XBT High Density Repeat Lines Information

---

Personnel Objectives Sponsor Rationale Implementation Results References Comments

To measure the upper ocean thermal structure in the center of the subtropical gyre in the North Atlantic (AX7 ), and the South Atlantic ( AX18 ), and ( AX25 to investigate the meridional structure at the subtropical gyre and Golf Stream in the North Atlantic( AX10 ), to characterize both the mean and the time-dependent upper ocean properties of the tropical portion of the Meridional Overturning Circulation and of the shallow Subtropical Cell in the Tropical Atlantic ( AX8 ).

NOAA's Office of Global Programs

Measuring the seasonal to interannual varibility in upper ocean heat content and transport is important to monitor and understand how the ocean responds to changes in surface temperature and improve our ability to predict important climatic fluctuations like the North Atlantic Oscillation (NAO).

Upper ocean temperature anomalies are known to have short (seasonal) and longer (decadal) signals in the Atlantic Ocean. As an example, the Figure below shows January SST anomalies from the COADS data set as analyzed by Hansen and Bezdek (1996). Red indicates warm anomalies, blue indicates cold anomalies, and yellow indicates areas where the data density is inadequate for mapping.

This figure shows that in the early 1950's a warm anomaly is seen just off the east coast of the United States. As the decade advances, the warm anomaly grows in size and begins to propogate northward around the subtropical and then subpolar gyres in the Atlantic. Later in the 1970's, a cool event invades the subtropical then subpolar gyres. In the 1980's and 1990's smaller warm anomalies can be seen advancing northward along the eastern United States. These sea surface temperature anomalies in the Atlantic have signatures in the deeper water as well (see the list of references below for just a few of the many papers examining these anomalies). Correlations between atmospheric variables have also been documented (see for instance Curry and McCartney, 1996 and references therein). Sutton and Allen (1997) followed several such anomalies that had their origins just off the east coast of the United States. Other authors have shown that the ocean is likely to impact the atmosphere in the small region just off the eastern coast, where the Gulf Stream liberates substantial heat to the atmosphere (see Swenson and Bezdek, 1998 and references therein).

As another example, the current system of the western Tropical Atlantic ocean is marked by an intense variability, apparently in response to seasonal changes in the wind stress pattern. This region is of particular interest for the large scale ocean circulation since it is a place where strong western boundary currents contribute to inter-hemispheric transport of properties (e.g., Stramma and Schott, 1996; Johns et al., 1998). Off the coast of Brazil the Meridional Overturning Circulation (MOC) carries warm water from the South Atlantic to the North Atlantic within a western boundary current, the North Brazil Current (NBC). Below the NBC colder, fresher Antarctic Intermediate water flows north in the North Brazil Undercurrent. In addition to the northward flow of the NBC a shallow Subtropical Cell (STC) carries subducted surface water from the southern subtropics to the equator, where it is upwelled to the surface. While the deep flows are characterized by time scales of decades or more the time scales of STCs are thought to be months to several years. Monitoring water mass properties as well as the velocity structure of the hypothesized pathways between the subtropics and tropics will provide the tools to begin to characterize both the mean and the time-dependent properties of the tropical portion of the MOC and the Atlantic STC. Similar investigations conducted in the Pacific have demonstrated that this type of study can provide substantial understanding of the linkage between the MOC and the STC.

NOAA/OGP funds five high-density XBT lines maintained by NOAA/AOML: 1) AX7 , located along 30°N extending from the Straits of Gibraltar to Miami, 2) AX10 , running between New York and Puerto Rico, 3) AX8 , sampling across the Tropical Atlantic with emphasis between 30& deg;N and 30°S, 4) AX18 , running between South Africa and Argentina along 35°S, and 5) AX25 , sampling between South Africa and Antarctica. These five XBT lines have been chosen to capture and monitor thermal properties within the Atlantic. The AX7 and AX18 lines have been selected to monitor the net meridional flow in the upper ocean. AX10, AX08 and AX25 are meridional lines that were selected because they cross important highly variable ocean currents, namely the Gulf Stream, the numerous Equatorial Atlantic Currents and the Agulhas and Antartic Circumpolar Currents respectively. All XBT lines are valuable in providing estimates of the mean and time dependent temperature fields with sufficiently close sp acing to sample the mesoscale field (XBTs spaced between 30-50km). They all sample various aspects of the overturning ci rculation and hence provide useful data on heat transport and interbasin/cross equatorial exchanges. To date, more than 1 3,000 XBTs have been deployed in its high density mode in the Atlantic Ocean.

