North Atlantic dynamical response to high latitude perturbations in buoyancy forcing

Vassil Roussenov, Ric Williams & Chris Hughes

How changes in the high latitude thermohaline forcing communicate over the ocean?

Downstream response at lower latitudes along the western boundary involving:

• fast, wave-like responses propagating against the sidewalls and along the equator

• an intermediate response involving changes in local circulation

• slower advective response

Link with NERC RAPID monitoring programme,

which aims to detect propagating wave signals along the western Atlantic from the bottom pressure and density signals.

WAVE array (Western Atlantic Variability Experiment) is currently under way (Hughes, Marshall, Williams, Meredith & Foden).

POL scientists (Hughes, Meredith, Foden, Pugh) + UKORS staff (Waddington) deployed bottom-pressure recorders and moorings along 2 sections in August 2004 and with WHOI deploying equipment in April 2004 (Toole, Hughes)

Surface layer thickness after an overturning of 10 Sv is switched on at time t = 0 on the northern boundary of the domain (Johnson and Marshall, 2002, JPO)

Using an idealised model, Johnson and Marshall (2002) demonstrate how overturning changes are communicated through the propagation of fast Kelvin and slower Rossby waves.

Background

Kawase (1987) suggested, that the deep water spreading is accompanied by fast Kelvin waves, producing tropical sea surface temperature anomalies.

Is there any evidence of a similar response in more complex models or in the observations?

Model set-up

Model simulations carried out using an isopycnic model (MICOM).

Horisontal resolution is 0.28º (30 km at the equator & 15km at 60N), 6 isopycnal layers. Model initialised from Levitus and forced by annual winds for 44 years. Thermohaline forcing is simulated by relaxation of model isopycnals to Levitus within the northern part of the basin.

Initial adjustment twin experiment.

Twin perturbation experiments during the last 4 years, where interfaces raised 120m/10 days over northern relaxation zone. Model tracer released in the relaxation zone.

Model topography (ETOPO5)

Initial adjustment movie

Model SSH (left) and deep interface pressure (right) anomalies (reference – disturbed fields).

No wind forcing; adjustment to the initial vertical stratification. 2 model years movie, year 2-3.

Twin experiment movie

Model SSH and bottom pressure anomalies (reference – disturbed fields) from the twin experiment forced by annual mean wind forcing. 4 model years movie, year 41-44.

Snapshots

Model snapshots of the sea surface height anomaly and the depth/pressure of a dense isopycnal

• there is a rapid spreading along the western boundary• a slower spreading along the eastern boundary• the SSH and isopycnal height anomalies appear related, but with SSH signals on a slightly broader scale.

• slow advective response, marked by the Labrador tracer

day 5

Year 4 tracer

Year 1

Year 2

Year 3

Year 4

Twin experiment time series

green is default, black is perturbed model run, red is Labrador Sea tracer.

• fast wave response; range of frequencies; wave interactions, modulation

Kelvin waves, coastal Rossby, hybrid? • Interactions between waves and the circulation• an intermediate response involving changes in local circulation• slower deep advection

Propagation of signals along the western boundary

Hovmueler diagram along the western boundary

from years 40-44 (tracer in contours)

Correlation patterns

Correlation of high pass filtered altimetry everywhere with that averaged in the northern NE Atlantic (100 grid point section 35-36 marked in black dots). Places where correlation is not significant at the 95% level are left white (Hughes & Meredith),

Signals correlate on western N. Atlantic and eastern edge of the N. Atlantic

With no wind forcing, there is band of positive correlation all around the edge of the basin. With wind forcing, the correlation pattern is similar, but with an opposite sign on each side of the basin. Thus, the propagation of the anomalies has altered.

Model results show a broadly similar character to the analytical and altimetric diagnostics.

Role of background flow needs to be explored.

Model correlation patterns for SSH without and with a background circulation (left panel, after 1 y of model run with no forcing; right panel, after 44 y of model run with wind forcing).

+

Conclusions

How changes in the high latitude thermohaline forcing communicate over the ocean?

Downstream response at lower latitudes along the western boundary involving:

• fast wave responses propagating against the sidewalls and along the equator

• an intermediate response involving changes in local circulation

• slower advective response

• higher horisontal and vertical model resolution

• precise diagnostics to identify different responses, type of the waves, physical mechanisms

• semi-idealised experiments – real topography, simplified forcing

• real forcing simulations in order to link with and support the RAPID monitoring programme

Future work

The monitoring aims to detect propagating wave signals along the western Atlantic from the bottom pressure and density signals. Thus, the programme aims to understand how

overturning signals are communicated.

Map of deployment along 3 sections (thick line in left panel) and mooring schematic (right panel).