Lecture 11 Picking up pieces from previous lectures + – result of surface force balance – scales of motion – mesoscale systems: sea breeze, land breeze – synoptic to planetary scale: monsoons – seasonal cycle Another look at Rossby waves

Convergence/divergence Due to frictional force at sfc, air flows toward low pressure (across isobars) close to the sfc. At center of the low, have convergence Upward motion close to center Divergence (outward motion) at upper levels above the low Opposite applies to flow in highs

Low pressure areas are cloudy & wet, high pressure areas have clear calm weather

Scales of motion and importance of forces Microscale: < 1km in size, PGF, CENTF, FF important, not Coriolis (scale is too small) Mesoscale 1km<L< 100km Coriolis gets more important for the 100km range Synoptic scale 1000km (geostrophic) Planetary scale > 1000km (geostrophic)

Sea breeze (day), land breeze (night) Land heats more quickly than sea surface under sunny conditions. Creates a PGF from ocean to land at low levels, opposite direction at upper levels Important for precipitation especially when it affects islands or peninsulas (e.g., Florida)

Sea breeze circulation. Development from early morning to miday

Idealized Pressure Belts & Prevailing Winds Effects of continents on idealized circ. patterns

Macroscale Global Circulation

Seasonal variations ITCZ Position of the ITCZ in January and July. Notice that variations are greater over continents than oceans

Seasonal variations (not just a shift) More seasonal variations in continental areas than over the oceans The subtropical highs are particularly strong in summer over the western ocean basins In summer have thermal lows in subtropical continental regions In winter the subtropical highs move east and south and are less well defined In winter, the subpolar lows are strong, Icelandic and Aleutian lows.

Seasonal Pressure and Wind Patterns

Monsoons Circulation feature driven by seasonal differences in heating of land/ocean. Seasonal shift in the direction of wind. This results in different precipitation patterns depending on season. Indian monsoon Southeast Asian monsoon African monsoon Australian North American monsoon

Monsoons continued In spring and summer, heating of land surfaces more intense than ocean sfc Horizontal pressure gradient from ocean to land (remember the sea breeze) In fall and winter the opposite applies

Monsoon Circulation The Indian Monsoon

The Indian Summer Monsoon Note that the Coriolis force deflects the SH trades to the left and the NH trades to the right. The flow is cross equatorial in the lower troposphere.

North American Monsoon Most of the moisture comes from the Pacific in the form of tropical disturbances

Pressure Belts and Precipitation

January July Distribution of precipitation– compare to the idealized conceptual model

The “thermal wind” change in westerlies w. z

Jet Stream Seasonal variation of the jet stream

Midlatitude Stormtracks In the NH they are concentrated over the two ocean basins, Pacific and Atlantic, guided by the jet maxima over the two ocean basins In the SH form an almost continuous band in midlatitudes Collection of storms in various stages of development, usually cyclogenesis at the entrance and mature storms further on

Rossby Waves

Meridional and Zonal Flow

NCEP / NCAR Reanalysis

Rossby wave mechanism

Linear, dissipative and time dependent theory  absorption. Linear theory appears to work well in explaining observed longitudinal asymmetries, however……….

PV on 350K surface on 4, 5 and 6 July 1979 PV on 350K surface on 16, 17 and 18 Dec 1993

Nonlinear theory Linear propagation from midlatitudes to lower latitudes Waves break as they approach their critical latitude ( u =0 stationary waves) Rearrangement of PV field in the critical layer (advection around closed streamlines) Wave propagation Wave breaking