Core Lecture 3 - Moving water vertically, upwelling, and ENSO

Does salinity always increase w depth

• if precipitation and river input dominate, salinity increases with depth (eg. NE pacific)

• if evaporation dominates, the surface can be very salty and salinity can decrease with depth (North Atlantic)

why don’t icebergs drift in the direction of the wind

• go 100s of m beneath surface → affected by more than just wind

• ekman spiral

ekman spiral

• water layers interact with each other through friction

• coriolis deflects surface currents 45deg to right of wind

• friction between deeper layers reflects them further right - spiral

• effect weakens with depth

• integrated over spiral, net water transport is 90deg to right of wind in NH

upwelling

upward transport of water which brings deep cold water up to surface

downwelling

downward transport of water which mixes gases dissolved in surface layer into deep ocean interior

where does upwelling occur

where surface waters diverge

main scenarios of upwelling

• along north-south oriented west coasts of continents with equatorward winds

• in the open tropical ocean along the equator

coastal upwelling (NH)

• as ekman transport moves surface water to west, water upwells from below to replace it

• wind from north

coastal downwelling (NH)

• polar winds blowing along west coasts in NH results in downwelling

• wind from south

conditions for coastal upwelling and downwelling in SH

• reversed

• due to coriolis force

upwelling seasonal variation off vancouver island

• spring/summer - upwelling favourable winds

• fall/winter - downwelling favourable winds

four major eastern-boundary upwelling systems

• NH

  • california current

  • canary current (NW africa)

• SH

  • benguela current (SW africa)

  • Humboldt current (peru)

are the four permanent upwelling zones equal

• share basic physical quantities:

  • n-s oriented coastline

  • predominantly equatorward upwelling-favourable winds

  • on western sides of continents

• differ in terms of:

  • size of upwelling zone

  • upwelling intensity

  • nutrient concentration

  • biological productivity

where does upwelled water come from

• roughly 100-200 m

• significantly colder and usually has higher nutrient concentrations than surrounding surface water

concentration of dissolved inorganic nutrients profile

• generally increase with depth

why is chlorophyll concentration highest near coast

ekman transport causes upwelling of nutrient rich water

equatorial upwelling

• winds blow east to west across equator

• coriolis causes water on north side to bend to right, south side to left

• divergence causes deep water to upwell along equator

normal tropical pacific

• easterly winds push warm surface water westward, creating a warm-water pool

• thermocline is deep in west, shallow in eastern tropical pacific

• upwelling in eastern tropical pacific brings cold, nutrient-rich water to surface

el nino conditions

• trade winds weaken or reverse

• warm water flows eastward along equator

• eastern pacific thermocline deepens and supply of cold, nutrient rich upwelled water is greatly reduced

ENSO

• recurring pattern of variation in wind and SST that starts in tropical pacific

• affects upwelling off coast of south america

• disrupts weather patterns and biological production

two extreme states of ENSO cycle

• el nino: warm phase

• la nina: cold phase

how to look for departures from average conditions

anomaly plots

la nina conditions

• stronger trade winds

• enhanced upwelling

• cooling eastern pacific

• stronger version of normal conditions

relative oceanic nino index (RONI)

• measures how anomalously warm or cold the central equatorial pacific is relative to the tropical oceans as a whole

ENSO variations

• varies in timing, duration, and strength

• el nino and la nina represent the ends of a continuum

• vary in when they occur, but also intensity and duration

ENSO future

• el nino is still strengthening

• >90% chance of very strong event in NH fall and winter 2026-2027

• oct-dec 2025: 75% chance of historic event exceeding previous el ninos since 1950 (RONI >= +2.5%)

• median forecast peak: about +2.7C during its peak between oct-dec, although exact magnitude remains uncertain

el nino effects

• increased risk of drought in some reasons

• increased risk of heavy rainfall and flooding in others

• increased likelihood of extreme heat

• effects vary by region, season, and individual el nino event

what might a very strong el nino mean for vancouver island in 2026-2027

• milder winter

• more rain, less snow

• reduced mountain snowpack

• potential downstream effects