In 1991, a 50-km array of 5 inverted echo sounders (IESs) was deployed as part of the WOCE deep water station. IESs record the round-trip acoustic travel time from the sea floor to the sea surface, which is known to correlate well with dynamic height. The purpose of these observations was to resolve the synoptic variability which may not be resolved by the monthly sampling.

The IESs were named according to their location with respect to the center of Station ALOHA (C:center, N:North, E:East, SE:Southeast, SW:Southwest) and recorded data from February 1991 to March 1992 when they were recovered.

IESs N, SW, and SE were redeployed in June 1992 and recorded data until their recovery in May 1993.

Two IESs recorded data during 1994. The IES C was deployed on 19 May 1993 and recovered on 21 June 1994. The data from this IES were unreadable and only a short section of the timeseries was recovered. Another IES was deployed at Station Kaena (K) on 23 May 1993, and recovered on 18 June 1994.

New IESs were deployed at the center of Station ALOHA (C) and at Station Kaena (K) in June 1994 and recovered in October 1995. These two IESs were placed in their respective locations in order to span the Hawaiian Ridge Current.

A single IES was deployed at the center of Station ALOHA (C) on October 1995 and recovered on October 1996. Another IES (#126) was deployed at the same site on October 1996, but could not be recovered as planned in July 1997 during cruise 85. The IES was contacted with the hydrophone and it responded confirming the release signal, but it failed to release from its position at the bottom. Operations were conducted during the following cruise (HOT-86) to try to recover the instrument. A 100-m wire with weights attached to it was dragged along the bottom for about 10 hours to try to dislodge the instrument from its presumed position. These activites were unsuccessful and the IES could not be recovered. IES #127 was deployed 4 nm north from the center of Station ALOHA in July 1997 and recovered in July 1998. The same IES was re-deployed at the center of Station ALOHA in August 1998, and was to be recovered on August 12, 1999, but the recovery attempt was unsuccessful as the IES would not rise to the surface after the release command was sent to the instrument. Another unsuccessful attempt to recover the IES was done on November 21 on board of the R/V Ka'Imikai O Kanaloa by using the trawling winch.

CTD casts made during 25 monthly cruises to the deep-water station were used to calibrate the IES. The conversion factor from travel time to dynamic height was found to be -54 dyn m/s. The time-series from the IESs show that there are many fluctuations in dynamic height having timescales from weeks to months that are not well resolved with the CTD sampling.


StationStation location~Depth (m)Deployment dates
Kaena21� 50'N 158� 22'W2,500May 1993-June 1994, June 1994-Oct 1995.
N23� 00'N 158� 00'W4,800Feb 1991-Feb 1992, June 1992-May 1993.
C22� 45'N 158� 00'W4,800Feb 1991-Feb 1992, June 1992-May 1993, May 1993-June 1994, June 1994-Oct 1995, Oct 1995-Oct 1996, Oct 1996-Jul 1997 (instrument lost, IES #126), Jul 1997-Jul 1998, Aug 1998-Aug 1999 (instrument lost, IES #127).
SW22� 37'N 158� 15'W4,800Feb 1991-Feb 1992, June 1992-May 1993.
SE22� 30'N 157� 45'W4,800 Feb 1991-Feb 1992, June 1992-May 1993.
E22� 45'N 157� 54'W4,800Feb 1991-Feb 1992.