The Diurnal Cycle of Near-Surface Stratified Shear Flow at 0°N, 23°W Sc

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Jacob O. Wenegrat and Michael J. McPhaden University of Washington, Seattle and Pacific Marine Environmental Laboratory, NOAA, Seattle

Corresponding Author: wenegrat@uw.edu

Motivation

Acknowledgements

The 0°N 23°W PIRATA Mooring, provides a time-series spanning more than 14 years of atmosphere and upper ocean variability in the central equatorial Atlantic. In addition to the standard PIRATA instrumentation, the 23°W mooring provides detailed surface flux data as part of the OceanSITES project[3]. This data is used to study the seasonal modulation of the diurnal cycle of SST in the Atlantic.

Enhanced Monitoring Period (EMP): From 13 October 2008 through 17 June 2009, the 23°W PIRATA mooring was instrumented with a downward facing ADCP, resolving horizontal velocity between 3.75-35 m depth. This data is used to study the vertical structure of the diurnal cycle of stratified shear flow in the near-surface layer. See [4] for details of data processing and validation.

Data Overview:

Complex Demodulation, is used to isolate the diurnal SST signal. This method expresses a signal as a linear combination of a component oscillating at the frequency of interest, with slowly varying amplitude and phase, and additional variability at other frequencies which is removed by filtering[5]. The peak-to-peak diurnal SST signal is 2×A(t).

Sh2red – Reduced shear squared is used as an alternate formulation of the Richardson number to

quantify the susceptibility of shear flow to turbulent mixing, with positive numbers indicating sub-critical Richardson numbers (Ri < 0.25).

Av – Near-surface eddy viscosity is inferred using the surface boundary condition and observed wind stress and shear at 5.64 m, assuming uniform stress between the surface and 5.64 m. Discussed further in [4].

The diurnal cycle of sea surface temperature and wind-driven shear flow has been shown to fundamentally affect ocean and atmospheric variability on a range of timescales[1]. Improved understanding of how both large scale atmospheric conditions, as well as mixed-layer physics, influence the diurnal cycle in the near-surface ocean remains an important goal.

Here we present initial results on the diurnal cycle of upper ocean variability from the 0°N, 23°W Prediction and Research Moored Array in the Tropical Atlantic[2] (PIRATA) mooring. Diurnal variability in the Atlantic has not been as fully characterized as in the Pacific, and the 23°W location provides both a long time-series of detailed surface flux data, and a unique 8 month dataset of velocity data in the near-surface layer.

We would like to thank Ren-Chieh Lien for very helpful input and discussion on this work, Patricia Plimpton for initial processing of the ADCP data, and Paul Freitag and the TAO Project Office for data collection.

0°N,  23°W  Study  Loca2on  

References [1]  Shinoda,  T.,  2005:  Impact  of  the  Diurnal  Cycle  of  Solar  Radia2on  on  Intraseasonal  SST  Variability  in  the  Western  Equatorial  Pacific,  J.  Climate,  18.  

[2]  Bourlès,  B.,  and  Co-­‐Authors,  2008:  The  Pirata  program:  history,  accomplishments,  and  future  direc2ons.  Bull.  Amer.  Meteor.  Soc.,  89  (8).    

[3]  hWp://www.pmel.noaa.gov/tao/oceansites/overview.html  

[4]  Wenegrat,  J.O.,  M.J.  McPhaden,  and  R.-­‐C.  Lien,  2014:  Wind  stress  and  near-­‐surface  shear  in  the  equatorial  Atlan2c  Ocean,  Geophys.  Res.  Le:.  In  Press.  

[5]  Cronin,  M.F.,  and  W.S.  Kessler,  2002:  Seasonal  and  interannual  modula2on  of  mixed-­‐layer  variability  at  0°N,  110°W,  Deep  Sea  Res.  I,  39.  

