Deep convection over the Gulf Stream

and its potential role in ocean atmosphere coupling

Luke Sheldon (Imperial College London) Supervisor: Arnaud Czaja (Imperial College London)

Motivation:

Minobe et al. 2008 Evidence of deep convection over the Gulf Stream

Deep convection is defined here as up to the tropopause

Motivation:

Minobe et al. 2008 Questions:

• Understand what deep convection is occurring over the Atlantic? Upright? Slantwise?

• How frequently convection is occurring?

• What controls the variability of convection?

• What role could the ocean play in modulating convection?

Method:

• Create a simple diagnostic of deep convection – for both slantwise and upright convection

• Use Era-Interim re-analysis data from 1979-2010

• Verify diagnostics in Era-Interim data and then look at occurrence

Stability to vertical displacements: vertical entropy gradient

Tropopause

Sea surface

z

Entropy Buoyancy frequency

As entropy is conserved, by raising parcel we can see if it would be unstable at it’s new level by comparing entropy with that of environment

Diagnostics Slantwise diagnostic: Upright diagnostic:

Gravitational instability Occurs when no gravitational and inertial instability. Essentially an inertial instability along isentropes

By using SST in Ssurf

it allows ocean-atmosphere coupling to be investigated

Absolute momentum (M):

Upright diagnostic (top) – which involves SST – shows similar sensitivity to convective precipitation as CAPE (bottom)

Average % of winter of that is upright unstable

Upright convection appears to be concentrated in the western boundary currents and south of Iceland In Gulf Stream and Kuroshio This climatology has great inter-annual variability (30% of mean)

First EOF accounts for 30% of variability of upright convection in the N Atlantic This EOF is highly correlated with the NAO (-0.82) EOF unchanged even when a fixed SST is used. Appears the inter-annual variability is coming from the storm track variability

Diagnostics Slantwise diagnostic: Upright diagnostic:

Absolute momentum (M):

Gravitational instability Occurs when no gravitational and inertial instability. Essentially an inertial instability along isentropes

By using SST in Ssurf

it allows ocean-atmosphere coupling to be investigated

Z

Slantwise diagnostic is only calculated when a front is detected and only in the transverse plane (grey) due to 2D nature of slantwise instability

Slantwise diagnostic:

Composite of circulation at fronts

All fronts

Difference between composite of fronts with only slantwise instability and composite for all fronts

Shading shows where significant to 99% confidence

Slantwise unstable fronts have greater upward circulation at the front, and stronger return flow on the cold side

Cold side Warm side

Tropopause

Composite of pressure velocity at fronts

All fronts Fronts with only slantwise instability

Slantwise diagnostic:

Slantwise unstable fronts have greater ascent at the front (front marked by an M surface in white)

Composite of Relative Humidity at fronts

All fronts Fronts with only slantwise instability

Slantwise diagnostic:

Slantwise unstable fronts are more moist (nearly 70% relative humidity along the whole front)

Average % of winter when 2D slantwise diagnostic is satisfied

Average % of winter when 3D slantwise diagnostic is satisfied (the condition that a front must be diagnosed at that time and location is lost)

Slantwise instability concentrated over the western boundary currents ~10 – 15 % in total

Lower Bound

Upper Bound

Ocean-Atmosphere Coupling Fraction of tropospheric stability reduced by decadal SST variability

Decadal SST variability

Sensitivity of surface entropy to SST change

Tropospheric stability

Upright:

Upright & Slantwise:

Amplitude of decadal SST change averaged from 4 SST datasets

Does this match tropospheric sensitivity to SST change (f) ?

Upright:

Slantwise & Upright:

% that tropospheric stability reduced by decadal SST change (f)

Difference: (ie. isolating increased sensitivity to SST by including slantwise instability)

Considering slantwise instability doubles the sensitivity to decadal ocean changes over the western boundary currents

Summary

• Simple diagnostics of deep upright and slantwise instability in Era-interim demonstrate circulation changes consistent with the release of the instability

• These diagnostics show that convection is concentrated over the Gulf Stream in the Atlantic, and as an upper estimate up to 50% of the winter deep convection (of some type) could be occurring over the Gulf Stream

• Decadal variability in the ocean has the potential to reduce the tropospheric instability considerably – slantwise instability significantly increases this sensitivity

Questions?

• What kind of affect is this convection (and it’s variability) potentially having on wider climate?

• With slantwise convection, to what degree are the results we are seeing a product of the resolution – is it just seen as the finer structure is not resolved which when it is produces very different circulation