Seasonal Variability of the Diurnal Cycle of Cloud Liquid ...
The Diurnal Cycle of Near-Surface Stratified Shear Flow at ... · The Diurnal Cycle of Near-Surface...
1
The Diurnal Cycle of Near-Surface Stratified Shear Flow at 0°N, 23°W S c h o o l o f O c e a n o g r a p h y U n i v e r s i t y o f W a s h i n g t o n Jacob O. Wenegrat and Michael J. McPhaden University of Washington, Seattle and Pacific Marine Environmental Laboratory, NOAA, Seattle Corresponding Author: [email protected] 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). Sh 2 red – 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). A v – 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 CoAuthors, 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 nearsurface 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 mixedlayer 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 0 0.1 0.2 0.3 0.4 SST Amp. (°C) 23°W 0 2.5 5 7.5 10 Wind (m s -1 ) 0 0.075 0.15 0.225 0.3 ΔSST (°C) 0 30 60 90 120 MLD, Z20 (m) 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 200 225 250 275 300 SWR, OLR (W m -2 ) a) e) d) c) b) X(t ) = A(t ) Amplitude cos(ωt −Φ(t ) Phase ) + Z(t ) 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 Depth (m) Reduced Shear Squared Sh 2 - 4 × N 2 0 2 4 6 8 10 12 14 16 18 20 22 0 5 10 15 20 25 30 35 s -2 -1 0 1 x 10 -4 Local Hour Depth (m) Reduced Shear Squared Sh 2 - 4 × N 2 0 2 4 6 8 10 12 14 16 18 20 22 0 5 10 15 20 25 30 35 s -2 -1 -0.5 0 0.5 1 x 10 -3 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. -200 0 200 400 600 800 Heat Flux W m -2 Mean Diurnal Composite 10/2008-1/2009 -.35 0 .35 SSTA °C Wind Stress, Current Shear Depth (m) Wind: 4 m/s Current: 20 cm/s 0 5 10 15 20 Temperature Anomaly ( °C) -0.5 0 0.5 0 2 4 6 8 10 12 14 16 18 20 22 10 -3 10 -2 Local Hour A v m -2 s -1 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 2medepth 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. -200 0 200 400 600 800 Heat Flux W m -2 Mean Diurnal Composite 1/2009-5/2009 -.35 0 .35 SSTA °C Wind Stress, Current Shear Depth (m) Wind: 4 m/s Current: 20 cm/s 0 5 10 15 20 Temperature Anomaly ( °C) -0.5 0 0.5 0 2 4 6 8 10 12 14 16 18 20 22 10 -3 10 -2 Local Hour A v m -2 s -1 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 u2 z 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 Sh 2 red (Fig. 5, below). The diurnal cycle of mixing is evidenced in both Sh 2 red , and near-surface eddy viscosity (A v , 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 Sh 2 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 A v and Sh 2 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 A v 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 Sh 2 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] . 0 0.1 0.2 0.3 0.4 SST Amp °C a) -100 0 100 200 300 Heat Flux W m s -2 b) Net Heat Flux SWR -LWR -Sensible HF -Latent HF -10 -5 0 E-P cm c) -12 -6 0 6 12 Wind m s -1 d) U Depth (m) e) 0 20 40 60 80 m s -1 -1 -0.5 0 0.5 1 V Depth (m) f) 0 20 40 60 80 m s -1 -1 -0.5 0 0.5 1 Log 10 (Sh 2 ) Depth (m) g) 0 20 40 60 80 Log 10 (s -2 ) -6 -4 -2 Log 10 (N 2 ) Depth (m) h) 0 20 40 60 80 log 10 (s -2 ) -6 -5 -4 -3 -2 Sh 2 red Depth (m) i) Nov Dec Jan Feb Mar Apr May Jun 0 20 40 60 80 × 10 -3 (s -2 ) -4 -2 0 1 Normal Trades Warm Period Fig. 3: Overview of enhanced monitoring period, a) 5day average diurnal SST amplitude, b) 5day average net surface heat flux (black), defined posi2ve into the ocean, with contribu2ng terms as indicated in legend, c) daily evapora2onprecipita2on, 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 colocated 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 Sh 2 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 Sh 2 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. 0 2 4 6 8 10 m s -1 220 240 260 280 300 W m -2 J F M A M J J A S O N D 0.1 0.15 0.2 0.25 0.3 0.35 °C 23°W Climatology SST Amp. Wind Speed SWR Figure 2: Climatology of diurnal SST amplitude (black), wind speed (blue), and short wave radia2on (red), formed from monthly averaged values. Sh red 2 = Sh 2 − 4 × N 2 A v = τ ρu z *Missing data filled by regression with neighboring depths
Transcript of The Diurnal Cycle of Near-Surface Stratified Shear Flow at ... · The Diurnal Cycle of Near-Surface...
