & Foden). POL scientists (Meredith, Foden, Pugh) +UKORS staff (Waddington) deployed bottom-pressure recorders and moorings along 2 sections in August 2004 and with WHOI deploying equipment in April 2004 (Toole, Hughes) Correlation patterns The monitoring aims to detect propagating wave signals along the western Atlantic from the bottom pressure and density signals. Thus, the programme aims to understand how overturning signals are communicated. Model correlation patterns for SSH without and with a background circulation (left panel, after 1 y of model run with no forcing; right panel, after 44 y of model run with wind forcing). With no wind forcing, there is band of positive correlation all around the edge of the basin. With wind forcing, the correlation pattern is similar, but with an opposite sign on each side of the basin. Thus, the propagation of the anomalies has altered. Propagation of signals along the western boundary: green is default, black is perturbed model run, red is tracer signal. Propagation of anomalies in the model SSH Height of dense isopycnal Day 5 Day 365 Day 725 Day 1005 Day 1405 Model snapshots of the sea surface height anomaly and height of a dense isopycnal there is a rapid spreading along the western boundary a slower spreading along the eastern boundary the SSH and isopycnal height anomalies appear related, but with SSH signals on a slightly broader scale. Hovmueler diagram along the western boundary from years 40-44 (tracer in contours) Model results show a broadly similar character to the analytical and altimetric diagnostics. Role of background flow needs to be explored. Map of deployment along 3 sections (thick line in left panel) and mooring schematic (right panel).