Research over the last two decades has shown that the relation between ocean surface temperature and climate patterns, as defined by jet stream patterns, is a major driver.

The various peaks and throughs define regions of high and low pressure which look like this, when mapped on to the surface:

Abnormal regional weather is generally the result of seasonal displacement in "normal" jet stream patterns. In general, the position of the jet stream determines the local weather for a period of 3-5 days.

In the summer, the Pacific Northwest is usually dominated by a very stable, high pressure ridge, resulting in a north wind and dry, sunny conditions.

August 12, 2007

The effect of the oceans, however, is to change the placement of the jet stream pattern for muliple reasons:

• Oceans store heat and release it later, and often in a different place.

• The oceans transport heat in amounts comparable with atmospheric transport. The top 2.5 metre depth of water can store as much heat as the entire atmosphere. This is the fundamental physics of heat transport. If your to be climate literate, this is what you should know

• Because of the high heat capacity of water, oceans take a long time in the seasonal cycle to warm up and reach their maximum warmth around Sept 10 (not, suprisingly, the peak of the Hurricane Season).

• One consequence of the ocean's ability to absorb more heat is that when an area of ocean becomes warmer or cooler than usual, it takes much longer for that area to revert to "normal" than it would for a land area it is this mechanism that influences jet stream patterns.

• When heated, the ocean responds by storing some of the heat and by increased evaporation. This increased evaporation has profound effects on the atmosphere and on climate.

• Horizontal currents, particularly those moving north or south, can carry warmed or cooled water as far as several thousand kilometres. The displaced water can then warm or cool the air which effects the nature of the jet stream.

Ocean Cycles:

The most well known cycle is El Nino/La Nina. These events are driven by changes in the trade wind pattern that ultimately determine the equatorial surface temperature (warmer - El Nino; cooler La Nina) of the oceans.

The formation of El Nino is linked with the cycling of a Pacific Ocean circulation pattern known as the El Nino Southern Oscillation or ENSO. In a normal year, low atmospheric pressure develops over northern Australia and Indonesia, with an anticyclone or high pressure over the equatorial Pacific. Consequently, trade winds over the Pacific move from east to west. The easterly flow of the trade winds carries warm surface waters westward bring convective storms to Indonesia and coastal Australia. Along the coast of Peru and Ecuador, cold bottom water wells up to the surface to replace the warm water that is pulled to the west.

In an El Nino year, low pressure over northern Australia is replaced by high pressure, whilst air pressure falls over large areas of the central Pacific and along the coast of South America. This change in pressure pattern causes the normal easterly trade winds to be reduced and sometimes reversed. This allows warm equatorial water to flow or "slop" back eastward across the Pacific, and accumulate along the coastlines of Peru and Ecuador. The warm water off the equatorial South American coast cuts off the upwelling of cold deep ocean water. The unusually warm water on the eastern side of the Pacific creates large moisture-laden convection currents in the atmosphere, leading to drastically increased rates of precipitation and flooding. In contrast, the high pressure and cooler waters around Australasia preclude the formation of major rain storms, leading to drought, and sometimes extensive bush fires as the vegetation dries up.

After an El Nino event, weather conditions usually return back to normal. However, in some years the easterly trade winds can become extremely strong and an abnormal accumulation of cold water can occur in the central and eastern Pacific. T his event is called a La Nina. The cold La Nina events sometimes (but not always) follow El Nino events.

Since 1950 (see below) there have been 18 El Nino Events and 12 La Nina events as defined by the areas above the red line or below the blue line

El Nino has a pattern of repeatibility on time scales of 3-5 years.

El Nino/La Nina is a subset of the larger Southern Oscillation Index where negative values indicate El Nino and Positive values indicate La Nina. We are currently in a relatively strong El Nino Period:

During El Nino, the PNW is usually characterized by the maintenaince of the high pressure ridge so its warmer and drier.

During La Nina, the PNW is subject to a trough and therefore cooler and wetter conditions. The La Nina season of 2007/2008 is similar to the strong La Nina PNW conditions of 1970-72 these years produce relatively large snowpacks.

Note the cluster of strong La Nina events in the 1970s which is statistically anamolous (relative to 100 year baseline). During that time there was significant snowpack in the Cascades and quite wet weather in Western Washington. But this period did not represent a "normal climate".

However, the multi-decadel oscillations seen in the Pacific Northwest Index (see above) are not consistent with the El Nino, La Nina timescales.

Enter now the Pacific Decadel Oscillation (PDO) which does have the correct timescale:

We also now have the Aleutian Pacific Index, which is a measure of the mean baromoteric pressure of the winter time Aleutian Low Pressure system. This basically determines the rainfall amounts in the PNW during winter time. Blue is intense low pressure. These variations are closer to the El Nino timescales than the PDO time scales.

Since these cycles have different timescales, there can be occurences of mutual reinforcement or mutual cancellation. That is the biggest driver on annual PNW snowfall.

Of course, there is also an oscillation in the Atlantic called the Altantic Mean Oscilliation:

THe PDO + AMO cycles are not in phase:

And these is a major driver of US precipitation Patterns

as well as regional ones (Colorado Drought Index)

Bottom line: Detecting systematic climate change is really quite difficult given these now established multi-decadel baseline shifts in regional climate patterns. We can now identify this from tree ring data going back at least 500 years: