# Presentation: 394455

## The 3-d Structure of the Tropical Cells in the Central Equatorial Pacific Ocean

- Renellys C. Perez and William S. Kessler
- NOAA/PMEL

-    Observations of the TCs and TIWs
-    Zonal wind anomaly experiments
-    Large-scale ocean response
-    Ageostrophic response along 140W
-    Linearity of ageostrophic response
-    Ongoing Work: Model-observation comparisons of TCs

- Perez and Kessler (2008, accepted by *JPO*)

## Observed Mean at 136W

- Mean circulation from ADCP transects (reproduced from Johnson et al. 2001)

- Nine years of shipboard ADCP data
- Near-surface currents linearly extrapolated from 20m - 30m shear
- Polynomial fit used to estimate mean u
- Averaging over 75 degrees longitude to estimate mean v
- Mean w estimated by downward integral of continuity equation (additional smoothing of v in 2 degree latitude bins)

*Gray shading = positive*

## Observed Mean at 136W

- Mean circulation from ADCP/CTD transects (reproduced from Johnson et al. 2001)

- Despite large error bars and smoothing, mean picture of TCs:
- Equatorial upwelling max = 1.6 m/day
- Surface poleward flow (-9 cm/sec south, 13 cm/sec north)
- Off-equatorial downwelling (-0.8 m/day south, -1.4 m/day north at 50 m)
- Subsurface equatorward flow (5 cm/sec south, -4 cm/sec north)

*Gray shading = positive*

## Observations of TC Variability at 0N, 140W

- Figure from Lyman et al. (2007)

- Poleward divergence and equatorial upwelling strongest in late boreal summer to fall when SE Trades were strong (Weisberg and Qiao 2000)
- TIW fluctuations (±50 cm/sec for v, ±20 m/day for w) >> TCs (WQ)
- TIW v can exceed ±75 cm/sec (Kessler et al. 1998; Kennan and Flament 2000; Lyman et al. 2007)

- O(±75 cm/sec)

## Wind-Driven Variability along 140W

- SE Trades strongest in fall and weakest in spring
- Ekman theory does not explain TC asymmetry (and blows up at equator)
- Add linear drag to momentum equations (e.g., Wittenberg 2002) for an equatorially-modified Ekman model

*Risien et al. (2008) QuikSCAT climatology*

## Wind-Driven Variability along 140W

*Risien et al. (2008) QuikSCAT climatology*

- V

- w

## Numerical Model

- Modular Ocean Model (MOM4; Griffies et al. 2003)
- Similar to OM3.1 global ocean implementation (Griffies et al. 2005)
- IPCC model
- KPP vertical mixing scheme
- Neutral physics
- Some important modifications:
- 120oE to 60oW, 40oS to 40oN
- 5/8o x 1/3o (stretches to 1o at 40o latitude) x 49 z-levels
- Constant anisotropic Laplacian viscosity
- QuikSCAT wind climatology (Risien et al. 2008)
- OAFlux/ISCCP heat flux (Zhang et al. 2004; Yu and Weller 2007)
- SST and SSS damped to climatology
- T, S restored to climatology in 15o sponge layer

## Model vs. Observed Mean

- Mean circulation from ADCP/CTD transects (reproduced from Johnson et al. 2001)

*Gray shading = positive*

- Model v

- Model u

- Model w

## Zonal Wind Anomaly Experiments

- Weakened easterly Trades
- Wind patch centered on equator, 140oW
- 15o nominal zonal width, A=0.025 N/m2
- 8 day ramp-up, 45 day constant anomaly, 8 day ramp-down

## Zonal Wind Anomaly Experiments

- Wind Patch

## Zonal Wind Anomaly Experiments

- Weakened easterly Trades
- Wind patch centered on equator, 140oW
- 15o nominal zonal width, A=0.025 N/m2
- 8 day ramp-up, 45 day constant anomaly, 8 day ramp-down
- Different phases of annual cycle and TIW regimes
- Weak Trades, TCs, TIWs: Spring/Summer
- Strong Trades, TCs, TIWs: Fall, Winter
- Anomalous response = weakened Trades run - climatology run
- 15o zonal filter removes anomalous TIWs

## Spring/Summer: Large-Scale Ocean Response

- Eastward anomaly
- Weakened zonal shears

- (Anti)symmetrically
- spins down TCs

- Little change east of patch

- Kelvin wave

- Wind Patch

**u****L****(5m)**

**v****L****(5m)**

**w****L****(60m)**

- 45-day mean of near-surface anomalies

## Fall: Large-Scale Ocean Response

**u****L****(5m)**

**v****L****(5m)**

**w****L****(60m)**

- Eastward anomaly
- Weakened zonal shears

- Asymmetrically
- spins down TCs

- Large modification east of patch

- Kelvin wave

- Wind Patch

- 45-day mean of near-surface anomalies

## Ageostrophic vs. Equatorially-Modified Ekman Response

- Ageostrophic response = anomalous currents - geostrophic currents
- Geostrophic currents adjusted to avoid equatorial singularity (e.g., Picaut and Tournier 1991)
- Horizontal currents integrated over upper 60m, w evaluated at 60m

*Gray shading = positive*

- 45-day mean of anomalies along 140W

## Equatorially-Modified Ekman Response

- For purely zonal wind anomaly, equatorially-modified Ekman transports and vertical velocity simplify to

