The waterways of the Indonesian seas link the tropical Pacific Ocean with the Indian Ocean. The resultant interocean transfer of seawater from the Pacific to the Indian Ocean, referred to as the Indonesian Throughflow (ITF), is ten times greater than all of the river flow of the Earth. The ITF has a major impact on the heat content and sea surface temperature patterns of the Indian Ocean, and also affects the western Pacific. As shifts in sea surface temperature patterns are so closely linked to El Niño and the Asian monsoon, both of which have a major impact on society, understanding the ITF relationship to the regional and global climate system is essential in predicting climate change. To advance understanding and our ability to model and predict ITF coupling to climate and marine ecosystems requires sustained observations of the ITF behavior.
Directly after the recovery of the NSF funded INSTANT western Makassar mooring in November 2006, a NOAA funded mooring (NOAA-MAK) was deployed at the same site (2 51 S; 118 28 E). We now have a 5.5-year continuous time series of the Makassar Throughflow that carries 80 to 85% of the ITF. The NOAA-MAK time series from late November 2006 to the end of May 2009 spans a predominately La Niña period (when the throughflow is expected to increase) reaching a peak in strength in early 2008; a weak El Niño (when the throughflow is expected to decrease) in the latter half of 2006 (top fig.). The Indian Ocean Dipole Mode Index (DMI) switched into a positive mode during 2006, remaining so throughout the NOAA-MAK time series. Positive DMI is related to lower sea level in the eastern tropical Indian Ocean and an increase in the ITF. Three consecutive years with a positive DMI have not been detected in the historical SST records. This unprecedented long period of +DMI is likely linked to the shift in the Makassar throughflow and ITF.
The Makassar throughflow change observed by the NOAA-MAK (late 2006 to mid-2009) relative to the INSTANT period (2004 to late 2006) (bottom fig.) was: (1) The thermocline flow was intensified, with a maximum speed near 75 m composed of warmer upper thermocline water, rather than at 140 m marking the mid-thermocline; (2) Increase of the Makassar throughflow transport from the 2004-2006 average of 11.6 Sverdrup (Sv = 106 m3/s) to 13.5 Sv in 2007-2008, with an increase in transport weighted temperature from 15C to 18C. The +DMI is likely related to these significant changes. We expect that the change in the ITF profile will eventually affect the Indian Ocean surface layer temperature. As the observational time series lengthen we will develop a more quantitative grasp of the ocean and climate coupled system.
This research was funded in full or in part under the Cooperative Institute for Climate Applications Research (CICAR) award number NA08OAR4320754 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the author(s) and do not necessarily reflect the views of the National Oceanic and Atmospheric Administration or the Department of Commerce.