Publications

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2013
Purkey, SG, Johnson GC.  2013.  Antarctic Bottom Water Warming and Freshening: Contributions to Sea Level Rise, Ocean Freshwater Budgets, and Global Heat Gain. Journal of Climate. 26:6105-6122.   10.1175/JCLI-D-12-00834.1   AbstractWebsite

Freshening and warming of Antarctic Bottom Water (AABW) between the 1980s and 2000s are quantified, assessing the relative contributions of water-mass changes and isotherm heave. The analysis uses highly accurate, full-depth, ship-based, conductivity-temperature-depth measurements taken along repeated oceanographic sections around the Southern Ocean. Fresher varieties of AABW are present within the South Pacific and south Indian Oceans in the 2000s compared to the 1990s, with the strongest freshening in the newest waters adjacent to the Antarctic continental slope and rise indicating a recent shift in the salinity of AABW produced in this region. Bottom waters in the Weddell Sea exhibit significantly less water-mass freshening than those in the other two southern basins. However, a decrease in the volume of the coldest, deepest waters is observed throughout the entire Southern Ocean. This isotherm heave causes a salinification and warming on isobaths from the bottom up to the shallow potential temperature maximum. The water-mass freshening of AABW in the Indian and Pacific Ocean sectors is equivalent to a freshwater flux of 73 +/- 26 Gt yr(-1), roughly half of the estimated recent mass loss of the West Antarctic Ice Sheet. Isotherm heave integrated below 2000 m and south of 30 degrees S equates to a net heat uptake of 34 +/- 14 TW of excess energy entering the deep ocean from deep volume loss of AABW and 0.37 +/- 0.15 mm yr(-1) of sea level rise from associated thermal expansion.

2012
Purkey, SG, Johnson GC.  2012.  Global Contraction of Antarctic Bottom Water between the 1980s and 2000s. Journal of Climate. 25:5830-5844.   10.1175/JCLI-D-11-00612.1   AbstractWebsite

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world's deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (theta) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below theta = 0 degrees C at a rate of -8.2 (+/- 2.6) 3 10(6) m(3) s (1). This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are seen along three of the four main northward outflow routes of AABW. Volume and heat budgets below deep, cold theta surfaces within the Brazil and Pacific basins are not in steady state. The observed changes in volume and heat of the coldest waters within these basins could be accounted for by small decreases to the volume transport or small increases to theta of their inflows, or fractional increases in deep mixing. The budget calculations and global contraction pattern are consistent with a global-scale slowdown of the bottom, southern limb of the MOC.

2010
Purkey, SG, Johnson GC.  2010.  Warming of Global Abyssal and Deep Southern Ocean Waters between the 1990s and 2000s: Contributions to Global Heat and Sea Level Rise Budgets. Journal of Climate. 23:6336-6351.   10.1175/2010JCLI3682.1   AbstractWebsite

Abyssal global and deep Southern Ocean temperature trends are quantified between the 1990s and 2000s to assess the role of recent warming of these regions in global heat and sea level budgets The authors 1) compute warming rates with uncertainties along 28 full depth high quality hydrographic sections that have been occupied two or more times between 1980 and 2010, 2) divide the global ocean into 32 basins defined by the topography and climatological ocean bottom temperatures and then 3) estimate temperature trends in the 24 sampled basins The three southernmost basins show a strong statistically significant abyssal warming trend with that warming signal weakening to the north in the central Pacific western Atlantic and eastern Indian Oceans Eastern Atlantic and western Indian Ocean basins show statistically insignificant abyssal cooling trends Excepting the Arctic Ocean and Nordic seas the rate of abyssal (below 4000 m) global ocean heat content change in the 1990s and 2000s is equivalent to a heat flux of 0 027 (+/- 0 009) W m(-2) applied over the entire surface of the earth Deep (1000-4000 m) warming south of the Subantarctic Front of the Antarctic Circumpolar Current adds 0 068 (+/- 0 062) W m(-2) The abyssal warming produces a 0 053 (+/- 0 017) mm yr(-1) increase in global average sea level and the deep warming south of the Subantarctic Front adds another 0 093 (+/- 0 081) mm yr(-1) Thus warming in these regions ventilated primarily by Antarctic Bottom Water accounts for a statistically significant fraction of the present global energy and sea level budgets

2008
Johnson, GC, Purkey SG, Bullister JL.  2008.  Warming and Freshening in the Abyssal Southeastern Indian Ocean. Journal of Climate. 21:5351-5363.   10.1175/2008JCLI2384.1   AbstractWebsite

Warming and freshening of abyssal waters in the eastern Indian Ocean between 1994/95 and 2007 are quantified using data from two closely sampled high-quality occupations of a hydrographic section extending from Antarctica northward to the equator. These changes are limited to abyssal waters in the Princess Elizabeth Trough and the Australian-Antarctic Basin, with little abyssal change evident north of the Southeast Indian Ridge. As in previous studies, significant cooling and freshening is observed in the bottom potential temperature-salinity relations in these two southern basins. In addition, analysis on pressure surfaces shows abyssal warming of about 0.05 degrees C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1 degrees C with freshening of about 0.005 in the abyssal Australian-Antarctic Basin. These 12-yr differences are statistically significant from zero at 95% confidence intervals over the bottom few to several hundred decibars of the water column in both deep basins. Both warming and freshening reduce the density of seawater, contributing to the vertical expansion of the water column. The changes below 3000 dbar in these basins suggest local contributions approaching 1 and 4 cm of sea level rise, respectively. Transient tracer data from the 2007 occupation qualitatively suggest that the abyssal waters in the two southern basins exhibiting changes have significant components that have been exposed to the ocean surface within the last few decades, whereas north of the Southeast Indian Ridge, where changes are not found, the component of abyssal waters that have undergone such ventilation is much reduced.

Johnson, GC, Purkey SG, Toole JM.  2008.  Reduced Antarctic meridional overturning circulation reaches the North Atlantic Ocean. Geophysical Research Letters. 35   Artn L2260110.1029/2008gl035619   AbstractWebsite

We analyze abyssal temperature data in the western North Atlantic Ocean from the 1980s-2000s, showing that reductions in Antarctic Bottom Water (AABW) signatures have reached even that basin. Trans-basin oceanographic sections occupied along 52 degrees W from 1983-2003 and 66 degrees W from 1985-2003 quantify abyssal warming resulting from deepening of the strong thermal boundary between AABW and North Atlantic Deep Water (NADW), hence a local AABW volume reduction. Repeat section data taken from 1981-2004 along 24 degrees N also show a reduced zonal gradient in abyssal temperatures, consistent with decreased northward transport of AABW. The reduction in the Antarctic limb of the MOC within the North Atlantic highlights the global reach of climate variability originating around Antarctica. Citation: Johnson, G. C., S. G. Purkey, and J. M. Toole (2008), Reduced Antarctic meridional overturning circulation reaches the North Atlantic Ocean, Geophys. Res. Lett., 35, L22601, doi: 10.1029/2008GL035619.