Changes in water properties and transports along 24°N in the North Pacific between 1985 and 2005

Abstract

[1] We conducted a trans-Pacific hydrographic section along 24°N in 2005 to investigate the ocean structure and its changes from previous observations in 1985. We detected significant basin-average water property changes from 1985 to 2005. Apparent oxygen utilization increased below the thermocline by up to 6 mmol kg À1 around the density of the central mode water (around 600 m). It appeared that the North Pacific intermediate water (around 800 m) was less dense in 2005 than in 1985 because of warming. From the decrease of the zonal gradient of the temperature and salinity around the North Pacific deep water (2500-4000 m) and lower circumpolar deep water (<4000 m), we suggest that northward bottom water and southward deep water transports became weaker from 1985 to 2005, consistent with the speculation from the observed temperature increase of the bottom water along its main path in previous studies. Although these water property changes suggest a slowdown of the meridional overturn in the North Pacific and large transport changes in the deep layers (below 4000 m) are estimated from an inverse method, significant heat transport changes were not detected. The estimated temperature transport change of 0.1 PW between the two sections was mainly due to shallow overturn changes, especially changes in the Kuroshio. To describe variability due to the Kuroshio changes, we estimated mass and heat transport changes from long-term observations of the Kuroshio in the Okinawa Trough, and we determined decadal variability of temperature transports, which was consistent with the variability estimated from sea-surface flux data sets.

State of the Climate in 2012

“…In the Northern Hemisphere, inverse model calculations in the Pacific using data from repeat hydrographic sections along 24°N suggest that the deep northern transport of waters of Southern Ocean origin lessened by 1.5 Sv between 1985 and 2005 (Kouketsu et al 2009), consistent with results in the South Pacific. In the North Atlantic Ocean at 24°N, one analysis based on geostrophic calculations inferred a remarkable slowdown of northward flow of 4 Sv from four hydrographic sections occupied between 1981 and 2004 ; although another set of inverse estimates using different reference levels, integration areas, and data extrapolation into u-sampled areas found a slowdown of only 1.3 Sv over those same times, with a resurgence of 0.4 Sv between 2004 and tions are described.…”

Deep ocean heat content changes estimated from observation and reanalysis product and their influence on sea level change

“…The uncertainties of temperature and salinity measurements are about 0.001°C and 0.002, respectively [e.g., Kawano et al , 2010]. The uncertainty in temperature measurements was less than the difference detected in the deep ocean (typically below 3000 m) in the comparison between WHP cruises and revisits along many sections, whereas the salinity changes in the deep ocean were too small to detect along many sections [e.g., Kawano et al , 2006; Kouketsu et al , 2009]. Therefore, for this study, we describe the temperature and HC changes in detail and note the effect of salinity changes on the steric height change as well as that of the temperature changes in the deep ocean, focusing on the areas where large salinity changes were detected.…”

A review of global ocean temperature observations: Implications for ocean heat content estimates and climate change

“…Thus, regional deep ocean temperature variations could impact the global ocean on timescales much faster than advection: decades as opposed to centuries. Finally, there are suggestions in the South Atlantic and South Pacific [ Kouketsu et al ., ], the North Atlantic [ Johnson et al ., ; Frajka‐Williams et al ., ], and the North Pacific [ Kouketsu et al ., ], as well that the MOC associated with AABW may have slowed, consistent with the near‐global contraction of this water mass [ Purkey and Johnson , ].…”