Long-Term Changes in Northern and Southern

Annular ModesPart I: Observations

Christopher L. Castro

AT 750

Trends in Annular Modes: Outline

• Part I: Observations

How have annular modes changed in the last decades and how is this related to observed climate change?

Castro

• Part II: Mechanisms

Ozone and/or greenhouse gas forcing?

Tropical Pacific SSTs?

Ahlgrimm

Introduction: Highlights of Reviewed Papers

NAO Perspective (Hurrell 1995)

• NAO to its high phase over recent decades and corresponding climate changes in North America and Europe.

NAM (or AO) Perspective (Thompson et al. 2000)

• Similar high index polarity of the AO, most apparent during active (winter) season. • AO explains most of long term trends in atmospheric observations. • Relationship between AO variability and ozone trends.• Consider also stratospheric variability.

SAM Perspective (Thompson and Solomon 2002)

• High index of SAM related to photochemical ozone loss and corresponding climate change southern hemisphere.

NAO Perspective: Index Trends

NAO index shows downward trend from 1940s to early 1970s, but sharp increase from late 1970s to present

Change in NAO has lead to higher mean SLP over Europe and lower over the pole. This, not surprisingly, looks a lot like the AO.

Also, a general pattern of cooling over oceans and warming over continents (not shown).

NAO Perspective: Large-Scale Climate Changes

Pacific

• Deepened and eastward-shifted Aleutian low• Southward shift in storm tracks and synoptic eddy activity• Changes in sensible and latent heat fluxes

Most likely these changes are related to ENSO-like decadal variability in the Pacific governed by tropical Pacific SSTs.

Atlantic

• Northward shift of jet stream and storm track• Warm over Northern Europe, cold over Greenland and Eastern Mediterranean• Continental scale changes in European rainfall

Atlantic changes are not related to tropical SSTs, in the Pacific or Atlantic.

NAO Perspective: Moisture Flux and Divergence in Atlantic Sector

High NAO index winters: Strong jet stream and storm activity in northern Europe

Low or normal NAO index winters: Weaker jet stream, further south

Coherent changes in the moisture flux divergence and rainfall over Europe. High NAO favors more rainfall in northern Europe and drier around the Mediterranean.

AO Perspective: Index Trends

Using various definitions of the AO based on surface air temperature, precipitation, and sea level pressure, basically same trends as Hurrell’s NAO index.

Again note the dramatic increase in the AO after approximately late 1970s.

WHAT IS GOING ON HERE??!!

AO Perspective: Wintertime Trends in

SLP and SAT

As with NAO index:

Strong warming over continentsDecrease in SLP near pole

Most trends explained by AO index.

Residual reflects ENSO-like decadal variability in the Pacific.

NOTE: Look at winter (active) season of the AO here because that is when trends are most statistically significant.

AO Perspective: Precipitation Trends

Same trends as observed for the NAO in Europe, except now note the changes over North America.

The North American rainfall trends show drying in Canada and northern US and wetter in the southern US. These trends are consistent with ENSO-like decadal variability in the Pacific and are not related to shift in AO.

AO Perspective:

Stratosphere Trends

Stratosphere trends show an equivalent barotropic anomaly.

AO trend explains 40% of stratospheric cooling and 70% of geopotential height falls poleward of 60 N.

Thompson et al. also use a AO* index based on leading PC of zonal mean geopotential height from 60-90 N, and they essentially get same results as with AO index.

AO Perspective: TOMS Ozone and

Tropopause Pressure

TOMS Ozone and tropopause pressure are closely related enough that one may be used as a proxy for the other.

AO index trends associated with decreased ozone concentration overall in the northern hemisphere over recent decades.

Regionally, increases in ozone where tropopause sinking (Greenland) and decreases where rising (Siberia).

AO Perspective: Long Term Ozone Trends

The plot of inverted ozone concentration at a long term Swiss observing site versus trends in the AO and European surface air temperature.

Ozone decreases are being observed in the most populated, industrialized areas of the world over the last few decades, not just Antarctica. The same trend presumably exists in the US.

AO Perspective: Latitudinal Depth Profiles of Wind,

Temperature Trends

Strengthening of westerly flow poleward of 45 N which extends all the way to the surface. Weakening of westerlies south of 45 N.

Cooling maximized in the lower polar stratosphere, warming in subtropical upper tropopshere. Suggests weakening of meridional circulation in lower stratosphere.

MMC changes in the same sense as the Ferrel Cell.

Hadley cell strengthened and extends farther north.

AO Perspective: Summer Trends (SLP and Ozone)

Summer trends are of the same sign, but not as strong or statistically significant. Changes in summer ozone reflect the memory of the previous winter season.

SAM Perspective: Latitudinal Depth Profiles of Temperature and Geopotential Height Trends

Similar trend as with NAM with some important differences:

Strongest signal is not in winter but (SH) summer, corresponding the time when flow allows for propogation of waves from the troposphere (recall earlier Thompson et al. 2000 paper). A weaker secondary maximum in fall.

Trends of strengthened polar vortices in periods of stratosphere-troposphere coupling are reducing the sensitivity to breakdown of the polar vortex in both hemispheres.

SAM Perspective: Ozone

Variation in spring ozone (when photochemical decay most rapid) is felt through the summer months through consistent variation in the SAM.

SAM Perspective: Geopotential Height and Temperature Trends

As in NH, majority of atmospheric trends in recent decades explained by variation in the annular mode.

These trends are related to coherent climate changes observed in the southern hemisphere—in particular Antarctica and southern South America.