Longitude-Time Plot

From more than 4 years of T/P SLA mapping, it is fairly easy to extract the time series of SLA along a given line of latitude, for all the longitudes of the maps.

(59 Kb)		Then, a time (y axis, in julian days, from October 8, 1992 to January 31, 1997) longitude (x axis, in degrees) diagram can be constructed by contouring the extracted time series of SLA for each longitude. The exemple here corresponds to the SLA extracted along 30°N. One can see the successive periods of high and low values of the SLA over the 4 years, which are mostly related to the steric annual signal. That is, heating/raising and cooling/lowering of the sea level. One can also remark westward propagation of alterning positive and negative sea level signals (i.e. to the left-up side of the diagram).
(55 Kb)		To better distinguish these possible wave propagations, the annual signal is removed by a sine/cosine fit. It is then possible to estimate the propagation velocity (i.e., the slope) and the length of this waves. The periods of this waves vary from 0.5 to 2 cycles per year, and the length from 1 to 5 degrees. Most of these waves are eddies propagating to the west. Some could be explained as first baroclinic Rossby waves (e.g., Le Traon and De Mey, 1994).

If we consider these propagating features as Rossby waves, we can analyze the wave propagation with Longitude-time plots where the annual signal is adjusted and removed. Then deduce the dominant propagating velocity. We are presenting here plots every two degrees of latitude (one degree of latitude at the vicinity of the Azores Current). The propagation velocity of the waves tend to increase from the north to the south (i.e. the wave slope decreases), which suggest an interpretation in term as Rossby waves, as was also shown recently by Cippolini et al., (1997).

44°N  (69 Kb) 42°N  (66 Kb) 40°N  (65 Kb) 38°N  (60 Kb) 36°N  (66 Kb) 35°N  (68 Kb) 34°N  (74 Kb) 33°N  (69 Kb) 32°N  (62 Kb) 31°N  (58 Kb) 30°N  (54 Kb) 28°N  (49 Kb) 26°N  (47 Kb) 24°N  (45 Kb)

We have estimated the wave velocity (solid line) and compared it to the theoritical Rossby wave propagations (phase speed) for the first baroclinic mode, taking into account the effect of the Azores Current as a 10 cm/s mean current at 34°N, for wavelength of respectively 100, 300 and 600 km (dashed line). There is a good agreement between the observed and theoritical velocities: the decreasing ratio with latitude and the mean current effect are well represented, but there is an amplitude difference of about 50%. This difference could be generated by altimetric data processing artifacts, but recent studies (e.g., Chelton and Schlax, 1996) suggest a deficiency in the free propagating, linear, baroclinic Rossby waves theory. Moreover, field experiments like SEMAPHORE-93 (Eymard et al., 1996 ; Eymard, 1998) have shown that eddies instead of waves were propagating to the west and the south-west. These eddies were interacting with the Azores current (Tychensky at al., 1998). It has also been shown that some surfaces eddies were associated with Meddies underneath (Richardson and Tychensky, 1998 ; Tychensky and Carton, 1998). In that case, the Longitude time plots no longer witness waves propagations, but real vortices advecting westward water masses.