[6]  hWp://www.esrl.noaa.gov/psd/data/gridded/data.interp_OLR.html  

[7]  Shudlich,  R.R.,  and  J.F.  Price,  1992:  Diurnal  Cycles  of  Current,  Temperature,  and  Turbulent  Dissipa2on  in  a  Model  of  the  Equatorial  Upper  Ocean,  J.  Geophys.  Res.,  97.  

Dataset

Seasonal Modulation Mixed  Layer  Dynamics  

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Depth Frequency Velocity (ADCP) 3.75 – 35 m in 0.75 m vertical bins Hourly Wind Speed +4 m Hourly Salinity 1*, 5*, 20, 40, 60, 80, 100, 120 Hourly Temperature 1, 5*, 10, 13, 20, 23, 40, 60, 80, 100, 120 10 min Short/Long Wave Radiation +4 m Hourly Precipitation +4 m Hourly

Diurnal SST amplitude is modulated seasonally by atmospheric variability. In boreal spring, decreased wind stress and increased near-surface stratification result in maximum monthly averaged diurnal SST amplitudes that reach higher than 0.3°C. This maximum coincides with the annual minimum in solar heating, suggesting the control of diurnal SST amplitude by wind-driven mixing and upwelling[5].

The converse is true in boreal summer, where trade wind conditions lead to deep mixed layers, a weakly stratified surface layer, and maximum diurnal SST amplitudes of approximately 0.15°C.

A climatology based on the total time series is shown in Figure 2 (right).

Local Hour

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Figure  1:  Detailed  flux  data,  a)  diurnal  SST  amplitude,  averaged  monthly,  b)  monthly  wind  speed,  c)  monthly  averaged  temperature  difference  between  1  m  and  5  m,  d)  monthly  Isothermal  mixed  layer  depth  (ΔT=.5°C,  blue  solid  line)  and  depth  of  20°C  isotherm  (black  dashed),  a  proxy  for  the  thermocline,  e)  monthly  averaged  short  wave  radia2on  (blue  solid)  from  the  mooring  and  outgoing  longwave  radia2on  (black  dashed)  from  NOAA  interpolated  OLR[6].  Ver2cal  solid  lines  indicate  the  enhanced  monitoring  period.  

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Mean Diurnal Composite 10/2008-1/2009

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Fig.  4:  Composite  diurnal  cycle  for  trade  wind  condi2ons,  a)  net  surface  heat  flux,  b)  SST  anomaly,  c)  wind  vectors  ploWed  along  the  z=0  line,  current  vectors,  rela2ve  to  the  20  m  currents,  are  ploWed  at  the  observa2on  depths,  temperature  anomaly,  rela2ve  to  the  2me-­‐depth  average,  is  shown  in  the  color  scale,  and  MLD  (dashed,  Δρ  =  0.015  kg  m3).  All  vectors  are  oriented  with  North  up,  d)  eddy  viscosity[4],  calculated  using  the  surface  wind  stress  and  shear  at  5.64  m.  

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Figure  6:  As  in  Figure  4,  but  for  the  warm  period.    

Primary Findings

Fig.  5:  Composite  diurnal  cycle  of  Sh2

red, formed  from  geometric  means  of  u2z  and  N2.  

Posi2ve  values  indicate  flow  unstable  to  shear  instability,  nega2ve  numbers  indicate  stability.  MLD  is  also  shown  (dashed  line).  

Normal Trades: Solar heating of the near-surface layer increases stratification, leading to a sheared diurnal jet aligned with the wind stress (Fig 4, left). Maximum surface shear and temperature anomalies are found at 14:00 local. Diurnal heating is spread through the near-surface layer, limiting diurnal SST anomalies to less than 0.2°C.

Near-surface shears reach as high as 23 cm s-1 relative to 20 m currents, increasing late afternoon values of Sh2

red (Fig. 5, below). The diurnal cycle of mixing is evidenced in both Sh2

red , and near-surface eddy viscosity (Av, Fig. 4, bottom).