The Diurnal Cycle of Near-Surface Stratified Shear Flow at 0°N, 23°W Sc
ho
ol o f O c e a n o g
ra
p
hy
Un
iv
er s i t y o f W a s h
i ng
t
on
Jacob O. Wenegrat and Michael J. McPhaden University of Washington, Seattle and Pacific Marine Environmental Laboratory, NOAA, Seattle
Corresponding Author: [email protected]
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
0
0.1
0.2
0.3
0.4
SS
T A
mp
.
(°C
)
23°W
0
2.5
5
7.5
10
Win
d
(m s
-1)
0
0.075
0.15
0.225
0.3
!S
ST
(°C
)
0
30
60
90
120
ML
D,
Z2
0(m
)
1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014200
225
250
275
300
SW
R,
OL
R
(W m
-2)
a)
e)
d)
c)
b)
X(t) = A(t)
Amplitude cos(ωt −Φ(t)
Phase)+ Z(t)
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
Dep
th (
m)
Reduced Shear Squared
Sh2 - 4 ! N
2
0 2 4 6 8 10 12 14 16 18 20 22
0
5
10
15
20
25
30
35
s-2
-1
0
1x 10
-4
Local Hour
Dep
th (
m)
Reduced Shear Squared
Sh2 - 4 ! N
2
0 2 4 6 8 10 12 14 16 18 20 22
0
5
10
15
20
25
30
35
s-2
-1
-0.5
0
0.5
1x 10
-3
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.
-2000
200400600800
Hea
t F
lux
W m
-2
Mean Diurnal Composite 10/2008-1/2009
-.350
.35
SS
TA
°C
Win
d S
tres
s, C
urr
ent
Shea
rD
epth
(m
)
Wind: 4 m/sCurrent: 20 cm/s
0
5
10
15
20
Tem
per
ature
Anom
aly (°
C)
-0.5
0
0.5
0 2 4 6 8 10 12 14 16 18 20 22
10-3
10-2
Local Hour
Av
m-2
s-1
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.
-2000
200400600800
Hea
t F
lux
W m
-2
Mean Diurnal Composite 1/2009-5/2009
-.350
.35
SS
TA
°C
Win
d S
tres
s, C
urr
ent
Shea
rD
epth
(m
)
Wind: 4 m/sCurrent: 20 cm/s
0
5
10
15
20
Tem
per
ature
Anom
aly (°
C)
-0.5
0
0.5
0 2 4 6 8 10 12 14 16 18 20 22
10-3
10-2
Local Hour
Av
m-2
s-1
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].
0
0.1
0.2
0.3
0.4
SS
T A
mp
°C
a)
-100
0
100
200
300
He
at
Flu
x
W m
s-2
b)
Net Heat Flux
SWR
-LWR
-Sensible HF
-Latent HF
-10
-5
0
E-P cm
c)
-12
-6
0
6
12
Win
d
m s
-1
d)
UD
ep
th (
m)
e) 0
20
40
60
80
m s
-1
-1
-0.5
0
0.5
1
VD
ep
th (
m)
f) 0
20
40
60
80
m s
-1
-1
-0.5
0
0.5
1
Lo
g1
0(S
h2)
De
pth
(m
)
g) 0
20
40
60
80
Log
10(s
-2)
-6
-4
-2
Lo
g1
0(N
2)
De
pth
(m
)
h) 0
20
40
60
80
log
10(s
-2)
-6
-5
-4
-3
-2
Sh
2 red
De
pth
(m
)
i)
Nov Dec Jan Feb Mar Apr May Jun
0
20
40
60
80
! 1
0-3
(s-2
)
-4
-2
0
1
Normal Trades Warm Period
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.
0
2
4
6
8
10
m s
-1
220
240
260
280
300
W m
-2
J F M A M J J A S O N D0.1
0.15
0.2
0.25
0.3
0.35
°C
23°W Climatology
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