Ancillary programs that monitor the Florida Current transport have been further augmented by this program to include temperature measurements. In an associated NOAA/OGP program, AOML monitors cable voltages and determines transport measurements to develope a transfer function to predict transports from the cable voltages. Up to 12 times per year AOML sends a small boat to measure Florida Current transport with a instrument called a dropsonde. These measurements are used to verify and calibrate the transport measurements derived by monitoring voltage on abandoned submarine telephone cables. AOML has modified the dropsonde to include a temperature sensor so that up to 12 temperature sections of the Florida Current can also be obtained.

These XBT lines require the aid of Volunteer Observing Ships (VOS) willing to have a scientist aboard to deploy XBTs. These lines (designated AX7, AX8, AX10, AX18 and AX25) meet WOCE criteria for high resolution deployment providing temperature profiles every 50 km in the open ocean and between 10-30 km near boundary currents down to a depth of about 800 meters. Such fine horizontal sampling requires a scientist on board the ship and the use of an autolauncher designed at AOML that can load and launch up to 6-8 XBTs fired remotely via computer-controlled software.

Typically an AX7 line takes 12 days of ship time and about 240 XBTs. The AX8 line takes approximately 20 days and about 215 XBTs. A typical AX10 line takes only 4 days and about 100 XBTs, and the AX18 line takes approximately 11 days and about 190 XBTs, and the AX25 line takes 12 days and about 190 XBTs.

Several high density XBT sections have been completed. For XBT cruise details, position summaries, maps and temperature sections for each high density line please see the individual home pages.

AX7 Line: Has been operating since 1995 with a brief false start in 1994. Sampling started as twice per year, then was augmented to four times per year in 1996. As of October, 2002, 28 high density sections have been completed .

AX10 Line: Has been operating since 1997. One cruise in December of 1996 was suspended due to extremely heavy weather (the Sealand Cruisader exterienced 60 knot winds and lost several large containers while in transit). Sampling started as four times per year. As of October, 2002, 22 high density sections have been completed .

AX8 Line: Has been operating since December 2000. Starting in 2002, cruises are scheduled four times per year. As of October, 2002, 5 cruises have been completed .

AX18 Line: Has been operating since July 2002. Sampling is scheduled for twice per year. As of October, 2002, only one cruise has been completed .

AX25 Line: Has been operating since February 2004. Sampling is scheduled for twice per year.

Baringer, M. O. and J. Larsen, 2001. "Sixteen Years of Florida Current Transport at 27 °N". Geophysical Research Letters, 28, 16, 3179-3182.

Baringer, M.O, 2002. "Historical record of Florida Current observations". AGU 2001 Fall Meeting, San Francisco, CA, December 10-14, 2001. Supplement to EOS, Transactions, American Geophysical Union, 82(47):F681-F682, OS52C-09.

Baringer, M. O. and D. Mayer 2002. "On The Mean and Fluctuating Temperature Field along a section of the SubTropical North Atlantic Ocean". J. of Geophysical Research, under revision.

Goni, G. J. and M. O. Baringer, 2002. "Surface currents in the tropical Atlantic across high density XBT line Ax08". Geophysical Research Letters, in press.

Mayer, D. A., R. L. Molinari, M. O. Baringer, and G. J. Goni, 2001. "Transition regions and their role in the relationship between sea surface height and subsurface temperature structure in the Atlantic Ocean". Geophysical Research Letters, 28, 20, 3943--3946.

Mayer, D. A., M. O. Baringer, R. L. Molinari and Gustavo J. Goni, 2002. "Comparison of Hydrographic and Altimetric Estimates of Sea Level Height Variability in the Atlantic Ocean". Interhemispheric Water Exchange in the Atlantic Ocean, Elsevier Oceanographic Series, to be published in March 2003. (submitted)

Molinari, R. L. , R. Lusic, S. L. Garzoli, M. O. Baringer and G. Goni, 2002. "Benchmarks for Atlantic Ocean Circulation". CLIVAR Exchanges, 7, No. 3/4, September 2002.

Bibliography

qi.yao@noaa.gov