5:15 PM - 6:30 PM
[ACG06-P07] Phase locking of equatorial Atlantic variability through the seasonal migration of the ITCZ
★Invited papers
Keywords:equatorial Atlantic, phase locking, ITCZ
The equatorial Atlantic is marked by significant interannual variability in sea-
surface temperature (SST) that is phase-locked to late boreal spring and early summer.
The role of the atmosphere in this phase locking is examined using observations, reanaly-
sis data, and model output. The results show that equatorial zonal surface wind anomalies,
which are a main driver of warm and cold events, typically start decreasing in June, de-
spite SST and sea-level pressure gradient anomalies being at their peak during this month.
This counterintuitive behavior is explained by the seasonal northward migration of the
intertropical convergence zone (ITCZ) in early summer. The north-equatorial position of
the Atlantic ITCZ contributes to the decay of wind anomalies in three ways: 1) Horizon-
tal advection associated with the cross-equatorial winds transports air masses of compara-
tively low zonal momentum anomalies from the southeast toward the equator. 2) The ab-
sence of deep convection leads to changes in vertical momentum transport that reduce the
equatorial surface wind anomalies. 3) The cross-equatorial flow is associated with in-
creased total wind speed, which increases surface drag and deposit of momentum into the
ocean.
Previous studies have shown that convection enhances the surface wind response to
SST anomalies. The present study indicates that convection also amplifies the surface
zonal wind response to sea-level pressure gradients in the western equatorial Atlantic,
where SST anomalies are small. This introduces a new element into coupled air-sea inter-
action of the tropical Atlantic.
surface temperature (SST) that is phase-locked to late boreal spring and early summer.
The role of the atmosphere in this phase locking is examined using observations, reanaly-
sis data, and model output. The results show that equatorial zonal surface wind anomalies,
which are a main driver of warm and cold events, typically start decreasing in June, de-
spite SST and sea-level pressure gradient anomalies being at their peak during this month.
This counterintuitive behavior is explained by the seasonal northward migration of the
intertropical convergence zone (ITCZ) in early summer. The north-equatorial position of
the Atlantic ITCZ contributes to the decay of wind anomalies in three ways: 1) Horizon-
tal advection associated with the cross-equatorial winds transports air masses of compara-
tively low zonal momentum anomalies from the southeast toward the equator. 2) The ab-
sence of deep convection leads to changes in vertical momentum transport that reduce the
equatorial surface wind anomalies. 3) The cross-equatorial flow is associated with in-
creased total wind speed, which increases surface drag and deposit of momentum into the
ocean.
Previous studies have shown that convection enhances the surface wind response to
SST anomalies. The present study indicates that convection also amplifies the surface
zonal wind response to sea-level pressure gradients in the western equatorial Atlantic,
where SST anomalies are small. This introduces a new element into coupled air-sea inter-
action of the tropical Atlantic.