## Ekman Response to Weakened Easterly Trades (140W)

- Equatorward
- flow

- Eastward
- flow

- Down

- Up

- Up

## Spring/Summer: Mean Ageostrophic TCs (140W)

- Ageostrophic response (green) exceeds opposing geostrophic response (red)
- Downwelling/upwelling dominated by convergence/divergence of meridional transport
- In spring/summer, ageostrophic TCs similar to equatorially-modified Ekman model (black)

## Fall: Mean Ageostrophic TCs (140W)

- Ageostrophic response (green) exceeds opposing geostrophic response (red)
- Downwelling/upwelling dominated by convergence/divergence of meridional transport
- In fall, ageostrophic TCs equatorially asymmetric with sharp upwelling peak near 4N

## Spring/Summer: Ageostrophic TC Spin-Down (140W)

- EOF structure of VAG and wAG versus equatorially-modified Ekman model
- VAG (green) vs. V (black)
- wAG (blue) vs. w (black)
- EOF time series versus wind anomaly time series (black)

## Fall: Ageostrophic TC Spin-Down (140W)

- EOF structure of VAG and wAG versus equatorially-modified Ekman model
- VAG (green) vs. V (black)
- wAG (blue) vs. w (black)
- EOF time series versus wind anomaly time series (black)

## Linearity of VAG

- Strengthened Trades (blue) vs. weakened Trades (red)
- EOFs highly correlated with wind anomaly time series
- Within a given season, response nearly linear
- Large interseasonal differences, enhanced asymmetry and deviation from Ekman in fall/winter

## Linearity of wAG

- Strengthened Trades (blue) vs. weakened Trades (red)
- In spring/summer, response is nearly linear and well correlated with wind anomaly time series
- In fall/winter, sharp peaks with small meridional scales and lower correlation with wind anomaly time series
- Nonlinear in fall/winter

## Choice of rs

| rs-1 (days) computed from: |  |  |  |  |  |
| --- | --- | --- | --- | --- | --- |
| SEASON | (1) | (2) South | (3) South | (2) North | (3) North |
| Spring | 1.99±0.06 | 2.14±0.19 | 2.57±0.09 | 1.65±0.22 | 2.78±0.03 |
| Fall | 1.67±0.26 | 2.00±0.24 | 2.35±0.09 | 1.29±0.06 | 3.78±0.24 |
| Winter | 2.01±0.06 | 1.65±0.38 | 2.41±0.17 | 1.29±0.06 | 3.26±0.31 |

- (1)

- (2)

- (3)

## Summary and Conclusions

- Weakening Trades in any season rapidly weakens TCs, decreases shear of zonal currents, reduces TIW amplitudes
- In fall and winter (background TCs and TIWs seasonally strong), enhanced equatorial asymmetry in anomalous TCs and evidence of nonlinear rectification by modified TIWs east of patch
- Anomalous ageostrophic TC response is similar to equatorially-modified Ekman solution with rs = (2 day)-1
- Results essentially linear within a given season, with relatively large interseasonal differences due to nonlinear interactions with background TCs and nonlinear rectification by TIWs
- Cross-equatorial velocity measurements with fine meridional and temporal sampling are needed to resolve both the TCs and TIWs

## Ongoing Work: Double Off-Equatorial Upwelling Max?

- Vertical velocity from 15 consecutive years of OSCAR u, v at 30m
- Top: w(140W)
- Bottom: w(125W)
- Evidence of off-equatorial upwelling maxima
- Large differences with TIW zonal scales

## Ongoing Work: Double Off-Equatorial Upwelling Max?

- Top: v(125W)
- Bottom: dV/dy(125W)
- No evidence of equatorial convergence
- Meridional spacing of moorings too coarse
- Ongoing work: reanalyze shipboard ADCP data with these features in mind

## Ongoing Work: Analysis of Shipboard ADCP data

- From simple averaging of v along repeat transects (black line), mean equatorward flow near equator along 140W
- In “frontal coordinates” (blue line), mean flow towards front along 140W
- Large differences in mean v with TIW zonal scales

## Extra Slides

## Geostrophic Adjustment

- Geostrophic currents away from and along equator given by:

- Add adjustment terms that ensure continuity of currents across the equator and preserves the non-singular solution at the equator.

## Spring/Summer: 45-day Mean

## Fall: 45-day Mean

## Spring/Summer: 45-day  of Small-Scale Anomaly

**Std of u****S****(5m)**

**Std of v****S****(5m)**

**Std of w****S****(60m)**

- Weak anomalous TIWs,
- Little rectification onto mean flow

- TIWs modified by Kelvin wave

- Weak background TIWs

- Wind Patch

## Fall: 45-day  of Small-Scale Anomaly

- Strong anomalous TIWs,
- Large rectification onto mean flow

- TIWs modified by Kelvin wave

**Std of u****S****(5m)**

**Std of v****S****(5m)**

**Std of w****S****(60m)**

- Strong background TIWs

- Wind Patch

## Ongoing Work: Double Off-Equatorial Upwelling Max?

## Ongoing Work: Double Off-Equatorial Upwelling Max?

- Possible to compare contribution of meridional gradient of v (dv/dy) to w with recent TAO data
- Test mooring at 2N, 140W during 2004-2005
- Cross-equatorial current measurements during 2006-2007
- No evidence of minimum (maximum) in v north (south) of equator in TAO

## Ongoing Work: Double Off-Equatorial Upwelling Max?

- No evidence of equatorial convergence in TAO
- However, meridional spacing of moorings still too coarse to rule out off-equatorial maxima
- Ongoing work: reanalyze shipboard ADCP data with these features in mind (with Meghan Cronin and Billy Kessler)