During nighttime hours, the surface heat flux changes sign, and the mixed layer deepens and cools rapidly due to convective overturning.

Figure  7:  As  in  Fig  5,  but  for  the  warm  period.  Note  increased  color  scale  range.  

-  Diurnal SST amplitude is maximum during periods of low wind stress, despite reduced short-wave radiation, reaching amplitudes greater than 0.4°C. During this period the near-surface layer is highly sheared due to vertical advection of the EUC, but Sh2

red indicates stability throughout the diurnal cycle.

-  During normal trade wind conditions, solar heating is increased, however the amplitude of the diurnal cycle is less than 0.2°C, highlighting the control of diurnal SST amplitude by mixed layer dynamics. Diurnal composites indicate a sheared diurnal jet with decreased daytime Av and Sh2

red, increasing in late afternoon and at night. Temperature anomalies are well mixed throughout the near-surface layer.

- Diurnal variability of SST responds to both large scale atmospheric forcing, as well as locally and remotely forced mixed layer dynamics.

Warm Period: Maximum diurnal SST amplitudes are found in January-May, reaching approximately 0.35°C. During this period solar heating is reduced, however weak wind stress leads to reduced mixing. Accordingly, the mixed layer is shallow, with diurnal temperature anomalies isolated in the upper few meters, remaining near maximum values until shortly after surface heat flux changes sign at 17:00 local. Near-surface Av shows little diurnal variability, and is an order of magnitude smaller than during trade wind conditions.

Vertical advection of the EUC results in a near-surface layer that is highly sheared throughout the diurnal cycle. High stratification leads to a stable surface layer (Fig 7, below), with isopleths of Sh2

red, oriented horizontally. The increased shear in this regime results from basin scale ocean dynamics, rather than local forcing, and has been shown to modify the local diurnal SST response[7].

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Fig.  3:  Overview  of  enhanced  monitoring  period,  a)  5-­‐day  average  diurnal  SST  amplitude,  b)  5-­‐day  average  net  surface  heat  flux  (black),  defined  posi2ve  into  the  ocean,  with  contribu2ng  terms  as  indicated  in  legend,  c)  daily  evapora2on-­‐precipita2on,  d)  daily  Wind  vectors,  oriented  with  North  up,  e)  daily  zonal  velocity,  f)  daily  meridional  velocity,  g)  daily  shear  squared,  h)  daily  N2,  i)  daily  reduced  shear  squared.  Tick  marks  inside  lem  axis  indicate  observa2on  depths,  and  black  dashed  line  indicates  mixed  layer  depth  for  (e)-­‐(i).  Ver2cal  lines  demarcate  the  two  composi2ng  periods.  A  co-­‐located  upwards  facing  ADCP  provides  velocity  data  below  35  m.  

The EMP spans the two seasonal regimes. Normal Trades: From Oct. – Dec., the diurnal SST amplitude is low, wind stress is high, and high shear on the upper flank of the equatorial undercurrent (EUC) leads to increased Sh2

red above 50 m. Warm Period: From Jan. – May, diurnal SST amplitude increases, along with increased cloudiness (decreased SWR), precipitation, and near-surface stratification. The mixed layer is shallow, and vertical advection of the EUC, creates a thin layer of near-critical Sh2

red directly below the MLD. This high shear region modifies the diurnal SST response through mixing[7], allowing basin scale ocean dynamics to interact with the locally forced response.

To examine the role of mixed layer dynamics we form composite diurnal cycles of the two time periods indicated on Figure 3.

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SST Amp.Wind SpeedSWR

Figure  2:  Climatology  of  diurnal  SST  amplitude  (black),  wind  speed  (blue),  and  short  wave  radia2on  (red),  formed  from  monthly  averaged  values.    

Shred2 = Sh2 − 4 × N 2

Av =τρuz

*Missing  data  filled  by  regression  with  neighboring  depths