Interannual to decadal salinity variations observed near Hawaii

R. Lukas¹, F. Santiago-Mandujano¹, D. Stammer²

¹Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, HI 96822

²Center for Marine and Atmospheric Research, University of Hamburg, Institute of Oceanography (IfM) Bundesstr. 53, 1st. floor, Hamburg, D-20146

Abstract

Measurements of surface salinity and temperature off Koko Head (Oahu) were initiated in 1955 by Gunter Seckel of the Honolulu Laboratory of what is now the National Marine Fisheries Service of NOAA. Observations continued into 1988, with at least weekly sampling. The Hawaii Ocean Time-series (HOT) was initiated at Station ALOHA, 100 km north of Oahu, just as the Koko Head time-series was discontinued for lack of funding. The two time series have been merged into a 50- year-long Hawaii surface salinity record, which exhibits variations over a range of nearly 1 psu. Rapid interannual increases in surface salinity near Hawaii are often associated with droughts during El Nino winters. Decadal variations are also present, but are less obvious. The highest observed salinities in the Hawaii record occurred during 1998-2002, correlated with significant surface salinity anomalies throughout the Pacific Ocean from the ECCO analysis, and with rainfall anomalies apparently associated with the unusually long and strong La Nina event. Subsurface salinity variations have only been observed in the recent 17 years of HOT measurements, but pronounced signals with decadal and longer time scales are already obvious in the thermocline. In the salinity maximum (24.2-25 sigma-theta, 80-160 m average depth range), one oscillation with approximately 15-year period and 0.1 psu amplitude has been observed. In the main thermocline (25-26 sigma-theta, 160-310 m average depth range), the dominant feature is a trend with a 0.2 psu salinity decrease from 1991 through 2005. These variations have been related to large-scale rainfall variations in the North Pacific storm track, through subduction and subsequent advection of the associated remote mixed layer salinity anomalies. The corresponding isopycnal temperature anomalies have a range of 1 deg C, which could be large enough to affect the overlying atmosphere where they eventually reemerge from the thermocline.