P. Vélez-Belchi (1), A. Hernandez-Guerra (2) and E. Fraile-Nuez (1)
Corresponding author : P.Vélez-Belchi
- Instituto Español de Oceanografía, Centr o Oceanográfico de Canarias,
- Facultad de Ciencias del Universidad de Laalmas de Gran Canaria ,Spain
Full paper: Velez-Belchi, P., A. Hernandez-Guerra, E. Fraile-Nuez and V. Benitez-Barrios, 2010: Changes in the temperature and salinity tendencies of the upper subtropical North Atlantic Ocean, Journal of Physical Oceanography 40: 2546-2555, doi:org/10.1175/2010JPO4410.1.
The Atlantic Meridional Overturning Circulation attenuates the climate in Europe through the northward ocean transport of 25% of the global heat flux, reaching its maximum of 1.5 PW at around 24.5°N. Consequently, strong interest in multi-decadal changes in ocean temperature and heat transport resulted in the North Atlantic hydrographic transect along 24.5ºN, being occupied five times since 1957, more than any other ocean crossing transect in the world. Sections at this latitude have become a benchmark to monitor long-term changes in temperature in the Atlantic, in order to understand the nature and causes of climate change.
In this study, five oceanographic sections at 24.5°N are used to show that a mean warming of 0.27°C occured in the upper ocean (600-1800 dbar range) during the 1957-1998 period but a mean cooling of -0.15°C of the upper ocean is found for the 1998-2004 period. Salinity shows a similar change in the tendency, with freshening since 1998. For the upper ocean, 1998 is the warmest and saltiest year since 1957. Data from the Argo network corroborate these tendencies, showing a -0.13°C between 1998 and 2006 and revealing interannual variability between 2005 and 2008 to be much smaller than the decadal variability estimated using the transect.
Data and Method
Five hydrographic sections at 24.5°N are used for this study (figure 1a). The first section has been done during the International Geophysical Year in october 1957 (Fuglister, 1960), bisecting the North Atlantic Subtropical Gyre ; the second one in 1981 (Roemmich and Wunsch, 1985) ; the third and the fourth ones have been done during the WOCE experiment in 1992 (A05-Parrilla et al., 1994) and 1998 (AR01-Baringer and Molinari, 1999) ; the last one has been executed in 2004 (Cunningham and Alderson, 2007). Given the different sampling grid for each transoceanic section, temperature and salinity data from each survey have been linearly interpolated to 101 pressure levels between the surface and 2000 dbar and bilinearly each 0.5° in longitude.
Figure 1. Station positions for transatlantic hydrographic sections at 24.5°N (a)- locations of the hydrographic sections at 24.5°N in 1957, 1981, 1992,1998 and 2004. (b) - locations of every Argo profile in the period between January 2003 and December 2008. The 2006 Argo «zonal» synthetic section obtained is the black line. Only casts between 23°W and 70°W latitudes are taken into account for this study.
In addition to these five oceanographic sections, all good quality temperature (T) and salinity (S) data available from Argo network for 6-years (January 2003 - December 2008) are used, representing 2048 quality-controlled profiles within the region of interest [20°N-40°N, 80°W-8°W] (figure 1b). They were objectively interpolated onto a hypothetical zonal section at 24.5°N (Fraile-Nuez and Hernandez-Guerra, 2006), every 0.5° between 23°W and 70°W at 101 pressure levels between the surface and 2000 dbar. This synthetic Argo « zonal » section, computed to the mean date 2006, is the sixth occupation of 24.5°N (black line in figure 1b).
Contoured sections of (T,S) differences on isobaric levels, for the periods 1998-57 (ie between 1957 and 1998), 2004-57, 2004-1998, 2006(Argo)-57 and 2006-98, have been computed (figure 2). They show clearly, for the first time, that there has been a significant cooling (-0.15ºC) in the upper ocean (600-1800 dbar range) during the last seven years (1998-2004), in contrast to the continuous warming (0.27ºC) observed in the previous forty years (1957-1998). Salinity shows a similar change in tendency to that found in temperature, with freshening since 1998. For the upper ocean at 24.5ºN, 1998 was the warmest and saltiest year since 1957. Data from the Argo network is used to confirm the strong cooling and freshening since 1998, showing a -0.13ºC cooling in the period 1998-2006.
Vertical profiles on pressure surfaces of zonally averaged (T,S) differences (figure 3) confirm and quantify the changes in the tendency observed in the basinwide sections. A warming between the first occupation in 1957 and the sections sampled in 1981, 1992, 1998 is observed. The maximum warming shifted from 900 dbar in 1981 to 1000 dbar in 1992 and back to 900 dbar in 1998. The maximum warming with respect to 1957 occurs in 1998, with and averaged warming of 0.27°C for the upper ocean (600-1800 dbar), 0.24°C for the thermocline waters (300-800 dbar) and 0.27°C for the intermediate waters (800-1800 dbars). From 1998 to 2004, the zonally averaged temperatures decreased, with the maximum cooling of -0.3°C found at 800 dbar. The thermocline waters cooled by -0.15°C, whereas the intermediate waters cooled by -0.13°C. The observed cooling between 1998 and 2004 is statistically significant and represents almost 50% of the warming found from 1957 to 1998.
These results clearly show, from the available hydrographic sections, that the upper ocean in 1998 was signiﬁcantly warmer and saltier than in any transoceanic measurements at 24.5°N since 1957. An analysis of the isobaric change decomposition shows precisely that, between 1957 and 1998, the warming in the upper ocean was mainly caused by deepening of the isoneutral surfaces, whereas the cooling found between 1998 and 2004 (2006 for the Argo synthetic section) was mainly due to shallowing of the isoneutral surfaces. The present dataset does not permit discrimination between the two mechanisms that contribute to heaving; therefore, the forcing for the observed changes since 1957 could be either wind-driven ocean forcing or changes in the renewal rates at the formation areas.
The results also demonstrate that Argo is an invaluable tool to observe the oscillations in the tendencies of the ocean. Argo data show that the changes in the 24.5°N hydrographic zonal section are a good proxy for decadal changes, because the interannual variability in the period with Argo synthetic sections (i.e., from 2005 to 2008) was small; therefore, the interannual variability aliased by the decadal sampling period is likely negligible. Without regular observations, oceanographers have little understanding of the scales of variability in water mass properties that can mask evidence of anthropogenic climate change.
- Baringer, M. O., and R. Molinari : Atlantic Ocean baroclinic heat ﬂux at 24 to 268N. Geophys. Res. Lett., 26, 353–356, 1999
- Cunningham, S., and S. Alderson : Transatlantic temperature and salinity changes at 24.58N from 1957 to 2004. Geophys. Res. Lett., 34, L14606, doi:10.1029/2007GL029821, 2007
- Fraile-Nuez, E., and A. Hernandez-Guerra : Wind-driven circulation for the eastern North Atlantic subtropical gyre from Argo data. Geophys. Res. Lett., 33, L03601, doi:10.1029/2005GL025122, 2006
- Fuglister, F. : Atlantic Ocean Atlas of Temperature and Salinity Proﬁles and Data from the International Geophysical Year of 1957-1958. Woods Hole Oceanographic Institution, 209 pp, 1960
- Parrilla, G., A. Lavin, H. Bryden, M. Garcia, and R. Millard : Rising temperatures in the subtropical North Atlantic Ocean over the past 35 years. Nature, 369, 48–51, 1994
- Roemmich, D., and C. Wunsch : Two transatlantic sections: Meridional circulation and heat ﬂux in the subtropical North Atlantic Ocean. Deep-Sea Res., 32, 619–664, 1985