Regional Scale Variability in Eastern Pacific: Relevance to SPURS-2 Campaign

Janet Sprintall, Scripps Institution of Oceanography

MoorSPICE Cruise, Solomon Sea

SIO La Jolla

Westpac Vietnam

Sorry I can’t join you today but I am on travel!

Wind Forcing

Kessler, W. The circulation of the eastern tropical Pacific: A review, PiO, 2006.

Positive Curl: Upwelling in NH

Note significantmeridional component

• Surface layer variability in the eastern Pacific is influenced by the seasonal migration of the ITCZ (most northerly in ~November).

• In the far eastern Pacific, the ITCZ interacts with wind jets that blow through gaps that produce strong wind stress curl dipoles (+ve on left flank; -ve curl on right) .

• Gap winds are strongest in winter; vary on short (weekly) time scales and generate coastal eddies

Precipitation

Fiedler and Talley. Hydrography of the eastern tropical Pacific: A review, PiO, 2006.

• ITCZ: P>> E with a maximum slightly west of Gulf of Panama

• Also, high river runoff off western slope of Andes

• Also, MJO intraseasonal variability high in winter drives heavy rainfall

• Also, “temporales”, week-long periods of continuous rain but weak winds maybe related to interaction of ITCZ and Caribbean systems.

Near Surface Currents

Sprintall, Kennan, Kim and Niiler, 2009Wind-driven ageostrophic transport in the NECC of the Eastern Pacific., JPO.

Mean surface velocity from drifters 1997-2006

Mean surface velocity from drifters June-July 1997-2006

• Interaction of this local and remote wind forcing results in a complex current system in the far eastern Pacific!

Examples:• the upwelling curl of the ITCZ

drives the eastward flowing NECC that can impact coastal regions particularly during boreal summer.

• wind stress curl from gap winds can change the regional thermocline slope and set up cyclonic flow in Gulf of Panama and Costa Rica Dome

• Western Pacific wind bursts can generate Kelvin waves that impact the Eastern Pacific

Surface Properties: SSS• Similarly, the interaction of local and

regional wind forcing influences the seasonal cycle of SSS and SST in the warm, fresh pool of the far eastern Pacific

• Summer (JAS) rainfall results in a fresh pool that is confined to the coast because of the stronger eastward flowing NECC. In winter, the gap winds kick in and drive upwelling of saltier water so that the fresh pool is destroyed in spring (AMJ).

• Note small “bumps” west of Gulf of Panama with opposing SSS sign. These are persistent features, but it is unclear if caused by interaction of regional currents (NECC/SEC) and/or gap wind forcing and/or correspond to precipitation maximum (see previous slide)Fiedler and Talley, 2006.

Hydrography of the eastern tropical Pacific: A review, PiO.

Surface Properties: Barrier Layers• In eastern Pacific boundary region

the mixed layer (MLD) is extremely near to surface, and 10-20 m shallower than thermocline depth (ILD), resulting in formation of barrier layers

• Differences in annual cycles of MLD and ILD cause temporal variations in barrier layer -> stronger during boreal winter, when rainfall is high, but the gap winds also strong (and hence may erode the barrier layer)?

Barrier LayerNDJ

Fiedler and Talley, 2006. Hydrography of the eastern tropical Pacific: A review, PiO. Sprintall and Tomczak, 1992. Evidence of the barrier

layer in the surface layer of the tropics, JGR.

Questions of InterestFocus is on understanding the role of the regional scale forcing and circulation in driving the seasonal patterns of salinity in SPURS-2 area.• How does the regional ocean circulation (i.e. NECC; SEC) impact the

local coastal circulation including eddy variability?• What interplay between regional currents, local upwelling driven by

gap wind and/or regional wind forcing, eddy variability and precipitation causes the distinctive SSS “bumps” in the far eastern Pacific?

• What mix of local (wind-driven upwelling, precipitation, background stratification etc.) and remote (Kelvin waves, regional advection etc.) forcing drives the seasonal variability in barrier layer formation?

Approach• Addressing these links of the regional/large-scale circulation to

the local circulation will consist of analysis of existing remotely sensed (winds, SSS, SST, precip etc.) and in situ (Argo, XBT/CTD etc) data sets to map the climatological local and regional fields

• Field work (CTD, TSG, hull-mounted ADCP and side-mounted high-freq. ADCP to resolve near-surface currents) required to examine the T-S-V characteristics of the upper ocean stratification (incl. MLD, barrier layer etc.) and circulation (incl. the very near-surface wind-driven Ekman transport)

• I look forward to future discussion and interaction• Contact me at jsprintall@ucsd.edu