Deep Western Boundary Current Variability of the Deep Western Boundary Current at 26.5°N Christopher S. Meinen, Silvia L. Garzoli, and Molly O. Baringer NOAA/Atlantic Oceanographic and Meteorological Laboratory With thanks to Rigo Garcia, Qi Yao, Uli Rivero, Pedro Pena, Andy Stefanick, Kyle Seaton, Carlos Fonseca, Robert Roddy, and Bill Johns

Observational programs Western Boundary Time Series (since 1982) RAPID-MOCHA (since 2004) Courtesy D. Rayner Technologies used: Submarine cable Dropsonde & XBT (Pegasus in 1980s) CTD, SADCP & LADCP Tide/bottom pressure gauges Inverted echo sounders (IES/PIES/CPIES) Technologies used: CTD, SADCP & LADCP Pegasus (in 1980s) Inverted echo sounders (IES/PIES/CPIES) Bottom pressure gauges Dynamic height moorings Current meter moorings

A B C D E Focus here will be on the Deep Western Boundary Current. Basically one can think of the combination of data sets working as follows: - Travel time plus 2-D GEM fields provides the baroclinic structure relative to the bottom - Bottom pressure provides the barotropic signal (where barotropic here is defined as the bottom velocity) A B C D E

How well does the PIES technique work for measuring the transport? Absolute transports How well does the PIES technique work for measuring the transport? Meinen et al, (2006) A B C D E

DWBC variability DWBC transport will be defined here as between 800 and 4800 dbar and between the continental shelf & site E A B C D E Mean transport = -44.2 Sv Standard deviation = 28.3 Sv Integral time scale = 57 days

Time scales of DWBC variability Energetic time scales: 12 days, 32 days, 51 days, 180 days

Time scales of DWBC variability Annual climatology suggests a quasi-semi-annual signal with the largest southward transports in July and December. Historical current meter data from the region has found a more annual signal (Lee et al., 1996), suggesting that this signal may include a bit of aliasing from other time scales.

Time scales of DWBC variability High frequency (< 2 month) variance = 26.37% of total variance Subannual (2 to 11 month) variance = 20.01 % of total variance Annual (11 to 13 month) variance = 0.02 % of total variance Interannual (> 13 month) variance = 53.60 % of total variance Identifying sources of DWBC variability So what is all this variability? Previous studies in this region using the STACS current meter array (e.g. Lee et al., 1996) have argued that the variability is strongly driven by onshore-offshore meandering, while other work (e.g. Chave et al., 1997) has suggested that the variability is dominated by along-slope ‘pulsation’ of the transport.

Identifying sources of DWBC variability: Evaluating spatial structure Attribution/understanding of these signals may benefit from a more fully resolved velocity field, particularly offshore. Additional instruments D3 & D4 were available for 2 years

Identifying sources of DWBC variability: Evaluating spatial structure Empirical orthogonal function analysis of the individual travel time records demonstrates a clear demarcation between the D and D3 sites. Within the two groups of instruments, the records tend to vary in a fairly coherent manner. Hovmoller diagrams show little indication of meandering so far…

Comparison of DWBC variability to MOC variability DWBC and MOC are not correlated (r = -0.19) with zero lag. Best correlation found with MOC leading DWBC by 289 days, however this correlation (r = +0.41) is not statistically significant at the 95% confidence limit.

Preliminary Conclusions The Deep Western Boundary Current is exhibiting a very high level of variability (stnd. dev. ~ 28 Sv) at time scales of a few days to years. The deep transports integrated out ~450 km from the continental shelf below 800 dbar exhibit variations roughly a factor of 5 greater than the basin-wide integrated MOC. There is no clear correlation between variations in the basin-wide MOC and variations in the DWBC. This is true for any lag-lead relationship that can be evaluated with a 4-year record. There is little indication of onshore-offshore meandering of the DWBC with the higher resolution observations collected in 2006-2008. The physical mechanisms behind the DWBC variability observed are still to be identified. Stay tuned…