Moored Mixing Measurements at the Equator

Long χpod time series in the Pacific cold tongue

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We have been deploying χpods on the TAO mooring at 0, 140W since September 2005. The measurement is simply of temperature, but fast enough to detect the changes due to turbulence. To date, our efforts have focused on:

  1. Demonstration and verification of the moored measurement (Moum & Nash, 2009; Zhang & Moum, 2010; Perlin & Moum, 2012);
  2. Investigations that target the physical processes directly leading to mixing through concentration on the short timescale variability of equatorial turbulence and small-scales (Moum et al., 2011; Smyth et al., 2011; Smyth et al., 2013); and
  3. Analyses of long term dependencies that clearly show the dominant role that vertical mixing plays in seasonal cooling of the equatorial Pacific cold tongue (Figure 3; Moum et al., 2013) and how the underlying seasonal cycle in mixing is linked to a varying state of marginal instability below the equatorial mixed layer (Smyth & Moum, 2013).

i. χpods and how we know they work

To sample turbulence (considered here as the mechanical contributor to mixing) over the periods of time necessary to resolve its effect on long timescale phenomena requires creative observational techniques. The difficulties in making turbulence measurements on oceanographic moorings are indicated by the relative lack of such measurements. Surface moorings in the open ocean are continually pumped by surface waves that transmit a range of motions down the cable (Moum & Nash 2009). To gain a complete understanding of these motions and how they affect the environmental signal requires a full suite of acceleration measurements, preferably over-sampled and stored for further analysis. Only recently have we been able to reap the benefits of technical advances that have accompanied cellular telephones and digital cameras; specifically low-power surface mount electronics, high capacity batteries, and extensive data storage using solid-state devices. These advances have permitted development of χpods: vaned, internally-recording instruments that measure temperature at 10 Hz and temperature gradients at 100 Hz using fast thermistors (Moum & Nash 2009). The instruments were designed for year-long deployments on oceanographic moorings. We have maintained a near-continuous deployment of 3–6 χpods in the upper equatorial Pacific on NOAA’s TAO mooring at 0, 140W since September 2005.

One of the reasons for our 2008 process experiment at 0, 140W was to directly compare turbulence quantities (χΤ, εχ) computed from thermistors on χpods with those computed directly from thermistors and shear probes on our turbulence profiler, Chameleon. Comparisons were made with χpods at several depths and on two equatorial moorings separated by 10 km. This field work permitted an evaluation, for the first time, of the tremendous influence of TIWs on equatorial mixing (Moum et al., 2009). Because the turbulence quantities that we measured using Chameleon were so much larger than previous experiments, we went to great lengths to assure ourselves that sensors were properly calibrated and processing of data was correct. Gratifyingly, comparison between completely independent measurements from Chameleon and χpods on the two moorings (Perlin & Moum, 2012) indicates that mean values of χΤ and εχ above 100 m and the 2 orders of magnitude decrease in ε below. Further analysis suggests that differences on shorter timescales can be reasonably accounted for by natural variability between measurement locations (10 km apart, Fig. 1).

ii. Physics of turbulence and instability
From the χpod measurements arises a new depiction of the full frequency spectrum of temperature fluctuations. For example, the narrowband signal identified in wavenumber (Moum et al., 1992) and frequency spectra (McPhaden & Peters 1992) and previously associated with the deep cycle (Lien et al., 1995) was recently linked to the advection of shear instabilities (Fig. 2; Moum et al., 2011). The narrowband spectral peak coincides with the peak frequency determined from an ensemble of unstable modes computed by linear stability analysis using observed currents and stratification. The peak of the frequency distribution, determined as the ratio of predicted phase speeds (c) to predicted wavelengths (λ), is a statistical consequence of the product of two uniform distributions, and the value of f = c/λ near N is due to near-critical gradient Richardson numbers that fix shear to stratification (Smyth et al., 2011).

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iii. Seasonal variations in SST Cooling, Mixing and Marginal Instability
χpods were developed specifically for the purpose of making long time-series measurements capable of capturing mixing on annual to interannual time scales. Analysis of the 1st six years of data from 0, 140W has led to the clear demonstration that subsurface mixing controls boreal summer cooling of the sea surface in the Pacific’s cold tongue through increased heat flux divergence across the air-sea interface (Fig. 3; Moum et al., 2013).

An important additional finding, using both TAO and χpod observations, shows the state of “marginal instability” (a state defined by low Ri above the EUC core that sets the deep cycle) to vary seasonally (Smyth & Moum, 2013). Marginal instability is present for 9 months of the year but disappears in boreal spring, exactly the time period of peak sea surface heating (Figure 3) and weakest turbulence at 0, 140W. While the absence of marginal instability is associated with weak turbulence, the presence of marginal instability does not provide sufficient information to predict variations in the intensity of the turbulence; both high and low turbulence are found when marginal instability exists.

The resolution of several ENSO cycles has begun with the time series in hand. The record encompasses the time frames of identified El Niñs in 2006/2007 and 2009/2010 and La Niñas in 2007/2008 as well as the most intense La Niña yet observed in 2010/2011. Over time, moored χpods will hopefully sample a sufficient number of oscillations between El Niño and La Niña phases of ocean state to reveal how ocean variability on large scales modulates mixing, and vice-versa. These particular measurements will be invaluable for improving climate models.

Moum, J.N., A. Perlin, J.D. Nash and M.J. McPhaden, 2013. Seasonal sea surface cooling in equatorial Pacific cold tongue controlled by ocean mixing. Nature, 500, 64-67, doi:10.1038/nature12363. [Nature Letter][News and Views]

Moum, J.N. and J.D. Nash, 2009. Mixing measurements on an equatorial ocean mooring. J.Atmos. Oceanic Technol. 26, 317-336. [pdf]

Moum, J.N., J.D. Nash and W.D. Smyth, 2011. Narrowband oscillations in the upper equatorial ocean. Part I: Interpretation as shear instabilities. J. Phys. Oceanogr., 41, 397-411. [pdf]

Perlin, A. and J.N. Moum, 2012. Comparison of thermal dissipation rate estimates from moored and profiling instruments at the equator. J.Atmos. Oceanic Technol., 29, 1347-1362.[AMS]

Smyth. W.D. and J.N. Moum, 2013. Marginal instability and deep cycle mixing in the eastern equatorial Pacific Ocean. Geophys. Res. Lett., 40, 1-5, doi:10.1002/2013GL058403. [pdf]

Smyth, W.D., J.N. Moum and J.D. Nash, 2011. Narrowband oscillations in the upper equatorial ocean. Part II: Properties of shear instabilities. J. Phys. Oceanogr., 41, 412-428.[pdf]