Can the Subtropical North Atlantic permanent thermocline be observed from space?

Abstract

The analysis of remotely sensed altimeter data and in situ measurements shows that ERS 2 radar can monitor the ocean permanent thermocline from space. The remotely sensed sea level anomaly data account for ∼2/3 of the temperature variance or vertical displacement of isotherms at a depth of ∼550 m in the Subtropical North Atlantic Ocean near 32·5°N. This depth corresponds closely to the region of maximum temperature gradient in the permanent thermocline where near semi-annual internal vertical displacements reach 200 to 300 m. The gradient of the altimeter sea level anomaly data correlates well with measured ocean currents to a depth of 750 m. It is shown that observations from space can account for ∼3/4 of the variance of ocean currents measured in situ in the permanent thermocline over a 2-y period. The magnification of the permanent thermocline displacement with respect to the displacement of the sea surface was determined as −×650 and gives a measure of the ratio of barotropic to baroclinic decay scale of geostrophic current with depth. The overall results are used to interpret an eight year altimeter data time series in the Subtropical North Atlantic at 32·5°N which shows a dominant wave or eddy period near 200 days, rather than semi-annual and increases in energy propagating westward in 1995 (west of 25°W). The effects of rapid North Atlantic Oscillation climate change on ocean circulation are discussed. The altimeter data for the Atlantic were Fourier analysed. It is shown how the annual and semi-annual components relate to the seasonal maximum cholorophyll-a SeaWiFS signal in tropical and equatorial regions due to the lifting of the thermocline caused by seasonally varying ocean currents forced by wind stress.

Ocean structure and climate (Eastern North Atlantic): in situ measurement and remote sensing (altimeter)

“…Figure 3). Pingree et al (2002) showed that temperature measurements in the permanent thermocline at the position of Rig 155 correlated with the sla data such that Depth of 10:08C isotherm(m) ¼ 631 sla(m) þ 788 m (1) with a correlation coe⁄cient squared value of r 2 ¼0.6 and this relationship is valid for both cyclonic and anticyclonic eddies or wavelike structure at this location. The maximum correlation coe⁄cient between isotherm displacement and sla occurred in the permanent thermocline where the correlation coe⁄cient reached a value of r¼0.8, at a depth of 500 m. The highest correlation between sla or rather sla gradient occurs with in situ ocean current measurement.…”

“…At 32.58N, where we have the most complete coverage of in situ observations, the wavelength for the mode 1 Rossby Wave would be about $ 200 km, much shorter than observed $ 500 km. The group velocity would be zero, whereas in Pingree et al (2002) it was shown that the group velocity at 32.58N was comparable with the westward speed of the sla eddy-wave-like structure. Further work shows that the westward speed of the observed structures can be matched or even exceeded at the latitudes considered here if ocean models are used to provide a detailed description of the barotropic contribution in addition to the mode 2 baroclinic £ow ¢eld on the theoretical speed of extratropical planetary waves (Killworth et al, 1997).…”

“…Some workers only use sla distributions and there is an extensive sla literature so the methods here provide insight into what the radar altimeter surface measurements are revealing about the internal structure of the ocean (e.g. Kelly & Watts, 1994;Pingree et al, 2002). The data processing methods and sla tracking techniques are given in Pingree & Sinha (1998) and Pingree et al, (1999a) and Fourier analysis of sla in time has been used to determine the periods of dominant structure at di¡erent latitudes and longitudes.…”

“…However, the adjacent ocean was also depressed with levels at $ 75 cm, considerably lower than in the winter of 1995/ 1996 (see previous section). In Pingree et al (2002), it was shown that the altimeter has a seasonal signal that relates to a thermodynamic or a non-dynamic heat storage or buoyancy (with fresh water conributions) content. This buoyancy signal can be estimated from the local seasonal net heat balance and the balance of precipitation (or run o¡ in the proximity of a freshwater source) over evaporation.…”

“…Numbers refer to the ¢gures where the data are examined or analysed. Climate and structure on the extended line (dashed) at 32.58N is described in Pingree et al (2002). The track of HMS 'Hecla' from Norfolk to the 32.58N latitude is shown dotted.…”