General circulation in the tropics

Outline: Global circulation and tropical regions Winds in low and high levels Hadley Cell and horizontal (trade winds) and vertical movements Concept of convergence of mass

Tropical Circulation in a Global Context Mean surface pressure and winds during DJF Easterlies Easterlies Westerlies Westerlies NE trades NE trades NE trades NE trades SE trades SE trades Westerlies Westerlies

TRADE WINDS ARE EASTERLY WINDS Mean surface pressure and winds during JJA Westerlies Westerlies NE trades NE trades NE trades SE trades SE trades SE trades Westerlies Westerlies

Sketch of the global circulation Polar cell Ferrel cell Hadley Cell Ferrel cell Polar cell

How is the circulation in high levels in January?

How is the circulation in high levels in July?

THE HADLEY CELL NE Trades Low level SE Trades North Pole Fc Radiative cooling High level Subtropical Jet Radiative cooling NE Trades Equator East West Fc High level Subtropical Jet Low level SE Trades Fc=Coriolis Force – perpendicular to the wind – left of the wind SH, Right NH

Now including Ferrel cell and polar cell Subtropical Westerly Jet Polar W Jet

Changes in the Hadley cell The Hadley cells show seasonal variations in their intensity, geographical extent and latitudinal position. They are not restricted to their respective hemispheres but display regular cross-equatorial incursions into the opposite hemisphere Let’s examine the cross section of the atmosphere (vertical components exaggeration) Vertical velocity maximum represent the mean position of the intertropical convergence zone (ITCZ)

Vertical motion Vertical velocity w is defined as p- 4dp z p- 3dp dz An ascending volume (parcel) experiences changes in pressure and density p- 1dp p The hydrostatic equation states that: Where : ρ is the air density and g is gravity

Vertical motion Vertical motions in synoptic systems (such as low pressure systems) are typically on the order of a few centimeters per second. Routine meteorological soundings, however, measure wind speed to an accuracy of only about a meter per second. Thus, in general, the vertical velocity is not measured directly but must be inferred from the fields that are directly measured For this purpose, we use the isobaric coordinate system so that we measure the vertical movement as the total time derivative of the pressure: Where w is the vertical velocity in Cartesian coordinates, and ρ is the density of the air z z w < 0 and ω >0 w >0 and ω<0

Vertical velocity (Omega) – Negative (Positive) values indicate ascending (descending) movement Annual

The vertical profiles for these two seasons reflect the seasonal behavior of the Hadley cell In the Southern Hemisphere summer, the strongest ascending motion occurs between 10 – 20S with maximum vertical velocities centered on 15S. During the Northern Hemisphere summer (JJA) the zone of maximum vertical velocities is displaced north of the equator with strong ascending motion centered on 5N while low latitudes of the SH become dominated by sinking motions. These are especially intense around 10 – 15S. Poleward an intense SH winter anticyclonic system almost cover continuously the entire SH. Trade winds emanate from these centers

Decomposition of horizontal winds into zonal and meridional components Y NORTH Any wind can be treated as a vector that can be decomposed into two components: zonal (‘x’) and meridional (‘y’) + Meridional component From South to North: SOUTHERLY AND ARE CONSIDERED POSITIVE Zonal component From West to East: WESTERLY AND ARE CONSIDERED POSITIVE X + - WEST EAST - SOUTH

Decomposition of horizontal winds into zonal and meridional components Any wind can be treated as a vector that can be decomposed into two components: zonal (‘x’) and meridional (‘y’) Y NORTH + Meridional component From North to South: NORTHERLY AND ARE CONSIDERED NEGATIVE X + - WEST EAST Zonal component From East to West: EASTERLY AND ARE CONSIDERED NEGATIVE - SOUTH

In other words… Northeasterly trades Southeasterly trades Have negative meridional (Northerly) component and negative (Easterly) zonal component Southeasterly trades Have positive meridional (Southerly) component and negative (Easterly) zonal component NE TRADES SE TRADES Easterly Northerly Southerly Easterly

Meridional profile of the horizontal wind velocities: mean speed of meridional wind components. Negative values are northerly, positive values are southerly July mean (1979-2009) January mean (1979-2009)

Training concepts in group The figure below shows the long term mean of the meridional component of the wind in October. What latitudes are dominated by northerly and southerly winds? What height (in mb or hPa) are these winds more intense? Where is the upward/downward branches of the Hadley located? Use arrows to represent wind vectors. How the patterns of winds compare with the winds in July and December?

Annual mean

Meridional profile of the horizontal wind velocities: mean speed of meridional wind components for a range of latitudinal bands. Southerly: positive values Northerly: negative values On an annual basis, poleward of 5N and south of the equator, surface winds are opposite in direction. Between 22N and 5N the winds are northerly (negative) These are the northeast trades of the NH South of 5N to ~ 25S the winds are southerly and flowing towards the north (SE trades of the SH)

Both equatorward flowing streams of air are the lower branches of the Hadley cells. Geographically, the southeast trades dominate and manage on an annual basis to push across the equator into the northern Hemisphere, which explains the asymmetry in the lower branches of the Hadley cell. Velocities in the lower branches of the Hadley cells decrease in the direction of the equator This deceleration is what is causing convergence in the trade flows. HOW?? = Convergence V2 V3 V1

On annual basis, the depth of flow in the lower branches of the Hadley cells is shallow, only reaching up to approximately 700mb. In the upper atmosphere the poleward returning winds of the upper branches of the Hadley cell are found in both hemispheres

Training concepts in group… The figure below shows a latitude/height cross section of the annual mean OMEGA (vertical velocity in Pa/s). Find regions with maximum ascending/descending motion, and relate these regions to convergence/divergence in low and high levels , high and low pressure systems at the surface in association with the Hadley cell.

Training concepts in group… Here we show two images obtained on October 07/2009 over Northwest Pacific and over the Atlantic and Caribbean. They show a snapshot of the convective activity in both places. Clearly, there are more storms over Northwest Pacific than over the Caribbean region. In 2009, the El Nino phenomena created unfavorable conditions to form storms in the Caribbean region over the entire summer season. Let’s take a look at the implications to the atmospheric dynamics in the region. Northwest Pacific Atlantic and Caribbean

Below are averages of ‘omega anomalies’ during September 2009 from -60S to 60N. Analyze these figures and answer the following questions Omega anomalies over Northwest Pacific Omega anomalies Atlantic and Caribbean How to interpret negative and positive anomalies of ‘omega’? Can you use these figures to show in what region the ITCZ is more active? What level in the atmosphere anomalous ascending movement is observed near the Equator? Any explanation for that? In what latitudes could we expect more subsidence (descending air) comparatively with the average for the month of September?

1000mb Question: 1) find regions with dominant easterly and westerly winds in January and July (draw wind vectors in these regions to help your analysis). What are the main differences between the two seasons (pay attention to the magnitude of the winds)? 2) What is approximately the region dominated by trade winds?

100mb Question: 1) find regions with dominant easterly and westerly winds in January and July. What are the main differences between the two seasons? 2) Locate the approximate region of the subtropical jets

Question: These figures show the magnitude of the zonal and meridional winds (annual mean climatology) Compare the magnitudes of the components and discuss where meridional and zonal components are more important. Try to relate these observations to the existence of the Hadley and Ferrel cells.