skip to main content


Title: Tropical Instability Waves and Wind‐Forced Cross‐Equatorial Flow in the Central Atlantic Ocean
Abstract

Based on velocity data from a long‐term moored observatory located at 0°N, 23°W we present evidence of a vertical asymmetry during the intraseasonal maxima of northward and southward upper‐ocean flow in the equatorial Atlantic Ocean. Periods of northward flow are characterized by a meridional velocity maximum close to the surface, while southward phases show a subsurface velocity maximum at about 40 m. We show that the observed asymmetry is caused by the local winds. Southerly wind stress at the equator drives northward flow near the surface and southward flow below that is superimposed on the Tropical Instability Wave (TIW) velocity field. This wind‐driven overturning cell, known as the Equatorial Roll, shows a distinct seasonal cycle linked to the seasonality of the meridional component of the south‐easterly trade winds. The superposition of vertical shear of the Equatorial Roll and TIWs causes asymmetric mixing during northward and southward TIW phases.

 
more » « less
Award ID(s):
2048631
NSF-PAR ID:
10444120
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Volume:
49
Issue:
19
ISSN:
0094-8276
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Equatorial Internal Wave Experiment observations at 0°, 140°W from October 2008 to February 2009 captured modulations of shear, stratification, and turbulence above the Equatorial Undercurrent by a series of tropical instability waves (TIWs). Analyzing these observations in terms of a four‐phase TIW cycle, we found that shear and stratification within the deep‐cycle layer being weakest in the middle of the N‐S phase (transition from northward to southward flow) and strongest in the late S phase (southward flow) and the early S‐N phase (transition from southward to northward flow). Turbulence was modulated but showed less dependence on the TIW cycle. The vertical diffusivity (KT) was largest during the N (northward flow) and N‐S phases, when stratification was weak, despite weak shear, and was smallest from the late S phase to the S‐N phase, when stratification was strong, despite strong shear. This tendency was less clear in turbulent heat flux because vertical temperature gradients were small at times whenKTwas large, and large whenKTwas small. We investigated the dynamics of shear and stratification variations within the TIW cycle by using an ocean general circulation model forced with observed winds. The model successfully reproduced the observed strong shear and stratification in the S phase, except for a small phase difference. The strong shear is explained by vortex stretching by TIWs. The strong stratification is explained by meridional and vertical advection.

     
    more » « less
  2. Abstract

    Tropical instability waves (TIWs) are identified in three multiyear equatorial mooring records in Pacific and Atlantic cold tongues to evaluate how TIWs modulate turbulence. At 0°, 140°W in the Pacific, TIWs are present in 43% of observations, and are associated with elevated vertical shear and a 40% average increase in turbulence dissipation rates (ϵ) above the Equatorial Undercurrent. Zonal shear is greatest when currents are southward while buoyancy is greatest later in the TIW cycle, leading to greater potential for instability and elevated turbulence before and during the southward flow maximum. This suggests that TIW vortex stretching contributes to enhanced shear and turbulence. In the Atlantic, TIWs are found in 38% of observations at 0°, 23°W and 16% of observations at 0°, 10°W. TIWs at 23°W increaseϵby 18% where turbulence is likely modulated by vortex stretching and, near the surface, by the seasonally wind‐forced equatorial roll. At 23°W and 140°W, TIWs with strong meridional velocity fluctuations are associated with the strongest turbulence. Contributions of seasonal variations are removed by considering only periods when TIWs are climatically active. During these periods, mean values ofϵin the presence of strong TIWs are elevated by 61% at 140°W, 29% at 23°W, and 36% at 10°W. At 10°W, where our identification scheme may include wind‐forced oscillations in the same frequency band, increases inϵare not consistent in the presence of TIWs and do not contribute significantly to multiyear averages.

     
    more » « less
  3. Abstract

    We used Fabry‐Perot Interferometer (FPI) observations at Jicamarca, Nasca, and Arequipa, Peru, from 2011 to 2017 to study the nighttime zonal and meridional disturbance winds over the Peruvian equatorial region. We derived initially the seasonal‐dependent average thermospheric winds corresponding to 12 hr of continuous geomagnetically quiet conditions. These quiet‐time climatological winds, which are in general agreement with results from the Horizontal Wind Model (HWM14), were then used as baselines for the calculation of the disturbance winds. Our results indicate that the nighttime zonal disturbance winds are westward with peak values near midnight and with magnitudes much larger than predicted by the Disturbance Wind Model (DWM07). The premidnight equinoctial and June solstice westward disturbance winds have comparable values and increase with local time. The postmidnight westward disturbance winds decrease toward dawn and are largest during equinox and smallest during June solstice. The meridional average disturbance winds have small values throughout the night. They are northward in the premidnight sector, and southward with larger (smaller) values during December solstice (equinox) in the postmidnight sector. We also present observations showing that during the main and recovery phases of the April 2012 and May 2016 geomagnetic storms the zonal disturbance winds have much larger magnitudes and lifetimes (up to about 48 hr) than suggested by the HWM14. These observations highlight the importance of longer‐term disturbance wind effects. The large and short‐lived (about 2 hr) observed meridional wind disturbances are not reproduced by current climatological empirical models.

     
    more » « less
  4. Abstract

    Recent sea surface height (SSH) trends in the South Pacific are substantially greater than trends in the North Pacific. Here, we use the Estimating the Climate and Circulation of the Ocean Version 4 Release 4 ocean state estimate and the Ocean Reanalysis System 5 to identify the forcing and mechanisms underlying that meridional asymmetry during 2005–2015. Thermosteric contributions dominate the spatial structure in Pacific SSH trends, but contributions from local surface heat fluxes are small. Wind stress trends drive a spin‐up of the South Pacific subtropical gyre and a northward shift of the North Pacific subtropical gyre. A reduced gravity model forced with reanalysis winds qualitatively reproduces the meridional seesaw in sea level, suggesting that asymmetric trends in subtropical wind stress drive a cross‐equatorial heat transport. A reversal in forcing associated with this process could impact near‐term rates of coastal sea‐level change, particularly in Pacific Island communities.

     
    more » « less
  5. Abstract

    The North Equatorial Countercurrent (NECC) simulated by a coupled ocean‐atmosphere model and its oceanic component have been investigated and compared against oceanographic observations. Coupled model simulations using the Community Earth System Model version 2 are compared against ocean‐ice simulations forced by the second phase of the Coordinated Ocean‐ice Reference Experiments (CORE) data set. The modeled circulation biases behave differently to the west of and to the east of 120°W: the CORE‐forced ocean model largely underestimates the NECC transport to the west and the coupled model underestimates it to the east. Further analysis suggests that the surface wind stress and its curl is the most important forcing term for correctly simulating the NECC in both models. West of 120°W, the NECC biases in the ocean model are attributed to the southward movement of the maximum easterly trade winds in the Northern Hemisphere and the associated wind stress curl (WSC) pattern; east of 120°W, the NECC biases in the coupled model are attributed to the weak northward cross‐equatorial winds and southwestward gap winds, which lead to a weak WSC gradient at the latitude of NECC. Further analysis confirms that the WSC biases comes mainly from the zonal wind bias, which may in turn relate to the protocol of CORE‐II of adjusting reanalysis winds toward satellite data, which include the relative wind effect.

     
    more » « less